Unsteady idle

[todras]

Ok here are the symptoms. Coming to a stop off of high rpms when the clutch is put in the tach drops like a rock. Engine goes to 600 rpm and shoots up to 1000, then 500 and up to 1100 then on the third try the car stalls. I've replaced the TPS to no effect, I cleaned the IAC (which wasn't dirty) but that doesn't mean it's not faulty, the MAF seems fine. I pulled codes 114, which would go off because the car was slightly cool, 157 because I forgot to plug the MAF back in after playing with it, 543 because the car stalled. We thought it might be plugged cats but on furter review the fuel pressure reg. (FPR). I'm going to atempt to plug the vac line to see what she does under rich. Suggestions welcome.

 

[projectSHO89]

Check for a vacuum leak.

Run the KOER test if you haven't already.

Look for codes related to the ISC.

I suspect your ISC/BPA/IAB valve is not performing properly.

Steve

 

[sdpatt]

projectSHO89:
I suspect your ISC/BPA/IAB valve is not performing properly.

I concur.

 

[haydenm315]

I had the exact symptoms with my car after I did the 60k, put stage II cams, and ud pullies. I ran codes and came up with nothing out of the ordinary. My iab was recently cleaned. I reset the idle as described on this forum and shotimes and it did the same exact thing. Does your car do this only when it has warmed up fully? Mine seemed to idle fine until it warmed up.

After the first start of my car after 60k, the idle was all messed up and I didn't know about the idle reset procedure. I did notice the setscrew on the throttle body and fiddled with that until my car seemed to idle. Later I found out it's known as the base idle set screw and I was going about setting the idle all wrong. Just for grins I increased the position of the base idle set screw a little bit and reset my idle. After I reset the idle, I never had the stalling problem when depressing the clutch again. Maybe that screw came loose somehow?

<small>[ February 21, 2003, 10:56 PM: Message edited by: haydenm315 ]</small>

 

[Huntervf]

One thing Todd forgot to mention is that the car idles fine. Start it up, let it set and no problems all day long. Drive easily all day long and no problems. The only time there is trouble is when the engine comes down from high RPM, either revving it out of gear or pushing in the clutch after a WOT event. Another SHOowner with a 93 MTX had a very similar problem that nobody could figure out...took it to Perf. Plus and it took them 5 minutes to diagnose the FPR.

I suggested FPR since there were no codes being set and the car runs fine under most circumstances. FYI I did a 60k on this car last fall & it also recently had a fuel filter. Would it be possible for the IAB to malfunction only after coming down from WOT?

Hey Todd, how's it doing with the FPR out of the loop?

 

[todras]

I think we might be onto something with the FPR. I pulled the vac line off and plugged it. A golf tee fits perfect BTW. She seems to be better. I took her for a 4-5 mile ride and jammed on the brakes trying to replicate. It did dip low once but it didn't flutter. It just came back up and stopped. The car seems to idle at 1100 now with the FPR uncorked. I just came back from a long drive so I'll report back after some cold driving.

 

[projectSHO89]

Dashpot control is one of the functions of the Idle Speed Conrol/Bypass Air (ISC/BPA) solenoid.

This one isn't working properly when the engine comes off throttle.

I would suggest a swap or replacement.

Steve