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Engine starts up at very high idle

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Engine starts up at very high idle

Post #1 by cpbean818 » Thu Sep 05, 2019 11:07 am

First, sorry to the moderators if I am posting this in the wrong place

I have a 1996 Ford F150 4.9L with a 5spd manual. When I start the truck, it starts up a very high rpm for a few seconds before dropping down to normal.

I checked for vacuum leaks, and I have found none. I put a vacuum guage on it, and it is right in normal range both running and not running.

I replaced the IAC valve last year. I suspected that it went bad. I put a meter on it, and it registers resistence at a solid 9.0 (should be between 7 and 13 according to Haynes). I metered the voltage coming to the IAC valve from the PCM, and it is right at 12v (which is what it should be according to Haynes).

The IAC valve is only a year old with about 6k miles on it, so I cannot imagine that it is carboned up and stuck. I also pulled the throttle body at that time last year, cleaned it up and reinstalled it with a new gasket. Correct me if I am wrong, but I am thinking that if it were the throttle body sticking, the problem wouldn't just be there for a few seconds at start up.

The PCM is new, and this problem existed before the new PCM and has remained after the installation of the new PCM (I replaced the PCM for other reasons - namely fuel/air mixture error codes, O2 sensor error codes, and other error codes that would appear and reappear - and those problems were solved with the new PCM).

I am pretty much out of ideas as to what is causing this - I don't want to just throw parts at it. Thanks for whatever help you can provide!
1996 F-150 Reg Cab Long Bed 4.9L M5OD 2WD Eddie Bauer

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Re: Engine starts up at very high idle

Post #2 by wallen7 » Fri Sep 06, 2019 12:04 pm

Check and make sure your ECT sensor is working correctly. If the ECT sends a signal to the PCM that is reading cold it will cause to idle fast even if the engine is warm. Also check IAT sensor. Some of the plugs on the harness are the same aand I have found that if someone has had the engine apart they were swapped by mistake.

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Re: Engine starts up at very high idle

Post #3 by MechRick » Fri Sep 06, 2019 4:41 pm

If the base timing has been adjusted higher than the stock 10 degrees, it can create this symptom. I ran mine 12-14 degrees and lived with it.

If you don't have one already, one of the cheap code pullers that are capable of displaying data are useful. When the engine is warm, accessories off, look for IAC counts in the 28-35 range. Adjusting the throttle plate closed will result in low cold idle and higher IAC counts (35-40). Open too far and IAC counts drop and cold idle will be high.

Looking at IAC counts will tell you if there is a vacuum leak. A big one causes IAC numbers to drop to zero.
1994 F150, 4.9L/ZF 5 speed, C-Vic police driveshaft
EFI head w/mild port work, 3 angle valve job
1996 long block, stock pistons, ARP rod bolts
Stock cam, aluminum cam gear
Hedman header, full mandrel bent duals, crossover, super turbos
Bronco II with a 2.3L swap
1988 F250 2x4, 460 ZF 5 speed.

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Re: Engine starts up at very high idle

Post #4 by cpbean818 » Sun Sep 08, 2019 5:36 pm

Thanks for both of those tips. I don't think anyone ever took this engine apart, I know the symptom started only about 6 weeks ago, and I did set the timing around that time (to the stock 10 deg)

MechRick - that is really helpful. I will get one of those code pullers that can display data. In the meantime, I will fool around with getting that timing in the 12-14 range and see if it does the trick.

I really appreciate the replies!!
1996 F-150 Reg Cab Long Bed 4.9L M5OD 2WD Eddie Bauer

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Re: Engine starts up at very high idle

Post #5 by Wesman07 » Sun Sep 08, 2019 10:24 pm


I think you miss read Mechrick. I believe he is saying that going higher than 10 degrees will cause the problem you are having.

How high is it going?
In-lines we trust

86 f150 300 efi with advanced stock cam. Np435, Dana 60/ 10.25, 35" BFG's, four link front suspension with 12" travel fox coil overs, custom deaver leaf pack in the rear.

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