A while back the CEL on my 1999 4.0 2wd auto came on. I had Autozone pull codes and got a PO171 and PO174 (lean on both banks). Of course Autozone wanted to sell me two O2 sensors but I had replaced one aboput 6 months ago and I am skeptical that both would be bad at the same time. I cleared the codes and the CEL did not come back on for about 4 weeks. then it came on again. I did a visual inspection to see if I could find any noticable vacuum leaks and found none. The engine runs fine, just the same whether the CEL is on or not. Again I cleared the codes and no CEL for about 3-4 weeks. Then on again. Clear the codes and off. Any ideas what might cause this?

 

You have a 4.0L...

I would suspect an intake mani gasket leak, but on both cylinder banks? Hmmm.

You could also be getting a case of a bad fuel pump, bad fuel pressure regulator, dirty MAF sensor....lots of things you have to check for.

Type the following into Google and I get: lean o2 sensor codes p0171 p0174

http://autorepair.about.com/cs/faqs/l/bl175h.htm

Good luck.

 

I would be inclined to retorque the lower intake manifold and clean the MAF sensor. I'd also inspect the EGR to ensure that there is no possibility that it's not seating snug and leak free. Happily, these are all freebies.

 

I think it is the "intermittent" nature that is throwing you off. Understand that for that code to occur after you clear it requires you to complete a drive cycle. It may take several rides to clear the P1000 code that is present after you clear all the codes.

http://www.obdii.com/drivecycleford.html

Thus, the problem most likely didn't just dissappear while the light was out, you're truck just hadn't run the necessary tests yet. In addition, I believe I read that the P0171/P0174 codes are set off when the rich limit adjustment is hit in 2 consecutive drives. So it may take even longer for you to hit that code.

I also had this problem with my 98 4.0L. I spent a lot of time trying to hunt it down (tried a smoke machine, sprayed a lot of carb cleaner on vacuum areas, etc). Finally, I broke down and just replaced the Upper Intake Gaskets after reading posts here and getting advice from several experienced Ford Techs that it would most likely solve my issue. Cost me less than $15 for the new gasket set at the dealership, the replacement took me only a few hours in my driveway, and the codes haven't come back for several months and a few thousand miles. I have a hunch that it doesn't take much of a vacuum leak to cause these codes.

-Jim

 

Well I don't know how long a drive cycle is but I have put about 800 miles and almost 4 tanks of fuel through the truck since I last cleared the codes and the CEL has not come back on yet. This is the way it has been for about 3-4 months. It will go for a long time with no CEL then it will come on. When I have had Autozone pull the codes they are PO171 and PO174. I am at a loss as to what might be the problem. I read in a previous post that an owner had a similar problem and the CEL only came on in cold weather. Once it got warm outside the CEL quite coming on. I suppose that could be similar here although over the weekend it got down to 20 degrees.

 

It might be worth pulling the upper intake and checking the gasket(s). They are cheap and easy to replace if there's a problem.