I recently purchased an '88 IROC Z-28 w/ 305 TPI and 4 speed auto at auction. It was a repo, so I have no info on the history of the motor and what mods may have been performed on it. Everything seems stock, but it's really a bit of an unknown.

It runs great, but I've got a couple of issues that I think may be related:

1. The tach reads high, I'm guessing roughly 1.5-2x the actual RPM
2. I'm getting a trouble code 36, which from trouble code lists appears can happen because of extra ignition pulses.

The engine appears to idle normally, though showing around 1500RPM, and shift points seem normal though the tach spins clean off the top when I put my foot into it!

I suspect this could be an caused by a noisy electrical connection in the ignition, causing a doubling of the ignition pulse to be read. My other concern is that this could be caused by a modified ignition system, such as a non-stock coil or ignition module that is not compatible with the stock tach and/or computer.

Any advice?

Welcome to the Club! You might pop open the distributor cap and see if an aftermarket module and/or coil is installed. If an Accell coil is installed, the OE TPI coil cover will not fit do to the increased height.

An OE module starts with a small ignition pulse, read dwell, and increases in dwell as rpms rise. Some aftermarket modules, I believe, function with a "set" dwell time throughout the rpm range.