So here's the background.... My 94 Z28 has had a surging idle problem for the past year or so. After the car got up to operating temperature and came to a complete stop, the tach got shaky, and the engine would rev up on its own. This happened in any gear, including park, but it was a relatively minor intermittent problem. A few weeks ago, the car suddenly had a severe stumbling and hesitation problem when accelerating from a stop. So I took the car to a shop and they changed the stock wires to MSD Super Conductors and put in a new MSD cap and rotor for my stock opti. That completely fixed the hesitation problem... but now the idle surging problem went from minor to severe. I brought the car back to the shop, and they tried swapping the computer as well as the ignition coil, but the revving at idle was still there. Now they're telling me they think it's the distributor (not the cap and rotor) that's causing it, and they need to replace the rest of the unit. Of course I flipped out, and I decided to get out of there and get your opinions on the problem instead. In hindsight, I should've just ordered a jason cromer OEM opti and put it in myself, but now it's too late. Here's a log of about a 4 mile drive with a few stops. You can see where it gets really bad at the end of my drive when in Park around cell 14726. No trouble codes thrown.

I really appreciate any help that you can give, especially because I know it takes time to analyze logs.

Here's the log file

 

I had the exact same problem. It is the opti - the bearing goes out and the high resolution pulse gets messed up caused a spurious rpm signal. If you scan it you'll see way more variation in the scanner rpm than the tach will show. A new opti will fix it.

 

Thanks for the quick response. That's kind of what I was afraid of (the opti).... Are there any other opinions, or anything in my log that might verify the culprit before I jump out and buy an opti? I also already replaced my intake elbow and checked for leaks.

 

Surging is usually caused by running lean. A marginal ignition being replaced by one that is firing better would tend to exagerate the problem. The very next thing to do is have the fuel injectors cleaned while they are still on the rail (professionally) of course. 3M makes some of the best cleaner that I have encountered. Change the fuel filter also.

Then go from there. Usually if the feedback portion of the opti is giving problems. It will throw a code. Usually.

 

Okay, I just replaced my IAC and cleaned out the TB passages. That seems to have eliminated most of the surging. There is still light surging (550 up to around 800 RPM) once in a while, but it tends to happen when the voltage gauge drops down to or below 12V. When I turn off the AC and stereo, the voltage goes up a little and the surge seems to go away. Is it possible that my weak alternator is causing the surge? I know it's going bad anyways, cause according to FreeScan, it's running at around 12.4V during normal driving.