Interesting Behavior on a 2001 Z28 LS1
03-24-2012, 06:19 AM
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Interesting Behavior on a 2001 Z28 LS1 Last September, I bought a stock 2001 Z28 with some problems. Among them were a mess up rear (replaced with a junkyard rear) and persistent P0420 code - driver's side catalytic efficiency problem. I replaced the driver's side exhaust manifold, cat, O2 sensors with all new parts, plus new plugs and wires. Since that time, I have put about 11,000 trouble free miles on the car, though the idle has never been butter-smooth.
Over the last couple of weeks, we've had uncharacteristically warm temperatures. Early on in this weather, I was driving without the AC and just enjoying the weather. In the mornings and prior to driving with the AC, the car seemed normal and threw no codes. However, in the evenings (when it was warmer), I would get a "Check Engine" light at exactly the same spot on my drive home - after driving about 20 miles on the Interstate, I exit and dump myself into suburban hell with strip malls and lots of traffic and red lights. Once the car sits enough at the second traffic light, I get the "Check Engine" light. I get the P0420 code again, but this time it is for the passenger side catalytic converter.
Now for the interesting part...
While I was deciding my next move, I also started driving with the AC on since the newness of the warm weather wore off and I want to be comfortable more that anything else. Now, when I get off the Interstate and sit at the second traffic light with the AC on, I don't get a "Check Engine" light. If I don't use the AC, I get it.
So, I started getting this code once the weather warmed up. However, when the AC is engaged, I don't get it in essentially the same weather.
Things that I think are relevant are:
1) According to documentation I have read, the PCM runs a cat diagnostic test by varying the air/fuel ratio between lean and rich when the car sits at idle for a given period of time and under specific conditions (warmed up, has been running for a while, intake temperature range reading within a tolerance, etc. etc.). I have a 4-page PDF that details this code and self test.
2) I have been told in the past (and I have no reason not to believe it), that when the AC is running, timing is advanced.
Obviously, I would very much like to avoid another swear-fest replacing the passenger-side pieces to get the code to go away for good.
Since I can control whether or not I get the code based on whether or not I have the AC on, I am wondering if there isn't a simpler way to eliminate this code. Is it possible that one of the "less important" sensors (e.g. intake air temp, barometric pressure, etc.) are reading funny but won't directly throw a code or is there another approach I haven't thought of?
Just a strange thing that I noticed and was interested in opinions from folk with more PCM knowledge than I have...
Over the last couple of weeks, we've had uncharacteristically warm temperatures. Early on in this weather, I was driving without the AC and just enjoying the weather. In the mornings and prior to driving with the AC, the car seemed normal and threw no codes. However, in the evenings (when it was warmer), I would get a "Check Engine" light at exactly the same spot on my drive home - after driving about 20 miles on the Interstate, I exit and dump myself into suburban hell with strip malls and lots of traffic and red lights. Once the car sits enough at the second traffic light, I get the "Check Engine" light. I get the P0420 code again, but this time it is for the passenger side catalytic converter.
Now for the interesting part...
While I was deciding my next move, I also started driving with the AC on since the newness of the warm weather wore off and I want to be comfortable more that anything else. Now, when I get off the Interstate and sit at the second traffic light with the AC on, I don't get a "Check Engine" light. If I don't use the AC, I get it.
So, I started getting this code once the weather warmed up. However, when the AC is engaged, I don't get it in essentially the same weather.
Things that I think are relevant are:
1) According to documentation I have read, the PCM runs a cat diagnostic test by varying the air/fuel ratio between lean and rich when the car sits at idle for a given period of time and under specific conditions (warmed up, has been running for a while, intake temperature range reading within a tolerance, etc. etc.). I have a 4-page PDF that details this code and self test.
2) I have been told in the past (and I have no reason not to believe it), that when the AC is running, timing is advanced.
Obviously, I would very much like to avoid another swear-fest replacing the passenger-side pieces to get the code to go away for good.
Since I can control whether or not I get the code based on whether or not I have the AC on, I am wondering if there isn't a simpler way to eliminate this code. Is it possible that one of the "less important" sensors (e.g. intake air temp, barometric pressure, etc.) are reading funny but won't directly throw a code or is there another approach I haven't thought of?
Just a strange thing that I noticed and was interested in opinions from folk with more PCM knowledge than I have...
Last edited by KeithO; 03-24-2012 at 06:24 AM. Reason: typos
01-05-2013, 05:15 PM
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Ever get any resolution here? I have what appears to be a similar issue. Drove the car home from the dealer with absolutely no problems, 2nd time I drive to work after idling when I got home for a minute to open a bay door on my shop, got the SES with the code. No strange driving, etc. Code was admitted to being present before, but all cats were replaced w/ oem (have documentation for it). I'm getting slightly lower fuel economy than I would have thought (could be excessive go pedal)... so I'll be scanning (HPTuners) sometime in the next few days to see what I have in the way of irritated sensors.
01-06-2013, 07:08 AM
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I am up to +30,000 miles on this car now. Shortly after I posted the original thread, the "Check Engine" light stopped coming on and has never come on since that time. I have no credible explanation.
My only theory is that I got a dose of bad gas? Other than that, I have no explanation. I have seen 90 degree outside temperatures and 10 degree temperatures since that time and have had no more "Check Engine" lights.
My only theory is that I got a dose of bad gas? Other than that, I have no explanation. I have seen 90 degree outside temperatures and 10 degree temperatures since that time and have had no more "Check Engine" lights.
01-06-2013, 08:33 AM
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Ok now that's crazy. Anyone think bad gas theory is sound? I did watch (and cringe) when the salesman filled the car up with 87 Octane gas. I have since put 91 (best I can get) in it. I suppose it could be a possibility. I brought my scanning equipment to work with me, so I'll be scanning it on the way home to watch for issues.
01-08-2013, 09:45 AM
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I ran a 20 min or so scan clip and reviewed it at home. At initial idle everything looked good, as soon as I got any engine speed, one of the rear O2s went high and effectively stayed there (it dipped a few times after letting off throttle, but went right back up). Also, my LTFTs were all over the place, but generally adding tons of fuel.
I'm going to put it up on my lift and check everything out when I get back home. Had to leave the state for a few days for family stuff. I'm betting I have an air leak due to the LTFT being affected- which don't look at the post-cat O2. After I've checked out any leak paths, I'll swap the rear sensors as well to see what that does. I have HPTuners, so eventually it will be tuned out. I hadn't thought of looking at the cat efficiency test, but as my front O2s are being affected by the problem and making me run WAY fat, I'm fairly sure it has little to nothing to do with my cats being bad. I do however want to get whatever is adding so much fuel fixed...19mpg with mostly highway driving is NOT ok!
I'm going to put it up on my lift and check everything out when I get back home. Had to leave the state for a few days for family stuff. I'm betting I have an air leak due to the LTFT being affected- which don't look at the post-cat O2. After I've checked out any leak paths, I'll swap the rear sensors as well to see what that does. I have HPTuners, so eventually it will be tuned out. I hadn't thought of looking at the cat efficiency test, but as my front O2s are being affected by the problem and making me run WAY fat, I'm fairly sure it has little to nothing to do with my cats being bad. I do however want to get whatever is adding so much fuel fixed...19mpg with mostly highway driving is NOT ok!
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01-13-2013, 12:23 AM
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I put it up on the lift and found a loose union on the drives side between the catted pipe and the Y-pipe. i haven't scanned again to see if that solved the problem, but the O2 sensor is within inches of that union, so if it was getting any air inleakage I would imagine it would see it.
