Hello,

I've read dozens of threads which attempt to troubleshoot 4.9 high idle issues and have been unable to fix my own instance of this problem. I'd appreciate the help if anyone has any suggestions. Here's the background:

Model: 1992 F-150
Engine: 4.9L L6
Transmission: Manual M5OD
Mileage: 286,000
Emissions: Stock and intact
Codes:
KOEO: 522, 111
KOER: 311, 411

The warm engine idle speed is roughly 1100 RPM. I say "roughly" because the idle speed sometimes hovers around 1500 RPM, particularly between shifts.

An inspection of the throttle linkage does not reveal any trouble. Also, the idle set screw still has its paint and does not appear to have been tampered with. It appears to be bottomed out.

The EGR tube behind the intake manifold does not appear to have any holes are leaks. Also, the exhaust is in sound condition.

A fuel pressure test was run and the fuel rail is consistently maintaining 52 PSI. The fuel pressure regulator vacuum line did not smell of fuel.

A tuneup was done recently with new plug wires, plugs, cap, rotor, coil, and PCV which had no affect on idle speed.

A compression test was run and all cylinders are within 5% of 150 PSI. The 6th cylinder was sooty and the engine's acceleration uneven, so the fuel injectors were replaced with new motorcraft units purchased from RockAuto. The engine accelerates much more smoothly, however, the new injectors had no effect on the idle speed.

I pulled a replacement MAP sensor, IAC valve, and throttle position sensor from the local salvage yard and tried replacement without effect. In particular, I connected the spare IAC valve to its plug while the engine was running and confirmed that the solenoid is moving as engine speed changes.

I've used propane to search for a vacuum leak. Tested were the injector o-rings, throttle body, EGR, and manifold gaskets, the PCV, as well as all visible and accessible hoses. None of the testing with propane resulted in an increase of RPM.

I disconnected and plugged the vacuum ports on the intake manifold's vacuum tree one at a time. When plugging the port which leads to the vacuum reservoir (the coffee can) the idle immediately dropped to around 900 RPM. I've traced the vacuum reservoir's other line to the TAB/TAD solenoids.

Having reached this point, I'm not sure if plugging the port to the vacuum reservoir would naturally result in lower RPMs. In other words, it may not be indicitive of a vacuum leak. I used propane on the TAD and TAB solenoids with their vacuum re-established and did not notice a change in RPM.

At this point, I plan to change out the hoses for the TAB/TAD and vacuum reservoir. If that does not work, I may pull another vacuum reservoir from the salvage yard and try to swap it. If there is still trouble, I plan to temporarily remove the IAC and seal its ports with duct-tape to see if it drops the idle.

Does anyone have any other suggestions?

Appreciated.

 

High idle can also be caused by worn Throttle body shaft, and blades when closing in the bore...everytime they close, a little wear occurs against the aluminium bores. Also, the EGR can be a vac leak, either big or small, but if leaking is usually brings in exhaust gases and causes stumbling. Remove the TB and check for daylight when placed between a good light source. If there is a gap, daylight showing where the blades close, it can be your problem. It's just a few thoughts...

 

511 isn't a sign either. I would be inclined to pull PCM and visually inspect the board. Many 4.9 owners are experiencing capacitor failure, and I did. May not apply, specifically because of code 511, but I would inspect.

 

Welcome to FTE. Check your vacuum canister for rot or holes on the bottom side. That'll definitely cause a vacuum leak and high idle.

 

Sorry, I'm actually getting 522, not 511. All the other codes are correct.

I've updated the original post with KOEO code 522.

 

I'll check it out, thanks.

 

Thanks. I'm in Arizona so there isn't much in the way of rust, but bypassing the reservoir definitely lowered the idle. I'm turning my attention to it...

 

As an update, I pulled a vacuum reservoir from the salvage yard and installed it but the high idle was not corrected. I'll try new vacuum lines this weekend.

 

You called it. Picked up a throttle body from a '88 or '89 bricknose at the salvage yard. Cleaned it up and installed it and the truck is now idling perfectly. It was dark by the time finished so I couldn't check for daylight in the old TB. That may have been the issue, however, I noticed that the two vacuum ports on the top of the TB went straight through the bore on the original TB, whereas on the older TB the vacuum ports open either in front of or on top of the blades. In short, the vacuum plumbing is different on the newer TB and that may have contributed to the high idle problem.

If other 4.9 owners on this board are having trouble with high idle after trying the items listed in the original post, then they may want to try swapping the throttle body, provided they have access to a cheap salvage yard (the TB cost me $20).

Thanks again for the advice timbersteel.