My wife drives a 1999 Pontiac Grand Am GT (3.4L V6) and, luckily, hadn't had any major problems with it since she purchased it in 2002.
Then things got ugly.
Back in the summer of 2006 it started drinking coolant. I tore the engine down and took the head in to get milled and replaced all the gaskets from the head gasket up. No more problems that year.
Then in the summer of last year my wife and I drove it to Wisconsin for a friend's wedding and noticed poor gas mileage during the trip. The car ran fine the whole way, but after the rehearsal and dinner I was driving back to my friend's house and the car starting surging and stalled while at a red light. I started it back up and it did the same thing...surging (or "searching" as some might say) at idle, sometimes RPMs dropping so low it would stall. I limped the rest of the way with some fancy footwork to keep the RPMs up at stoplights and decided to look into it the next morning. The next day my friend read the DTC causing the MIL/SES lamp to come on and it said something about an O2 sensor (can't remember what the code or description was now, just that it was O2 sensor related). I started it up and it ran and idled fine, so I decided to risk the 300 mile drive home before I did any more investigation. Only I never did any more investigation, because besides poor gas mileage, it has run fine ever since with no check engine light coming on.
Until this past week...
My wife was driving back from work and noticed it was doing the surging at idle again. I took it for a drive and noticed it ran ok as long I kept the RPMs up over 1000, but when it came back down to idle it would surge, but then the surging would eventually even out to a steady idle. My first guess was the EGR valve was sticking, so I took it off and sure enough, the pintle was sticking! Aha! I cleaned out the EGR valve with carb cleaner and made sure the pintle moved up and down smoothly without sticking and re-installed it. I then took the car for a 20 minute test drive around town and on the highway and it ran great. My wife drove it the following day and didn't have any problems.
Then this morning she came back from the store upset because it was surging again. The SES light was back on and a quick check of the EGR valve showed that it was working properly without sticking. I checked for vacuum leaks...none. I also put in a new air filter. When I did the head gasket in 2006, I replaced the PCV valve, spark plugs and wires, etc, so it shouldn't be any of those. I then checked fuel pressure at the fuel rail and measured 41psi at key on, 37psi at idle...both within spec. Not a fuel delivery problem. Then I cleaned out the MAF sensor and throttle body. That didn't help either. I also cleaned out the IAC valve, but it didn't appear that dirty or sticky anyway. At this point, the car was running so poorly at idle it would start and immediately stall. As a last resort before giving up, I decided to believe the code scan from last year and replace the upstream O2 sensor. After driving it for 30 miles, the light has gone off and I'm back to a smooth idle.
Was it really that simple? Did the ECU report a code that was right on the money, not one that was being caused by something else in the emissions system failing/operating poorly? Can a bad O2 sensor really cause an engine to run that poorly? Or will the car run fine for a while and start acting up again?
Any thoughts, previous experiences, technical expertise would be more than welcome.
Then things got ugly.
Back in the summer of 2006 it started drinking coolant. I tore the engine down and took the head in to get milled and replaced all the gaskets from the head gasket up. No more problems that year.
Then in the summer of last year my wife and I drove it to Wisconsin for a friend's wedding and noticed poor gas mileage during the trip. The car ran fine the whole way, but after the rehearsal and dinner I was driving back to my friend's house and the car starting surging and stalled while at a red light. I started it back up and it did the same thing...surging (or "searching" as some might say) at idle, sometimes RPMs dropping so low it would stall. I limped the rest of the way with some fancy footwork to keep the RPMs up at stoplights and decided to look into it the next morning. The next day my friend read the DTC causing the MIL/SES lamp to come on and it said something about an O2 sensor (can't remember what the code or description was now, just that it was O2 sensor related). I started it up and it ran and idled fine, so I decided to risk the 300 mile drive home before I did any more investigation. Only I never did any more investigation, because besides poor gas mileage, it has run fine ever since with no check engine light coming on.
Until this past week...
My wife was driving back from work and noticed it was doing the surging at idle again. I took it for a drive and noticed it ran ok as long I kept the RPMs up over 1000, but when it came back down to idle it would surge, but then the surging would eventually even out to a steady idle. My first guess was the EGR valve was sticking, so I took it off and sure enough, the pintle was sticking! Aha! I cleaned out the EGR valve with carb cleaner and made sure the pintle moved up and down smoothly without sticking and re-installed it. I then took the car for a 20 minute test drive around town and on the highway and it ran great. My wife drove it the following day and didn't have any problems.
Then this morning she came back from the store upset because it was surging again. The SES light was back on and a quick check of the EGR valve showed that it was working properly without sticking. I checked for vacuum leaks...none. I also put in a new air filter. When I did the head gasket in 2006, I replaced the PCV valve, spark plugs and wires, etc, so it shouldn't be any of those. I then checked fuel pressure at the fuel rail and measured 41psi at key on, 37psi at idle...both within spec. Not a fuel delivery problem. Then I cleaned out the MAF sensor and throttle body. That didn't help either. I also cleaned out the IAC valve, but it didn't appear that dirty or sticky anyway. At this point, the car was running so poorly at idle it would start and immediately stall. As a last resort before giving up, I decided to believe the code scan from last year and replace the upstream O2 sensor. After driving it for 30 miles, the light has gone off and I'm back to a smooth idle.
Was it really that simple? Did the ECU report a code that was right on the money, not one that was being caused by something else in the emissions system failing/operating poorly? Can a bad O2 sensor really cause an engine to run that poorly? Or will the car run fine for a while and start acting up again?
Any thoughts, previous experiences, technical expertise would be more than welcome.