have a 04 envoy that when idleing it drags down and revvs up. it idles down to around 200-300 rpm when jumps up to 1200-1400. its my gf's car and the other day she was on the interstate and it died on her and when she started it back it wouldnt go over 40mph and died. i dont know whether she was out of fuel cause it was pretty low on fuel to cause the interstate incident but i changed the fuel filter and put some 93 in it and its still doin the revving thing. anyone have any ideas?

Those engines use a drive by wire throttle that can get very erratic when it gets gummed up. Take the air ducting off the throttle and use a old soft bristle tooth brush and some throttle body cleaner to wash out the bore and clean the throttle blade. They also are speed density type fuel injected and don't have a mass airflow sensor. All load calculations are based on Manifold Air Pressure and Air Charge Temperature sensor inputs. The PCM determines fuel needs based on those and your throttle pedal position, engine speed and load etc. If the actual airflow through the throttle is out of the range the PCM expects it will go into a reduced power mode to protect the engine but still allow you to limp to a service facility. That's probably what happened to your GF the other day. That engine managment system is trying to do to much with to little and it's very temperamental and easy to confuse compared to a common MAF based fuel injection. After cleaning the throttle do this when you put it back together and start it, turn the key to on then hold the throttle pedal all the way down, crank the engine continuously until the engine starts then let off the pedal. It should idle high for a while until the engine control starts to adjust the throttle blades idle position to reflect the higher air flow of the clean throttle. It may take an hour or so of driving to become completely normal again. If you know what the MAP sensor is it would be a good idea to pull it out of it's hole and spray some throttle cleaner into the hole when cleaning to remove any film from it's diaphram thereby allowing it to read more accuratly. Just a little then just shove it back into place.

Vernon

man that was an excellent write up. thanks so much. she was driving it today and it wont go over like 30mph so id say it whats you are describing. if you try to run over 30 it will run like 50 for a min then start cuttin out. im familar with map sensors but not familiar where it would be on her car. is it easily visible or hard to find? what tools will i need to get it out since she left her car at a truck stop and im gonna attempt to fix it there. i appreciate your help

Map sensor is common GM style, it's easy to find, on top near the back of the intake. By spray in the hole I mean the hole on the sensor, not the hole it sits in.

Vernon

P0171 "Fuel Trim System Lean Bank 1

I have tried everything you said. here is the code im gettin. any ideas?

That code means the O2 sensor is observing excessive oxegen in the exhaust. Either there's a vacuum leak that's allowing air into the intake or exhaust system after the throttle body or a sensor is out of calibration. Usually a sensor problem will have a code for it also but like I said this system is kinda fundimental and so problems might not be so easy for it to identify with less inputs to compare against. Could be a bad MAP or bad O2 sensor (bank 1,#1). The EGR valve is digital so it would have a code if it was stuck open. Look for leaks in the intake and exhaust, spraying the intake with carb cleaner will help find one. I can't remember if this one has a external PCV valve but if it does pull it out and check to see if it's stuck open.

Vernon

Is it still limited to 30-50 MPH? I just remembered that a stopped up exhaust can set that code with those symptoms. Use a rubber mallet to rap on the catalatic convertor and muffler, if you hear pieces rattle inside it then the catalyst has desinigrated and is clogging up the exhaust system. If it's not running worth a dam take out an O2 sensor in front of the convertor and see if it will suddenly drive up the street fine. If so you know it's got a resticted exhaust. Don't drive to far with hot exhaust blowing at things not meant to handle the heat. If necessary you can drill a 1/4" hole in the pipe pre-cat pointing strait down and that will get you to a muffler shop just fine. Don't drive over flamables though.

Vernon

well the last time we tried to drive it. it started up and ran down the road 70mph for about 3-5 miles then just started losing power and everntually slowed to a 10 mpg crawl. durning this crawl it would eventally just stop and you would have to kill it. wait just a little bit and take back off. but when you would take back off you might get up to 15 or 20 then itd slow back down to 10.

Can you say "Fuel pressure test"

turned out to be a fuel pump an 847 venture to the stealership