New 5.0L built, low compression and horrible MPG

[1997XLTRollover]

39.15 Key on engine off (pump running)
With engine running the pressure will change according to engine vacuum.
The formula for vacuum to psi is..
1 in. HG = .49 psi
so if you have 20 inches of vacuum you should have 9.8 lower fuel pressure
29.35psi

You were driving with the fuel pressure gauge hooked up under the wiper blade, that's what a diagnostic technician would do. Did the fuel pressure drop at wot or did it stay at aprox 39 psi?

Also O2 sensors only sense oxygen, not fuel. High voltage means rich(lack of O2), low voltage means lean(lots of O2 in exhaust). (a mixture is rich or lean on fuel) The normal operating range of a Ford narrowband O2 sensor is 0-.9 (they should vary at least from .3 to .8)
The .88 was a rich reading but you can not say the vehicle runs rich by looking at the split second in time that you captured that O2 reading. That is why we have Fuel trims. STFT and LTFT should be 0 plus or minus 5. So -5 to 5 would be fine. Between 5-10 you have a small issue. 10-15 it is a noticeable issue, 15-20 is a large issue. I have seen codes set on 1996 on up vehicles from 10 and up, but if you don't have a code it doesn't mean you don't have a problem.

If the problem might be A/F mixture related, you need to look at STFT and LTFT at Idle, and 2,000 rpm's in park.
Could you post screens shots of those please?

You want a hose for compression and leak down testing that works, try SnapOn. I have not found a vehicle that mine does not work on yet. (outside of the 3 valve heads with different size threads).

Also to check for restricted catalytic converters you would need an Exhaust back pressure tester, Snap On has those too, they are about $100.00-$150.00. I have video of mine in use at WOT on youtube(https://www.youtube.com/watch?v=O-AY7SKhYX0). I had at least 11 lbs of back pressure, Ford says no more than 8. Idle should be no more than 2-3lbs.

I was driving with the vac gauge under my wiper not fuel pressure. Next will be fuel pressure I just have to find a way to hook my fuel pressure gauge up to a longer hose so I can get it out from under the hood. My LTFT's are anywhere from 4-8 depending on load and rpm, STFT of course kind of bounce all over sometimes adding as much as 9-10 but that's rare, usually it's adding 5-7 and then pulling in the negatives when I let off the throttle. This is only when it's adding fuel, a lot of times it's around zero now that it's learned but my LTFT are still 4-8.

My fuel pressures were listed in a previous post, I'll see if I can find them.

Found it:

Engine off key on its 30psi, engine idling 30 psi, rev engine and it spikes to maybe 40 psi. Let off gas and it drops to maybe 27psi and steadys back at 30. Truck in gear brake on running wot it pins 39 psi and holds steady.

Oh and if I pull the vac line off the regulator when it's idling, it spikes to 40 and holds steady.

In an earlier post you said you disconnected the vacuum line and plugged it, that would cause a code for no purge.

I had this purge flow code before I pulled any hoses off. I pulled the battery and have driven maybe 200 miles and so far it didn't come back but I'm guessing it will at some point.

 

[4pointslow]

Fuel pressure

Whoops, I saw gauge under the wiper blade and my mind thought fuel pressure.

The wiper trick works with both types of gauges, and if it doesn't stay there with just the wiper holding it down then an X of electrical tape across the back off it strapping it to your windshield will do the trick.
making sure it stays at 40 when at WOT under load will help us rule out the fuel pump/filter, or not.

Not picking on you, but this does not help me help you ("My LTFT's are anywhere from 4-8 depending on load and rpm, STFT of course kind of bounce all over sometimes adding as much as 9-10 but that's rare, usually it's adding 5-7 and then pulling in the negatives when I let off the throttle. This is only when it's adding fuel, a lot of times it's around zero now that it's learned but my LTFT are still 4-8.")

What I would need to see is the LTFT and STFT on a warmed up engine in park, at Idle(stable for 10 seconds) and then at 2,000 rpms(stable for 10 seconds). Here is what they are used to diagnose if there is a problem with them.

STFT and LTFT high at Idle and not so bad at 2,000 is a vacuum leak.
STFT and LTFT high at 2000 and not so bad at Idle is Fuel starvation (pump/filter)
STFT and LTFT Low at 2000 and not so bad at Idle is a dirty air filter
STFT and LTFT Low at Idle and not so bad at 2000 is excessive fueling, like high fuel pressure from stuck closed fuel pressure regulator.

