1998 Ford Expedition Engine Codes

On Mon, 10 Dec 2007 21:33:37 -0600, Jake Jaschob rearranged some electrons to say:

This could indicate a vacuum leak, or perhaps a defective IAC.

For an Expedition, a common cause of the P0171 and P0174 codes is a contaminated MAF sensor. I'd try cleaning the MAF sensor. See:

With a System Lean report for both banks, I'd be thinking of a vacuum leak at a location that is common to both sides of the intake manifold. Vacuum is vacuum, and there are ports on one side or the other to provide vac to various components. I think that if there is a vac leak on one side or the other, you would see a lean report that has to do with that side, since you have a lean report for both sides, I'd start my search closer to to the throttle body, where a leak can lean the mixture to both sides.

The computer looks at the throttle plate (and a couple of sensors) to see how much air is going in, then adjusts the injector timing to affect changes to the air/fuel ratio (mixture). Your computer is telling you that the injector timing has been set fully rich, but the mixture is still lean. One cause of such a condition is a leak that is letting in air at a point downstream from the throttle body that the computer does not monitor/control, and the resulting air can not be made richer by the addition of more fuel.

My guess is that you may be experiencing poor mileage too because the computer is trying to add more gas to make the mixture richer, but the error report you have says the effort is not working.

You could have failing O2 Sensors too. These look at the mixture leaving the cylinders, and this feeds back to the computer which is looking at the throttle plate and a host of other things to make decisions on injector timing. But, there are specific codes for O2 sensor faliures, and I would expect one of these codes ahead of a Too Lean (or Too Rich) report. The O2 Sensors are in the troubleshooting chart for your issue, but they fall well below the vacuum leak.

If you have had any work done recently -- even an oil change -- then I'd be looking for cracked hoses in the immediate vicinity of where the work would be expected to be performed.

Your response suggests these codes P0171 and P0174 specifically happen on an EXPEDITION due to MAF problems, but doesn't the MAF affect all itterations of the engine the same way? That is, wouldn't my F150 be affected the same way?

The most common hose to fail on an Expedition is the PCV hose elbow at the rear of the engine (hard to find unless you know to look for it). A failure of this hose can cause the lean codes, but I still think the problem is most likely a contaminated MAF sensor.

See:

Definitely. I said Expedition because the OP was asking about an Expedition. This problem has come up several times among members of the Expedition Owners Mailing List. The two most common causes are a contaminated MAF and a cracked PCV Hose Elbow. Cleaning the MAF sensor is cheap. The PCV hose isn't too bad either.

Ed

I tend to agree. I _think_ the PCV elbow crack would favor a lean on one bank before it affected both banks, depending on its physical location in the intake manifold. The MAF would give a false reading that will cause the computer to detect a lean condition on both banks.