Rough when cold, no start when hot

207RAcing

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Jun 11, 2004
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I have a Bronco with a 90 Mustang 5.0 and Mass Air EFI on it. I just put the EFI system on and I am having 2 issues. I have replaced the fuel pump, pressure regulator, plugs and wires. I am getting no codes from the computer. When cold, it idles rough and has a hard time running at low RPM. Once warmed up it runs great. When you shut if off, it wont start again for about 10 minutes. It will crank over great just wont start. I think the cold problem is the coolant temp sensor, but what the heck is the hot starting issue? If you unplug the Fuel pump relay it will fire, but cant start obviously since there isn't any fuel after it fires.

Need to get this licked as I am going on vacation in a couple weeks and going to be pulling a boat. I would hate to kill the motor on the boat ramp and not be able to start it again.

Thanks
Scott
 
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If your ECT (engine coolant temparature sensor) is bad, you will have hot starting problems as well. I went through the same thing on an '88 Grand Marquis. Ran rough when cold, wouldn't start for 5-10 minutes when hot. I replaced the ECT and it has run fine ever sense.
 
You would have to rig something up to measure the voltage/resistance through the sensor at the different temperatures it is designed to measure. I don't know exactly how to do it, if you want to test your "spare" someone else may be able to post instructions to test it.
 
Code 21 – ECT sensor out of range. Broken or damaged wiring, bad ECT sensor. Note that that if the outside air temp is below 50 degrees F that the test for the ECT can be in error.

The ACT & ECT have the same thermistor, so the table values are the same

ACT & ECT test data:

Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate readings.

Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

Pin 25 on the computer - ACT signal in. at 50 degrees F it should be 3.5 volts. It is a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower intake manifold, the voltage readings will be lower because of the heat transfer.

Voltages may be measured across the ECT/ACT by probing the connector from the rear. Use care in doing it so that you don't damage the wiring or connector.

Here's the table :

68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61

Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.
50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms

I doubt the ECT is your problem since you got no codes, so here's the checklist.

Cranks OK, but No Start Checklist for Fuel Injected Mustangs

1.) Remove push on connector from starter solenoid and turn ignition switch on. Place car in neutral or Park. Remove coil wire from distributor & and hold 3/8” away from engine block. Jumper the screw to the big bolt on the starter solenoid that has the battery wire connected to it. You should get a nice fat blue spark.
Most of the items are electrical in nature, so a test light, or even better, a voltmeter, is helpful to be sure they have power to them.
No spark, possible failed items in order of their probability:
A.) Coil
B.) TFI module
C.) PIP sensor in distributor
D.) ECC relay next to computer - look for 12 volts at the fuel injector red wires
E.) Fuse links in wiring harness - look for 12 volts at the fuel injector red wires
F.) Ignition switch - look for 12 volts at the fuel injector red wires
G.) Computer

2.) Spark at coil wire, pull #1 plug wire off at the spark plug and check to see spark. No spark, possible failed items in order of their probability:
A.) Moisture inside distributor – remove cap, dry off & spray with WD40
B.) Distributor cap
C.) Rotor
D.) Spark Plug wires
E.) Coil weak or intermittent - you should see 3/8" fat blue spark with a good coil

3.) Spark at spark plug, but no start.
Next, get a can of starting fluid (ether) from your local auto parts store: costs a $1.30 or so. Then pull the air duct off at the throttle body elbow, open the throttle, and spray the ether in it. Reconnect the air duct and try to start the car. Do not try to start the car without reconnecting the air duct.
Two reasons:
1.) If it backfires, the chance for a serious fire is increased.
2.) On Mass Air cars, the computer needs to measure the MAF flow once the engine starts.
If it starts then, you have a fuel management issue. Continue the checklist with emphasis of fuel related items that follow. If it doesn’t ,then it is a computer or timing issue: see Step 4.

Clue – listen for the fuel pump to prime when you first turn the ignition switch on. It should run for 5-20 seconds and shut off. To trick the fuel pump into running, find the ECC test connector and jump the connector in the lower RH corner to ground. See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html for a description of the test connector. If the relay & inertia switch are OK, you will have power to the pump. Check fuel pressure – remove the cap from the schrader valve behind the alternator and depress the core. Fuel should squirt out, catch it in a rag. Beware of fire hazard when you do this. In pinch you can use a tire pressure gauge to measure the fuel pressure. It may not be completely accurate, but you will have some clue as to how much pressure you have.