Looking at your previous pictures it looked like you had a vacuum leak and it seems like it is fixed now. But I am not fully sure until I see Idle and 2000 done one after the other on a warm engine.

If you see STFT% go to a steady 0 the pcm is in open loop, if 70% throttle or more you will most likely see commanded A/F. All other times STFT should be switching trying to keep the O2 sensors switching near stoichiometric. If the O2 voltage goes below .45(lean) the STFT% will go up adding fuel. If the O2 voltage goes above .45(rich) the STFT% goes down to remove fuel. STFT doesn't learn, if you shut the vehicle off or the vehicle goes into open loop then the STFT will automatically go back to 0%. STFT bases its movements off O2 voltage.

LTFT is a learned value. It bases it's movements off STFT.
If STFT is high for a while, then LTFT will go high and make STFT go back to switching around 0. If STFT is low for a while then LTFT will go low and make STFT go back to switching.
What ever is learned in closed loop will remain when the vehicle is shut off or goes into open loop. So if LTFT learns to add 8%, and the engine gets shut off, when restarted LTFT will go right back to adding that 8% (until PCM has it's memory erased).

Sorry if this is too much information, I just really love drivability diagnosis.

Let us know if the Evap code comes back and if it does what number it is.

 

[1997XLTRollover]

Whoops, I saw gauge under the wiper blade and my mind thought fuel pressure.

The wiper trick works with both types of gauges, and if it doesn't stay there with just the wiper holding it down then an X of electrical tape across the back off it strapping it to your windshield will do the trick.
making sure it stays at 40 when at WOT under load will help us rule out the fuel pump/filter, or not.

Not picking on you, but this does not help me help you ("My LTFT's are anywhere from 4-8 depending on load and rpm, STFT of course kind of bounce all over sometimes adding as much as 9-10 but that's rare, usually it's adding 5-7 and then pulling in the negatives when I let off the throttle. This is only when it's adding fuel, a lot of times it's around zero now that it's learned but my LTFT are still 4-8.")

What I would need to see is the LTFT and STFT on a warmed up engine in park, at Idle(stable for 10 seconds) and then at 2,000 rpms(stable for 10 seconds). Here is what they are used to diagnose if there is a problem with them.

STFT and LTFT high at Idle and not so bad at 2,000 is a vacuum leak.
STFT and LTFT high at 2000 and not so bad at Idle is Fuel starvation (pump/filter)
STFT and LTFT Low at 2000 and not so bad at Idle is a dirty air filter
STFT and LTFT Low at Idle and not so bad at 2000 is excessive fueling, like high fuel pressure from stuck closed fuel pressure regulator.

Looking at your previous pictures it looked like you had a vacuum leak and it seems like it is fixed now. But I am not fully sure until I see Idle and 2000 done one after the other on a warm engine.

If you see STFT% go to a steady 0 the pcm is in open loop, if 70% throttle or more you will most likely see commanded A/F. All other times STFT should be switching trying to keep the O2 sensors switching near stoichiometric. If the O2 voltage goes below .45(lean) the STFT% will go up adding fuel. If the O2 voltage goes above .45(rich) the STFT% goes down to remove fuel. STFT doesn't learn, if you shut the vehicle off or the vehicle goes into open loop then the STFT will automatically go back to 0%. STFT bases its movements off O2 voltage.

LTFT is a learned value. It bases it's movements off STFT.
If STFT is high for a while, then LTFT will go high and make STFT go back to switching around 0. If STFT is low for a while then LTFT will go low and make STFT go back to switching.
What ever is learned in closed loop will remain when the vehicle is shut off or goes into open loop. So if LTFT learns to add 8%, and the engine gets shut off, when restarted LTFT will go right back to adding that 8% (until PCM has it's memory erased).

Sorry if this is too much information, I just really love drivability diagnosis.

Let us know if the Evap code comes back and if it does what number it is.

No No if I say something useless by all means tell me haha you won't hurt my feelings. Hell like I said at this point I'd take a kick to the nuts to get the truck fixed. I'm supposed to be working on getting my Mustang ready for summer, not working on my beater truck non stop. I'll try to get that stuff tonight and post it up ASAP. Thanks for taking the time to help I really do appreciate it.


From what I remember just from driving around I think my fuel trims are high (adding fuel to correct for a lean condition) at 2-2500 and at idle they are more normal. So I may have a fuel issue. What's odd and probably unrelated is that my fuel gauge has been acting weird lately. The check gauge light will pop on for a second and sometimes the gauge will drop and then it will come back. Maybe I have a bad pump ground or electrical connection somewhere.

