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Old 04-01-2016
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Icon5 Why would LTFT soar? This makes no sense.

I've been chasing a PO171 code in my '97. I changed vacuum hoses, PCV, did the smoke test, used carb spray, and all the other tricks many here have used. Thanks to all, especially RonD, by the way.

So, I had to make a trip to San Jose Wednesday, about 30 miles via freeway, and decided that was a nice trip to reset my sensors, as I'd just cleared codes.

All was well until I was getting back onto the freeway on the return trip. I hit the gas in second and third to accelerate up the ramp and merge, and the CEL illuminated. I got home and copied down the freeze frame data for the CEL, and this is what it said:

Closed loop
Load: 89.4
ECT: 85 c
STFT: 13.3
LTFT: 25+
RPM: 2700
Speed: 35 MPH

Now, I had the scantool hooked up for the whole trip, and read the live data when I pulled into the parking lot on arrival. Both fuel trims were within reasonable limits (+ or minus about 6) and every live reading was normal.

I can understand why the STFT elevated, I was jumping onto the freeway and accelerating, so more fuel demand, but the LONG Term trim jumps up?! What the heck is going on here? I simply don't understand, and this gremlin is apparently what's setting my code.

I currently am swapping out intake gaskets and plan to change plugs, wires, and clean the fuel injectors. This vehicles does NOT act like it has a vacuum leak. It starts, stops, idles, doesn't buck or stall, and has no problem accelerating. I have a fair amount of experience as a car guy (50 years) and I can't figure it out.

All theories and responses welcome, and thanks.
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Old 04-01-2016
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The +25 LTFT set the P0171

Quick read here about fuel trims: Fuel trim: How it works and how to make it work for you - Automotive Service Professional

PCV system can be the source of high numbers, dip stick not pushed down all the way or missing, loose oil filler cap, cracked vent hose.
PCV system expects a sealed engine.

Vacuum leak is one thing, but any air leak between MAF sensor and throttle plate is the same as a vacuum leak, a ported vacuum leak in that case.
Like on a carb, ported vacuum above throttle plate, regular vacuum below throttle plate.
And like on a carb, ported vacuum increases as throttle plate opens and regular vacuum decreases.

I don't think you can clear LTFT, it is stored in memory that requires a Ford scanner to clear, but things may have changed, lol, I am old too.

See if it clears next time you do that.
If it doesn't then it will take a while for it to come down assuming problem is found.
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Old 04-01-2016
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Thanks for the info on "ported vacuum," Ron. I had the battery out for a while and my LTFT did reset to zero. On the trip up I mentioned in my post, the LTFT and STFT were both within normal limits, plus or minus about six.

The code set when I got on the freeway and opened the throttle quite a bit to accelerate. Then, the LTFT maxed out and set the CEL. As I said, I don't know why the LTFT suddenly zoomed without a corresponding leap in STFT.

I still don't understand how that makes sense. Any explanation you can think of?
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Old 04-02-2016
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Icon4 I think I've located at least part of the problem...

After I pulled the upper intake yesterday, I decided to go ahead and have a look at the plugs, even though there has never been a misifire code set nor a miss I felt.

I have a strong suspicion these are the original plugs at 218K miles. Not only are they Motorcraft, as was the fuel filter, but I had to use a breaker bar to get them out.

The narrowest gap is .065. The rest are around .080, and one is over .100! Yeesh. Got a new set of APP104s, per the recommendations here, and a nice set of Belden wires from NAPA.

I can slobber this sh!tbox back together as soon as I clean the injectors and get my new IAC on Tuesday. (It flunked the ohmeter test, although no code.)
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Old 04-02-2016
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Holy cow.

I can see why idle should have been OK but higher RPMs an issue, the higher RPM misfires dump unburned oxygen into the exhaust, O2 sensor sees that as Lean burn, a False Lean, so would spike the fuel trims up, the +25 and beyond

Spec gap is .044
Spark plug gaps are set at average use, you can tailor that gap for specific use.

If you want better low RPM performance you can use up to a .01 wider gap, so .054, this would give better cold starting and lower RPM loading.
This wider gap makes for a stronger spark but coil(s) has less recovery time so best for lower RPM use, under 3,000rpm average, strong spark is needed to ignite cold air/fuel mix and when pulling a load which tends to be a richer mix.

For better High RPM performance then narrow the gap, so .034 although I would only go .039 on the 2.3l it already has a narrower gap at .044
Racers use very narrow gaps as RPMs remain high for long periods, it gives the coil(s) better recovery time and weaker spark still ignites the warmer air/fuel mix the same as stronger spark.

The 2.3l is a higher RPM engine, max horse power is at 4,800rpm, max torque around 2,600rpm, so has the narrow stock gap spec.

I run my 4.0l OHV at .059, I rarely go above 3,000rpms
spec is .054, it has peak power at lower RPMs

Last edited by RonD; 04-02-2016 at 12:17 PM.
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Old 04-03-2016
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Icon3

Yeah, as soon as I saw the plugs, I figured I had a false lean condition. I'm quite familiar with that from my '99 and '97 Bonnevilles.

Getting ready to pull the lower intake today and swap gaskets.

Say, Ron, my gasket set has two upper intake gaskets. One is metallic and one gasket material. Any idea why? Use both, and if not, which one?
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Old 04-03-2016
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No, not sure on that one, I wouldn't think both.

What years are covered by the gasket set?
Could be for carb with riser plate, but just guessing
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