Hi folks,
Have a 2001 Grand Prix with about 160k miles on it. It has a cylinder 4 misfire that is very consistant, but only at an idle, and when the motor is warmed up. In fact, it can even be warmed up, but the missfire will go away when the car is shut off for a few minutes. Restart car, and miss is gone for about 15 seconds until idle goes down to around 800rpm and then miss returns. Cylinder will miss a few cycles, then fire a few, in a rythmic pattern, and you can hear the miss and watch the tach climb and fall in about a 4 second pattern. This is my daughters car and I only get it a few hours on weekends. Here is what is done so far.
New plugs and wires (AC Delco and custom Bosch wire set) They were definately due, but made no change in problem.
Compression check, all cylinders looked good.
switched coil pacs.
Thought I heard a vacuum whistle on one start up so I sprayed whole top of motor with carb cleaner and no change in rpm anywhere.
Checked fuel pressure at rail and was good and jumped up 10 psi when vac hose removed.
Changed fuel injector cylinder 4 today.
Getting desperate, cleaned MAF sensor today.
Now it's gonna get weird.
Bought a code scanner some time ago to see if problem moved to another cylinder while trouble shooting. Had scanner hooked up today to reset code after replacing fuel injector. At about the same time I unplugged what I think is the first O2 sensor to see what effect it would have and somehow blew key code out of computer. Car started once, and ran about 5 seconds, and stalled. Figured car could not run without that sensor. Plug it back in and start, and same thing, run 5 secs and stall. Takes me about 15 minutes to figure out it is key code problem, and not computer shutting down fuel because I blew up the computer by running engine with no O2 sensor connected. Whew!
Then half an hour to reprogram key. Wild huh? Well that was the end of my trouble shooting window for today.
It really sounds and feels to me like that cylinder is running lean after warm up. The plug is white and dry when you pull it after idling with the miss. I do not fully understand the role of the O2 sensors and wonder if one of them may be the problem. Are they just a fuel trim device after it is warmed up and ready(closed loop??) and are they not in play when engine is cold? I can read O2 voltages with my scanner and show OSS11 at .100 volt and O2S12 at .785 volt while the problem is happening. What should I be reading on these at idle, and are the O2 sensors indicating the problem, or causing it?
Sorry about the long post, I am getting pretty frustrated.
Anyone have any ideas or willing to explain O2 sensors role at idle when warm.
One more thing, car runs great otherwise, plenty of power and engine is smooth off idle.
Help!!