 

[4pointslow]

Fuel Sending Unit Ground

Here is the ground for the fuel sending unit.
There are a million things that go through that ground, good news is it is under the hood near the radiator.
If the ground is ok it is probably the sending unit, and if the pump has not been replaced recently I would replace it with the sending unit.
Better to do it when you have a dry warm place to work on it and after you have run it low on fuel, the alternative is when it fails when you least expect it and have to put a pump in it outside in the cold or wet.

 

[1997XLTRollover]

Here is the ground for the fuel sending unit.
There are a million things that go through that ground, good news is it is under the hood near the radiator.
If the ground is ok it is probably the sending unit, and if the pump has not been replaced recently I would replace it with the sending unit.
Better to do it when you have a dry warm place to work on it and after you have run it low on fuel, the alternative is when it fails when you least expect it and have to put a pump in it outside in the cold or wet.

Thanks! Is that the one near the battery? If so I've already cleaned that ground off. Can I test it all somehow? I'm guessing I need to get at the fuel pump to do that huh?

 

[1997XLTRollover]

Whoops, I saw gauge under the wiper blade and my mind thought fuel pressure.

The wiper trick works with both types of gauges, and if it doesn't stay there with just the wiper holding it down then an X of electrical tape across the back off it strapping it to your windshield will do the trick.
making sure it stays at 40 when at WOT under load will help us rule out the fuel pump/filter, or not.

Not picking on you, but this does not help me help you ("My LTFT's are anywhere from 4-8 depending on load and rpm, STFT of course kind of bounce all over sometimes adding as much as 9-10 but that's rare, usually it's adding 5-7 and then pulling in the negatives when I let off the throttle. This is only when it's adding fuel, a lot of times it's around zero now that it's learned but my LTFT are still 4-8.")

What I would need to see is the LTFT and STFT on a warmed up engine in park, at Idle(stable for 10 seconds) and then at 2,000 rpms(stable for 10 seconds). Here is what they are used to diagnose if there is a problem with them.

STFT and LTFT high at Idle and not so bad at 2,000 is a vacuum leak.
STFT and LTFT high at 2000 and not so bad at Idle is Fuel starvation (pump/filter)
STFT and LTFT Low at 2000 and not so bad at Idle is a dirty air filter
STFT and LTFT Low at Idle and not so bad at 2000 is excessive fueling, like high fuel pressure from stuck closed fuel pressure regulator.

Looking at your previous pictures it looked like you had a vacuum leak and it seems like it is fixed now. But I am not fully sure until I see Idle and 2000 done one after the other on a warm engine.

If you see STFT% go to a steady 0 the pcm is in open loop, if 70% throttle or more you will most likely see commanded A/F. All other times STFT should be switching trying to keep the O2 sensors switching near stoichiometric. If the O2 voltage goes below .45(lean) the STFT% will go up adding fuel. If the O2 voltage goes above .45(rich) the STFT% goes down to remove fuel. STFT doesn't learn, if you shut the vehicle off or the vehicle goes into open loop then the STFT will automatically go back to 0%. STFT bases its movements off O2 voltage.

LTFT is a learned value. It bases it's movements off STFT.
If STFT is high for a while, then LTFT will go high and make STFT go back to switching around 0. If STFT is low for a while then LTFT will go low and make STFT go back to switching.
What ever is learned in closed loop will remain when the vehicle is shut off or goes into open loop. So if LTFT learns to add 8%, and the engine gets shut off, when restarted LTFT will go right back to adding that 8% (until PCM has it's memory erased).

Sorry if this is too much information, I just really love drivability diagnosis.

Let us know if the Evap code comes back and if it does what number it is.

Ok here are the snap shots. It doesn't look good. At least to me. Looks like there's nothing wrong! As of now I still have the 97 PCM installed. I'm going to swap my 96 PCM back this weekend because swapping it hasn't seemed to fix anything at all and I think it's bugging out my GEM because I have a bunch of GEM errors, maybe that's why my fuel and alt gauge have been freaking out lately. I did have some GEM errors on my old 96 PCM too but no where near this many.

I also have a P1450 code, no CE light but the code is there. I don't think that was the same code I had before but this is also a different PCM and IIRC it sounds like the same problem, no purge I think was the old code.

Anyway here are the snap shots. Took it for a 10 minute drive to warm it up fully and then let it idle/rev for a minimum of 10 seconds and took the snap shots.

