I have a 1998 F150 2WD 4.2l with 238K on it. The truck has started to die after I downshift and slow down to turn off the highway. I have had both 171 and 174 lean codes for a while now even after finding a leak on an elbow at the PVC valve and taking care of it. I have replaced all of the fuel injectors since I had a bad #4 and cleaned out the EGR ports while I had the plenum off. The EGR valve has been replaced since I couldn't clean it good enough and could blow through it. Since it was time for a plug change, I also changed the wires and the coil pack since I was getting borderline resistance readings. All of the above took place over the past 6-12 months. I have sprayed all around the engine trying to locate any vacuum leaks and have even tried Scotty Kilmer's cigar smoke test and have found nothing. I have a K&N filter and have sprayed the MAF sensor with electrical contact cleaner. Just this week I replaced the original fuel pump since I was getting a pressure reading of around 39 and I couldn't really hear the pump whir like it used to. That got rid of the roar the truck was making when first started until it warmed up and gave it better acceleration when going up hills now. I thought that might also stop the stalling as I slow down to turn but it didn't (thought it might get rid of the lean codes, too). Guess I should test the MAF, IAC and DPFE now. Any ideas where to steer me to next? Thanks.

 

I'd start by cleaning the IAC valve.

 

,You need to look at the live data with a Scanner, and what codes are listed.
Analyze, then correct what's out of limits.
I would be looking at the Long Term Fuel tables first.
They can still be just within limits but cause a stalling issue and should be corrected.
You have been all over the motor and now have to start from a different point because there are still issues were not lucky with.
Good luck.
.

 

Last time I had a scanner with live data display I had the truck running in park at 800 rpm and STFT 1 was 6.3-7.0 and STFT2 was 10.9-12.5. At 2000 rpm STFT1 was 3.9-6.3 and STFT2 was 3.9-7.0. LTFTs were both 25 or above. Lean codes accurred at 9 m.p.h..

 

"Lean" codes are triggered when the sum of the short and long term trims on the same bank exceeds +35%.

The 4.2L engines of that era were infamous for intake manifold leaks due to gasket and isolator bolt failures. You still have a severe vacuum leak which is preventing proper operation of the IAC valve.

Spraying and listening is usually a useless endeavor, the computer adjusts things too fast. You have to watch the short term fuel trims or the upstream O2 sensor voltage outputs in order to get something useful.

 

Don't ignore the crankcase as a possible the source of the vacuum leak. I once worked on a BMW 6 cylinder that used a plastic valve cover with leans codes and positive O2 readings. A smoke test didn't reveal any vacuum leaks in the intake system or vacuum hoses. But when I smoke tested the crankcase smoke started coming out all sorts of places around cracks in the valve cover. Now Ford doesn't use plastic valve covers here but the point is leaks in the crankcase can affect the PCV system and cause lean codes and O2 sensors to go positive.

 

I had a case of 174 code with bank 2 not out of normal limits.
Usually an intake leak shifts the tables both sides on a leak 'common' to both banks..
BUT, this time it was a head pipe to manifold leak being sensed by the OX sensor.
This leak not in the intake, affects the total system differently.
Attempts to find a leak using the spray method would not work at all in this case.
Not only is the system able to sense separately but the balance as well even when both tables have not reached limit..
Both signals are also applied to a summing logic to be able to do this and set code for the difference detected that should not be allowed for normal operation.
It has a noticeable effect on fuel mileage because that Ox sensor has to sense it over all operating conditions and still provide an offset signal for fuel and timing correction control, full time...
Scanners are good but you must have enough knowledge behind using them to recognize extended causes and effects to the data seen,..
They are the only way you have to look into and assess the system.
Good luck.

 

I'm glad you brought up crankcase leaks alloro. I have seen and heard that bad O-rings on dipsticks and filler caps can cause lean codes. I have a bad leak at the seal on the oil pan which I was going to take care of during the next oil change. To me, if the dipstick can cause lean codes, why not a bad seal of the oil pan? I may have a leak on a valve cover but someone here posted that won't cause lean codes as long as the PCV valve is ok. Don't know whether to believe that or not.

 

Crankcase leaks should be controlled if the PCV system is using the correct valve and is otherwise intact and functional. Use only the correct Motorcraft PCV valve, not an aftermarket replacement, as some of the aftermarket parts will allow excessive amounts of air to be drawn through the system and will, in fact, then cause "lean" codes.

r

 

Probably the easiest way to test for a crankcase vacuum leak is to remove the dipstick and connect a vacuum test gauge to it; also disconnect and block any breather hoses going to the crankcase or valve covers, but leave the PCV system intact. With the engine running there should be a couple of inches of vacuum being pulled. Failure to pull vacuum can mean a leak or excessive piston ring blow-by.

 

Thanks to both of you and that might be the problem projectSHO89. I didn't think an aftermarket PCV valve would cause a problem and can't remember if the last time it was replaced it was a Motorcraft or not. I'll check it out and replace if I need to. That's a simple enough job, well not too simple considering its location, but easier than some jobs I've done!