Lean on both banks (P0171 & P0174)

billr

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Oh, wonderful... this has "variable" runners; I had not noticed that.

I'll do some more visible/audible inspection, but will probably leave it alone until more specific symptoms pop up. There is occasional idle struggling and the codes, of course, but it is still reasonably drivable.
 

grcauto

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You have LOW fuel pressure. Fix that and your problem will go away.
 

billr

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What do you think the pressure should be??? Give me the "WOT" reading, no vac to the FPR. (It is 39-40 psig on this particular truck right now)
 

grcauto

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I thought you had 30 at idle. Should be close to 40 at idle with vac line attached. I think the book has 35 to 45. Should go up with vac line off.
 

billr

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OK, so what is the specified pressure range for vac off?
 

billr

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Sure, I have made a cursory check of the PVC and brake booster lines, as well as listening all around with the stethescope. However, I have not yet tried blocking off the lines to the PCV, booster, EGR, or MAP/BARO sensor; and have not yet blocked off the EGR ports.

I have been focused on the fuel pressure and TPS reading. I have finally found some definitive statements that FP should be 40-50 psig with vacuum off of the FPR. Is the "39" I'm reading really 40 and OK, considering the limited accuracy of cheap mechanical gauges? Although it is probably marginal, I would rather see the more common 44 (3 bar), I am still not convinced FP alone is the problem. Even if I decide to just throw parts at it because of the marginal pressure, which parts do I throw at it? The pump, or a filter, or an FPR? Or, maybe the pump wiring/relay? I want to do more testing, now that I feel confident nominal FP should be more like 44 psig.

I'm still getting no comment about the TPS reading that won't go lower than 20%. That seems very queer to me, but may be the way Ford does things... I would like to resolve the TPS question, since it should be so easy to do, before tapping into the fuel system in various places to get more definitive pressure readings. Same with making a smoke machine to find vacuum leaks in various places, or removing lots of parts to check thoroughly for bad hoses/gaskets/etc.
 

Mobile Dan

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You could do some of these tests to see "what the sensor says" and not "what your scanner says". BONUS!! The TPS is easy to reach.

Yes, it is OK to question the accuracy of your fuel pressure guage. Just had a crazy idea. What if you put 40 # in a tire and then attached your guage? Ford pressure Tap size looks a lot like a tire valve stem.
 

billr

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Oh, I can and will back-probe the TPS to look at its "raw data". However, that won't tell me if "closed throttle" is supposed to be 0% or 20%. The PCM could be programmed to work OK either way, but that 20% seems so odd to me that it is a "flag" I would like to resolve before going on to more-difficult-to-reach stuff.

Yes, the fuel test port is a standard tire-stem thread, the ever-popular .305"-32. The only source I have found for screw-on couplings like that is McMaster-Carr, and I'm not even sure that has a "valve stem depressor". Right now we are using a "clip-on" chuck from a FLAPS, the kind with a little finger to engage part of one thread on the stem; not what I like for gasoline, especially if when we close the hood to drive around with the gauge taped to the windshield. I suppose I could take out the valve stem, slip a hose over the OD of the port and secure it with a band-clamp.
 

Mobile Dan

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So, it sounds like you could "calibrate" your fuel guage by setting tire pressure at a "known #40" by using a trusted tire pressure guage, and then clipping your fuel pressure guage onto the tire to see what kind of reading you get.

In my experience, TPS data info on a scanner never goes 0%- 100%.
Youtube video. what you want to see starts about 2:30.
 

billr

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I think this one is done! I did check the FP gauge against four others. The gauge we are using is probably reading a bit low, and reads right at 40 with the vac line off, so actual FP must be above 40, in-spec.

A lot more exploring, testing, and reading was done, but the culprit turned out to be a little rubber elbow in a vacuum line, just below the TB. the elbow had a crack. It was a bit tight in there to use a simple piece of hose, and the elbow connects two different sizes of lines anyway. So, a boot from the coil-end of an 8mm GM ignition wire was used. The wire end is 5/16" and the coil-tower end is about 7/16"... just right!
 

Mobile Dan

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Glad you found that! I have used coil wire boots for similar reasons. Some boots are pretty soft, sometimes I put a short length of metal or plastic tubing inside to prevent collapse.
 

JackC

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Ahh, the dreaded. common vac leak. It always seems simple AFTER we find the culprit. But, nice work Bill. You probably saved a few hundred dollars in trial parts and labor at a shop.
 
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