Ahnenerbe

Hey guys, I have a bit of a mystery I am trying to wade through. A few times when I have started my 89 with the L98 I will go to back it out of driveway and as I am backing out it all of a sudden the idle drops to 4 or 5 hundred and the engine light comes on.

When I pull it back up and turn it off, it fires right up, lights out, and idles fine. If it's warmed up I have no problem, only seems to do this intermittently when cold. I retimed the engine to 6tdc and even advanced it a bit more to see if that would help, but it still throws the code once in awhile. Any ideas on what to look for?

A FB forum friend sent a testing procedure from the manual, and is this the only route to go? Thanks

billschroeder5842

Because this only happens when your engine is cold, I'd start by 1) checking your coolant temperature sensor and 2) your IAC valve.

Your cold engine is leaning out and will run poorly when cold. You coolant sensor may be sending false reads to your ECM and not giving your cold engine enough gas. Back in the day we would say your carburetor choke is not working.

Also, don't trust your RPM dash gauge so put on a "real" tester. You need to be running at 700ish IMHO.

Kevova

DTC 42 is Electronic Spark Timing fault. It is guessing diagnosis really, modules on each end of a wire. The rpm drop is caused by spark advance going away. ECM grounds, set timing connector, 4 wire connector wires too close to spark plug wires. disconnecting and reconnecting sometimes will correct condition if just slight corrosion. If your stuck guessing ICM has more heat cycling so I would try it first. Make sure set timing wire isn't chafing on booster

Ahnenerbe

I'll take the connector apart and clean it good. I started her last night and backed out and it did not do this. I am blessed with the intermittent problems lol.

Ahnenerbe

Dude, you were right on, started her up and started playing with wire and connector and boom she started running rough and code came on. Clean connector good ans so far no issues.