high idle sometimes w/ built vortech engine

whitenblue65

New Member
Apr 29, 2010
2
0
0
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 braking). 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!
 
  • Sponsors (?)


I had the same problem after I put a new throttle body on. After messing with the tps a 1000 times and everything else it was the throttle body. It turns out the blade inside the throttle body was off center letting to much air thru when the blade was closed. Unless you just got a new throttle body this is probably not your problem, but you never know. Check out the IAC. What the hell is compression braking?
 
well first off some pictures of what you are talking about with the discharge valves would be helpful. if you have a draw through maf sensor than you have to have all metered air(air read by the sensor) stay in the intake system. you cant have any of it going into the atmoshphere. that will cause running issues. the most common cause for hanging idle on our cars is the IAC valve. try cleaning it or just replace it. it sounds to me like you may have a couple of issues here. is your car tuned? i cant believe your tuner wouldn't tell you that that discharge setup is wrong if you have had it tuned.

oh and SWL987.....compression braking is kind of like a "jake brake" on semi's. on a manual car when you decel without pushing the clutch in the engine will actually slow you down some. that is the natural compression of the motor with the throttle closed creating a resistance helping you slow down.
 
I have a 90mm Lightning MAF, good point about all metered air having to stay in the system, logic says that would throw things off. The strange thing is that it used to run fine with the same set up. The car was tuned by NRP in Las Vegas but everything was done with the previous owner so I cannot be sure if everything is still exactly the same or not.

I have cleaned the IAC several times and even swapped one from another car for a few days but it did not help at all.

It is hard to get good pictures of the discharge valves, but here is an album of my attempts: Picasa Web Albums - whitenblue65 - engine In picture #7 you can see the two black rubber hoses with a green stripe which both have discharge valves connected (actual valves can't really be seen though). The one on the left is venting to atmosphere and one on the right goes into the intake.