P0171 and P0174 Lean both banks (Solved)

[myf16]

My 40.L SOHC 1995 has these codes. I already sprayed the MAF and changed the IAC valve which had generated its own error code.

I have attached the FORScan app screen grabs showing fuel trim at idle and the error codes and explanations.

A year ago when I had a similar problem I checked everywhere for a vacuum leak with propane and found nothing. I plan to repeat the vacuum leak test with starter fluid. If I find nothing, I will probably change the intake manifold gaskets anyway. Or would DPFE sensor would be a better second step?

 

[koda2000]

There was no SOHC until '97.

 

[myf16]

It's just the base 4.0 liter, which I guess is OHV?

 

[koda2000]

It's just the base 4.0 liter, which I guess is OHV?

Could be a sizable vacuum leak effecting both banks. When's the last time you replaced your fuel filter?

 

[myf16]

Could be a sizable vacuum leak effecting both banks. When's the last time you replaced your fuel filter?

I'm ashamed to realize: Never. Not in the 50k miles since purchase. Doubtful in the 110ki miles before that. I'll put fuel filter replacement at the top of my list.

Under the vehicle I noticed a wet transmission pan which was not that way a year ago. The fluid level is down. I think I need to buy a pan with a drain plug, replace the pan and change the fluid once or twice.

 

[koda2000]

I'm ashamed to realize: Never. Not in the 50k miles since purchase. Doubtful in the 110ki miles before that. I'll put fuel filter replacement at the top of my list.

Under the vehicle I noticed a wet transmission pan which was not that way a year ago. The fluid level is down. I think I need to buy a pan with a drain plug, replace the pan and change the fluid once or twice.

You should start by replacing your fuel filter. Ford recommends every 30,000 miles, but I'd say you can get away with every 50k. Then, if the lean condition still exists, start looking for vacuum leaks. While the old way of spraying starting fluid or propane works sometimes, there are so many places that vacuum leaks can happen you don't always locate them that way. A smoke test will work better, but PCV hoses/grommet and lines are good place to start with visual inspection. Anywhere vacuum is used can leak on a 24 year old vehicle. You may need to replace all the vacuum hoses in the engine compartment and then there are also vacuum lines used in the rear of the vehicle on the evaporation canister, though leaks there should throw a different code. Intake manifold gaskets are also a possibility. There are no easy answers, but a smoke test would be worth the money.

You can suck most of the old fluid from the trans pan out through the trans dipstick tube with an inexpensive transfer pump and then add a $12 B&M universal drain plug to your trans pan if you want. Use a Motorcraft or WIX brand filter.

 

[masospaghetti]

If they are original I bet your intake manifold gaskets are bad and causing air to leak past the MAF and throttle body.

 

[410Fortune]

fuel filter and intake plenum gaskets you need to do the O rings and gaskets above and below the fuel rail. Excellent time to replace or check/clean and re O ring your injectors as well

 

[gmanpaint]

Both banks lean (HO2's) = Blown Lower intake gaskets. Always.


Edit.... guess not always!! Lol

 

[myf16]

I changed the fuel filter, which was Motorcraft and looked original, 23 years old. I cleared the error codes and so far (50 miles of driving) they have not returned.

Edit: I also sprayed about half a can of starting fluid everywhere under the hood. The idle rpms never changed. Maybe, just maybe, I don’t have a vacuum leak.

 

[koda2000]

I changed the fuel filter, which was Motorcraft and looked original, 23 years old. I cleared the error codes and so far (50 miles of driving) they have not returned.

Edit: I also sprayed about half a can of starting fluid everywhere under the hood. The idle rpms never changed. Maybe, just maybe, I don’t have a vacuum leak.

The fuel filter was probably the issue. If fuel can't get through the engine has nothing to run on but air and a little bit of fuel (= lean as seen by the O2 sensors). Your Forscan results listed possible fuel filter as the problem. The owner's manual recommends replacing the fuel filter every 30,000 miles. You can push it to 50,000 but anything more than that is a problem waiting to happen.