idle staying at 2000rpm?

blackcrystal90 · Apr 25, 2012

hey guys. i recently rebuild my engine. my car will start right up but when it does the RPM's will stay at about 2,000rpm then slowly drop to about 1,200 rpm. while driving it will do the same thing. if i push the clutch in and stake it off of gear, the idle will stay at 2,000 rpm while coasting to a red light or stop. then it will slowly go down while i am stopped but it wont go back down to normal idle. it will only go back down to about 1,200 rpm. i have replaced IAC, TPS, EGR, MAP, o2 sensors, checked timing and checked for vacuum leaks. any ideas why it would be doing this. i have a holley systemax II intake manifolds if that makes a difference. thanks.

HISSIN50 · Apr 25, 2012

Have you pulled codes?
Though you said you have checked, I would continue to look for vacuum leaks.

FWIW, a high rolling idle is normal and by design. Yours is higher than normal but that's probably a result of the underlying issue.

If you have not done so, the surging idle checklist has a lot of info which is germane to your issue.

blackcrystal90 · Apr 25, 2012

yeah i have checked for vacuum leaks but threirs a small one i cant seem to locate under the egr valve in the lower intake manifold. i ran codes and i replaced everything it said. i seem to have gasoline in the upper manifold. any ideas why? and what do you mean by underlying issues?

HISSIN50 · Apr 25, 2012

The rolling idle is high but once you fix your idle issue, that rolling idle should come down.

What codes did you get and what codes do you get now? The codes don't relate to parts to replace. For instance, o2 codes often relate to a vacuum leak or MAF issue , not necessarily bad O2 sensors.

Is it raw gas or the effects of blow-by in your plenum?

You might also want to list the year of car and modifications.

blackcrystal90 · Apr 25, 2012

its a 90 gt 5.0 . 306. e303 cam. gt40x heads. holley systemax II. #24 injectors. bbk 75mm maf. 75mm acufab tb.
it gave my the codes 22,33,41,91.
i havent re-runned for codes i will do that tomarrow.
it was more effect bucause it was moist of it and smelled like gas and was all in the plenum.
and the idle will also start at about 2k rpm when cold them slowy move down to about 1200rpm

MFE92 · Apr 25, 2012

Classic symptom of bad connections at the 10-pin connectors, aka the "salt and pepper shakers" clipped to the back of the intake throat. Disconnect them, tweak the terminals for a tight fit, and reconnect them.

blackcrystal90 · Apr 25, 2012

okay cool. will try that that first thing in the morning. im open to all options at this point. thanx

larrym1961 · Apr 25, 2012

MFE92 said:
Classic symptom of bad connections at the 10-pin connectors, aka the "salt and pepper shakers" clipped to the back of the intake throat. Disconnect them, tweak the terminals for a tight fit, and reconnect them.

I had the very same problem with my Mustang after I installed my new motor and fought the hanging Idle issue for 5 years and replaced IAC, TPS with no luck. I could not find any vaccume leakes anywhere either. Until a Stangnet member told me of the "salt & Pepper shaker" issue. I cleaned and opened up the pins on the 10 pin connectors applyed dielectric grease and have not had the hanging idle issue in over a year and a half now.

jrichker · Apr 26, 2012

Fix the codes before you do anything else, The code 22 can cause code 41/91 so fix the code 22 first.

If you still have problems after fixing the codes, start looking for vacuum leaks.

MAP/BARO sensor operation and code 22

Revised 19-Jul-2011 to add functional descriptions for MAP and BARO operation.

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 acts to sense 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.

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

Baro or MAP test using 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.

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.

