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Random RPM surging

Wynco92

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I have a 1990 Supercab XLT that I have been restoring, Engine rebuilt, 5 speed Tranny rebuilt, All new parts on the engine. I am still breaking in the engine I have about 260 miles on it so far. I am experiencing random high rpm surges, sometimes when idling and often when I shift gears it will climb to 3000 rpm and slowly come down. This 2.9 doesn't have a Mas flow on it. any thoughts on the subject would be appreciated.
 


Bgunner

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I am not all that familiar with the 2.9 so does it have an IAC? Idle air control valve. This could cause your issues if it is leaking. Call this a shot in the dark.

EDIT: just as a note for future ease could you go into your forum settings and add the 2.9L motor to your truck description so we can just look at your description of your truck and know what you have with out you constantly typing that you have the 2.9L in your truck. Thanks. :)
 
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rusty ol ranger

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IAC, TPS, or vaccuum leaks are the usual suspects on the 2.9.
 

Wynco92

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I am not all that familiar with the 2.9 so does it have an IAC? Idle air control valve. This could cause your issues if it is leaking. Call this a shot in the dark.

EDIT: just as a note for future ease could you go into your forum settings and add the 2.9L motor to your truck description so we can just look at your description of your truck and know what you have with out you constantly typing that you have the 2.9L in your truck. Thanks. :)
I have looked and can't seem to locate where to add to the forum settings?
 

RonD

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Change PCV valve if its not new, its a controlled vacuum "leak", if it gets sticky then not so controlled, lol

Also on a 1990 the power brake booster is pretty old, so can be a random vacuum leak
Start engine, foot OFF the brake pedal
Shut off engine
Press brake pedal down 3 times, should feel power assist each time, then try 4th and 5th time should lose the power assist as vacuum is used up
If you are only get 1 or 2 presses then booster is leaking, may not be your idle surge issue but booster is leaking
 

Wynco92

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Change PCV valve if its not new, its a controlled vacuum "leak", if it gets sticky then not so controlled, lol

Also on a 1990 the power brake booster is pretty old, so can be a random vacuum leak
Start engine, foot OFF the brake pedal
Shut off engine
Press brake pedal down 3 times, should feel power assist each time, then try 4th and 5th time should lose the power assist as vacuum is used up
If you are only get 1 or 2 presses then booster is leaking, may not be your idle surge issue but booster is leaking
Both PCV valves are new, however the Brake booster is not, I will look into that...thanks for the suggestion
 

JerrySab

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I had a really similar issue, with spiking RPM. Turned out to be failing TPS. It would spike to 3-5v with no pedal input, which would then cause resultant RPM spike. I had the benefit of decent shop nearby that discovered by using an oscilloscope. Easy fix but really enormous headache, as the TPS was relatively new. Wiring looked fine, computer looked fine, part swap solved the problem. Youtube link below is not my truck, but this was the gremlin hunting process.

 

Bgunner

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2WD
Tire Size
225/70/R15
My credo
If it's not broken Don't Fix It!
I have looked and can't seem to locate where to add to the forum settings?
Click on your name in the upper right corner, then click on account details, scroll down and you will see where you can add your trucks specs.

Hope this helps.
 

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