Hey Fellas,

I have a 98 Expedition 4X4 with a 5.4 engine. After startup and when I first pull out of my drive or a parking lot, as soon as I give it gas it will almost die for half a second and then go on fine. This only happens about once a week. I have no other issues with the engine. I have replaced plugs, coil packs, throttle position sensor, temp sensor, cleaned mass air meter, and none have helped problem. Any thoughts would be a great help.

 

Fuel filter maybe, or an issue with atleast one fuel injector or the fuel pressure reg.

 

Is it at a particular time, like say every Monday morning after the vehicle has been sitting all weekend, or after it rains?

 

No particular times that I have noticed. You might be right about the rain because it has rained here 3 out of the last four days and it has done it 3 times. Only after initial startup and when you give it gas to pull out off from red light or drive way.

 

I have changed the fuel filter as well and had a fuel pressure gauge on it while driving but pressure has been consistent. Haven't had fuel pressure gauge on it when it has stumbled though.

 

In that case you might have a MAP sensor that reads good to the on-board computer during normal driving or weather conditions. With rain comes a change in atmospheric pressure. This change might be enough to push your MAP sensor out of range and cause the air/fuel mixture to run slightly lean. During acceleration a slightly rich mixture is desired.

Here's a good article on how the sensor operates: Manifold Absolute Pressure MAP Sensors

 

Alloro, don't these rigs use a MAF (Mass Air Flow) sensor?

 

They use both a MAP sensor and a MAF sensor.

 

Thanks! I guess I'm not so old that I can't learn some more!

Where is the MAP sensor located? Does it use vehicle power or does it use vref, and is it digital or analog?

 

Actually you were correct, I was in error. In my mind I could see a MAP sensor under the hood, but what I was really picturing was the DPFE sensor that is used to regulate the EGR valve. It has a 3-wire connector going to it and hoses that look just like what would be going to a MAP sensor.

 

Okay, got it.
To the OP (tang1987) might want to take a closer look at the MAF sensor. Some put on those aftermarket air filters and they can end up with some lint coming of the filter surface (or air filter oil) and contaminating the thin wire of the MAF sensor.

Also, if the "bog" only happens when giving it gas from almost a standing start, I have seen where the engine torques over a little bit in its mounts and might open up a small leak in the intake hose between the MAF and the throttle body. This can allow more air into the engine than the MAF sensor "sees" and can cause a lean bog. Once the engine bogs, the engine rolls back to where it usually is and closes the leak and then runs fine.

These intermitant issues can be hard to figure out. Good luck with it.

 

To all of the Doubting Thomases, see that I do admit it when it happens. It doesn't happen often, but it does happen.

 

Thanks to both of you for your input. I have since cleaned the MAF again and the IAC valve. Rubber hose going into throttle body was a little loose fitting, so I put a hose clamp on it. Hasn't stumble any more but it has only been since the weekend. Thanks again fellas