Idle problem

DANIELW

MEMBER
1989 CHEVROLET SILVERADO
V8
2WD
AUTOMATIC
20,000 MILES

All sensors on the throttle body are new, tps, map sensor, iac, new fuel pump, new fuel filter. When you first start the truck whether it is hot or cold it idles up and down and will clear out instantly without notice. Sometimes it clears out immediately or a few seconds later. When it is doing the surge at idle you can lightly mist some carb cleaner over the throttle body and it runs perfect. When you unplug the map sensor it runs ok but smells rich. With the map unplugged it does not surge at idle and the truck runs well. When the map is plugged in it does the surging again.

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

You have a vacuum leak. If you can spray the cleaner and it runs better, then that means you are pulling in this fuel source and the engine smooths out.

Basically, it is running lean due to the added air until the engine warms up and the PCM starts reading the o2 sensor data and it compensates for the additional air.

Here are a couple of guides that will help with this:

https://youtu.be/4ktw1X4W_-k

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

I would suggest running through all the rubber vacuum lines and look for cracks. However, we need to start to pinpoint the leak so that means we need to use more of a stream and not a mist when testing. This is covered in the guides above so please let us know what you find with this.

Thanks