OH! And one more thing, there's a SMALL exhaust tick/leak on the passenger side from the header/y pipe joint. Now correct me if I'm wrong but I believe that's bank 1 which explains the small amount of extra fuel being injected on that bank as seen in the snap shots.

Idle:

2000 RPM:

 

[4pointslow]

Pcm

Put the correct PCM back in and retest please.
Clear all the codes in the GEM after reinstalling the correct PCM, if your scan tool lets you do that. Then retest and check for codes again.

P1450 is EXCESSIVE FUEL TANK VACUUM. Could be caused by a bunch of things like a stuck open canister purge valve(vapor management valve), or the wrong PCM.

Let me know if the code comes back with the correct PCM installed.

 

[4pointslow]

Exhaust Leak at Manifold to Pipe connection

A buddy of mine had a Mountaineer with the 5.0L (1998).
STFT on Bank 2 was around 15% and it was setting a code for upstream O2 sensor being lean. Bank 1 was fine.
I also heard the exhaust leak where the manifold connects to the pipe. The bolt was all rusty and skinny looking but to my surprise It came right out. I found a better bolt in my spare parts drawer and installed it. Once tightened all the way, there was no more noise and STFT went back to switching around 0%.

You hit the nail on the head with exhaust leaks, they cause false lean readings at the O2 sensors and the PCM will add fuel the engine doesn't need. There goes fuel mileage! Of course you fuel trims don't show that as being a problem right now but lets see what happens with the correct PCM back in there. If it still looks good you can rule out A/F mixture problems.

Bank 1 is the passenger side, Bank 2 is the drivers side.(on our fords)

 

[1997XLTRollover]

Put the correct PCM back in and retest please.
Clear all the codes in the GEM after reinstalling the correct PCM, if your scan tool lets you do that. Then retest and check for codes again.

P1450 is EXCESSIVE FUEL TANK VACUUM. Could be caused by a bunch of things like a stuck open canister purge valve(vapor management valve), or the wrong PCM.

Let me know if the code comes back with the correct PCM installed.

A buddy of mine had a Mountaineer with the 5.0L (1998).
STFT on Bank 2 was around 15% and it was setting a code for upstream O2 sensor being lean. Bank 1 was fine.
I also heard the exhaust leak where the manifold connects to the pipe. The bolt was all rusty and skinny looking but to my surprise It came right out. I found a better bolt in my spare parts drawer and installed it. Once tightened all the way, there was no more noise and STFT went back to switching around 0%.

You hit the nail on the head with exhaust leaks, they cause false lean readings at the O2 sensors and the PCM will add fuel the engine doesn't need. There goes fuel mileage! Of course you fuel trims don't show that as being a problem right now but lets see what happens with the correct PCM back in there. If it still looks good you can rule out A/F mixture problems.

Bank 1 is the passenger side, Bank 2 is the drivers side.(on our fords)

I'm using Forscan. Does that have the ability to clear the GEM? I've just started using it and I don't know much about it short of just some playing around.

Far as my exhaust leak it's very small and the bolts are tight, it's just the way the pipe is positioned I think that's causing it to not seal due to the way my catback is angled. I'm also running torque monster headers, not sure how much that may effect things.

Far as my exhaust leak causing the computer to falsely inject fuel, my plugs show a sickly lean condition. So mechanically the thing is lean lean lean. Now, I haven't checked it since I fixed my vacuum leaks because I wanted to get some miles on it to give the plugs a chance to color up. So I'll pull a plug as soon as I can and see how they look now.

 

[4pointslow]

Forscan

I have no idea about the forscan, but maybe when you disconnect the battery to change the PCM the codes will clear. Most modules clear out when power is lost, but some don't.

 

[1997XLTRollover]

Ok I got the 96 PCM back in and here are the idle and 2000 RPM fuel trim readings.




Idle:

2000 RPM:

 

[1997XLTRollover]

I also managed to do my redneck fuel pressure test while driving. Everything seems ok. As I tip into the throttle pressure comes up, anything past 30% throttle gives 40 PSI and holds steady. Let off and jump back in it, it drops to 30 then spikes back to 40. Ran it all the way to redline and pressure never dropped below 40 or spiked above it, it held perfectly even. Between idle and 30% throttle I was running 37-38 psi give or take as vacuum came down.

NOW. Here's the part I found weird. I of course removed the schrader valve for this test and put a rubber hose on to extend the fuel pressure gauge. When I initially let the pressure out of my rail I got a huge burst of fuel. Then once pressure was out I went back and pressed it again and it had pressure again, and again and again. Less every time but I would still get a little puff of pressure relief. I found this a bit odd because any other system I ever worked with never pressurized again after the initial pressure release.

