Rough Idle - Ford Mustang Forum

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post #1 of 12 (permalink) Old 04-11-2002
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Rough Idle

My car has developed a rough idle. On cold starts my engine starts to run rough for 5 to 10 seconds and then it starts to idle like it's supposed to. Sometime it switches off when I try to apply the accelerator to increase the rpm. I don't know if it's fuel related or is it electrical. I don't know if it's the injectors, fuel filter, pressure or blocked fuel lines. When the car warms up it runs fine even under hard accelaration. Could this have anything to do with the Throttle Position Sensor?
I don't think it's the idle screw on throttle body because the EEC controls the idle.
Is there anyone that has experianced the same problem.

Any help would be appreciated

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post #2 of 12 (permalink) Old 04-11-2002
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Re: Rough Idle

it could be the idle air control valve

post #3 of 12 (permalink) Old 04-11-2002
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Re: Rough Idle

What year car is it? If it does it when you hit the gas, its more than likely a fuel issue, TPS,dirty MAF,Dirty Throttle body, dirty injectors, dirty filter, basically a good tune up should fix it, run redline fuel injector cleaner thru it and change the filter if its been a while or you mightve gotten bad gas.....
post #4 of 12 (permalink) Old 04-11-2002
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Rough Idle

The car is a 93 Mustang 5.0L HO with the unmodified engine with the exception of 73mm MAF, K&N air filter, headers, dual 2 1/2" exhaust and the timing has been set to 15 degrees.

I don't think it's the gas because the gas station because i've been getting the gas from the same gas station for a while and i never had any problems.

The filter has been changed about a month ago and I have tryed using the redline fuel injector cleaner and it didn't help unless the injectors a faulty. As for the TPS i am not sure. Is there a way to test the TPS and if so how? Also how would you go about cleaning the MAF & the throttle body? Any special chemicals involved and is there a special procedure that I should follow?

Could it be that it's because I am using a MAF that has been set for 24lb injectors and i am still using 19lb injectors? If so is there i way that i can modify the MAF my self or do I need to take it to someone to get it calibrated.
post #5 of 12 (permalink) Old 04-11-2002
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Re: Rough Idle

The MAF sounds like the problem then, if its for 24#ers and you're still using stock, put your old one back on and it should fix it.... either way, you want the MAF and injectors to match...If that doesnt work, leave the stock one on, and use some Throttle body/carb spray and spray the TB butterfly then open it and spray it good in there. it might not start right away but it will and then burn the carbon out.... its always a good thing to do at tune-ups.
post #6 of 12 (permalink) Old 04-16-2002
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Rough Idle


Before Electronic Engine Control (EEC) systems with electronic fuel injection were installed on cars, the function of allowing a car with a cold engine to start and idle properly was performed by a mechanical choke. This was a butterfly valve located inside the carburetor that closed off a portion of the air flow to the engine while leaving the fuel flow the same. This richens the Air/Fuel ratio enough to allow a cold engine to idle smoothly until the engine warms up and the choke gradually returns to full open.

Cars with EEC systems and electronic fuel injection don't have a carburetor and therefore don't have a choke. What they do have is a sensor located in the coolant lines on the engine that reads the current temperature of the coolant circulating through the engine and reports that reading to the EEC computer.

On most Fords that sensor is called the Engine Coolant Temperature (ECT) sensor. If your ECT is electrically out of range (effectively busted!) it can no longer report the correct engine temperature to the EEC computer. And, therefore, the EEC computer can no longer send a signal to the fuel injectors to stay open longer on each fuel injection pulse to richen the A/F ratio because the engine is sill cold.

So, if your engine does not idle correctly with a cold engine but idles fine once warmed up, the most likely culprit is a bad ECT sensor! Go to the Ford parts counter and buy one for your particular engine and install it. (About $35.00 or so.)

If you don't want to pay the $35.00 without knowing for sure that your ECT is out of range, then go to the local book store and buy a copy of Ford Fuel Injection & Electronic Engine Control, 1988 to 1993, by Charles Probst (covers all Ford EEC IV systems, not the newer EEC V). Read that book and learn how to run the computer Quick Checks on your car. The quick checks allow you to read the trouble codes that are stored in the EEC IV computer. If your ECT is out of range, the trouble code for a bad ECT will be output when you run those checks. You don't need to go out a buy a fancy STAR tester to read the trouble codes (though its easier). You can read them out directly from the flashes on the check engine trouble light (MIL) on the dash board of your car. If you are going to do your own maintenance on a Ford with and EEC system, you owe it to yourself to learn how to do this. It will save you hundreds of dollars at the Ford dealer if you do.

PS) There is another temperature sensor that can affect engine performance, thought it does not have anywhere near as much effect on engine idle as the ECT. This one is the Air Charge Temperature (ACT) sensor. It is usually located in the inlet air path ducts coming into the upper intake manifold before the throttle body. It senses the ambient temperature of the outside air coming into the intake manifold and reports that to the EEC computer. The EEC computer adjusts the A/F mixture accordingly. If you don't seem to have quite the right A/F mixture at part to full throttle (engine surges or runs lean/rich) the ACT may be to blame.

There are a dozen or so sensors that send various signals to the EEC computer so that it can control your engine correctly. These are just the two that have to do with operating temperatures.

Good luck with your troubleshooting and repair.

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post #7 of 12 (permalink) Old 03-07-2006
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for me the rough idle was a bad spark plug

Check my post in this thread:
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post #8 of 12 (permalink) Old 09-01-2006
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I have a 89 lx 5.0 ho and i had the same problem i changed the spark plugs helped some [o by the way the best spark plugs for the car has to be MOTORCRAFT SPARK PLUG 32c i belive some reason ford motors relly like the motorcraft plug] After that i chaged the Coolant Temperature Sensor it could cause the car to run rough untill it warms up but no luck for me the thing that fixed the idle was the mass air sensor good luck

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post #9 of 12 (permalink) Old 11-25-2008
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I just went through a similar issue with rough idle and hunting 500-1000 plus rpm.

My fix was crc maf cleaner to EGR VALVE/IAC/
Checked TPS module running at 1.38-1.45 volts max is .99voltsto .98 so replaced TPS.

Curb idle set to 900 rpm with IAC Unplugged 20 minutes battery disconnected for 20 mins reconnect bat started with IAC FOR 2mins for EEC TO LEARN shutdown for another 2 minutes.

Started back up ran it around for EEC to learn driving habits etc.. for 12 miles or so idels perfect at 850-900 no deviations. stopped rough idle and tps change stopped hunting check your tps mod multimeter on 20k ohms pierce the green wire use red probe ground black to battery no need to pierce 2 wires volts max .99 no more than 1.00 preferred experts say .99-.98.

"Remember this weather its MAF OR Speed Density regardless of the system if any one of many electrical signals are out of spec to the Computers especially S/D I have learned through my research both systems will cause numerous problems at best.
Best to learn how to test these components before spending $$ if you do not you will have spent over 200 bucks in electrical components by the time you find a 33.49 tps was an issues the whole time" Just my advice....

Hope this helps

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This ancient thread should still help somebody out there.

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DO you think they figured this one out, yet?

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mine does the same thing good luck id like to hear what it is

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