ok so i've been chasing this gremlin for a while and can't seem to pin it down.



symptoms are:

high idle (as well as hunting) but not all always high. the hunting is constant however.



when i leave the car in gear and let off the gas (engine brake) the car jerks like its going brake-coast-brake-coast



i have an aem FIC installed but all it's doing is scaling back the injectors (440cc green tops) the car is pretty much stock power-adder wise. i tried lowering the TPS voltage to see if it would help and it didn't change the engine brake issue, did bring the idle down a bit though. i don't think this is the way to do it. i have replaced a lot recently, including:

tps

iac

spark plugs and wires

o2 sensors



also ran a vacuum leak test, all good there.



car is not throwing any codes so i don't know what to look for next. any suggestions?

 

How high is your high idle

 

it ranges from 1200 up to just over 2000 rpms. on a cold start it almost always idles at 2krpm. i also tried adjusting the idle screw on the throttle body, it's as far back as it will go and still won't bring the idle down.



just had a thought: could the coolant temp sensor affect the idle? i'll check that next to be sure.

 

What sensor are you scaling back to adjust your injectors? Do you have a CEL?

 

i'm scaling the injector signals back directly using the FIC. no CEL's.

 

I reckon it is the intake manifold gasket leaking, letting in too much air. I have seen a few of these.

Solution: new gasket.

 

I think you may be right, it sounds a lot like a vacuum leak and its hard to check the IM gasket area (the way I do it) I'll throw a new one on and a throttle body gasket while I'm in there. couldn't hurt right?

 

do you smoke? or have access to a smoke machine?



blow smoke down a vacuum line



look for smoke

 

I use incense sticks. Pressurize the intake to about 2 psi and look for the smoke to move

 

that works



ive seen people mix a lil florescent dye with some top end cleaner and use a spray bottle to spray the crap out of the intake track then use a uv light to look for the dye



i still say smoke works the best