Engine Arrghhh! Cold Start, Surging/stalling Problem

tca7291 · Jan 29, 2015

Okay, '87 Mustang SD 5 speed. Mods are minimal, off road H, 2.5" cat back, 12* base time, otherwise stock. Pulled codes, only get a code for the thermactor solenoid.

My car runs fine, after warm, but at a cold start (less than roughly 50* ambient), she surges horribly, and will stall out if I don't feather the gas. After running for about 1-2 minutes, she will level out and run properly. I've replaced the ECT, ACT, cleaned S&P shakers, cleaned IAB, checked for vacuum leaks, ect. The O2's look pretty new, and are probably what is making the engine run smooth after they warm up (I believe, correct me if I'm wrong).

I've read/worked through the surging idle checklist, and all seems good, but just still won't idle when cold. She runs like a carburetor is on the engine on cold start (ambient temp. below ~45*), then runs fine after 1~2 minutes.

Thanks in advance!

Noobz347 · Jan 29, 2015

Have you looked at swapping the MAP sensor? Outside of the things that you mentioned, although who can really ever rule out vac leaks.

SD relies on it as heavily as it does O2 sensors during closed loop and even more when open loop. Might also be worth looking at the condition of the line going to it. Not just for dry rot but for collapse.

mikestang63 · Jan 29, 2015

wondering if the egr or thermactor tubes are clogged up with carbon

tca7291 · Jan 29, 2015

@Noobz347 , that was next on my list, replace the BAP sensor. ECT was fried due to a , replaced that (it threw a code), thinking it is temperature related, replaced the ACT with no changes. (Looking back at the surging idle checklist, I was able to test the old ACT, and it was in range, waste of $20...)

EDIT: problem was happening before slight overheat/ECT out of range code.

@mikestang63 , the EGR is smooth and functional, but as far as carbon buildup, I dunno. I know you're leaning towards vacuum leak through the tubes, but wouldn't that cause some issues across the whole board, and not just at a "freezing" cold start issue?

Noobz347 · Jan 29, 2015

You know the one in the back of the block is. Not sure I've ever seen one that wasn't.

tca7291 · Jan 29, 2015

@jrichker , or any others, is there a test/procedure to check the BAP sensor? It has never thrown a code, but I'm thinking that it could be out of range. Thanks!

jrichker · Jan 30, 2015

Yes...

The problem is that not everyone has the tools to do all of it....

MAP/BARO sensor operation and code 22

Revised 14-Nov-2014 to add wire colors for frequency & voltage testing and engine sensor wiring diagrams.

On a Speed Density car, the MAP/BARO sensor is connected to the intake manifold and acts to sense the manifold pressure. Lower vacuum inside the intake manifold when combined with more throttle opening measured by the TPS means more airflow through the engine. As airflow increases, fuel flow through the injectors needs to increase to keep the air/fuel ratio where it needs to be. When manifold vacuum increases, the engine is either decelerating or idling, and it needs to reduce the fuel flow through the injectors.

On a Mass Air car, the MAP/BARO sensor vents to open air and actually senses the barometric pressure due to changes in weather and altitude. Its purpose is to set a baseline for the computer to know the barometric pressure. As barometric pressure decreases, it leans out the fuel flow to compensate for less oxygen in the air. When the barometric pressure rises, it increases to add fuel since there is more oxygen in the air. The fuel requirements decrease as altitude increases, since the atmospheric pressure decreases.

Disconnecting the MAP or BARO sensor will set code 22.

Misconnecting the BARO sensor to vacuum on a Mass Air car will cause the computer to lean out the fuel mixture.

Code 22 or 126 MAP (vacuum) or BARO signal out of range. The MAP or BARO sensor is pretty much the same sensor for both Mass Air & Speed Density cars. The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum. Its purpose is to help set a baseline for the air/fuel mixture by sensing changes in barometric pressure. The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it. There a very few DVM’s with a price tag under $40 that will measure frequency, but there are some out there.

Map sensor wiring:
black/white - ground
orange/white or +5 volts power
white/red signal out.

Measure the +5 volt supply using the orange/white and black/white wires
Measure the signal using the black/white and white/red wires.

The MAP/BARO sensor is mounted on the firewall behind the upper manifold on 86-93 Mustangs.

Baro or MAP test using a real frequency meter - run the test key on, engine off. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout. And oscilloscope is very useful if you have access to one or know of someone who does. With an oscilloscope, you can see the waveform and amplitude.

If it is defective, your air/fuel ratio will be off and the car’s performance & emissions will suffer

Some basic checks you can make to be sure that the sensor is getting power & ground:
Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 1.5 ohm.

The following power on check requires you to turn the ignition switch to the Run position.
Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or EGR sensors. Use the black/white wire for the ground for the DVM.

