Well the weather has changed and I am now chasing the infamous P0171 (lean bank 1) code and resulting check engine light on my '97 AWD 4.0. I haven't had this problem all summer, but it started as soon as things cooled down weatherwise.

I have cleaned the MAF, made sure all the connections are airtight.
I did the O2 sensor less than a year ago, have replaced the temp sender, air filter and the idle speed valve. The fuel filer has been checked as well.

The van runs fine. Just that darned light! I have been driving the van with the code reader attached, scanning for codes with the van in use and this is what I found.

When I first start it up in the morning everything is fine. Once it warms up and the computer takes over I will not get a light (or code) as long as I am driving at a steady rate. Under this condition the code never shows up.

But if I stop and idle for a bit, like at a traffic light, the code will appear when I go to accelerate away from the light. Of course once the code shows for the second time the light comes on. If I drive the van in the morning and then come home, clear the code and not let let van cool down and use the van off and on through out the day the code won't show up again all day. But if I let the van sit for the whole day I will get the codes again in the afternoon un the same warm up conditions as the morning.

I am completely lost on this one.

 

soounds like an almost good 02 sensor. My 93 3.0 acts like that when it is going bad or getting weak.

 

hey man I have been running my 1991 4.0 for over a year now with the check engine light on, in that time I have replaced the cylinder head gaskets, alternater, starter motor, ignition pack, starter module, computer brain, water temp sender units, 02 sensor, front and rear oil seal's on tranny, water temp stat, battery, fuel relay and starter relay and the stupid c.e.l is still on, beat that!. But the van runs fine, next time I take the dash out I will pull that bulb.
Cheers.


edited for language by FTE

 

Berry, do you have one O2 sensor or two? My. 4.0L has two. I have the same code and I ordered both sensors. I'll let you know how much (or how little) they help.

Give it a couple of weeks. Military mail take a while.

I'll post after replacing them.

 

It could be the fuel pressure regulator. I was getting the same code, and replacing this with one I got from a junkyard finally did the trick.

 

I have two O2 sensors, one before and one after the CAT. According to Ford there is no way the one after the cat can cause the code. The leading one might though.
And in response to the other answer I have considered the fuel regulator and will try that after the holidays. I have found that if I run the van without stopping when I first start out in the morning I don't get a code anytime until the van sits over night.
Then the last couple of days here there have been no codes.
I will let you all know what I try and what happens.

 

A long time in posting this update, but, it has also been a long time in fixing the problem.

I FINALLY did the intake gasket replacment and just like everyone told me, that was the problem.

I just kept putting it off due to the dread of working in such cramped conditions as exist under the "hood" of the aerostar but a few skinned knuckles later all is well.

Thanks to all those who pointed me in this direction so long ago.

I also replaced the MAF, the O2 sensor, the fuel pressure regulator and fuel filter.

The darned van is so smooth at idle now that I have to keep doing a double take to see if it running!

 

Thanks for the follow-up, Berry. Were you able to find where the intake gasket was leaking? I rebuilt the top of my 3.0, and I always worry about the rubber gaskets on the front and rear of the intake manifold.

Good job!
M

 

it always amazes me how quiet these 4.0L V6 OHV Fords are....until one tinkers with the air filter box opening and air dam in front

zoom zoom zoom

 

It was on the side of the upper intake. One of the o rings was pulled it a bit. ( not really sure if you should really call it an o ring or just part of the gasket set up but it looks like an o ring.)
I found it by spraying wd40 on the joints to the manifolds while the engine was running. The leak wasn't huge but the engine speeded up just a bit when I hit the sweet spot.

While I had it down that far I did the intake manifold as well. Had to move alot of stuff to get to everything but it worked out.

 

... or the IAC goes bad and the van starts doing it's foghorn impression...