My father in-law has a 1998 Lincoln Navigator with 160k miles. He brought it to my house so I could change his P/S pump which was making noise at low RPMs when turning the wheel. No biggie, right? So, all I do is move intake plastic out of the way, take off the fan/shroud, remove belt, pull off P/S pulley, disconnect P/S pressure hose, remove P/S pump, install new pump and new pressure hose, install pulley, put on new belt, put on fan/shroud, replace intake system, start truck. Here's the problem.

The truck starts for a split second like normal and then just dies. If I give it gas, it will run at whatever RPM I give it, so it is getting gas and has good ignition. When I let off the gas, it drops to around 100-200 RPM and will sometimes jump up to 1000-2000 RPM for a second like a surge and then back down to the 100-200 RPM range and then just die. No CEL when its running, either. So, now the truck is immobile at my house and I look like a crappy son-in-law know-it-all shadretree mechanic!

I searched the internet and found a few fixes for this description, so this is what I tried:

I disconnected the battery and reconnected to try and reset the idle trim, no luck.

I removed the IAC valve and cleaned it and moved the mechanism back and forth, no luck.

I removed the PCV hose system that goes from the passenger valve cover then Y's to the either side of the throttle body and inspected for leaks, none found. The PITA one on the back of the intake where it goes from skinny to wide hose is OK too, and mine doesn't make an elbow, it just reduces straight.

I wiggled and disconnected and reconnected my crank position sensor, no luck.

I cleaned my MAF with MAF cleaner, no luck.

Now, without a CEL, I am kind of at a loss here. Any suggestions?

I've posted this in the Expedition/Navigator forum as well...

 

Did you make sure you connected the MAF sensor back up and/or any other sensors you unplugged?

 

I never disconnected the MAF sensor, I just moved the whole MAF housing off to the side. The only thing I unplugged was the temp sensor on the downstream of the MAF, which I plugged back in. Wouldn't I get a CEL if something was unplugged though?

 

Not for a couple dozen miles at least.

 

It does sound like the symptoms of a failed IAC.

 

99.99% chance it's the IAC valve on the back of the intake. Pull it out and either clean it out with brake cleaner or I'd just put a new one in for 39.99+tax.

Mike

 

exact same symptoms as my 98 Expy 4.6L when the IACV crapped out.

 

Well I cleaned the IAC the other day when all this started and it had no effect. Tonight I wiggled the connection at the MAF and it has started to idle. It is still surging though, like it can't figure out how to idle right. Does that have anything to do with the "learning" process of the computer?

 

OK so as an update:

MAF plugged in: could not start the truck without it dying immediately
MAF unplugged: truck runs but not very well. I drove it around and must have low vacuum because brakes were almost nonexistent

So, I replaced the MAF thinking it was bad. Wrong. Next step is IAC? I am trying to avoid the "throw parts at it" approach.

Some interesting tests I did:
With truck running (unplugged MAF), I sprayed for leaks with starter fluid and also soapy water, and found nothing. I unplugged a couple of vacuum hoses and felt for suction, although I don't have a vacuum gauge to measure it. If I dismantly the air cleaner assembly where the MAF is, and I remove the MAF from the breather, the truck will start and run with the MAF plugged in. If I move the MAF into the breather while the truck runs, it shuts down. If I move the MAF into the breather with my finger over the hole that leads to the MAF filaments, it still runs. Soon as I remove my finger, it dies.

Still at a loss and feeling dumb right about now.

 

Before you buy a new IAC try unplugging the connector for the IAC. If the engine still has the same problems then there is a good chance the IAC is bad, if the engine behaves differently then more than likely the IAC is working.
It kind of sounds like you have a problem with the wiring for the MAF. I would recommend getting a pinnout of the MAF connector and check for the proper signals to and from the MAF. Seeing that you just worked on the power steering, which is in the same corner of the engine compartment as the MAF, I would suspect that either the connector or the wiring harness for the MAF has been damaged.

 

Definitely sounds like the IAC.

Remove it, put some voltage on it, and reverse the polarity and see if it moves back and forth.

Also, check the resistance of the IAC. Is resistance between 6.0 and 13.0 ohms?

 

For those interested, I've been updating the other thread:

https://www.ford-trucks.com/forums/7...ive-gas-3.html

I've done all kinds of tests and I think the MAF is functional, although when it is being used, it kills the engine. I don't know where to go from here. I am NOT a shotgun part replacer, and I am trying to figure this out without resorting to that. I haven't replaced the IAC for that reason, although I have cleaned it.

 

Knock, Knock, Knock... is this thing on?

 

Just changed the IAC. No difference.

 

Jees... Gotta be a vacuum leak, or that MAF is not right - sure you don't have it in backwards?