This brings me to after my test run. When I got back to my shop and took the fuel pressure gauge off the end of the rubber hose, it of course bled fuel out and it sprayed as I would expect. Pressure was relieved and I can walk away right? Wrong. I let the hose hang off the side of my truck and 20 seconds later I heard a puff of pressure fire out the line and then a huge stream of fuel sprayed out. Is this normal or is there something wacky going on with my fuel system? Maybe this has something to do with the EVAP purge not releasing?

 

[1997XLTRollover]

One other thing I noticed was the truck runs noticeably rough at 1300 RPM. Idle seems off to me but it would seem ok to the untrained ear and so is the rest of the RPM band. But at 1300 RPM and I mean like 1250-1350 ONLY the engine just seems like it's missing a cylinder or misfiring. Anything above or below that seems to be "ok". It's very weird; any thoughts on this? What would make it miss at only one certain RPM?

I also pulled a spark plug again and it's still ghost white. I was going to try a leak down test too but my tester line I had "repaired" fell apart again.

 

[4pointslow]

Fuel trims

The fuel trims look great, I would not worry about the 6 or 7%, it is so minor. I would fix that exhaust leak though.

Fuel pressure holding at WOT indicates that that fuel pump is not dropping out under load. At this point I would rule out any A/F system problems.

I would like to see the results of an exhaust back pressure test, with the poor fuel mileage concern, it is something I would want to test. Of course you would have to have a back pressure tester. And for the record, I have used many cheap tools that do not work accurately and have been burned from doing that. Remember that any test you do is only as accurate as the test results you get from the equipment you use. The spec is no more than 8 psi at wot under load in gear while driving. Idle is no more than 2-3 psi. The test is done with an O2 sensor removed from in front of the cat, a fitting is threaded in and a low pressure gauge attached to that fitting.

OK so now you mention a new symptom, a misfire at 1250-1300 rpm only. What can cause a misfire just in that rpm range? I have no idea. So look at it this way, what can cause a misfire? "A" misfire would be one, meaning in one cylinder only.
Lean A/F mixture
Fuel injector
wiring
PCM
spark plug
ignition wire
coil
compression issue
Can you hold the engine at 1250-1300 rpm get it misfiring constantly?

As far as the fuel squirting out the hose, the hose is not supposed to be unhooked. Don't read into it. Pressure can easily build up in hot fuel rails/lines etc. Pressure can build up in the tank at certain times too, so can a vacuum.
Did the Evap code come back with the correct PCM in the vehicle?
If so was it P1405?

So now we have multiple symptoms if I am correct?
Misfire at 1250-1300
Poor fuel mileage
Are there any more?

 

[1997XLTRollover]

The fuel trims look great, I would not worry about the 6 or 7%, it is so minor. I would fix that exhaust leak though.

Fuel pressure holding at WOT indicates that that fuel pump is not dropping out under load. At this point I would rule out any A/F system problems.

I would like to see the results of an exhaust back pressure test, with the poor fuel mileage concern, it is something I would want to test. Of course you would have to have a back pressure tester. And for the record, I have used many cheap tools that do not work accurately and have been burned from doing that. Remember that any test you do is only as accurate as the test results you get from the equipment you use. The spec is no more than 8 psi at wot under load in gear while driving. Idle is no more than 2-3 psi. The test is done with an O2 sensor removed from in front of the cat, a fitting is threaded in and a low pressure gauge attached to that fitting.

OK so now you mention a new symptom, a misfire at 1250-1300 rpm only. What can cause a misfire just in that rpm range? I have no idea. So look at it this way, what can cause a misfire? "A" misfire would be one, meaning in one cylinder only.
Lean A/F mixture
Fuel injector
wiring
PCM
spark plug
ignition wire
coil
compression issue
Can you hold the engine at 1250-1300 rpm get it misfiring constantly?

As far as the fuel squirting out the hose, the hose is not supposed to be unhooked. Don't read into it. Pressure can easily build up in hot fuel rails/lines etc. Pressure can build up in the tank at certain times too, so can a vacuum.
Did the Evap code come back with the correct PCM in the vehicle?
If so was it P1405?

So now we have multiple symptoms if I am correct?
Misfire at 1250-1300
Poor fuel mileage
Are there any more?

