3800 Random multiple misfire but it doesn't seem like it? (Page 1/1)
Threedog DEC 27, 09:24 PM
I have a 3800SC in an 87 fiero with a 5 speed, getting a random/multiple misfire code.

Here is what happens:
- At seemingly random times, but most often when I am slowing down in 2nd or 3rd gear (foot off the gas petal and RPM between 1,500 and 3,500), the CEL light will start flashing. Sometimes for 20 seconds, sometimes for 40 seconds sometimes for a full minute. (With a random multiple misfire code).
- Sometimes I can get it to stop by putting it under load for 10-15 seconds, sometimes it just stops on its own.
- If I come to a stop and is flashing, it won't stop flashing until I get going again (even though it idles smooth while flashing)
- The weird thing is that it does not feel like a misfire. The light can be flashing under WOT and the car is not stuttering or anything. The light can be flashing under idle and the idles is still perfectly smooth. I have had misfires before and they are usually way more noticeable.


This problem started when I let the car sit for 6-8 months. It didnt do this before, it does it now. The things that I changed over that time:
- Harmonic Balancer
- Spark Plugs
- Wires
- Exhaust
- New pre cat O2 sensor


Stuff I have tried:
- New Coil Packs
- Two New Fuel filters (over the course of 4-5 tanks of gas)
- New wires
- New plugs
- Three different ICMs
One small thing that might help, the car doesn't "gurgle" when slowing down anymore. Before the hiatus, if I let off the gas at 2500+ RPM I got unburnt fuel exploding in the exhaust creating a "gurgling" sound. Now I don't get that anymore..
Dave E Bouy DEC 28, 07:52 PM
My guess is that since you changed the harmonic balancer the ECM needs to do a crankshaft varience relearn. I was getting the same codes 9n a new 3800 swap and while I've done number of 3800 swaps this was the first time I had to do this. In my case it was a matter of the freshly tuned ECM not working with this motor and it had to learn to read the signals it was getting from crank position sensor. Or something like that anyway lol. Now you may ask what does this have to do with me? While I was researching what a crankshaft variance was I learned that it is a procedure that may, (or may not) have to be done if you change ecms, flywheel, crankshaft position sensors or HARMONIC BALANCER. It's a pretty simple thing really. Connect scan guage to ALDL, set scan guage to crankshaft variance relearn and follow the prompts from the scan guage. It finishes with having to briefly mash the throttle pedal until it hits the redline and Bob's yer uncle!
Dave E Bouy DEC 28, 07:56 PM
Oh and I forgot to add that mine was also running like a champ at the time.

[This message has been edited by Dave E Bouy (edited 12-28-2020).]

Threedog DEC 29, 06:55 PM
Hmm, relearn, I might have replaced the cps sensor as well, I will certainly give it a shot! Thanks