Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Hi all, I have a 1998 2-door Tahoe with an L31 5.7 vortec that I’ve rebuilt into a 383. I can’t resolve an intermittent random misfire issue that I strongly believe is electrical and/or computer related. More on that in a minute.
Everything has been replaced gone through multiple times. It’s been a journey of learning.
I have a few observations that seem to be a possible anomaly, which might point to the source of the issue.
- I see 2.2 amps of AC current on the power feed line. I don’t have a scope. This is from a Klein AC/DC clamp meter. Makes me go hmm.
- CMP retard is bouncing around on scan tool over 1000 rpm. It’s still within spec, but don’t remember it ever doing that and yes the dizzy is tight.
- The battery voltage from the scan tool shows 1.1 volts below actual even though the power wire on the DLC shows actual battery voltage.
The intermittent nature of the misfires is 80/20. Mostly there, but as I troubleshoot I can make it go away for a moment leading to me believe I’ve fixed it…only for it to return.
Examples: most recent first
- unplug Maf, plug back in and misfires went away for 30 minute drive session. Started next day…misfires were back. Messed with wires at maf connector…car stumbled. “Yes, I found it”.
Replaced wiring and connector…still misfires.
- Saw 7amps of AC current on battery feed to fuse box. “Yes, I found it”. Replace alternator. Start up. No misfires. Start 20 minutes later…misfires are back. Now 2.2amps of AC current on feed wire.
- saw a voltage drop issue on ecm 1 fuse. Using a test light traced it to the o2 power feed wires. “Yes, I found it.” Ran all new power wires for sensors on the circuit. No misfires on startup. Started next day…they were back.
The misfires are random, but there is a concentration on 4. Plugs, wires, cap, rotor, plugs all gone through multiple times. No intake leaks. Compression is good.
On most recent test drive, misfires were resolved while under load…greater than 20% TPS…and return as I let off.
Fuel pressure is 60 on updated spider. I setup a test rig on fuel system when I had the intake off for 5th time…fuel pressure regulator did not leak.
LT Fuel trims have been -1 to -3%.
New update 9/15/2024:
unplug TPS. Misfires go away. Replace wiring on TPS and CTS outside of harness into PCM. Plug back in and misfires return. Try a different TPS…still misfires. Coolant temp is accurate and TPS voltage rises smoothly from .55v to ~5v. So confused.
There is so much more I could say, but maybe start here. Thanks in advance!
There's a lot to digest on this post, but this video came to mind when you said manipulating the MAF made it run better. Usually that isn't a connector issue, although it seems that way while flexing the connector.
SCANNER DANNER shows us it's the part that's faulty when this happens, not the connector.
Topic Author
