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2004 gmc envoy_slt
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Ok I have an 04 Envoy I6 with around 62,000 miles. I bought it used over 2 years ago. I had noticed MPG was down and power also. I had just replaced plugs in my sisters TB due to water getting under the seal of the coil. Thought I might have the same problem, so I put new plugs in’s. The following is what I have done. New delco plugs. I sprayed the throttle body with 3M carb cleaner. I wiped it down with a shop towel, and repeated the process. I did not remove the throttle body. I ran a bottle of fuel treatment / cleaner through it. I installed a new cam shaft sensor, the one on the side by the power steering pump as it was leaking oil into the connecter. I did clean all the oil I could from the connector. I do have the SES light on due to the secondary air pump problem. I know this by having the codes pulled at advance auto. I had the – side of the battery unplugged for almost an hour. Truck runs great but after about a week or two of driving, it has picked up a miss while I’m at a stop. I accelerate from a stop and it sputters a little but then goes. The power is back along with the MPG. I know from reading on the secondary air pump I have the voltage drop going on. From what I understand this should only happen for a short time after startup. I have seen this several times after the truck was at the 210 deg mark. And I had driven it around 20 miles in town. The miss at times gets better after I have driven it for about 20 mins. Any suggestion:confused:



FYI The plugs I took out looked ok, the center post and electrode, had a little white/ cream color to them, the outer part had a black color to them. All plugs looked the same
 

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The back side of the throttle body may be dirtier than you can get to from the front with a shop towel. As an experiment, turn the AC off and see if it idles better.

There may be other pending codes that haven't gotten to the severity yet to turn on the SES light. See if you can read any.

On the air pump voltage drop issue, if you sit still for 30 seconds after a cold start and the air pump system is working, it won't run the diagnostic later that causes the voltage drop. But if it isn't working, that's when the subsequent test gets scheduled. But the test is held off by the PCM until some conditions are met, such as engine run time over 200 seconds, ROM at least 1150, MAP less than 30 kPa, throttle position change less than 5%, and operating conditions in fuel trim cells 16 or 17. So your drive might not be satisfying those conditions until you are up to operating temp and 20 miles from home. Not unexpected.

Is your temp gauge at a good 210 degrees? Not under? A stuck open thermostat would decrease mileage, but I'm not sure it would cause a miss.
 
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