4.6L Dies at Closed Throttle

[2000StreetRod]

Last month my 2003 Centennial (Limited) with stock 4.6L V8 starting stalling when I released the accelerator. Since I have no past maintenance records on the vehicle I decided to replace the spark plugs. The old plugs looked good (no signs of detonation or fouling) other than having an excessive gap due to wear. I gapped and installed a new set of Motorcraft SP-493 AGSF32PM platinum plugs. That didn't fix the problem which has gotten worse. The engine runs fine at 800 rpm if I keep the accelerator slightly depressed and doesn't miss when accelerating. It acts like there's no idle air control (IAC) valve function.

I removed the IAC valve and cleaned the internals with throttle body cleaner. I can move the valve shaft with a flat blade screwdriver. When I remove the screwdriver the spring returns the shaft to the original position. I can blow thru the valve, its connecting hose, and the associated main intake hose port and intake manifold port. There are no IAC valve related diagnostic trouble codes (DTCs). I didn't check the power source fuse because the same fuse provides power to the injectors that are working.

Our 2014 Jeep Grand Cherokee is at the dealer for the next few days awaiting arrival and replacement of a failed driver seat air bag wiring harness. So this afternoon I adjusted the Explorer throttle stop screw for the warm engine to run at about 700 rpm with the accelerator released. I measured the resistance of the IAC valve solenoid to be about 10 ohms which is comparable to a Mustang valve I have that is too long to fit in the space available. Tomorrow I'll try energizing the solenoid with my battery tender to see if the shaft moves. I'll check the mating connector electrical path to the PCM and Battery Junction Box. I'll also leave the connector disconnected to see if that triggers an IAC valve related DTC. I'm rapidly running out of diagnostic ideas.

 

[Turdle]

IS there a throttle position sensor on this engine? What is it's idle reading after the throttle adjustment?

 

[2000StreetRod]

IS there a throttle position sensor on this engine? What is it's idle reading after the throttle adjustment?

Yes. I haven't checked it yet but I suspect it increased from about 18% relative to about 20% relative based on the amount of throttle stop movement.

This morning I energized the Mustang IAC valve solenoid with my battery tender and the unit made a hum while displacing the valve and shaft against the spring pressure. Performing the same process with my Explorer IAC valve resulted in the unit simulating a loud buzzer so I decided to purchase a new one. I installed the old unit leaving the electrical connector disconnected and drove about 2.5 miles to the grocery store to pick up a couple items. While not moving the engine idled at 600 rpm in drive with the A/C off. Next I drove about 0.5 miles to Advance Auto and purchased a BWD 31077 IAC valve for $57. Then I drove the 2.5 miles back to home. When I checked for new DTCs P0511, IAC circuit malfunction was reported. Since that DTC had not been reported before I assumed that the electrical wiring and connector was functional. After letting the engine cool for an hour I replaced the IAC valve and cleared the DTCs. Upon start the engine cycled thru its normal cold start routine starting at about 1800 rpm and rapidly dropping to around 1000 rpm. Idle speeds were normal in Park, Reverse, Neutral and Drive with the A/C on or off.

I lambast Ford for the totally inadequate IAC valve diagnostics implemented in my 2003 PCM strategy. In older models the PCM compared the commanded engine idle speed to that reported by the crankshaft position sensor. When there was a significant difference between the two the PCM increased or decreased the commanded rpm until the reported rpm was achieved. The PCM computed the delta between the two and utilized that in subsequent commanded engine speeds. The PCM stored the delta in nonvolatile memory for the next engine start. When the PCM was unable to achieve the commanded engine speed due to a defective IAC valve the PCM set DTC P0506 Idle Control System RPM Lower Than Expected or P0507 Idle Control System RPM Higher Than Expected as appropriate. It is possible the diagnostic capability was deleted in anticipation of elimination of the IAC valve when converting to electronic throttle control.

There are two other DTCs I want to eliminate before pulling the engine: P0152 02 Sensor Circuit High Voltage (Bank 2 Sensor 1) and P2198 O2 Sensor Signal Biased/Stuck Rich (Bank 2 Sensor 1).