Z4Ccoupe

My water pump was going out. Not bad enough to see puddles, but enough to drown my original Optispark. I had great success with MSD in the past so I decided to go with the MSD 83811 for my 96 SS with the stock LT4 motor. Everything went together easily and it started immediately. No codes. And the car runs significantly better above 5000 rpm. However, when it gets warm, the idle will occasionally surge upward. Sometimes it will rev to 1800 rpm in neutral. When I was putting everything back together, it seemed like I reconnected fewer fittings and connections than I took apart. But after a thorough search, I can’t find any thing that isn’t put back together correctly. Any suggestions? Thanks.

Injuneer

I've run into that problem 3 times recently. Using a data log, we tracked the problem to a sudden, impossible (single frame reading of ~3,500 RPM) spike in engine idle RPM, followup by the IAC count running up. First case was an MSD Opti that had just been rebuilt by MSD, returned and installed. We suspected an erratic high resolution signal from the Opti cam position sensor. Owner replaced the Opti with an AC Delco, problem went away.

The other two cases were on MSD and one AC Delco (or Delphi).

Watch the tach needle for a slight "twitch", just before the RPM starts to increase. That seemd to be the common denominator in the three cases. A data log would also tell you if the RPM signal (from the high res signal) is becoming erratic.

mzgp5x

I had a similar problem @ engine running temp 160*F. I recalibrated the IAC valve count. I had replaced my TB with a large mono_blade. I lowered the count value with a PCM flash. That fixed it. B.

Z4Ccoupe

Some additional back info. When I installed the LT4 someone had previously cranked the idle stop screw in a ton. I couldn't get it to turn back out. When I first started it the car idled around 2000 rpm. In spite of numerous attempts, I couldn't budge the screw. So I did a field expedient repair, bending the throttle stop arm. Probably not the best solution but I did get the idle down to 1000 rpm. The high idle code went away and no idle problems with the original optispark. But when I replaced the optispark with the MSD, I would get this occasional high idle surge. That was my original post on this thread. I later decided I still wanted a lower idle, more like the 700-800 rpm of my last Z28. So I bent the throttle stop arm more. To my surprise the idle stayed at 1000 rpm but I no longer have the occasional high idle surges. I don't know enough about the systems but I guess the TPS is a partial clue to the high idle. Can I get my idle down to 700-800 rpm? Or is 1000 rpm programmed into my tuned (pcmforless.com) PCM?