I have a 2002 GMC Envoy, recently the service engine light will blink, usually when accelerating, when I let off of gas it stops. I took it to a mechanic and the computer said that nothing was wrong. They told me that it might have something in the fuel lines or bad gas and it processed it through. I changed the fuel filter and put in fuel injector cleaner, but it still does it. Sometimes it will run fine and others will I start it up it will have a rough vibrating feel. It will also hesitate when I am at a complete stop and take off. Any ideas?
I have a 2002 GMC Envoy, recently the service engine light will blink,?
- Thread starter kwhite84
- Start date
Molson02536
New member
The VVT is causing the "check engine light" due to the fact that it is also doubles as an EGR system. The more the cam is retarded, the more exhaust is entering back in through the intake valve, heating up the intake charge.
At Idle the cam angle is 0 which is fully advanced, and then from there it moves around based on several paramaters. Now in PE mode, from 2800 to 5200 the cam angle is 12* for that whole range, that seems pretty generic to me. I don't fully understand the whole vvt sytem completely but it seems that within a 2400 rpm range, that there would be different angles at the different rpm ranges.
That makes me believe that GM may have this table set more for emissions purposes than power purposes. The valve overlap is most useful at higher Rpms to use the exhaust pressure wave pulse to pull air through the intake valve to improve cylinder filling. Unfortunitely, GM may have done a pretty good job of 'tuning' the VVT for stock engines.
The EGR system was introduced to cool the combustion temperatures just enough to lower NOx emissions. I'd use caution when reducing EGR, as it should raise temperatures and could possibly cause damage to the converter by overheating it. Before we get flamed, the theory is that exhaust gas is basically inert and will not burn again, thus reducing the amount of air/fuel being burned by the power stroke, which in turn reduces the combustion temperatures. Its the intense heat of combustion that results in O2 in the water vapor combining with the Nitrogen in the air to form NOx.
Hot air is less dense but the O2 sensor will account for any leaning of the charge. The lean charge will last for a very short time until the ECM corrects it. I mean by the charge temp, taking the EGR out of the equation will be the greater of two evils. I tried this before on a few other EGR vehicles, the ECM controlled and for a short while there was a small increase in felt power but soon followed by pre-ignition and a "check engine" light from a very hot combustion chamber.
Start with the simple and try flushing out your internal engine by adding Marvel Mystery Oil for the next 250-400 mile then give it a Oil change with the recommended oil weight. 5W-30. Anything heavier may cause your check engine light to come one again.
Long story short, dirt in your engine oil and sludge build up may cause your VVT to stick causing your check engine light to come on during your acceleration phase due to the valve not being able to change there position. This in turn will cause a conflict to your PCM causing it to trigger your Check engine light to come on due to misfires or VVT timing issues.
This is just a good guess i can give you without a Tech2 scanner in front of me to address the issues your Envoy has.
Please keep us posted and good luck.
http://gmcowners.com/gmc-forum/showthread.php?t=13239
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