I've got a 1997 E-150 with the 4.6l (right under 120k on the clock, if that matters). A couple weeks ago it started idling very high (I'm guessing around 1,500-2,000?) and I went hunting for vacuum leaks using the water-spray method and the hose-a-scope method. Found nothing. I tried replacing the IAC with one that I knew was good and that didn't solve it either. I noticed that the PCV valve was making a ton of noise so I replaced that and it got a little better. Thinking that perhaps in my troubleshooting I had confused the PCM, I disconnected the battery for twenty minutes and that finally brought the idle back down to normal.

Everything was fine for a couple weeks and a couple hundred miles but then yesterday, it started doing it again. I had the folks at AutoZone pull the codes and it returned none at all. I picked up a can of CRC MAF cleaner and I'm going to try that. I also picked up some carb cleaner, and I'm going to go hunting for vacuum leaks with that method.

I'm not sure if this is "normal", but there are two vacuum solenoids on the rear of the engine, one appears to be for the EGR system and I'm not sure what the other one is. When I shut the van down, both of them flutter and make a rather loud groaning noise as the vacuum dissapates. It's been doing that for the last 20,000+ miles at least, so I don't know that the problem is there, but I thought it might be relevant.

Any ideas or areas that I should be focusing on to track this down?

 

Here's some weirdness on this problem: I left the battery disconnected for about 30 minutes and then reconnected it and started up the van. I did absolutely nothing else to the vehicle. The PCM reboot, however, caused the idle problem to go away. I can see that my TPS is reporting a little more than 17% travel when my foot is off the throttle and about 94% travel when it's depressed fully (and I've checked that the throttle cable isn't getting hung up anywhere), but other than that, all the PIDs are looking pretty much normal and there are no other issues (idles fine, doesn't stumble, doesn't hesitate, no missing, no noises, pulls strong, shifts fine). Because it wasn't very difficult to do, I did spray the MAF with the CRC MAF Sensor cleaner, but other than that, I suppose I'm just waiting for the problem to reappear.

 

I'm open to any guesses on this one, as it is still a mystery to me. I'll admit that I didn't do a full cleaning of the MAF sensor -- I sprayed it while it was still on the car, as opposed to removing it and following the directions on the back of the can to the letter. The problem came back again, and this time there was a pending P1151 code, although no MIL. The PCM says that the idle is 1500 RPM and change, as opposed to the normal 760 and change, and as per usual, there are no drivability concerns other than wearing out my leg holding the brake down at stoplights and the clanking of the transmission when it has to shift at 1500 RPM.

I reset the codes and cleared all the monitors, but that didn't reduce the idle. Keeping the battery disconnected for 20 minutes, however, is still clearing up the problem, at least temporarily, but I'm getting tired of resetting my clock. Because resetting the computer seems to clear things up for a while, my W.A.G. is that some sensor is giving the PCM bad information and over time its "learning" something that is forcing the idle up and probably leaning things out (based on the P1151 code). I know that testing a MAF without a flow bench and an oscilloscope is a waste of time, but what sort of airflow does a "normal" 4.6's PCM report at idle? I'm thinking that maybe the MAF could be reporting a lower amount of air than is actually passing through it, resulting in more air in the manifold than the PCM knows to inject fuel for. I'm going to spray some carb cleaner all over the engine to hunt for leaks, but I'm not sure how resetting a PCM would also patch a hole in a vacuum line or plug up a leaky gasket...

Anyone?....

Bueller? Bueller?....