Rough start weird idle

Acmerica

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Need help solving issue with my 2v has issues starting only sometimes and stumbling idle on and off have had fuel trim in the 20s and now in the area of -8
Pcv valve seems to be fine as well as iac have smoke tested for vacuum leaks and found nothing leaking. Hoping to get some insight. Fuel issue or something else?
 

96blak54

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Iac can play games. Be sure its not really dirty.

Leaky injectors cause rough starts. Dirty injectors cause rough starts too.
 
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Acmerica

Acmerica

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Iac can play games. Be sure its not really dirty.

Leaky injectors cause rough starts. Dirty injectors cause rough starts too.
I’ve “tested” the iac as I’ve seen others on forums say to try and it didn’t seem to be the problem but I’ll check it again
 

96blak54

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In a ways, thats a common modular mustang thing. I dont agree with the harder starts after the engine was just running, because I know exactly what you talking about. I have a few theories on it. A combination of tuning and mechanical that helps the perspective, but long story short its falls on start up tuning. And that thought coincides with properly functioning fuel system and sensors. Just my .02 input
 

I_LIKE_TURTLEZ

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When you say hard start I assume you mean it doesn't want to fire right away but the starter is spinning freely and able to illuminate a test light brightly.


If your car thinks it's running rich then it likely is, there aren't many things that can cause that.

-Check the air filter and clean it out(yes seriously)
-Check the MAF data PID(you expect to see about 1g/s per liter)
-Hook up a fuel pressure gauge and run the car, then turn it off. If pressure rapidly decreases you have a stuck open injector. You could also use a stethoscope on each injector, one stuck open would make no mechanical clicking.
-Check upstream o2 data; they sometimes go lazy with age.


If you meant the car thought it was lean, most often it's a vacuum leak. It's a 20+ year old car, I can assure you it has leaks, most are too small to make a noticeable difference until they get worse.
-All of the above applies.
-Check fuel pressure.
-Check for an exhaust leak, you may not hear it from inside the cabin.


You would expect to see a positive short term fuel trim on a cold start, followed by a slightly negative once warm; variance of +/- 10 is considered acceptable depending on RPM.
Since the car is stumbling from throttle input while trying to add fuel, personally I'd be going straight to the fuel pressure testing.
 
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Acmerica

Acmerica

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Since the car is stumbling from throttle input while trying to add fuel, personally I'd be going straight to the fuel pressure testing.
I will check out everything later thank you I just wanted to add the car doesn't stumble from throttle input only rough idling and starting
 

I_LIKE_TURTLEZ

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I will check out everything later thank you I just wanted to add the car doesn't stumble from throttle input only rough idling and starting
If it's idling rough on cold start then it wouldn't be an o2 problem since it's in open loop, that helps narrow it down. Check the basics like fuel and air and it will lead you to the problem.
 
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Acmerica

Acmerica

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Just wondering but the purge valve solenoid should be closed when the car is off correct?
 

I_LIKE_TURTLEZ

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Just wondering but the purge valve solenoid should be closed when the car is off correct?
Purge valve should always be closed until it's commanded on by the PCM when the canister becomes full, on a side note any 20+ year old canister is going to be full of gassy mud and junk, you can tell because it will weigh like 10lbs.

To easily test the purge valve, remove it and try to blow through it with your mouth, if you can it's bad.
 

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