The setup:

GM847, ported heads, full bolt-ons
Running an old Speed inc full tune for a Comp XE 224/230

The problem:

My Actron CP9145 scan tool shows long term fuel trim on Bank 2 is pegged at 25%, Bank 1 is substantially leaner (around 5%). I have two brand new front O2 sensors. In addition, the car is showing a random multiple cylinder misfire code... even though it pulls like a bat out of hell and seems to drive just fine.

This sounds like a similar issue to this thread. I've seen one method for fixing the problem outlined here but I'm not sure if it applies for an OBD2 car.

Any hints? Thanks!

 

Seems logical. Misfire causing it to run rich. I would expect the bad cylinder to be on bank 2. Start with the plug wire. Then go for the plug.

 

Check for misfire history codes on each cylinder. Sounds like wires or plugs. Go from there. You may even want to check the wires to the O2 sensor for connectivity on that bank at the PCM, its pretty easy to do.

 

I don't think you will ever be able to get rid of the false misfires detected because of that cam. So, it's going to be hard to know when you have real misfires.

 

Ok, did some more driving / scanning after I cleared the codes.

Misfire on cylinder #3. After more driving, I get a multiple cylinder misfire code. With each reset of the computer I get similar behavior... always #3.

I'm going to pull the plug and make sure that it's not fouled. If that isn't it, am I looking at a bad opti?

 

Are you still running OBD II? If so it, along with the CPS, is going to report misfires simply because of the cam lope. That doesn't mean you're really getting misfires--it is only measuring the position of the crankshaft, the jerkyness of the lope is very similar to a car that's misfiring.

The split BLM's at idle are something many of us just learn to live with. I switched to a PE idle, set in the PE table to make it run as lean as I can without getting a lean code (which would be false, it's still pretty rich) and lock the BLMs with BLM locker.

In short, it sounds like your engine is running perfectly, the computer just doesn't like it.

 

Played around with the car some more. Found a small exhaust leak by my EGR fitting on the Kooks, but sealing it up didn't change the split BLM issue. Part throttle cruise and acceleration seems ok so you guys are probably right... cam is just throwing off the computer.

Jon, what software do you use for tuning? Can you recommend any OBDII specific sites? A lot of the things I've seen seem geared towards the '94-'95 cars.

 

I got a random misfire code with my wienie LT4 hot cam. GM847 would probably be worse. LOL

 

same here w/ the puny 211/219 "RV" cam.

 

Sorry, I can't--I swapped to an OBDI PCM conversion from Ion M (Madwolf). I tune with Tunercat and TTS Datamaster for OBDI. OBDII was just obnoxiously expensive a couple years ago. It might be better now though, I haven't kept up.