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High Idle/ Coasting RPMs

DezertExpo92

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Hey all, scratching my head on this one...

Truck starts fine, fluctuating between 500-600. drive it around and notice the idle/cruising RPMs seem high. Come to stop and truck idles at 1000-12000 throw it in park and climbs to 1500-2000. I noticed I can stab the throttle and idle will drop. If its idling high and I shut off the truck, it restarts just fine with normal idle. Then starts having problems starting, doesn't fire right up anymore, crank for 5 seconds or so and nothing, stop and crank again, fires in a second or so but low, almost stumbles, and climbs up to normal.

Pulled codes:

KOEO: 111
CM: 111
KOER: 411 - uncontrollable idle, usually high (re test and got 111)

Disconnected IAC, lowers idle a bit, seemingly no more high idle, then comes back about 20 minutes later. Doubt its IAC, but clean anyway.

Cleaned MAF

Checked TPS - normal (.98-4.64 iirc)

Changed BOTH temp sensor and sender, gauge works.

New T-stat

Checked VAC with carb cleaner, no change, no leaks

New PVC

sprayed carb cleaner around intake, intake tube, everywhere and got nothing and no change in how the truck runs.

did notice a crack in the intake tube around the bellows after the MAF, but it "closes" since its in the inside of the bellows, reinstalled and sprayed area with carb cleaner, no change either. new tube on its way anyway.

truck runs fine otherwise, besides a bad fuel gauge or sending unit. And what im hoping is a bad servo in the trans (second gear not engaging) please let me know anything else I can check or ideas. Thank you in advance.
 


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2stroke

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If the idle changes with the IAC disconnected, its a vacuum leak. It can't be anything else.
 




DezertExpo92

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I thought that if the idle changes when I disconnect the IAC, it meant the IAC is doing what it's supposed to..
 




DezertExpo92

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I should also mention it's intermittent. Driving it now and when I come to a stop it hangs around 1000 for a second and then super slowly drops to about 600-700
 




Maniak

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Don't forget to check for a sticky throttle body. Our would hang the rpms some if I slowly let off the gas. If I did a quick stab at the throttle the rpms would come back down.

When I looked inside the throttle body I could see the throttle plate was rubbing on the body. It was quick/easy/cheap to just get another throttle body from the junk yard than it was to replace bushings in the old body.

~Mark
 




DezertExpo92

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I cleaned it up, and it seemed to operate smoothly, ill do a thorough inspection tonite and see if I can see anything more.
 




Maniak

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What I did was get it to have the problem and while the rpms were still up, walk to the front and push the throttle arm to close it more.. If it moves, well, you found the issue.. Then you need to find out "why" it's not closing all the way.

~Mark
 




DezertExpo92

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As of yesterday its not hanging long enough for me to get to the front to check, but that I guess is a good thing? However it is still hanging when I coast to a stop. It doesn't slow down past 1200-1500- RPMs or so unless Im on the brake. Then it downshifts hard and revs up pretty good. Once im stopped it sits at 1000 for a second or two, then slowly drops to about 600. I noticed a drop in fuel economy, but that could also be due to the way ive been driving it and fixing the AC (and testing it). Only thing I haven't changed is the FPR, Fuel Filter and Fuel Pump. I have a new filter that I was gonna throw on this weekend, I need to do a new pump assembly since the sender is bad anyway. But $100+ for an FPR that might not be the problem isn't sitting too well with me lol. I don't think (and hope) its the Intake Manifold gaskets as I sprayed the area and got nothing. Eventually I want to do the 95TM heads and cam, so I don't want to do half the work now then redo it al later when I get the heads.

In short, Im going to install new fuel filter, don't think its causing the problem but it should be done and I have it. and take a better look at the throttle body.
 




DezertExpo92

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Throttle body closes all the way.

Took the truck on a 500+ mile trip. Ran good but the high idle issue showed itself quite a bit.

13.5 mpg with 4.56 gears, 35" tires, fiberglass fenders, light bar, etc.

noticed that 55 mph was about 2000 rpm, 70 mph was about 2500 rpm iirc. If I let of the gas, rpms would drop to about 1500 and hold. would drop if I hit the brake but hold at whatever rpm I was at when I let off the brake.
 




2stroke

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What I meant was if you un-plug the IAC, the RPM's will drop. Once they drop, they should stay rock steady as there is absolutely nothing there to affect them. I doubt even a malfunctioning EGR would affect idle that much, but I could be wrong.
 




FR-425

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All sounds like a bad PCM temp sensor.

Hard starts when warmed up = PCM temp sensor. ("thinks' it's cold all the time)

Intermittent high idle = PCM temp sensor ("thinks' it's cold all the time)

Injecting too much fuel at idle.

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Yes, I read the part where you said you replaced it. Doesn't mean it's good or the correct one. It happens.. .. ..

Always disconnect the batt for a couple mins after replacing any engine/emission control device to reset the PCM so it will "relearn".

These old OBDI computers do have "adaptive" tuning (All be it limited) and just like any computer sometimes it just needs a reboot.
 




FR-425

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I also noticed you say the gauge is working then two sentences later you say it's not.

May be you put the sensors in the wrong holes, or mixed up the wires..?
 




FR-425

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Just throw'n out some different ideas to check

:salute:
 




DezertExpo92

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FR-425, Sorry for the confusion. The temp gauge is working and has always worked. The fuel level gauge is the one that's not working right now.

Ill order a new ECM temp sensor along with the sender needed for the oil pressure gauge mod and probably a new fuel pump while im at it. might as well get it all so I don't get hit with shipping.

any particular brands I should stay away from or use?
 




FR-425

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No, just be sure that part number you order is the one you get.

I bought Motorcraft from Rock Auto, but it looks like they currently don't have any. (EDIT: it is Motorcraft but I got it local at Oreilleys')

You may have to call to see if they are going stock any more.

I guess my next choice would be the AC Delco.
 




FR-425

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I also noticed you say the gauge is working then two sentences later you say it's not.

May be you put the sensors in the wrong holes, or mixed up the wires..?

Duh... can't cross up the wires huh, one's a plug the other is a push cap... my bad.
 




2stroke

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Motorcraft is the best for sensors, hands down. Looking at rockauto, there is nothing wrong with airtex/wells or standard motor products.
 




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