Low power at altitude - Please help!
November 14th, 2010, 5:29 PM
#1
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Location: Thousand Oaks, Ca
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Low power at altitude - Please help!
Hi All,
I have a 2000 (build date 11/99 in Texas) Tahoe Z71 4WD with 120,000 miles on it. I bought it lightly used in Texas with just 20,000 miles on it and have done most of the major repairs that can be expected with the miles I've put on it - timing belt, radiator leaks, fuel pump, serpentine belt, thermostat, etc.
One thing that bothers me is that it perfoms pretty badly at altitude - very low power! I live at sea level in the Los Angeles area, and whenever I go fishing, camping or visiting friends in the mountains, it just bogs. No check engine lights come on, and my mechanic finds no error codes when I return. He said that he has heard of some vehicles having this problem and that once at altitude if you turn the vehicle off and then back on it will run correctly - something is obviously causing the Mass Air Sensor to not be able to dynamically monitor the altitude/pressure correctly when driving, but will do so on start up. He gave me one of his computers to run tests on the Tahoe when I went up to visit a friend who lives in Tehachapi (at 6,000') this weekend. The Tahoe ran fine driving up the long grades from Aqua Dulce through Acton and Palmdale/Lancaster and on to Mojave and the valley floor in Tehachapi itself - but that is all from 1,000'-3,000'. As soon as I started up from 4,000' to my friends house at 6,000' it started to show the low power symptoms. I started up the computer and did about 3-4 snapshots while under power/load. Once at my friends house, I shut the car off and when I started it the next morning and drove further up above my friends house - it ran just fine!
What is the culprit here?
Do I have to replace the entire MAS?
Does the ECU just need some update it might have missed?
What else needs to be checked?
I'll return the computer and it's findings to my mechanic tomorrow, but just wanted to get the collective knowedge that's on this forum first!!
Thanks,
P
I have a 2000 (build date 11/99 in Texas) Tahoe Z71 4WD with 120,000 miles on it. I bought it lightly used in Texas with just 20,000 miles on it and have done most of the major repairs that can be expected with the miles I've put on it - timing belt, radiator leaks, fuel pump, serpentine belt, thermostat, etc.
One thing that bothers me is that it perfoms pretty badly at altitude - very low power! I live at sea level in the Los Angeles area, and whenever I go fishing, camping or visiting friends in the mountains, it just bogs. No check engine lights come on, and my mechanic finds no error codes when I return. He said that he has heard of some vehicles having this problem and that once at altitude if you turn the vehicle off and then back on it will run correctly - something is obviously causing the Mass Air Sensor to not be able to dynamically monitor the altitude/pressure correctly when driving, but will do so on start up. He gave me one of his computers to run tests on the Tahoe when I went up to visit a friend who lives in Tehachapi (at 6,000') this weekend. The Tahoe ran fine driving up the long grades from Aqua Dulce through Acton and Palmdale/Lancaster and on to Mojave and the valley floor in Tehachapi itself - but that is all from 1,000'-3,000'. As soon as I started up from 4,000' to my friends house at 6,000' it started to show the low power symptoms. I started up the computer and did about 3-4 snapshots while under power/load. Once at my friends house, I shut the car off and when I started it the next morning and drove further up above my friends house - it ran just fine!
What is the culprit here?
Do I have to replace the entire MAS?
Does the ECU just need some update it might have missed?
What else needs to be checked?
I'll return the computer and it's findings to my mechanic tomorrow, but just wanted to get the collective knowedge that's on this forum first!!
Thanks,
P
November 14th, 2010, 8:27 PM
#2
Administrator
Welcome to the forum.
A GM dealer can run your VIN and check your computers to see if there are any missing updates.
If I were to suspect something it would be the MAP sensor, not the MAF sensor. It senses air pressure changes.
A GM dealer can run your VIN and check your computers to see if there are any missing updates.
If I were to suspect something it would be the MAP sensor, not the MAF sensor. It senses air pressure changes.
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