Hello, I've got a car with a '93 LT1 in it that I've driven for 3,200 trouble free miles until the other night. I brought it home hesitating and popping through the pipe, but there were times that it would smooth right out for a few seconds, then go back to behaving badly.

Tonight, I checked it out with the Datamaster program, and in open loop the short term/long term numbers were right around 128 and the car runs normally. As soon as it goes into closed loop, the short term numbers on the right bank go to one (1) and it starts popping out the pipe. If I rev it up, the numbers come up some, but nowhere near 128. I also changed the 02 sensor on the right bank (last night).

I checked for intake and exhaust leaks, bad wires, plugs and connections. Alternator output voltage is 14.6 and the grounds are excellent. I have had the same madz28 tune in the car for 3,200 miles.

I read everything about this problem using the search function and I haven't seen a clear-cut answer to this problem. If someone can help me with this, I'd appreciate it.

Doug

 

Unfortunately, I cannot answer your question, but I thought I could point you in a good direction. You might want to check out www.speeddensity.org

It is a great place for specific questions about 93's.

 

Maybe try to get a datamaster log and post it on here. Make sure it is in .csv (something like that) format so everyone can see it. That could help us figure out what's wrong.

 

Either the O2 sensor is not correct (readings of 1.0V) and the ECM is simply reacting to that unrealistic high voltage, or something is suddenly telling the ECM to add a ton of fuel, the O2 sensor is seeing it, and the short terms are dropping like a rock.

What do the O2 sensors read when its in open loop? .....at the point where it transitions to closed loop? .....when it gets into closed loop?

In open loop, the short terms shouldn't be "right around 128", they should be exactly 128, since there's no feedback from the O2 sensors and learning is disabled.

 

I'm going to be away from the car for the next day or so, but I'll report back with more specifics. I appreciate the help.

Thanks -

Doug

 

check for air leaks on that side of engine--exaust or header perhaps even a intake manifold leak. excess air entering the system creates a lean condition which tells the ecm to add fuel.

 

Maybe there's some special mode where it can go to one, but as far as I know it cannot go below 108 unless it is reprogrammed to allow a different BLM range. And I can't imagine anyone would reprogram it to allow a value of 1.

Therefore I think you have a computer problem. Either there is an error in the logging, or the PCM hardware or software.

If you have the ability to reprogram the computer, then reflash with the madz28 tune.

First thing I'd do, though, would be to email Ion and ask him about the BLM=1 situation.

 

When a short term fuel trim drops to "1", it means the PCM has detected a RICH condition, and is reducing the amount of fuel.

Your confused. He's talking about the SHORT term fuel correction (integrator, or INT), and your talking about the LONG term fuel correction (block learn multiplier, or BLM). The BLM's can not go below 108. The short terms can cover the full range of the 8-bit field - 1-255

 

I changed the 02 sensor on that side and the counts are close to the other sides numbers and the car runs like it used to, or maybe better. Should I have a variation of two or three counts on both the short and long term counts when the car is idling?

Also, what makes these 02 sensors go bad? I'm fishing around to make sure I don't have this problem again.

Help appreciated -

Doug

 

Each bank of the engine is controlled independantly. The PCM intentionally "dithers" the A/F ratio in closed loop, using the INT's to drive it from slightly rich to slightly lean, and back and forth several times a second. In any given frame of scan data, the left bank INT could be radically different from the right bank INT, and it would mean nothing.

On the other hand, you don't want your BLM's to differ by more than a few numbers in any given Cell.

Have you read the A/F control section of this?

http://members.aol.com/InjuneerZZ/ScanMast.htm