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After reading live data I see the O2s seem to be working but the PCM throws the O2 low voltage codes. My scanner also says its operating temp is too low to run in closed loop. The other, second problem is the PCM won't turn on the cooling fans (all circuits are good and tested). The live data says the engine is/was at 225° but won't trigger the fans. I have traced all wiring and tested the circuits and fans. I have also tested for intake and vacuum leaks with no luck. Would the free flowing cold air intake and no cat exhaust cause a lean condition that the pcm can't correct and cause the O2s to read low voltage? I didn't think that would be a big change.
Hope I'm posting in the right section. Background, 1998 LS and 4L60E from my personal car. It ran perfectly fine with no codes when I pulled it. I left the entire harness on the engine and peeled the car apart around it. I power up the hot wires and switched wires and made the connections at the C100 and other connectors, not the actual PCM red and blue connectors. It fires right up but runs rough while driving. Sometimes it accelerates great and then shifts and starts stumbling. Also the fans won't turn on. I grounded the pins at the pcm and the run so I know my wiring and relays are good. I did have a friend (reyguy) tune out the VATS and emissions stuff. He says the fans are still set to the factory settings. So any ideas? I am checking for vacuum leaks right now and I checked all of thw grounds and power wires atr the PCM and all are good. It did throw codes on the first drive. They were P0131 and P0151. I cleared them and drove it a few more times and they haven't come back but it still stumbles at times all over the place.
Took it for a 15 mile drive (forgot to mention it's in a 1979 Trans am). At about the 10 mile mark the MIL came on and it ran worse. I checked the codes and it was the P0131 and P0151 again. The fans aren't running either. I did check to see if there were any vacuum leaks but couldn't find anything with the carb cleaner. It seems like the PCM is missing a ground or something. I double checked the grounds and the Blue #21 and #60 and the red #72 & #76 and they were good. I checked the ground and 12v at the O2 sensors and they were good.
Oh. It is completely stock like it was in the original car except the emissions delete, cone air filter in the 12" tube and no cats and no rear O2s.
Just checked fuel pressure and it's 58-59 psi from idle all the way through wot including when it's stumbling. Spark plugs are new too.
Any ideas?
Thanks for the reply. I was hoping someone would easily recognize what's up. Must be more than I thought.
I checked all of the grounds and bat+ and switched + at the PCM connectors. I also checked the 3 grounds on the heads as well as ran separate ground wires from the battery to these locations to rule that out. It starts and runs great until part throttle or once it goes into closed loop. It has been about 5 years of sitting and only being started but not driven. Do you think the MAF could just go bad? I doubt both O2S would go bad at the same time. I guess I'll know more once the elm317 scanner comes in and I can check the data while driving. Anyone have a app recommendation for this scanner?
My old actron can't scan live data. I order a ELM327 reader so I can see live data. I hope that will point me in the right direction. The PCM has the stock tune, someone said it will need to be retuned but the only change is free flowing PYPES exhaust (no cats) and a 4" aluminum pipe with a cone filter for the intake. I thought the stock tune could adjust for that.
My old actron can't scan live data. I order a ELM327 reader so I can see live data. I hope that will point me in the right direction. The PCM has the stock tune, someone said it will need to be retuned but the only change is free flowing PYPES exhaust (no cats) and a 4" aluminum pipe with a cone filter for the intake. I thought the stock tune could adjust for that.
Everything is stock. I peeled the 98 body off and dropped the engine ine in the car. I had to make the intake out of 4" aluminum and a rubberelbow. I checked for vacuum leaks there too.. The exhaust is the stock manifolds and down pipes connected to a pyoes exhaust. The front O2s are still in the stock down pipe so their location hasn't changed. Like this
So, with a scanner I took the car for a ride. It started out great running in a closed loop and ran really well. Then, part way through the drive it switched to open loop sighting insufficient engine temp but the engine temp on the same screen said it was 183° (I hot wired one fan on so it wouldn't over heat.)
It's like the PCM doesn't know what the engine temp is but yet it reports it...
Slim chance but double check. The GM tech that installed my engine flipped the wires for the O2 sensors. In other words not paying attention to the length of the sensor wires he plugged the rear sensor plugs into the front 02 sensors. I don't have any rear 02 sensors. The plugs ins for the rear's are much longer than the plugins for the front so IMO he should have caught his mistake.
It's possible that with the car sitting so long and perhaps the engine never brought up to operating temp (just idled and then turned off) that the sensors have failed.
I'm of the opinion since these cars are now nearly 20 years old that the sensors should be replaced. Might want to remove one and look at the tip.
They are still connected as i never unhooked them. The rears I did as i had them tuned out. Just watching the live data it doesn't seam like they are bad but it's possible. The PCM goes into open loop and that's when it runs like garbage. Also, I let the engine temp climb to 225 (watching the live data) and it still won't turn the fans on. I must be missing something here. Someone has to have an idea.
I know the connection is correct to the pcm and the fans run when I ground that pin. The pcm also throws a code when I unhook them, so it is seeing them....
So, I figured I'd replace the O2S. Which didn't help then I remembered hearing that the PCM starts reading the MAF over a certain rpm so i unhooked the MAF and it runs 1000% better, it's amazing. It also stays in closed loop. So, what does this mean? The live data shows the MAF is sending data to the PCM but maybe it's flawed and I need to replace the MAF? See video of data.
08-17-2020, 05:48 PM
1998 LS swap PCM PO131 and PO151 
