High idle help

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Jan 17, 2023 | 08:08 AM

Lt96

Thread Starter

5th Wheeling

Joined: Jan 2023

Posts: 25

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High idle help

Help needed
1996 f150 4.9 5spd
Iam trying to diagnose my high idle (1000) engine warm, checked for vacuum leaks with smoke machine (home made) propane and a 1/4 hose and brake clean none found. Just tried to replace the iac valve and clear kam memory no change, disconnect iac valve no change on new and old iac have voltage on red lead to iac. I have no engine codes checked with quality scanner. Any suggestions iam think of replacing the tps next to see if that helps
Thanks in advance

Jan 17, 2023 | 08:32 AM

jas88

Lead Driver

Joined: Feb 2002

Posts: 7,952

Likes: 589

From: Greater Austin, Texas

Is this truck new to you and came this way? Or did it develop over time?

Jan 17, 2023 | 08:42 AM

Conanski

FTE Legend

Joined: Jun 2006

Posts: 31,930

Likes: 1,499

From: Ottawa, Ontario

If unplugging the IAC makes no difference to idle speed then the motor either has a significant vacuum leak or the throttle blades are not fully closing. It seems you have already investigated for vacuum leaks so I'd suggest you take a closer look at throttle control. Possibilities include a binding throttle cable, a misadjusted throttle stop screw, and a warn or binding throttle linkage.

Jan 17, 2023 | 09:06 AM

Lt96

Thread Starter

5th Wheeling

Joined: Jan 2023

Posts: 25

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Truck came this way, Is it possible that previous owner installed tps wrong and its holding the throttle open ?. I plan on checking the tps tonight with multi meter. I know there is something about install and rotate anyone have a quick rundown on that thanks

Jan 17, 2023 | 10:19 AM

Prototypemech

Logistics Pro

Joined: Nov 2013

Posts: 3,921

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From: Mojave, Ca

Its very common for these trucks to have cracked vacuum hoses, which causes hi-idle. The PCV valve hose is a big offender. Carefully check all you vacuum hoses for breaks and cracks.

Jan 17, 2023 | 01:20 PM

jas88

Lead Driver

Joined: Feb 2002

Posts: 7,952

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From: Greater Austin, Texas

If it came that way, I bet the PO jacked the base idle screw out to compensate for a dirty IAC valve.

Jan 17, 2023 | 09:09 PM

Lt96

Thread Starter

5th Wheeling

Joined: Jan 2023

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Update might have found something

So tonight I changed the tps and check its volts .88 closed and 4.68 wide open not perfect but in the ball park (aftermarket part) cleared my kam memory dropped idle to 808 to 850 ish so definitely a improvement. I can still disconnect the iac with no change so started looking for vac leak again, think I found it on the intake gasket to head near the front two cylinders. My question is with the propane rims didn't jump alot just a bit how can I verify 100% and why didn't the smoke show the leak. I must introducing the smoke into the hose off of the master brake ....
thanks

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Jan 17, 2023 | 09:46 PM

Lt96

Thread Starter

5th Wheeling

Joined: Jan 2023

Posts: 25

Likes: 3

Thanks ill give that a try

Jan 18, 2023 | 07:07 AM

#10

rla2005

Fleet Owner

Joined: Oct 2005

Posts: 20,780

Likes: 1,746

From: Kentucky

Quote:

Originally Posted by Lt96

So tonight I changed the tps and check its volts .88 closed and 4.68 wide open not perfect but in the ball park (aftermarket part)

Anything between 0.67 to 1.0 VDC at closed throttle is text book perfect. The internet myth of the TPS has to be set to 0.99 VDC is just that...a myth. Leave the TPS alone and do not lose any sleep over a "not perfect" closed throttle TPS output. The WOT throttle value is good as well.

I sincerely hope that aftermarket TPS is not a short fuse in disguise.

Jan 18, 2023 | 09:08 AM

#11

jas88

Lead Driver

Joined: Feb 2002

Posts: 7,952

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From: Greater Austin, Texas

You need to try backing off the base idle screw with IAC valve disconnected to see if that makes any change. I am not sure about the idle spec on the 4.9L, but IIRC on the 5.0L with manual trans it is about 850-900 RPM.