KevinRoach

Hey Guys!

I know this may have been addressed a thousand times but I looked on the forum and couldn't find anything.

I have a 1994 Mustang 3.8l engine that when cold misfire, idles rough and when driving jerks. I have done some checks, cleaning and replaced parts and still the same thing. It runs a lot butter when warmed up or doesn't always do it. It is not throwing any code either.

I and the previous owner have replaced, egr valve, fuel filter, crank sensor, verified TPS is working correctly, cleaned and checked the MAF, fuel pressure is 30 ignition on/engine off, fuel pressure ignition on/engine on at 30-40 when I rev the engine, new plugs, new wires and new coil pack.

It seemed to be better after the coil pack but still doing it just not as rough misfire.

Any help and suggestions appreciated.
Thanks!
Kevin

corker

Check for a vac leak when cold. Engine operates in open loop when cold - PCM ignores certain sensor inputs when cold. Once into closed loop, PCM compensates for vac leak lean condition and engine will smooth out. If vac leak is bad enough, PCM should trigger CEL when fuel trims max out trying to compensate.
Just a thought,
Corky

KevinRoach

Thanks Corker!
I will try that, any suggestions on what to use for checking the vacuum leak and what my be typically at fault?

Kevin

corker

You can spray throttle body cleaner and listen for a rise in RPM - I prefer a safer method and spray water and listen for sucking noise and engine stumble. Spray all around plastic intake runners and check for dried out vac lines. Check brake booster by pinching off the large vac line going to it.
Good Luck

KevinRoach

Ok sprayed some throttle body/carb cleaner around and got some stumbling from a back hose across the firewall. I am confused though because it stumbled, so I pulled the vacuum line from the intake plenum and spayed into the plenum and it killed the engine.

Shouldn't it have speeded up if it was lean?

petrock

Not necessarily. You pulled the line and created a huge vacuum leak so the computer dumps a ton of fuel into the motor to correct the extreme lean condition. Then you spray more fuel in… effectively flooding the motor.

corker

Petrock is spot on, Sounds like you found your problem. Replace the hose and see how it runs when cold.