2000 Pontiac Grand Prix

**sturgillbd** · 01-15-2014

I have been working on a 2000 grand prix for a friend (3800 V6). It had the typical intake gasket troubles. Replaced lower intake gaskets and whole upper intake. Engine had been passing coolant into the upper plenum. Everything is sealed up now and took it for a test drive. I sets a code of bank 1 lean. It only has an 1 upstream and one downstream o2 sensor. Watching the sensor data during a test drive, the o2 swings from .9 down to .02 volts while driving down the road. I assumed the O2 sensor is poisoned from the coolant being leaked into the upper plenum so I bought a new O2 sensor and installed. The ecm never went into closed loop and the upstream O2 voltage was basically locked at .450 volts. It then set a code of little or no activity on B1S1 (upstream sensor). We pulled an O2 sensor out of a running car with the same engine (2 years newer) and installed. O2 voltage stayed at .450 like the sensor wasn't even plugged in. Re-installed the old sensor and it instantly started the wild swings between way rich and lean. Compared all of the sensors and all have the heaters, signal and ground in the same places on the plug. The original was an AC sensor. The new sensor was denso and the used one that was pulled from a running car was a delphi. I have traced wires with a meter from the O2 sensor to the ECM and all are connected well. Checked the grounds on the car and they are fine. The worst ground had 34mV between the battery post to that connection. Cleaned it and it dropped to 15mV. I am going to attach two screen shots of the scanner screen in live data view taken 12 seconds apart with vehicle in park and at idle. This one is just weird. I also get a P0300 (random multiple misfires) when on a test drive. Fuel pressure starts out at 50 with key on. Normal driving 43-45 psi. During hard acceleration pressure goes to 52-55 psi. During hard acceleration, it feels as though it goes way lean and the scanner shows this in the live data. MAF shows good flow numbers during test drives. TPS is faily linear with no dropouts on scanner. MAP sensor seems operational. Engine cold, the CTS and IAT are within 1 degree of each other and seem pretty accurate according to outside temps. Has anyone ever run into anything like this??? I have also sprayed carb cleaner around intakes looking for vacuum leaks and none have been found.

**pmkls1** · 01-18-2014

The wife's car had a lean code stored as a history code from before I replaced the lower intake gaskets and upper plenum, but they did not return. I've got a few ideas of where to look, but have not seen that exact problem. In your snapshots the MAF readings look a little low IIRC. I think that at idle you should be seeing closer to 6 g/s so I would try cleaning the MAF just to see if that changes anything. Don't use anything other than MAF sensor cleaner as many other spray cleaners leave a residue. I would also remove the sensor from the throttle body assembly for cleaning. I would also pay attention to what the actual MAP readings are as it may be reading but the numbers could be skewed (not likely but possible). There is also a possibility of the PCM being faulty, but that is also less likely. Aside from that, there is also a very good possibility that the ethanol content of the gas that is in the car is too high. I have seen this happen many times so I know that it does happen. Unfortunately, I do not know how to test the alcohol content of the fuel without the special tool that we used at the dealer. Your O2 sensor experience is odd, but I'm not sure as to it being related to your problem since the original O2 sensor does seem to read. So, I would not focus on that until after looking into the other possible causes for the lean codes. Out of the list of possibilities I would rank the MAF sensor and fuel contamination as the top two likely causes. HTH,
Phil

**sturgillbd** · 01-19-2014

I found the problem yesterday and repaired it. Just now got a chance to log back on. Never fully figured out the O2 problem with the other sesnors. I had a coil pack that measured fine using the specs in the service manual and using a spark tester. Found problem using a osciloscope and an inductive pickup on the plug wires. On cold start, It missed somewhat but randomly. Measured voltage on each wire and noticed when it would miss, the two wires on that coil pack were definitely lower. Engine seemed to cut up when cold or under heavy load.I had already cleaned the MAF when I replaced plugs/wires. Cleaned throttle body / IAC when we replaced the upper intake plenum and lower intake gaskets. The problem showed up randomly but mostly when cold and under load (going up steep grade). The coil was breaking down but not fully dead. It was just an odd problem. Replaced coil packs and all is well. Thank you for the reply. I ended up leaving the original o2 sensor in it and viewed live datastream after replacing the coil packs. No more codes are being set. I went ahead and replaced all three coil packs because if one was going, the others were probably not far behind. Upstream O2 sensor doesnt have the wild swings now. Going to drive it a while and see how it performs.

**busta9876** · 01-21-2014

some year vehicles used case ground vs isolate ground oxygen sensors. The case ground sensors relied on the sensor being screwed into the exhaust for the PCM to get a ground reference on the tan wire to which to measure the 0-1 volt on the purple wire. Later years used isolate ground o2 sensors, which the PCM supplied a low reference to the tan wire, and the tan wire was than 'isolated' from the case of the sensor. If you install an isolate ground sensor in place of a case ground sensor, the voltage will be stuck at 0.450. This is why you should always use the correct o2 sensor for that specific application. On the GM V8 vortec 5.3 engines GM was nice enough to change the connector around so you could not plug in the wrong sensor, however, fwd v6 I have found you can mix them up...which can certainly cause some confusion.