89 TBI races after 10 minutes

[beezer]

My 89 engine starts to rev high after about 10 minutes running. I have replaced the TBI unit for a reconditioned unit from rockauto and the EGR/PCV valves also.

I note from some of the threads to check for vacum leaks...I replaced a lot of the vacum hoses and the brake booster too as I thought it was making a hissing sound when I pressed the brake. If there is one clue, it might be that pressing the brake pedal makes it rev a tiny bit faster...is that normal?

Starting to run out of ideas and things to replace . Has anyone experienced this high/eratic idle?

Thanks,
Dave.

 

[63chevyll]

I would try pinching off the van hose or van source to the booster just to eliminate the booster. But yes, sounds like lean, vac leak issue

 

[Fordum]

I seriouslly doubt that this will help your problem. But I had the exact same problem with my 89 Ford.
The difference was, that it did not do it all the time.
I would be sitting at a red light, and suddenly it would go to about half throttle. I almost drove over several cars.

I tinkered with it for about 3 months. My mechanic and I changed the computer, egr valve, air idle control, TPS, everything we could think of.

I parked it in his shop with the engine idling. Suddenly it would rev up to bout 3K, stay there for a minute or so, then idle back down.

Every diagnostic we could run said that the computer was commanding it to do that, but no idea why.
Finally, reading through the code in the computer, the only time I could see when it was supposed to rev like that was when the engine was overheating at idle.
We swapped the temp sensor, and it went back to normal.

We had monitored that line while it was acting up, and saw nothing. Our best guess is that it was glitching really fast, and the computer was seeing it but our meter was not.

I'm not sure if your truck even has that command.
You might check the TPS though. A bad Throttle Position Sensor might be telling it that you have pressed down on the throttle.

 

[4xcrazy]

My recent '87 is doing the same thing, it'll idle alittle high at cold start-up, like they usually do, then idle down, but once running for a bit, it picks up to about 1500-1600 rpm, haven't really dug into it yet to find out, i was thinking maybe it's running too cold or something, one of these days, i'll get to it.

Although, mine only started doing this after I removed the TBI spacer that the previous owner installed, that never let the throttle cable to rest back to full idle, so they added an aux spring to help pull it back, and well, after two+ years of that, it wore out the original throttle body base unit/shaft, I recently replaced the base and removed the spacer.

 

[British Slave]

Hate to recommend replacing another part, but coolant sensor MAY be the problem. If it runs smooth and idles right for ten minutes or after it warms up, then revs up, it's not a vacuum leak. vacuum leak will cause problems all the time.

 

[beezer]

Appears that changing out the coolant sensor has cured the problem. Thanks for the tip!

 

[4xcrazy]

I thought I had fixed mine with a simple vacuum leak from the previous owner installing the wrong PCV valve, noticed it making noise one day and heard alittle more hissing than normal. I replaced it, it did run better and now idles alittle lower and normal, but still raises up to about 1200rpm every now and then.

Drive alittle more and it's back to normal, drive some more, back up,, so it varies. Guess I'll have to try the CTS since it's not a horribly expensive part anyways.