Rough idle, stalling when changing gears and turning at idle.

[Anthony Metzger]

I have a 1998 Ford F-150 5.4L vin code L r4100 auto transmission extended cab step side 4×4 with the off road and lariat package. I purchased the truck two weeks ago.

The issue I’m having is when the truck is idling and you turn the wheel in either direction it almost stalls and occasionally does stall. The rpms dip way down to around 300-500 rpms. Also when I shift gears while idling the same issues occurs. It mostly stalls when turning the wheel at idle and changing gears at the same time. Like when parallel parking. Additionally the truck shakes at idle like it’s misfiring, the exhaust smells of gas, and the gas mileage is not the best.

Here is a list of things I’ve replaced.

Fuel filter
Oil replaced with 5w-20
Oil filter Oem motorcraft
Coil packs
Spark plus replaced with oem I also checked the gap. (old ones looked like normal wear)
Idle air control valve
Throttle position sensor
Bank 2 sensor 2 oxygen sensor
Dpfe sensor
Egr valve
Egr solinoid
Engine air filter

I removed the throttle body and cleaned it including the egr ports. The throttle body was a little dirty but not bad. The egr ports were not blocked.
I removed all the vacuum lines from the throttle body’s and I plugged all of them except one. I attached a vacuum pump and the vacuum lines and the vacuum revisor holds vacuum.
I attached vacuum pump to the egr valve and it holds vacuum. The purge valve has three hoses that attach to it. One bigger hose goes from the purge valve to the fuel tank. One bigger hose goes from the purge valve to the throttle body. One smaller hose goes from purge valve to vacuum Reservoir I attached vacuum pump to the purge valve small vacuum line. It vents to atmosphere. It’s a one way valve as I can not blow into it. The two bigger hoses will hold vacuum. One I can blow into and the other I can not.
I attached vacuum pump to the intake via a vacuum line and used a vacuum line t to attach the pump. I then started the engine. Engine vacuum is 13 inches of mercury at idle. When I raise the idle the engine vacuum is 10 inches of Mercury.
I have checked the exhaust for leaks and there are none. The intake manifold bolts are all there and I don’t feel or here any exhaust leaks.
I attached a fuel pressure gage and checked the fuel pressure with the engine off key on The fuel pressure is 40 psi. I started the engine and the fuel pressure is 32 psi. When I rev the engine fuel pressure goes to 28 psi and returns to 32 when I go back to idle. I removed the vacuum line on the fuel pressure regulator while the engine is running and fuel pressure is 40 psi. No fuel leaked out of the fuel pressure regulator.
I swapped the mass air flow sensor with one from another truck I have that ran fine so I know it’s good.
I vacuum checked the pcv hoses and there’s no leaks there. I also tested the pcv valve operation and it’s working as it should.
The engine temp gage on the instrument panel is a little lazy meaning it reads a little below half way when the engine is up to temp. When I attach a scan tool the engine temp is 194-198 when driving, and 203 when idling.
There are no check engine lights on or malfunction indicator lights on, or service engine soon lights. Also there are no stored diagnostic trouble codes stored or pending. So there’s nothing to freeze frame.
Additionally the engine is not lacking in power. I can drive it on ther highway at 75 mph without any issues
The long term fuel trim is high, and both downstream oxygen sensor are reading higher voltage. I tend to see around 0.750 on the downstream Oxygen sensors after idling for 20 minutes when the engine is up to temp.
Sorry about the long post. I just wanted to layout what I’ve done and what I’ve tested to try to resolve this issue.
What am I missing? Any help you can provide would be greatly appreciated.

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