i live at 5000 foot elevation. I drive 200 miles to an elevation of about 2500-3000 feet elevation.

Car runs fine. Runs fine the entire time I am at that elevation.

I come back from 2500-3000 foot elevation to 5000 feet and it seems as soon as I am back in my home town

of 5,000 feet, I get a check engine light for a lean code.

Seems to do it everytime I make this journey. Car runs fine, pulls the elevation changes coming back and all.

I get in the 35 mph main street traffic and badda bing........check engine light.

Maybe the air is too clean here in the mountain.......I realize there's a difference in density but why won't the computer

adjust for that?

Once I get the code turned off, it runs fine here....no codes nothing..........life is sweet again.

 

What year car?

If you're referring to driving nonstop from a high elevation to a low elevation, then it's entirely possible that the car is not compensating for the altitude automatically. I'm not sure what year you have, but in the LT1/LT4 cars, the computer updates the barometric reading when the car is first started, and then again whenever the throttle goes to 100%. So if you drive the car nonstop without ever going wide open throttle, the barometric reading will never update, so it'll assume it's the initial value the whole time. That you say it runs fine after turning the car off and back on again leads me to think that might be all it is.

So the next time you do one of those drives, romp on it as you go to lower altitude, before the check engine light would come on. See if it still comes on when you hit main street.

 

How new is your O2 sensor?

 

Yes it is nonstop..........200 miles most of it highway............my C4 is a 1996 LT4 with 50K on it.

I'm not sure of the age of the oxygen sensors, I purchased the car from a deceasded owner's family with little to no information.

I'll give that a try. It amazes me that it is so consistent in it's happening and the timing and place where it happens. I can almost predict when the check engine light will appear.

Thanks for the reply. I'm not due to make that journey again for a fwe months but I'll give it a try.
Rob/aka AZ

 

Well since you've got an LT4, I'm fairly confident that the issue is just that the car is using the barometric pressure from your high altitude once you get back down to low altitude, which is screwing up your fueling. I actually had a look at the code that's being used, and it looks like you can force a barometric pressure update anywhere from 0~3600 RPM at anything over 37.5% throttle. So yeah, as long as you're under 3600 RPM, just give the throttle a good push and see if that does the trick.

But long-term, restarting the car also resets the barometric pressure, so it won't hurt anything to do that either.

 

Thanks, I have nothing to lose trying it except a pesky Check Engine Light. Thank you for your research...I'd have never known where to start. It's nice having intelligent help.

 

Which one? There are 4 on a '96?

 

2 are downstream so they monitor the cat. So which side is lean? If both I wonder about the MAF. I usually replace all 4 at once. If I was trying to be cheap, I'd replace the 2 upstream ones. What do you have for codes?

 

Again, the LT1/LT4 cars read barometric pressure at key-on, and reset the reading when the throttle is open enough for the MAP to be equal to the outside air pressure. If you drive nonstop between altitude extremes without triggering the BARO reset sequence, the car will be using the incorrect barometric pressure for fueling calculations. This would throw both banks off without the MAF or O2 sensors being bad.

The hint here is that OP's car throws zero codes when it stays at either elevation, throws a code when going from one elevation to the other nonstop, and then doesn't throw codes after the car is restarted.

 

Just a follow up, I just had the opportunity to drive the C4 today and when I got in the Service Engine light was still on. I had put a few miles and cycles on it
since the SE light appeared.

I didn't disconnect the battery because I wanted to see if the computer would make the adjustments to my altitude.

Sure enough, stopped at Subway and came out to go for a drive to Roosevelt Lake and NO SERVICE ENGINE LIGHT.

100 mile round trip and all is well.

I'll just need to remember the procedure to reset using this procedure........
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Well since you've got an LT4, I'm fairly confident that the issue is just that the car is using the barometric pressure from your high altitude once you get back down to low altitude, which is screwing up your fueling. I actually had a look at the code that's being used, and it looks like you can force a barometric pressure update anywhere from 0~3600 RPM at anything over 37.5% throttle. So yeah, as long as you're under 3600 RPM, just give the throttle a good push and see if that does the trick.

But long-term, restarting the car also resets the barometric pressure, so it won't hurt anything to do that either.
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ALL IS GOOD..........