BlackHawk23

Ive got a strange problem with my 95 z28, and my head hurts from scratching it so much lately. Heres the deal - after the car warms up and goes into closed loop, it runs fine, until i stop and it goes to idle. After a few seconds of idling at a stop light, the rpm's drop and i get a check engine light. When i get back to off idle, the light goes out and it seems to be fine. I keep getting a code for 02 sensor 1 and 2 too lean, and lo-res pulse failure. While in closed loop, the o2 sensors arent cycling like they should, they stay pretty constant at around 70 - 80 mv. As it turns out, when it stumbles at idle and the mil comes on, its actually popping back into open loop until i bring the rpm back up. Im not sure where to go from here, any ideas? Is this a common problem? Thanks guys!

Guest47904

Either you are actually running lean or it's not but the PCM thinks it is. However, you give us nearly no information. We have to guess at the codes and we know nothing of mods etc.....

If the low res code you are referring to is a DTC16, the engine would be shutting down. Even before you start investigating any thing else, you should look at the connections to the opti all the way to the PCM. Then a thorough investigation of the opti should be next.

Once you have that answered, then you have to figure out if the engine is actually running lean or if there is a condition that makes the PCM merely think it's lean. For example a burnt wire etc....

Typically you don't see damaged wiring to both sides of engine controls so start with things that are global to both sides. Fuel delivery, MAF, MAP and the like. Then after that move to the things that are split side to side like O2s.

If you had a fuel delivery problem where the pressure were low at the fuel rails, could account for lean running. Best to start by looking at fuel pressure at the rails. From there consider cleaning the fuel rails and injectors professionally either on or off the engine.

BlackHawk23

Ok, so this is the deal. I only have a problem at idle after i stop for a minute, it starts to run rough, and stumbles a little, then the check engine light comes on and i get codes 44 - left 02 lean, and 64 - right 02 lean and the pcm goes back into open loop. Then a few seconds after accellerating, the light goes out, the engine clears up, and goes back into closed loop mode. I only have the problem at idle, so im thinking maybe some sort of intake manifold vac leak? The opti and MAF are both brand new, and this problem came about after they were replaced. Also, when it happens, it smells like its running very rich. Any other thoughts? Its driving me crazy. Thanks a bunch.

sl2racer

what is your coolant temp sensor reading? not your radiator temp gauge...i am having a similar issue that every once in a blue moon the CTS decides it wants to read -40 degrees even though my gauge says 180. both o2 banks read lean and get the service engine light and then car stalls. wait about 10 minutes and it runs fine again. tends to only do this at idel.

BlackHawk23

Hmmmm, thats a good question. I know for sure the gauge is fine, but not sure about the cts. Ill put the scanner back on it and check it out.

BlackHawk23

Cts is good. Anybody have any other thoughts? Where should i go from here?

mrmint69

I would replace the O2 sensors if it was my car. If you had coated headers put on they could have fouled them out. If the sensors aren't cycling between 0xx and 8xx at idle then its a good chance they are bad. You should put a signature so we know what has been done to the car. If you have 100K mile + and your O2's are origional then a replacement is a good idea if anything seems out of wack.

BlackHawk23

The car is stock for now, no headers, the only thing it has is a flowmaster cat-back. I thought that maybe it could have been the 02's but it seemed a little strange that both banks would go out at the same time. I was thinking it could be something else, because the car runs perfect off idle. Ill check into getting some 02's for it, maybe that will help. Thanks.