No fuel pressure, possible failed items in order of their probability:
A.) Tripped inertia switch – Coupe & hatch cars hide it under the plastic trim covering the driver's side taillight. Use the voltmeter or test light to make sure you have power to both sides of the switch
B.) Fuel pump power relay – located under the driver’s seat in most stangs built before 92. On 92 and later model cars it is located below the Mass Air Flow meter. Look for 12 volts at the Pink/Black wire on the fuel pump relay.
C.) Clogged fuel filter
D.) Failed fuel pump
E.) Blown fuse link in wiring harness. Look for 12 volts at the Orange/Lt Blue wire on the fuel pump relay
F.) Fuel pressure regulator failed. Remove vacuum line from regulator and inspect for fuel escaping while pump is running.

Fuel pressure OK, the injectors are not firing.

A.) A Noid light available from Autozone, is one way to test the injector wiring.
B.) I like to use an old injector with compressed air applied to the injector where the fuel rail would normally connect. I hook the whole thing up, apply compressed air to the injector and stick it in a paper cup of soapy water. When the engine cranks with the ignition switch on, if the injector fires, it makes bubbles. Cheap if you have the stuff laying around, and works good too.

a.) Pull an injector wire connector off and look for 12 volts on the red wire when the ignition switch is on.
b.) No power, then look for problems with the 10 pin connecter (salt & pepper shakers at the rear of the upper manifold).
c.) No power and the 10 pin connections are good: look for broken wiring between the orange/black wire on the ECC relay and the red wire for the 10 pin connectors.


4.) Spark & fuel pressure OK.

A.) Failed IAB (no airflow to start engine). Press the throttle ¼ way down and try to start the car.
B.) Failed computer (not very likely)
C.) Engine ignition or cam timing off only likely if the engine has been worked on recently).
D.) Firing order off: HO & 351 use a different firing order from the non HO engines.
 
battyice said:
If your ECT (engine coolant temparature sensor) is bad, you will have hot starting problems as well. I went through the same thing on an '88 Grand Marquis. Ran rough when cold, wouldn't start for 5-10 minutes when hot. I replaced the ECT and it has run fine ever sense.

Well that wasn't it. I am really starting to suspect the computer! It is like it doesn't read the sensors input. That plus the WOT AC cutoff is stuck cutting off the AC, had to bypass it to get Air.
 
Ok, so the Broncos use a 2 fuel pump system, a low pressure high volume one in the tank and a high pressure one inline. The tank one is not doing anything. Hopfully it is just disconnected from when it had a carb on it.
 
Now I need help. First, the pump in the tank was not there at all, put one in, didn't really made a difference, probably becuase my front pump is new and was keeping up. Now, I have narrowed down the hot start problem. There isn't any spark while cranking over, as soon as you let up from the start circuit, I get spark again, but obviously too late to fire the engine. I am thinking TFI, but want to be sure, any one else run into this? I hate to spend yet another 50$ and have it not working still. Wierd thing is, I know the truck ran a few years ago when I took this distributer off. I am not sure however the computer is any good since it stayed with the vehicle for a few years that I did't have it. Who knows what happened.

Here is another clue, the rough when cold, smooths out if you unplug the mass air sensor, then as it warms up you have to plug it back in. It is like the computer is stuck on the engine warm strategy.
 
207RAcing said:
There isn't any spark while cranking over, as soon as you let up from the start circuit, I get spark again, but obviously too late to fire the engine.
ok, im confused. about the only way i know to check spark on a non-starting vehicle is to crank it while checking. how do you know you have spark once you stop cranking it?
im just confused, but trying to think of something for ya.
 
HISSIN50 said:
ok, im confused. about the only way i know to check spark on a non-starting vehicle is to crank it while checking. how do you know you have spark once you stop cranking it?
im just confused, but trying to think of something for ya.

I have a remote start switch hooked to the start solenoid, I have the coil wire about 1/2 inch from ground. I use the remote switch and watch for spark, no spark while cranking, as soon as I let up on the switch, I see spark. The engine spins a little after you let up.

Turned out to be a bad ECM relay, it was shutting off while cranking when the engine was warmed up. Must have enough crap on during the warmed up strategy to pull the voltage down that a bad realy would not stay on.

I still have the rough idle and off idle when cold issue. All my sensors are functioning properly. the only component not replaced so far is the ECM itself. Thet tied witht he fact the AC WOT is stuck cutting off the AC, I really think the ECM is toast. It runs great when warmed up though, so for now I will just let it warm up.