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Old 02-21-2019, 01:56 PM   #1
96mustangtx
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Default Ford 4.9 High Idle

I have been chasing a high idle of 1100-2000 RPMs on my 93 F150 4.9 for a few weeks now and any information would be greatly appreciated.

I have replaced all vacuum lines and spent a small fortune on carb cleaner looking for a vacuum leak. TPS voltage is normal, IAC is functioning as I have tried two Ford units and a new one from the auto parts store. EGR seems to be functioning normally when I apply vacuum.

The only code I have is 411, which is unable to control high idle, or something to that effect.

I have ruled out the throttle blades and shaft from being worn and feel it is still a vacuum leak. I am able to start the truck and completely block off the throttle body and the truck will still run. It will settle down and idle at about 850 RPMs.
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Old 02-21-2019, 03:39 PM   #2
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Do the brakes work fine or do you have a real hard pedal? If hard, your brake booster i bad and that'll cause the vacuum leak.
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Old 02-21-2019, 03:53 PM   #3
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The brakes work fine. I even removed the brake booster hose and looked for vacuum leaks at both sides. I have been driving the truck daily. Other than the high idle and bad gas mileage, it has been running pretty well.
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Old 02-21-2019, 04:29 PM   #4
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Id probably get a shop to smoke test it while you watch and then go fix it yourself. I bet you could talk someone into doing that for you pretty cheap.
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Old 02-21-2019, 04:31 PM   #5
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Unplug the IAC and see what it does. Unplug maf and see, same with the tps. One at a time. I assume this truck has all of those.
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Old 02-21-2019, 07:17 PM   #6
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Those motors are notoriously for intake gasket leaking. Usually toward the back cylinders and at the bottom of the intake where you have a hard time getting spray in there to find a leak.
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Old 02-21-2019, 09:40 PM   #7
96mustangtx
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Thank you for all the tips.

Unplugging the IAC makes no difference. That combined with the fact the truck runs with the throttle body 100% blocked really points to a vacuum leak in my opinion. Unplugging the TPS changes idle slightly and causes a check engine light. There is no MAF as this is a speed density system.

A smoke test is happening this weekend as I bust out my coffee can smoke machine again. If this does not work I will allow the engine to idle with the blocked throttle body and then look for leaks by spraying with carb cleaner around lines and gasket mating surfaces.
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Old 02-23-2019, 04:55 PM   #8
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Blocked off the throttle body with the engine still running. Found a huge leak at the throttle body. Put a new gasket in and everything was perfect for about 10 minutes.

Now idle is better than it was but I still have a leak. When I blindly spray carb cleaner toward the rear of the motor the engine starts to stumble. All vacuum lines are good. Best guess is the intake is leaking on the rear cylinders.

Sound about right?
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Old 02-23-2019, 09:39 PM   #9
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Pulled the upper intake and fuel rail, which was a fun treat since I was outside and it is a wonderful 7 degrees outside.

There is no way to see any kind of leak, but I did find something interesting. On the back of the motor there is only two culprits I can think of back there. The first is an actual intake leak at the head or a bad injector o-ring allowing air in.

The rear of the motor, especially near the #6 injector, is pretty dirty. None of the other areas around the injectors are near as dirty.

When spraying the carb cleaner earlier this afternoon it was directed at the rear of the motor, between the fuel lines. The intake and #6 injector were directly in the sprays path.

Heading to the auto parts store tomorrow for new o-rings and slapping it back together. If that does not fix the issue, I will drive the truck until it is much warmer and then do the gaskets.
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