Idling too high, sometimes...

Bish · October 7, 2005

Got a new problem. Vehicle is in sig and has 153K miles on her with a rebuilt trans about 8K ago. This just started happening.

Truck starts and idles fine. At some point later (25 minutes), it will start idling over 1000 rpms. If I turn off the AC, it jumps to 1300 rpms while in park or neutral and stays at around 900 in drive with foot on brake. Also it seems like the trans is holding shifts for longer during this time. Next time I start it up it is fine for the first 15-20 minutes then the problem starts again.

My first thought was to clean the throttle body and Idle Air control. THey were not too dirty. No change. I have no SES light and no codes stored. I do have a custom tune but it has been fine for a few thousand miles and now this. I'm thinking about swapping back my original PCM to see if that makes a difference.

Any thoughts?

Chad H · October 7, 2005

It kinda sounds like a vacuum leak. I would check all the hoses first. I doubt the coils would do that, so I don't think its anything ignition accessory related.

bobcat5544 · October 7, 2005

If it is doing this every time after 15-20 min, then it sounds like temp sensor is bad and telling the engine it is still cold, which would cause the pcm to keep dumping more fuel and advancing the timing-(higher rpms), and also would effect trans operation.

and thats just a guess WITH OUT ANY MORE INFO

Bish · October 7, 2005

If it is doing this every time after 15-20 min, then it sounds like temp sensor is bad and telling the engine it is still cold, which would cause the pcm to keep dumping more fuel and advancing the timing-(higher rpms), and also would effect trans operation.

and thats just a guess WITH OUT ANY MORE INFO

<{POST_SNAPBACK}>

That actually sounds plausable but I have a question. When the engine is cold (50 degrees overnight) it starts and idles fine. Wouldn't it act the same when legitimately cold and when it is warmed up but "thinks" it is cold due to a bad temp sensor? Also, the temp guage seems to work properly as it shows full cold in the morning and warms up to normal like it should.

I agree though that if it thought it was cold, it may keep the shift points high and that is what the trans feels like. But then again, when 50 degrees in the am, the trans shifts like it should and not with raised shift points.

Lastly, what other info could I provide to help diagnose? Thanks for the comments.

Regarding the vacuum leak, I did check everywhere I could and did not find any leaking vacuum lines.