Diagrams courtesy of Tmoss & Stang&2birds

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs

tca7291 · Feb 10, 2015

Update time.

So this is new. Got to work on her this past weekend, replaced the IAC, found a vacuum leak at the PCV on the back of the intake, repaired, reset the ECM, triple checked/ reset base timing to 10-11*, reset base idle, 550-600 rpm, and the poor car runs worse. Still have the "I don't wanna run because it's cold" problem, but it has gotten a lot worse. Now I'm getting bucking/misfires going down the road, at WOT I will have 60% power at best, then buck, and run right after a few seconds. I have gone ahead and pulled codes again, still a 67 for the tad solenoid in KOEO, 11 in CM, but NOW I'm getting a 41 and 91 on KOER, for low voltage/lean on my O2s. The exhaust smells like it is running rich, almost burning your eyes if you are behind it while idling.

So, after the work was done:
Fuel pressure:
34 psi idle w/FPR hooked up, 40 when throttle hit
42 psi with FPR disconnected and vacuum line plugged, holds well when throttle hit

Manifold vacuum:
holds at 16-18 in./hg at idle, no spikes or drops with issues under load

Any ideas, anybody? I hate fixing things just to make things worse.

Edit: Oh yeah, swapped my MAP sensor with one off of my man's '90, and that made zero difference.

jrichker · Feb 11, 2015

Code 67 –
Revised 2 Nov 2012 to add definition of the NSS functions for both 5 speed and auto transmissions

Cause of problem:
clutch not depressed (5 speed) or car not in neutral or park (auto) or A/C in On position when codes where dumped. Possible neutral safety switch or wiring problem. This code may prevent you from running the Key On Engine On tests.

External evidence from other sources claims that a code 67 can cause an idle surge condition. Do try to find and fix any issues with the switch and wiring if you get a code 67.

What the NSS (Neutral Safety Switch) does:
5 speed transmission: It has no connection with the starter, and the engine can be cranked without it being connected.
Auto transmission: It is the safety interlock that prevents the starter from cranking the engine with the transmission in gear.
What it does for both 5 speed and auto transmission cars:
The computer wants to make sure the A/C is off due to the added load on the engine for the engine running computer diagnostic tests. It also checks to see that the transmission is in Neutral (5 speed and auto transmission) and the clutch depressed (T5, T56, Tremec 3550 & TKO)). This prevents the diagnostics from being run when the car is driven. Key On Engine Running test mode takes the throttle control away from the driver for several tests. This could prove hazardous if the computer was jumpered into test mode and then driven.

The following is for 5 speed cars only.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the passenger side kick panel and then remove the plastic cover from the computer wiring connector. Use a safety pin to probe the connector from the rear. Jumper the safety pin to the ground near the computer.
Be sure to remove the jumper BEFORE attempting to drive the car!!!

Fix the code 67 and then run a cylinder balance test. With code 67 present, you cannot dump the engine running codes or do the cylinder balance test. The cylinder balance test will help find non functioning or poorly functioning cylinders that may cause the code 41/91.

Cylinder balance test: use this to find dead or weak cylinders:

Revised 25 March 2012 to add necessity allowing the KOEO tests to finish before starting the engine and the need for a properly functioning IAB/IAC to run the cylinder balance test.

The computer has a cylinder balance test that helps locate cylinders with low power output. You’ll need to dump the codes out of the computer and make sure that you have the A/C off, clutch depressed to the floor and the transmission in neutral. Fail to do this and you can’t do the engine running dump codes test that allows you to do the cylinder balance test.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C clutch depressed to the floor, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

Here's how to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and drivability problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, clutch depressed to the floor and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Cylinder balance test

If you have idle or IAC/IAB problems and the engine will not idle on its own without mechanically adjusting the base idle speed above 625-750 RPM, this test will fail with random cylinders pointed out every time it runs. The IAC/IAB must be capable of controlling the engine speed to run in the 1400-1600 RPM range. Playing with the base idle speed by adjusting it upwards will not work, the computer has to be able to control the engine speed using the IAC/IAB.

Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Let it finish the Key On Engine Off (KOEO) code dump. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. Remember to keep the clutch pedal (5 speed) depressed to the floor during the test. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure

Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent/loan. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the throttle wide open with a plastic screwdriver handle between the throttle butterfly and the throttle housing. Crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 PSI. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.

A battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Connect it up while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.

