I have a Montana, 3400 engine. I'm having the following problems, all of which happen sometimes.
When I start it, it idles fast. If I turn the key off and the on sometimes it idles normally, sometimes it continues to idle fast. When I'm driving and take my foot off the accelerator the car doesn't slow down and may even gradually speed up. From a stop it hesitates and bucks a little. On the highway the motor will suddenly rev for just a second then return to normal. Most strangely, twice in the last 6 months, on the highway, the gas gauge went from full to empty over a period of a few minutes. The first time it happened I got out and checked the gas tank for a leak. Again, these things only happen sometimes.
The idle problem and the fuel gauge problem MAY be corrected by updating the computer program in the Montana's computer. I have seen this be the problem on other GM vehicles in the past.
To verify that this is a computer problem, return to the dealer and see if they checked for an update. If they did check and there is not an update, then you MAY have two problems.
The erratic idling problem MAY just be a dirty throttle body and Idle Air Control Motor. The "IAC" controls the idle by making a controlled vacuum leak. If it is dirty it can't respond as quickly as it should. This may be why it idles too fast when you come to a stop.
What I recommend is for you to have your Throttle Plate cleaned as well as the Idle Air Control motor. This is something that most people can do without requiring special tools or training.
You will need to purchase an aerosol can of throttle body cleaner from your local automotive part store and have an old toothbrush handy.
Before you start the car up and the engine is cold, remove the air cleaner "snorkel" from the throttle body. Have an assistant push the gas pedal to the floor and hold it. You will see the "butterfly' or "throttle plate" move. This is what needs to be cleaned with the throttle body cleaner and the tooth brush.
After you have cleaned away all of the deposits on the throttle plate, release the gas pedal and start up your Montana. (It may be reluctant to start at first due to the Throttle Body Cleaner that the engine has ingested). After it does start spray the remainder of the Throttle Body Cleaner in the throttle body. This will help clean the Idle Air Control motor as well as the intake manifold.
Replace the air cleaner snorkel and see if the problem is solved. If not, find a local professional and have them go on a test drive with you to verify this problem.
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