P1506 Code High Idle

[TOMANDPAT]

I've got 17,000 miles on a remanufactured Jasper engine, 4.0 SOHC. A week ago, I've been experiencing a high idle both at start up and when warm. I'm talking 2500 rpm and above. I've searched the forum, put a new IAC valve on, checked for vacuum leaks,(found none) and still get a P1506 code. Last night, I disconnected the IAC, the idle dropped about 200 rpm (to 1000) but is smooth as glass. Most of the posts I've read here talk about a very rough idle when the IAC is disconnected. Tomorrow I'm going to remove the upper intake to get a better view of the vacuum hoses down there, and see if I can retorque the lower intake. I'm also going to replace the upper intake gaskets. Does anyone have any experience with this problem that can give me any insight? Any and all will be appreciated.
Tom

 

[2000StreetRod]

closed throttle plate?

P1506 Idle Air Control Over Speed Error

The PCM is commanding the IAC valve to a lower idle speed than what the crankshaft position sensor is reporting. The PCM is assuming that the IAC valve is at fault. Make sure that the throttle plate is fully closing.

 

[TOMANDPAT]

The throttle plate is fully closed key on or off. Would a defective IAT or ECT sensor cause this problem? I'm grasping at straws here.
Tom

 

[2000StreetRod]

intake air

A bad IAT or ECT sensor would affect the air/fuel ratio but not allow the additional air for a very fast idle. The intake air is either the result of a leak, a stuck open throttle plate, or the IAC valve excessively open. It may be possible that a bad TPS may confuse the PCM and cause it to open the IAC valve excessively. However, a 200 rpm drop after disconnecting the IAC valve is normal and the DTC indicates that the PCM is unable to utilize the IAC valve to lower the rpm to that desired. I suspect one of the vacuum connections has come loose, a hose is cracked or broken, or the intake gaskets are leaking. Intake manifold vacuum lines

 

[swetrid]

P1506 is a not a very common code but I have seen this code on escapes before and the problem was pcv valve or vacuum leaks. We would clamp off the pcv hoses and see if idle drops down.

 

[TOMANDPAT]

Today I pulled the upper intake, checked the gaskets, they looked brand new. I checked all the vacuum hoses for chafing, loose fitting connections, and the like, they all passed inspection. I checked the torque on the lower intake, put everything back together, and started it up. Idles at about 1000 rpm with IAC disconnected. When I plugged in the IAC, the engine roared, and I disconnected it. Now it's throwing a P1504 code, but the idle is smooth, and I'm inclined to leave it at that. I've pulled the PCV valve with the intention of replacing it, but I can't get the tee fitting off, and the parts store doesn't sell one with a tee. Could I just try eliminating the PCV and blocking the ports on the intake?
Tom

 

[2000StreetRod]

SOHC PCV Valve Mod thru 2000

SOHC PCV Valve Mod thru 2000

 

[TOMANDPAT]

Well, I've been driving for a week with the CEL on, and the IAC disconnected. It's been idling around 700 - 800 rpm. Today I got tired of the CEL and bought a new IAC, disconnected the battery, installed the new IAC, reconnected the IAC and the battery, and started it up. It jumps to about 2000 rpm's on start up, then settles around 1500 rpm's at idle. Could it be I have a PCM problem? I'm still grasping at straws.
Tom

 

[2000StreetRod]

Which vehicle?

Is the problem on the 1998 or the 2002?

When you swapped the IAC valve what was the condition of the gasket that looks like an "8"? It is very thin and easily broken.

Are there any DTCs with the IAC valve connected and clearing old codes by disconnecting the battery?

 

[TOMANDPAT]

I'm working on the '98. The new IAC came with a new gasket/o ring. I was showing a code P1504 with the old IAC disconnected, but the CEL is off now.
Tom

 

[Turdle]

Just a long shot-
Is the coolant level in the radiator full? Check the radiator, not the reservoir please.

