(Crossposted from LS1tech)

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. (But, to the best I can tell, they look OK, no fraying, burned wires, etc.)

What could it be? MAF? MAP? Knock sensor?

(I have a spare MAF I can try)

UPDATE:

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.

 

I had almost the exact same issue in my 95Z awhile ago. Scanning it I found that the passenger side 02 was stuck at 4 millivolts. It wouldn't switch at all. Does your check engine light come on?

EDIT: I know you changed the sensors, but, TBH I've never had good luck with any of the bosch sensors. I put a set of bosch in once just to get the car running and it made almost no change.

 

When the sensors are plugged in, there are no codes. At one point when it was missing really badly, it set a code but the SES light turned back off after only 15 seconds or so. I am guessing it was just a 'random multiple misfire' code though, to be honest.

I do wonder, however, if I have a wiring harness issue with the sensors. I maybe need to chase the wire runs this weekend and inspect them entirely. I know the loom is missing from one part of them though.

 

I would try and scan it and find out what the code actually was. When my O2 was acting up it was only tripping a "right bank lean" code. I only tracked it cuz I saw it wasn't changing voltage up and down. And if possible, trace your harness. I see you have LTs on the car, if its the one in your sig, I'd check your extensions too that I'm assumg you probably have on it.

 

Yeah I have Jet Hot LTs. I have an extension on the passenger side, but I think the drivers side was long enough stock. Although I may be mis-remembering that.

I plan on looking at it with Datamaster when I actually get my ALDL cable.

 

I have seen many posts on here, down through the years, that indicate Bosch O2 sensors do not perform well in the LT1.

 

Yea, I agree with moparman, I too have seen alot of that. And TheHeadFL if you have an autozone or an advance auto near you just take a ride over and get it scanned real quick while your waiting for your cable. You're OBDII, it should at least store the code in memory. And you keep saying ALDL, is that just from like ALDL cables or are you trying to get an old GM OBDI aldl? Cuz, that won't work on your 97.

 

I have an OBD1 conversion.

 

What brand O2s do people use?

 

There is no misfire detection in OBD-I. Get the code.

AC Delco/Delphi

 

Interesting... would there still be a code stored even if the SES light wasn't lit anymore? It's been a long time since I've messed much with my LT1 but I can never remember having a SES light that went away after 15 seconds or so.

EDIT: Also, I'm screwed for the moment since I had reset the PCM in the process of troubleshooting. Code is lost. There was no SES light after of driving around misfiring for like 20 minutes last night. I'm sure if I drive it long enough it may come back...

 

I'm not sure, it depends on the OBDI that Ion did in your tune. I'm fairly certain that my 95 stores codes even with the light being off. But if you pulled the fuse it probably cleared the memory.

EDIT: also, I was just browsing your page, and in the one section of pics, it looks like you have MSD wires, I had nothing but problems with them, a few sets actually. You've checked them to make sure it's just not a coincidence that the car is warm and in closed loop for the wires to get more arcing? Idk, just trying suggestions till your light/aldl cable comes in and you can get some data. lol

 

Sorry, I probably forgot to change it somewhere on my page.

I am running Taylor OTVC wires since I did my longtubes.

Also, I guess I am going to just eat it and buy Delco O2s... Even though the Bosch ones *might* work, its not worth having the variable. $100 down the drain. That is what I get for assuming O2s were O2s, lol.

 

All the PCM's store the codes as soon as they are set. They will retain them in storage until a specific number of "drive cycles" without the same problem recurring.

 

I would recommend just waiting to get the delcos until after you get your cable and such. Why not? Get some data and see the readings, go from there.