1996 Explorer XLT 4.0l: DTC=P0171

[jazzkeys]

Howdy y'all,

Check Engine was on and the code is P0171. FoMoCo published TSB 04-17-4 about which I found a reference:
http://www.aa1car.com/library/ford_lean_codes.htm

I cleaned the MAF sensor with Radio Shack spray solvent though it wasn't really dirty. Also checked the sensor signal at idle and it's within normal range at 1.71VDC.

Replaced PCV valve and checked for intake manifold leaks visually using the cigar-smoke (yuck) test by removing the PCV tube and blowing smoke into the intake manifold through a 3/8" ID vinyl tube from HD. No visible leak. Turned off Check Engine using my code reader.

I've driven a few miles locally so checked for a pending P0171. No DTC yet, but haven't really fixed anything so I'm expecting that it'll return. The code reader showed that evap and catalyst monitors aren't ready, but that might be because I haven't driven far since clearing the P0171.

Bi-annual smog check here in SoCal is due in a few days but this chump is stumped. Any ideas? Thanks much.

 

[Mike65]

Welcome to EF. Check the vacuum hoses to the pcv for any cracks/leaks, & also check your fuel pressure. I do not know how true it is but I have read that a fuel pump starting to go bad can cause this problem also. The monitors not reading ready will take some driving to set them. When I had to do a repair to my 00 Ranger & my wife's 99 Sport I had to drive them about 30-40 miles to get the monitors to reset.

 

[4D X]

The MAF sensor readings can be confused with IAC valve problems. This part doesn't really give off its own code and can cause several issues in the system. If anyone ever buys a 90s Explorer 4.0 you should just replace it right off the bat. It will save alot of headaches.
Also they make a special MAF sensor solvent kin to other electrical/contact cleaners.

 

[mark2016]

The MAF sensor readings can be confused with IAC valve problems. This part doesn't really give off its own code and can cause several issues in the system. If anyone ever buys a 90s Explorer 4.0 you should just replace it right off the bat. It will save alot of headaches. Also they make a special MAF sensor solvent kin to other electrical/contact cleaners.

Highly recommended to buy a Motorcraft IAC to avoid issues.

 

[jazzkeys]

Thanks much for your replies.

Checked the OBD-II state with the KOEO test and the P0171 DTC has not returned. However, I checked the long-term fuel trim for bank 1 and it's steady at almost 15%, so the PCM thinks that the O2 sensor is detecting a leaner mixture than it should and is injecting more fuel.

I also used my scanner to graph the bank 1 upstream O2 sensor voltage and, to my surprise, the signal actually dropped to 0V at points in the curve. Given that a lean A/F mixture should generate a signal of at least 200mV, I'm suspicious of the O2 sensor and will replace both of the upstream sensors. I think that the O2 sensors are original and our Exploder was assembled in January of 1996.

But I'm also gonna have a smoke test done to determine once and for all if there's a manifold leak. I'll report the results.

 

[jazzkeys]

BTW, no other obvious driveability issues like misfires or hard starts. Cold starts are immediate but idle after warm-up seems less smooth. If there's a vacuum leak, it's a small one.

 

[jazzkeys]

Vacuum leak: Detected and repaired

Sorry about the late solution post, but I hope that another Explorer owner can benefit from my experience. I took the Explorer to a shop that owns a Snap-On leak detector and they found it. The two-part intake assembly was leaking between the upper molded manifold and the plenum. The six seals, one for each cylinder, were replaced, and the problem was solved. P0171 DTC gone. Passed the CA emissions test with readings well below spec maximums. That's the good news. Bad news is that I don't have an excuse to replace it for coupla years. Runs as well as ever.

Thanks to those of you who replied.

 

[J_C]

Often you can just clean an IAC (including the filter pads under the plastic cap) with carb cleaner and let gravity work a drop of oil down the base of the shaft into the bushing it slides in.

 

[lobo411]

The lower intake manifold bolts have a habit of loosening themselves over time. My new biannual smog check procedure (aside from general tune-up stuff) is to retorque the LIM bolts and check the rubber upper intake manifold gaskets to see if they've gone hard. If you want to be safe, you can replace the rubber upper gaskets too at this time (they're cheap).

I just retorqued my bolts last week and they took a full turn and then some to get up to spec. Definitely put a bit more pep in the engine, and hopefully I'll pass fine.

 

[gatzdon]

I would get Forscan Lite and pull the extended codes. This would give you all the pending codes from the PCM and other modules in the vehicle.

With this, you can see the state of each individual emissions monitoring system. I don't know about California specifically, but many states will give you a "PASS" with up to two monitoring systems in the Not Ready state. This can at least buy you some time to find the underlying root cause without feeling rushed or pressured.

If nothing else, you can see that all the emissions tests have completing successfully and that your P0171 is not sitting there pending waiting to trip the check engine light.

 

[gatzdon]

Also, for future reference, don't clear the codes. Codes like P0171 will clear themselves when the condition goes away. If you do clear the codes, save the freeze frame data so you can look back at it later should the condition return.