I can look into doing a backpressure test but if I had clogged cats I would think they would be excessively hot no? My cats don't heat beyond 500 degrees. My muffler is also brand new (that doesn't mean it's good) but with my cats not coming to temp could there even be a clog somewhere else? I also have a perfectly clean tailpipe, no soot whatsoever and spark plugs that are white as a ghost. Everything points to a severe lean condition, which would account for the lack of power and poor fuel economy.

Also yes I can very easily reproduce the miss. The engine runs noticeably rough at 1300 rpm in neutral. Every so often I feel like the idle is worse then it normally is, but it still seems off daily, but does not run as rough as the truck does at 1300. Soon as I pass 1300 it smooths out, it's very weird. The engine also sounds "airy" to me. Like the fuel mixture is off and too lean.

That being said I have a little bit of a shaky vacuum reading when I test off my brake booster line. it jumps from 18-20 where as all my other lines hold perfectly steady at 19. This makes me think I may have a vacuum leak here but I just can't seem to find anything. What makes no sense to me is if there was such a severe vacuum leak wouldn't the fuel trims be much higher?




Lean A/F mixture - I think it's lean based on mechanical signs I see
Fuel injector - Injectors are brand new remans (I know they could be bad even new)
wiring - Possibly, not sure where to look because all 8 cylinders look lean based on the plugs and I have cleaned every ground I can find.
PCM - Swapped with no change to the 97 pcm (now back to running the 96 pcm)
spark plug - All brand new and tested with an otc spark tester and all spark well
ignition wire - See spark test above
coil - See spark test above
compression issue - all cylinders are dead nuts even at 135 with no deviation between cylinders.


I'm not sure about the EVAP code I'll have to check because I don't have a CE light, but if I didn't tell you the truck was down on power and running lean, you'd never know there was a thing wrong with it. Driveabily wise it's fine, no issues. Starts ok, runs ok drives ok and even pulls all the way to redline ok. I was wondering if I had a PCV problem but my PCV valve rattles and seems ok. I plugged the clean air line and put a vacuum gauge on my oil dipstick tube and it read 10 mmhg at idle. Is this normal?


Also could this be timing related? If I had my cam and crank timing off mechanically or my crank pulley slipped, could that show a false lean condition? The truck runs too well for it to be off by a lot but if it was off a few degrees could that cause this? I just feel like even if timing was off the truck wouldn't seem to run so lean, it would have the right mixture but just be down on power. But then again the computer seems to think the mixture is ok where as mechanically to me it doesn't seem right.

 

[beach]

You have way more patience then anyone I know, I would have parked it at the junkyard by now and just walked away lol.

 

[1997XLTRollover]

You have way more patience then anyone I know, I would have parked it at the junkyard by now and just walked away lol.

I've come close, but this truck has so many new parts on it, it would irritate me more to just scrap it then to keep chugging away. I also learn a hell of a lot troubleshooting on my own even if I do the wrong things first. So as much as I'm pissed off and sick of doing this, I'm learning a hell of a lot. I am kind of at wits end now though. I want to be working on my summer toys now, now this pile.

 

[1997XLTRollover]

Ok I checked the truck out again and no codes. Just P1000 so the EVAP code may need more time to come back since I just swapped the PCM a few days ago. I did clear all the garbage out of my GEM and all that so I have a clean slate. But here's a few shots of some info that doesn't really mean anything to me but might to someone else that knows what it all is.

Now the only other thing I thought of which I just mentioned recently was that my "check gauge" light keeps popping on randomly. It seems to either be low charge on the alternator or something with my fuel gauge. Yesterday my fuel gauge just dropped to E and then shot back up. Alternator may be going out too because it's not charging well at idle sometimes and it was acting funky a few weeks back. high charge, drive a few miles then low charge for just a few days and now seems fine apart from just tonight not charging well at idle until I hit the gas. I did check the voltage to be sure it wasn't really high and it was at 14.5. Also did a voltage drop test on the charging system which came back fine.

 

[1997XLTRollover]

I also while I was at it decided to get some screen shots at 1350 RPM. It seems the "miss" is worst at 1350 according to Forscan. I can feel a slight miss at idle all the way to 1500 rpm when it smooths out but it's definitely the worst right around 1350 RPM. The below are screen shots holding the truck steady around 1350 for longer than 10 seconds. Maybe these are just misses from the lean condition. I don't hear popping or anything out the exhaust and throttle response is fine if I snap the gas at 1350. No stumbling or anything, just seems like the combustion isn't as smooth as it should be.

 

[crunchie_frog]

Are you confident the throttle position sensor does not have a "dead" spot where you are seeing the misfire?