Self accelerate when heated

thundercat

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Its a Montana 2003. Some overheating but not is the main problem.

The main (BIG) problem is later 45 ~ 60 minutes driving (depending how much the day is hot and the trafic, and some time later the nail of the temperature gauge fluctuates over half) self accelerate and mainly I have to push brakes otherwise exceed the speed limit, or hit the other car ahead. In this condition at parking is not the usual 600 r.p.m. but more than 1500 r.p.m. Then (if I reach home) I have to turn off engine, lift the hood, and wait a few hours to cool.

I unsuccessfully did and changed (also thinking maybe related with overheating):

- TP sensor
- IAC valve
- PVC valve
- cleaned throttle body and the pipe to the EGR valve
- changed several broken air pipes and water hoses
- cooling temperature sensor
- thermostat
- cooling radiator (before deciding to change, flushing cooling system 2 times)
- water pump
- periodically purge trapped air by bleed screws (think always remain something)

Currently the scan tool not show OBD2 error codes. I really don't know what the next step to do.

Thanks,
 
Last edited:

Steelcity

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Pittsburgh,Pa
I'm having a hard time understanding ur problem, is ur engine reving up and that is causing ur overheating? Or do u have 2 separate issues engine over speed and over heating engine?
 

thundercat

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While the first 40 minutes it work like a charm, later when warm start the problem..

I know that the Dealer can fix it, but is sure the bill will be at least 1000 bucks, money that I not have.

Could someone suggest me some idea?
 

melsg5

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On the temperature problem, do the cooling fans come on when the engine gets hot?
Do the cooling fans come on when you turn on the A/C?
On the high revs, have you checked for a vacuum leak using throttle body spray?
 

thundercat

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On the temperature problem, do the cooling fans come on when the engine gets hot?

Yes

Do the cooling fans come on when you turn on the A/C?

Yes

On the high revs, have you checked for a vacuum leak using throttle body spray?

Not yet (while no warm at 40 or 50 minutes, I can drive at highway and high revolutions perfectly).

Might explain me how is the procedure please?
 

melsg5

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when the engine is showing high revolutions in park, spray around the throttle body, the intake manifold where it meets the heads, anyplace where you have vacuum lines. If the engine speed changes you have a vacuum leak.
 

thundercat

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Is a specific spray? What kind of spray I have to use?
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thundercat

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I did it, while holding sprayed over edges of manifold and around the throttle body, and I not perceived (hear), or I should use an appropriate tool? But not like a little significant variation in rotation speed.

Maybe since more than a year, I saw a permanent moisture shadow (grease?) over front side of the manifold (see picture), rear side no have this shadow. Question: This is showing me some important clue?
 

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thundercat

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If someone had the same self accelerate problem in a montana and then could repair, I would feel so grateful you share your experience. Thank so much for your attention.
 

Steelcity

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Pittsburgh,Pa
I never had an experience like this but you can also check other forums/cars that use the 3400 engine. Have you checked the vacuum lines ?
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melsg5

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you have spent a lot of time, money, and effort guessing at replacing parts. How about taking the vehicle to a shop that has a Tech II or similar tool to diagnose how the the throttle positioner, iac, and ecm are interacting with one another to determine the real cause of the problem.
 

melsg5

Staff member
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Assuming the IAC was replaced with a new one the other possiblity is the ECM but the vehicle should be checked with a Tech II or similar diagnostic tool to confirm this.
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