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Junior Charter Member
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239 Posts
Discussion Starter · #1 ·
This is off topic, but I think this forum gets the most looks so I will post it here. Moderators - feel free to move it to OT if needed.

I have a '97 Ford Expedition (5.4L V8, 4x4, 120K miles) with trouble codes PO171 and PO174. These show both banks lean. I just had 1 of the coils and 2 O2 sensors replaced after a heater core leak dripped on them and caused all kinds of problems. Now that the other codes are gone, these showed up. The car runs great but the idle drops a bit low (500 rpm) several seconds after coming to a stop.

What might be causing both banks to be lean? Could it be the fuel filter? Gas cap problem? Air filter? I really don't know where to start. I am trying to sell this SUV and don't want to pass a problem on to the new owner - not to mention they won't be very likely to buy a car with a check engine light on.

Thanks for the help!

Cheers,
Shack
 

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Junior Charter Member
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18 Posts
p0171, p0174 are lean both banks, this is caused by common problem, the mass air flow meter gets a fuzz build up on the wires, the map sensor is in the air meter and key up will take baro sensor, pull and clean blow off with compressed air. disconnect battery and hook both cable ends together this will discharge the capacitor and clear learned air flow, drive from 0-60 full throttle coast down to 40 no brake back to 60 coast 40 back to 60 all full throttle. the other common problem is the intake manifold gaskets plenumin is plastic the gaskets are very small and will leak on cold start up, may also cause dip down @ idle.
 

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section 8
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5,136 Posts
Seek proffessional help some one with a smoke machine and an up to date scanner and all data or mitchell on demand. With EFI lean is rich which probably indicates a vacuum leak . We fill the crancase with smoke and watch for where the smoke comes out. Usual culptrits intake gaskets (as stated) egr , or left off vacuum hose. I have seen one case where a weak fuel pump caused a lean misfire. A clogged filter can do the same thing . These are all actual case scenarios , it's best to pay for an hour of diagnostic ,rather than throw parts at it . If the maintenance for your mileage has not been done start there . Real case from yesterday, 1992 f150 302 .Misfire, stalling, poor running when running. Customer wanted to find the one thing that was causing his problems. I went out with him looked at the truck and saw ignition wires dated 1992 . So that makes them roughly 15 years old. So I explained the limitations of the eec4 and that a misfire will generally not show on a scan, so he has more than one problem. I also mentioned all of the fluids were 1/2 empty .Scan test reveals 15 codes . 1st MAP frequency off, change map clear codes , lots better but not right manually test tps and it tests bad , replace tps stalling gone runs well but still misses, Replace 15 year old tune up parts woo hoo ,land rocket!! Moral of the story the computer scanners are not a fix all . The Scanner with current updates will not tell you if you have a vacuum leak or bad tune up parts. BTW 4 of the ign wires had the boots crackes and split and were arching to the manifolds intermittently. Sorry for the rant . Bob
 

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Junior Charter Member
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239 Posts
Discussion Starter · #4 ·
Bob, don't apologize for the rant. I will try the "free" stuff like blowing out the MAF, and any others that folks might suggest. The car has been maintained pretty well, and until the heater core leak it ran great with zero problems. If none of the simple things fix it then I will seek professional help. (Sounds like I need marriage counseling.)

If you guys have any other words of advice I will gladly take them. I did have a guy who was interested in it suggest that the fuel pressure regulator might be the problem - does that sound plausible? Thanks again for the help - this forum saves me again.

Cheers,
Shack
 
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