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Discussion Starter #1
My 91 VR-4 has been doing this since last year and now that I am driving it often again it is driving me crazy.. At idle I have a random misfire and it can actually buck a little if cold. Also hear slight gargle/sputter on deceleration (no cats). I do not seem to notice this during moderate-load and high-load acceleration under boost.

I have also noticed gas mileage seems to have gone down noticeably as well and a slight hissing noise that seems to be coming from the plenum.

What I have done so far
Installed new fuel filter
Installed fuel pressure gauge on rail
Installed new injectors
NGK copper spark plugs gapped at .29
Put stock plug wires back on
Tested CAS with multi-meter
Tested FPR
Swapped ECU
Swapped PTU
Swapped coil pack
EGR block off plates
Swapped MAF
Unplugged ECS
Pressure tested over and over
Vacuum at idle about -18
Fuel trims all around 100
No CEL errors
Basic ignition timing set to 5º
Tried running with O2’s unplugged
Pulled front timing cover and exhaust and timing markers are perfect

What else should I be doing to figure this out once and for all??
 

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og
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4,293 Posts
have you checked for exhaust leaks when cold? have you pressure tested when both hot and cold?
 

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still here
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Discussion Starter #3
have you checked for exhaust leaks when cold? have you pressure tested when both hot and cold?
I pressure test it usually when cold but I can try it when hot... Would it matter if I can hear the misfire as soon as the idle stabilizes and the motor is basically still cold?

I do not hear any major exhaust leaks but what would be an easy way to test the exhaust?
 

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og
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I pressure test it usually when cold but I can try it when hot... Would it matter if I can hear the misfire as soon as the idle stabilizes and the motor is basically still cold?

I do not hear any major exhaust leaks but what would be an easy way to test the exhaust?
smaller exhaust leaks tend to be more exposed when the car is cold. once it warms up, things expand and sometimes seal up. this applies particularly close to the engine... hairline manifold cracks etc, which i've come across on our cars more than once.

the intake testing shouldn't really matter hot or cold unless you have an intake leak on the engine itself, which our cars are not known for at all. furthermore your vacuum is spot on for a system with no leaks, which is a positive sign.

you say you hear the misfire... i will also mention i had an issue on one of our cars where oil was leaking into the spark plug well through one of the well seals causing a misfire. this would happen when the car was cold since the oil had a chance to collect when the car was cooling down. once it warmed up the issue went away. may be worth checking into.
 

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Discussion Starter #5
I will look into that as I pulled each plug at the coil and the spark semed strong on all cylinders.

Today I installed new plugs & wires plus put a brand new throttle body on that included all new sensors. Set the SAS and the TPS tonight. Tomorrow will be the idle and A/F. I also dropped the dp and had a shop weld in the bung so I could install my wb02... They welded the bung on the wrong side arrgh!



smaller exhaust leaks tend to be more exposed when the car is cold. once it warms up, things expand and sometimes seal up. this applies particularly close to the engine... hairline manifold cracks etc, which i've come across on our cars more than once.

the intake testing shouldn't really matter hot or cold unless you have an intake leak on the engine itself, which our cars are not known for at all. furthermore your vacuum is spot on for a system with no leaks, which is a positive sign.

you say you hear the misfire... i will also mention i had an issue on one of our cars where oil was leaking into the spark plug well through one of the well seals causing a misfire. this would happen when the car was cold since the oil had a chance to collect when the car was cooling down. once it warmed up the issue went away. may be worth checking into.
 

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Did you ever fix your problem? I'm having basically the same problems with my car and im not sure what to do. Most parts are new like you have done. Not sure where to check now...
 
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