Closed Loop Misfire/Stumble
Thread Starter
On The Tree
Joined: May 2004
Posts: 183
Likes: 0
From: Orlando, FL
Closed Loop Misfire/Stumble So I've been trying to troubleshoot this problem and I'm looking for some ideas...
Here is the scenario:
Car runs perfectly fine in open loop, and when the O2s are unplugged, I can drive around for hours and never have a problem.
When I plug in the O2s, once the car goes into closed loop, I get a very bad part throttle misfire/stumble. It can be very violent, but sometimes it will straighten up and be OK for a few minutes. However, the idle is fine, and WOT is actually fine.
Replaced the O2s with brand new Bosch O2 sensors, cleaned off the weatherpack connectors with electrical parts cleaner, made sure everything was good, and reset the PCM (battery disconnected). Car ran fine through closed loop for probably an hour worth of driving, and then the misfire gradually returned.
I'm ordering an ALDL cable so I can log it with Datamaster, but the only thing I can come up with is that I have either a vacuum or exhaust leak, or some sort of bad sensor. Obviously the O2s are new, but I can't rule out the wiring harnesses for them.
What could it be? MAF? MAP? Knock sensor?
Here is the scenario:
Car runs perfectly fine in open loop, and when the O2s are unplugged, I can drive around for hours and never have a problem.
When I plug in the O2s, once the car goes into closed loop, I get a very bad part throttle misfire/stumble. It can be very violent, but sometimes it will straighten up and be OK for a few minutes. However, the idle is fine, and WOT is actually fine.
Replaced the O2s with brand new Bosch O2 sensors, cleaned off the weatherpack connectors with electrical parts cleaner, made sure everything was good, and reset the PCM (battery disconnected). Car ran fine through closed loop for probably an hour worth of driving, and then the misfire gradually returned.
I'm ordering an ALDL cable so I can log it with Datamaster, but the only thing I can come up with is that I have either a vacuum or exhaust leak, or some sort of bad sensor. Obviously the O2s are new, but I can't rule out the wiring harnesses for them.
What could it be? MAF? MAP? Knock sensor?
Thread Starter
On The Tree
Joined: May 2004
Posts: 183
Likes: 0
From: Orlando, FL
I am going to try resetting the PCM again to see if it temporarily solves the problem. Given that it took so long to misbehave, I'm thinking it was something the PCM 'learned'.
Also, I have a spare MAF I can try.
Also, I have a spare MAF I can try.
Thread Starter
On The Tree
Joined: May 2004
Posts: 183
Likes: 0
From: Orlando, FL
Took it out again, and it seems to need to run for about 15 minutes before it acts up.
Pulled the PCM fuse, unplugged the O2s, and it runs great again.
A little note, when it was acting up, there was a definite increase in 'lean popping' in the exhaust.
Pulled the PCM fuse, unplugged the O2s, and it runs great again.
A little note, when it was acting up, there was a definite increase in 'lean popping' in the exhaust.
Teching In
Joined: Mar 2012
Posts: 6
Likes: 0
I am having the same problem with mine It is a 99 ta motor stock except for short tube headers has a hot rod tune since it is in early 50s truck has the same harness that came out of the car with unneeded wires removed just got obdcom scanner today and 02 are nowhere running the same numbers still playing with it. I have 2 new o2 sensors 2 new knock sensors all ac delco any help appreciated
11 Second Club
Joined: Jan 2010
Posts: 1,707
Likes: 13
From: New Orleans, LA
https://ls1tech.com/forums/showthrea...5#post17564605 if you have longtube headers read this I hope this helps