See the link to my site for details on how to build your own blow down type compression tester.

tca7291 · Feb 11, 2015

Thanks @jrichker , but I posted the wrong code. still having an issue, but it is better. I haven't tried a cod start yet, but i sealed the vacuum line feeding the emission controls, which has yielded better "on throttle" responce, but still surging like crazy at idle. Still getting code 41 and 91 on a koer test...

jrichker · Feb 11, 2015

Do the cylinder balance test...

tca7291 · Feb 11, 2015

Ran the balance test twice, code 90 both times. Arrgh, I did a bad thing, turned back to the early '90's way of tuning, and capped off the tad, tab, egr, ect. Runs a hell of a lot better, I know I'm forcing open loop, but with a 41 and 91, isn't it anyway? It still smells pig rich, I have regained (close to) proper power at WOT, but still bucking while at 10-25% throttle. Anyway, throwing egr code now, with tad and tab solenoid codes, and still have the 41 91.

jrichker · Feb 12, 2015

Getting a 90 means all the cylinders are working like they should and a misfire is not the cause of your code 41/91. That pretty much eliminates ignition as a source of your problem.

The code 41/91 forces the engine into limp mode with reduced power and economy.

Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 20-Jan-2014 to add code 43 to test collection

Code 41 is a RH side sensor, as viewed from the driver's seat.
Code 91 is the LH side sensor, as viewed from the driver's seat.

Code 172 is the RH side sensor, as viewed from the driver's seat.
Code 176 is the LH side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.

Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.

Backside view of the computer wiring connector:

87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (L\RH O2 with a dark green/pink wire) and 43 (LH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.

87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 29

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27

There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

Code 41 can also be due to carbon plugging the driver’s side Thermactor air crossover tube on the back of the engine. The tube fills up with carbon and does not pass air to the driver’s side head ports, Remove the tube and clean it out so that both sides get good airflow: this may be more difficult than it sounds. You need something like a mini rotor-rooter to do the job because of the curves in the tube. Something like the outer spiral jacket of a flexible push-pull cable may be the thing that does the trick.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.

tca7291 · Feb 15, 2015

Ok, @jrichker , is the wiring the same for a DA1? I checked voltage at pin 43, and I'm getting a fluctuation of about .01v to about .04 volts. On pin 29 I'm getting about .05 to about .08 volts. I can't really check resistance in the harness until I can get some help next weekend. Do you think it sounds like my O2's are shot, and because of the vacuum leaks, it was allowing her to run better with the leaks? Everyone's help has been greatly appreciated!

Edit: Also I went ahead and hooked all my vacuum/emission controls back up, it ran better at first, but then I could tell a difference in a negative way. So, everything is back to the way it should be, minus the TAD solenoid fault (has always been there), capped off carbon canister purge vac line, and capped pcv valve line. Both capped at the intake to make sure of no vacuum leaks.

tca7291 · Feb 15, 2015

...and some more info, at the pins I was able to get a reading of over .1volt, but only during decel after revving the engine. It smoothed out like she does occasionally while I was taking readings on pin 29, and was holding about .09v while acting properly, then dropped with the rough/low idle coming back to .06-.07volts.

Would low temps and low backpressure also cause an issue? O2's not getting hot enough/staying hot enough?

jrichker · Feb 16, 2015

tca7291 said:
...and some more info, at the pins I was able to get a reading of over .1volt, but only during decel after revving the engine. It smoothed out like she does occasionally while I was taking readings on pin 29, and was holding about .09v while acting properly, then dropped with the rough/low idle coming back to .06-.07volts.

Would low temps and low backpressure also cause an issue? O2's not getting hot enough/staying hot enough?

The O2 sensors have built it heater, that heats anytime the ignition is in the Run position. One of the O2 sensor leads should show 12 volts to ground at all times. Make sure that the orange wire on the engine mounted fuel injection harness is grounded to one of the intake manifold bolts or to the back of the cylinder head.

tca7291 · Feb 16, 2015

Correct me if I'm wrong, but the ground for the sensors themselves gets continuity through the piping, right? I did test resistance between the computer ground and the passenger pipe, I know it wasn't the best connection, but I did scratch the pipe with my meeter lead, and was getting measurable resistance (> 1 ohm) I was thinking after work, I would hook up my jumper cables to one of the O2 bungs, and to the negative on the battery to see if that would bring my voltage up. (I still need to get under the car properly to check my connections at the O2 harness plugs). Just racking my brain for other possibilities.

tca7291 · Feb 16, 2015

Just checked, orange coming out of the FI harness is grounded at the bellhousing, on the upper right bolt. Too cold, and too wintry mix to do further testing at this time...

tca7291 · Feb 16, 2015

Perhaps I should call Bill @LaserSVT into this, if willing...

LaserSVT · Feb 16, 2015

Well my first thoughts were IAC and vac leak but I see you found those. J hit all the right testing. I suspect a faulty ECT. Need to know what its reading when cold. It could also be a worn engine. Worn rings take some heat to seal better. Also if you can check the TPS as maybe its acting up when cold or maybe its not set at .9v

I had a van a few weeks ago with an identical problem. Was ECT sensor.
Had a truck come in with the exact same problem. Was the TPS.
Had another truck come in with the same problem. Was a loose ground to the ECU.

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