Drivability problems have me stumped! Please help!
Thread Starter
Teching In
Joined: Jun 2006
Posts: 19
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From: Chicago
Drivability problems have me stumped! Please help! I have a 1987 Mazda RX7 GXL with a swapped in 2000 Camaro SS LS1 and T56 (40k miles). Mechanically, my swap is complete, but I’ve been fighting with a drivability problem since the new engine has been running.
Symptoms:
The car runs great (driving or idle) for 2-5 minutes. At this point, the idle becomes very rough and I can feel some misfiring when under acceleration.
Relevant info:
I have Autotap generic and have checked the codes and logged the sensors. I had the emissions and air-conditioning sensors removed from the PCM program since they are physically gone. Currently, there are no check engine codes. I have run heavy gauge grounds and have checked for vacuum leaks to the best of my ability. I have replaced the front 2 O2 sensors and all spark plugs. The rear two O2 sensor plugs have eliminators, although I do have cats. I have no exhaust leaks. I have good fuel pressure. Autotap is reading values of zero for vehicle speed in all cases, but I don’t feel this should affect anything if the car is not moving anyway. Long term fuel trims from banks 1 and 2 sensor 2 (both rear O2 sensors) is 99.2%. I was under the impression the rear O2’s don’t affect the program. The other values in autotap look reasonable, but I am out of my expertise when it comes to PCM and programming.
Does anyone have an idea or theory as to what is wrong? Is there anything I should try that could narrow down the possibilities? I tried to pressurize my engine with compressed air and sealed all the leaks I found. Is there anything else I should do?
Thanks,
Tom
Symptoms:
The car runs great (driving or idle) for 2-5 minutes. At this point, the idle becomes very rough and I can feel some misfiring when under acceleration.
Relevant info:
I have Autotap generic and have checked the codes and logged the sensors. I had the emissions and air-conditioning sensors removed from the PCM program since they are physically gone. Currently, there are no check engine codes. I have run heavy gauge grounds and have checked for vacuum leaks to the best of my ability. I have replaced the front 2 O2 sensors and all spark plugs. The rear two O2 sensor plugs have eliminators, although I do have cats. I have no exhaust leaks. I have good fuel pressure. Autotap is reading values of zero for vehicle speed in all cases, but I don’t feel this should affect anything if the car is not moving anyway. Long term fuel trims from banks 1 and 2 sensor 2 (both rear O2 sensors) is 99.2%. I was under the impression the rear O2’s don’t affect the program. The other values in autotap look reasonable, but I am out of my expertise when it comes to PCM and programming.
Does anyone have an idea or theory as to what is wrong? Is there anything I should try that could narrow down the possibilities? I tried to pressurize my engine with compressed air and sealed all the leaks I found. Is there anything else I should do?
Thanks,
Tom
TECH Regular
Joined: Jul 2004
Posts: 419
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any smoke at all?
with one bad O2 sensor my LS1FC ran rich as hell and stuttered at times, the car definitely had issues with acceleration... cruising was okay but anymore than partial throttle and the car bucked and coughed. no CELs but the driver side O2 sensor was loose in its housing (hex nut) that bolts to the manifold. replaced that O2 and the car ran great.
hopefully you just have an issue with your rear O2 eliminators or a sensor is bad.
or maybe your idea that the rear O2s dont effect the program is wrong, I would guess they were instrumental in fuel tables, at least when the PCM is looking for them.
with one bad O2 sensor my LS1FC ran rich as hell and stuttered at times, the car definitely had issues with acceleration... cruising was okay but anymore than partial throttle and the car bucked and coughed. no CELs but the driver side O2 sensor was loose in its housing (hex nut) that bolts to the manifold. replaced that O2 and the car ran great.
hopefully you just have an issue with your rear O2 eliminators or a sensor is bad.
or maybe your idea that the rear O2s dont effect the program is wrong, I would guess they were instrumental in fuel tables, at least when the PCM is looking for them.
Thread Starter
Teching In
Joined: Jun 2006
Posts: 19
Likes: 0
From: Chicago
The guy who flashed my PCM swears that the two rear sensors don't affect drivability, but are only to check the cats. I'm not saying he is right though. I tried unpluging the two front 02 sensors and I ended up replacing them with no fix. I just put them in, so I'm sure they are tight.
I will try testing my coils individually with my old timing light.
thanks guys! don't hesitate to post if you have other ideas, I've run out!
I will try testing my coils individually with my old timing light.
thanks guys! don't hesitate to post if you have other ideas, I've run out!
When I first started my car, I ran it without the O2 sensors for for a couple hours and at least 25-30 miles. It ran smooth as silk the whole time. If you suspect an O2 sensor is sending bad info causing the improper mixture, maybe you can just unplug them altogether and see if it still does the same thing. That at least would eliminate them as the cause.
Pat
Pat
Is the wiring okay?
You say that it runs fine for 3-5 minutes - that is because it is in open loop, and the PCM is running the car off of a predetermined set of tables and values. When you switch to closed loop, it starts relying on sensors for feedback, and responds accordingly. I had an o2 go bad and it was the same thing, ran great for 5 minutes then would not get out of its own way.
He is right, the rear sensors don't affect driveability, they are only there to measure the effectiveness of the cats...that's why when most people do headers, they get rid of the o2's since they skip putting cats back on the car.
Were the o2's brand new that you put in...check all o2 wiring.
You say that it runs fine for 3-5 minutes - that is because it is in open loop, and the PCM is running the car off of a predetermined set of tables and values. When you switch to closed loop, it starts relying on sensors for feedback, and responds accordingly. I had an o2 go bad and it was the same thing, ran great for 5 minutes then would not get out of its own way.
He is right, the rear sensors don't affect driveability, they are only there to measure the effectiveness of the cats...that's why when most people do headers, they get rid of the o2's since they skip putting cats back on the car.
Were the o2's brand new that you put in...check all o2 wiring.
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How did you check the fuel pressure? Best thing to do is keep the fuel pressure gauge hooked up then go for a drive and monitor it. You may be suprised at what you find. Just a thought.
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Verdad Gallardo Thread Starter
Teching In
Joined: Jun 2006
Posts: 19
Likes: 0
From: Chicago
I have checked the O2 sensors many times. They are new and installed properly. Autotap is also reading the values just fine, so I don't think that is the problem. I have also run the car with a fuel pressure gauge and get a steady 62 PSI. My fuel trims are very rich about -30 to -40% and the O2 sensors are reading 900 to 1000mV, which means they are telling the computer the mixture is rich. Why would the computer be adjusting to a rich mix if the O2 sensors are measuring an overly rich exhaust?
TECH Resident
Joined: Oct 2004
Posts: 800
Likes: 1
From: Simi VAlley,CA
You can't overlook the little things, mine ran great the first few minutes also, then started running rough, the idle was all screwed up and it seemed like the mixture was 'way off. As it turned out......the front O2 sensors were wired backwards in my harness, as the right side indicated rich and the PCM tried to lean it out, it was actually leaning out the other side and causing the reverse on that side. It took a bit of messing around but I finally got it straight. After 2000 miles I had a drivabilty problem also which turned out to be a bad fuel pump, these engines run like crap with only 30 psi of fuel pressure.
