whitenblue65

I have a '98 Cobra with a 4.6L with a vortech V1 and built motor. I'm having issues with the idle sticking around 1,500-2,000rpm but it will seemingly randomly return to a normal idle sometimes. If I turn the car off and on it will idle normally when re-started and always idles normally when first started. This is also affecting off throttle deceleration (compression breaking). It sometimes will compression brake until about 2,500rpm but then stop. Other times it will have compression braking at all rpm which is when it seems to idle normally when in neutral.

I adjusted the TPS from .8-1V and even swapped the TPS with another car but had the same issue. The IMRC has been removed and I cannot find any obvious vacuum leaks.

One possible problem could be that only 1 of the 2 discharge valves off the aftercooler go back to the intake while the other vents to the atmosphere since there is not a place for both on the intake. I have also heard the discharge valves can stick which could push boost back through the intake.

If anyone can help me I would truly appreciate it. I'm sure I left out some crucial information so please let me know what else you want to know. Thank you!

whitenblue65

Anyone?

Frostbitten

check your fuel pressure, ground wire connections, and vaccum leaks

Moonshine

Does it only idle high while driving it? Meaning will it do it if you start the engine and run it until warm. Using the foot throttle to wind the engine up. Versus using your hand on your throttle body.

wruno

Have you cleaned your IAC valve?

Z28KLR

You can try this if you like but don't expect too much. Ford is fairly explicit in noting that the IAC is not a serviceable part and should be replaced if not functioning properly. Are you getting a CEL??

uberstang1

It is due to you're IMRCs being deleted. 1500-2000 is still a little high, but out of the 3 supercharged 96-98 4 valves around here they all have the IMRCs deleted and each one idles 900-1100, can't get em to idle any lower with out stalling issues including mine.

whitenblue65

Thank you for the replies! The other day I tried gently slipping the clutch while stopped to bring the rpm's from about 2,000 where it is sticking down to around 1,000. After I did this it will sometimes stay at a normal idle...

Frostbitten: fuel pressure is fine (I have a gauge), it probably wouldn't be ground wires since it is a sudden fix when turned off/on, and I have checked for vacuum leaks without finding any but it is most likely not the cause either since it only happens randomly while vacuum leaks are usually consistent.

Moonshine: I have not tried starting from cold with only manually revving with the throttle body but will do so and get back to you. What exactly will this tell me if it works/doesn't work?

Wruno/Z28KLR: I have cleaned the IAC valve and even swapped one from another car but neither helped the problem at all.

uberstang1: The IMRC's are deleted, but I am considering 900-1100 as being my normal idle. The problem is that it sticks well above that at 1,500-2,000 which is very abnormal even without IMRC's.

Stevecooper

Can you data log the TPS at the time this happens? Perhaps the TB is sticking slightly.

whitenblue65

Great idea, I'll get on that!