 

[2000StreetRod]

No DTCs

If you have no DTCs then the PCM must think that the engine rpm is as commanded. Either the engine speed is being measured incorrectly (low) by the PCM or the PCM thinks the engine speed should not be at idle. One possibility is the TPS is reporting partial throttle demand to the PCM and the PCM is opening the IAC valve to respond to the demand. I suggest: TPS Test Procedure

 

[TOMANDPAT]

The coolant level in the radiator is full. I did the TPS test, and the voltage readings were good. What is the resistance supposed to be?
Tom

 

[2000StreetRod]

TPS resistance?

I don't remember what the TPS resistance should be. The PCM measures the voltage so if that's correct the resistance should be also. I'm running out of ideas. If the crankshaft position sensor is defective and not sending a signal every revolution then the PCM would think the engine speed is less than actual and try to increase it with the IAC valve. You might try disconnecting and reconnecting the crankshaft position sensor connector to clean the contacts.

Another possibility is faulty wiring or connection between the IAC valve and the PCM. Obviously there is some electrical connection since the engine speeds up when the IACV connector is attached. I believe the PCM controls the IACV via pulse width modulation - a varying on/off duty cycle. Perhaps the control signal is stuck on and the IACV is at its full open limit. I'll check my wiring diagrams tomorrow to see if the PCM activates the IACV with a low. If so, then the line could be shorted resulting in IACV constantly full on.

 

[2000StreetRod]

shorted IACV wire?

My 2000 wiring diagrams show battery voltage from the PCM Power Relay connected to one side of the IAC valve via a red wire. The PCM controls the IACV by periodically shorting the white/light blue wire to ground via PCM pin 83. If the wire is accidently shorted to ground then the IACV would be driven fully open. The physically adjacent pins to pin 83 are: pin 82, not used; pin 84, turbine shaft speed sensor; pin 56, EVAP canister purge valve; and pin 57, knock sensor input. If pin 83 was shorted to any of the other pins then there would be other DTCs and symptoms. I suggest that you disconnect the PCM connector and the IACV connector and test for a short to ground. If there is no short then inspect the PCM connector for abnormalities. Make sure pin 83 does not get pushed back when reconnected. Then measure for a short to ground with the PCM connector connected.

 

[TOMANDPAT]

I'll do that this weekend. Not enough hours in the day during the week. Thanks for all the help StreetRod, I'll keep you posted
Tom

 

[220richard]

Any new news

My explorer would stall when coming to a stop after a minute or two. I replaced the Mass Air Flow sensor, both C02, IAC, EGR, PCV, TPC, Fuel Filter, fuel cap, checked for vacuum leaks, new hoses, new K&N Air Filter, performed the fuel system cleaning for a cable driven throttle body 3 times (Sea Foam and throttle cleaner with tooth brush), and disconnected the negative battery terminal for 30 minutes to give the PCM a new learning strategy. While rebuilding the gas tank I found the screen at the bottom of the fuel tank had broken off and deteriorated in pieces. After I replaced all the components of the fuel tank the truck no longer stalled when at a stop but now it idles high, sounds like the same symptoms that you are having. When it is idling high the air filter box sounds like it is struggling to get air. It makes a whistling noise like it is trying to get air through the gap between the box and the lid to the box. Once I noticed the air filter box clamps had come loose and when I clamped them the truck stalled. I started it again and it idled high and still seams starved for air from the filter box. – Anyways, did you ever resolve this problem?

 

[TOMANDPAT]

I'm still working on it. Yesterday I made a gasket for the IAC out of solid rubber to close off the ports. I installed in under the IAC and started the truck, and it idled perfectly at around 700 rpms. Unfortunately, it can't be driven with that gasket in place. I was just checking to make sure I had no vacuum leaks. I got out the multimeter and checked the resistance between the signal and ground on the MAF, and got about 3 ohms. I did the same thing on my wife's 2002 with the sme engine and got 50 ohms. My daughter's 2000 showed 45 ohms. So it's off to Pick n Pull after work tomorrow for a replacement MAF. I'll let you know

Tom

 

[220richard]

Solved It

My New IAC though new and ohming correctly was faulty. Thanks