Code 33 - Insufficient EGR flow detected.
If someone has modified the EGR system, there may be vacuum lines disconnected.
Look for vacuum leaks, cracked vacuum lines, failed EGR vacuum regulator. Check to see if you have 10” of vacuum at the EGR vacuum connection coming from the intake manifold. Look for electrical signal at the vacuum regulator solenoid valves located on the rear of the passenger side wheel well. Using a test light across the electrical connector, it should flicker as the electrical signal flickers. Remember that the computer does not source any power, but provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.
Check for resistance between the brown/lt green wire on the EGR sensor and pin 27 on the computer: you should have less than 1.5 ohm.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif

EGR test procedure courtesy of cjones

to check the EGR valve:
bring the engine to normal temp.

connect a vacuum pump to the EGR Valve or see the EGR test jig drawing below. Connnect the test jig or to directly to manifold vacuum.

Do not connect the EGR test jig to the EVR (Electronic Vacuum Regulator).

apply 5in vacuum to the valve. Using the test jig, use your finger to vary the vacuum

if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.

if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.

if engine stumbled, connect EGR test jig to the hose coming off of the EGR Valve.
Use your finger to cap the open port on the vacuum tee.
snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
did the vacuum gauge show about 2-5 in vacuum?
if not the EVR has failed

EGR test jig

The operation of the EGR vacuum regulator can be checked by using a test light applied across the wiring connector. Jumper the computer into self test mode and turn the key on but do not start the engine. You will hear all the actuators (including the EVR vacuum regulator) cycle. Watch for the light to flicker: that means the computer has signaled the EGR vacuum regulator successfully.

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

Revised 20-Nov-2011 to include computer pin numbers for O2 sensor wiring for resistance checks

Code 41 is a RH side sensor,
Code 91 is the LH side sensor.

Code 172 is the RH side sensor
Code 176 is the LH side sensor

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.

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 (LH O2 with a dark green/pink wire) and 43 (RH 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 43

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.

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.
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up the driver’s side, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41. The cure is to remove the crossover tube and thoroughly clean the insides so that there is no carbon blocking the free flow of air to both heads.

Diagram courtesy of Tmoss & Stang&2birds

blackcrystal90 · Apr 26, 2012

I got to fix the codes and now I'm not getting any which is good. The idle seemed to not get stuck around 2000rpm anymore since cleaning out the 10 pin connectors but now will idle at about 1300 then down to 900 then back up again. And still seems to jerk a little while in gear at about 2000 rpm. Think it might be because I'm running too lean/rich?

jrichker · Apr 26, 2012

No code 22 or code 41/91? What did you do to fix them? Clearing the computer's stored memory does not fix sensor problems. Drive the car for a while and see if they come back. Be sure to dump the engine running codes as well as the Key On Engine Off codes.

See the "Surging Idle Checklist” for help with all your idle/stall problems. You can guess at the problem and throw parts at it, or you can use the checklist to help you find the problem quickly and inexpensively. It’s free and doesn’t cost anything: at last count there were more than 103,000 visits and still climbing

The quick and easy way to dump the codes is in there too, and all you need to do it is a paper clip! The first two posts contain all the fixes & updates. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.

blackcrystal90 · Apr 27, 2012

i just replaced my map sensor and o2 sensors. the idle seems to be better on cold but once it warm up it back to doing a rough and surge idle. i have some gas moisture in the intake manifold. any clues why?

jrichker · Apr 27, 2012

See the Surging idle checklist link I posted. Everything yoy need to fix and fix idle/stall problems is in there: 103,000+ views means that it works great if you work it....

blackcrystal90 · Apr 27, 2012

checked about almost everything on the check list and still is a no go. going to check fuel pressure and a couple of sensors. i will post my fix..if i get one. thanks a lot.

k00ksta · May 11, 2012

I see your running 24lb injectors have you gotten a calibrated MAF for that injector size?

blackcrystal90 · May 12, 2012

yeah i have venom 24lb injectors.new. i changed MAF with another one calibrated for 24lb injectors and it was still doing the same thing.

88LX5.Oh · May 13, 2012

Gas in the intake sounds like fuel pressure regulator took a dump on you.

idle staying at 2000rpm?

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