P1506 Idle Air Control System Overspeed Error

My 1999 Ford Escort 2.0l 4cyl Wagon’s computer produced the above info when interrogated at our local Autozone by some skinny dude with eyeglasses and his orange colored I-pod-lookin’ gizmo. My car has been running normally as far as I can tell, except for the “SERVICE ENGINE SOON” light being on and MAYBE our MPGs are down a bit - though this could be from all the short trips we make in the cold upstate NY winter.

Researched the problem and it seems to involve replacing a $60 part called the Idle Air Control Valve. Looks like an easy fix. Is there anyone out there with more insight than me who can say that the problem is worth the fix? Any imminent consequences from simply resetting the damn car computer and ignoring the problem?

Try spraying some throttle body cleaner in the throttle body first and see of the light will go off. (You’ll have to disconnect the air intake between the throttle body and the maf sensor)

The IAC valve is easy to change if needed.

You went so far as to have the DTC (Diagnostic Trouble Code) pulled from the engine control computer by “some skinny dude with glasses” with a scan tool. NO DTCs (check engine light codes) tell you that ANY component is faulty (ever!). It tells you there is a problem in a particular circuit. Certain diagnostic steps are performed to determine where the fault lies. We start with the code. DTC P1506 is “Idle Air Control Overspeed Error(see P0506)”. DTC P0506 is “Idle Air Control System RPM Lower Than Expected”.

Update: Did more research, and most opinions pointed toward cleaning the throttle body. So I found a good article on the Popular Mechanics website about this sort of thing. First I tested the valve with an ohmmeter. Valve responded properly. Then I followed Popular Mechanics’ instructions for cleaning the sensitive insides of Ford throttle bodies - use a soft toothbrush and a soft rag with 1:5 dilution gasoline/carb cleaner. Put everything back together, reset the service engine light, and 4 drive cycles later all is well. The car doesn’t clunk into gear now since I did this cleaning. Coincidence?

Update to the update: SERVICE ENGINE SOON light is lit again. Back to the drawing board…

Use the carb/throttle body cleaner spray, again. Be generous in its application. Let it soak for two hours. Start the engine and let run, at fast idle, until the smoke clears.

Update to the update to the update…

Took the IAC valve out, cleaned its insides, service engine light went off, mileage improved to 30+ mpg. Now, one month and 160 miles later, light is BACK ON! We just filled the tank. I think I will try switching to my spare gas cap… Owner’s manual suggests gas cap issues when the service engine light pops on… more later. May consult the local junkyard for wrecked Escorts with serviceable IAC valves in them.

I would have the computer rescanned to make sure that the P1506 code is still active. I am assuming you erased prior codes when you did the last service. It is possible the CEL is ‘on’ for another fault.

If the code is for the iac (idle air control valve) you need to remove the oil and carbon deposits from the air passage to the iac, and from the iac. Use steel round wire brushes, small picks, etc., and carb/throttle body cleaner to clean those passages. You can just spray cleaner, let it soak, to clean the throttle body bore and throttle plate.

Having done the cleaning already, per instructions from Popular Mechanics, and having the check engine light on again, I, as I posted earlier, switched gas caps.

The story behind the gas caps: Years ago, I left the factory cap on the roof of the car after filling up. I drove off, and off it slid, bouncing its way to the curb. I bought an aftermarket cap to replace it. Then, days later, I retraced my trail and found the factory original sitting at the road side. I saved it. Now, as I have been experiancing these engine light problems, I finally remembered I had the original cap stashed away in the garage cabinets somewhere. So I dug it up, and put it back on the tank, replacing the cheap aftermarket cap which I had been using for years by now.

Soooo, the CHECK ENGINE LIGHT IS NOW OFF AGAIN! Has been off for a couple hundred miles now. No resets involved.

I hope this is the end of the story! Thanks for your comments!

PS the error code has always been “p1506 IAC system overspeed error”.

A vacuum leak will also cause the “overspeed” error code. The only thing the computer knows is that there’s too much air getting into the engine and it’s causing it to idle too fast. The extra air could just as easily be coming from a disconnected or broken vacuum hose as from a stuck IAC Valve. Are there any hissing noises coming from under the hood while the engine’s running? Another possibility is if someone at some point messed with the curb idle screw, which would also let in too much air.

As nice as it would be that the gas cap would fix it, the gas cap wouldn’t cause this particular code.

The engine light remains off. We have gotten 40mpg over the past 3 tankfuls. No hissing. Idle sounds fine. IAC valve issue was addressed long ago (see previous posts). Though I will not rule out coincidence, I will continue to say my replacing the gas cap back to the factory original was the final nail in the coffin of this problem.

Engine light still off. The front brake rotors are so corroded (almost 10 years old with 80K miles driven in snowy salty upstate NY) that they need replaced. It’s always something…