My wife's 99 Expedition w/ 5.4L has had the Check Engine light come on (quite some time back, actually). It's been running fine, but it needed inspection. So I pulled up the codes on http://www.obd-codes.com. Based on the write-up there, I got some electrical cleaner and sprayed down the MAF. Afterward I left the battery unhooked for a bit and the put it all back together. Check Engine light was gone. Drove around for about 10 minutes and it was fine. Wife drove it this morning for a good 20+ minutes and then headed over to get it inspected. She pulled into the bay and just as the guy was getting in the light came back on.

So, annoying to say the least. Now I'm wondering if the MAF is bad, or I've got some other fun issue like a vaccuum leak somewhere that I'll have to find. I'm about to go buy a new MAF, but I'm not sure if I can return it to Autozone if that ends up not being the problem. It does have a K&N drop in. I was thinking that might have gummed the MAF last time I cleaned it, but it's been quite some time since that was done. So, it should be dry and not doing it again. Fuel Filter was changed at the dealer about 15K miles back. I was having a hell of a time with the clips so I let them do it that time.

My current gameplan:
Go buy a new MAF and plain air filter. Reset and watch to see if it throws the code?
If it does, try to take MAF back and take it in to someone to hunt down the leak. (Not what I want as it gets expensive at this point)

Thoughts?

 

Locate the PCV hose that routes to the back of your engine. On this hose, there's an 'L' shaped rubber boot on this plastic hose that goes into the back of your engine. Make sure this rubber boot is not cracked and or split. This is a common point of failure that can throw these codes and or make your idle rough depending on how bad the leak is. It's just another one of many things for you to check when troubleshooting this problem.

 

Any mods ?? My truck was throwing a P0172 and a P0175 when the bolt that held my AFI pipe to the bracket shook it's way out. Since my truck was custom tuned to such tight tolerances that little hole let enough air in to throw the code as I have returned the truck to stock, in the middle of this nice Northwestern winter, and no code for a week now. I knew I should have helped Marc Carpenter out when he put that new 3.5" pipe on when I was at Troyer for the November 06 Dyno day ! I learned a valuable lesson the cold hard way but now I have medium strength bolt fastener to prevent this from ever happening again. I am going to wait to things get above freezing and then I am going to get the custom set up back on my truck.

 

No mods. This is just a plain `99 Expy Eddie Bauer w/ 5.4L.

So, I guess first I'll check PCV valve, elbow, and hose. Reset KAM and see how it goes.

Then if still a problem. clean MAF again. and test.
Then if still a problem... re-group. Wish there was a diagnostic way to specifically test the MAF.

 

I had the same problem with my wife's '99 Expy w/5.4. Turned out that a couple of the vacuum lines on the passenger side just above the valve cover had cracked at the bends. I replaced those hoses and everything has been fine for 2 years now until I recently got the "insufficient flow to egr valve" code...

 

I had the same problem and for some reason my bad DPFE tubes had set off the trouble code. Change the tubes coming off the exhaust to the DPFE and all is well.

 

Well, after looking around, I found that on the vacuum lines connecting the PCV one had caved and eventually cracked, and another one that has a 90 degree bend in it cracked as well. So, I replaced them , put a new PCV valve on, and reset KAM. Engine light hasn't come back and we've put about 100 miles on it. So, here's hoping that's all it was.

Thanks for the help, guys.

 

There is. In a Haynes manual, it shows a procedure to backpin the connections on the MAF plug, and check voltages with a mulitmeter.

Glad you found the actual problem though