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'86 B2 2.9 miss at ~3200 RPM


Uncle Gump

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Is it a Motorcraft module?

I'm not so sure I'd trust the aftermarket to get the color of the module correct.

So does it miss at 2500 on up... or does it go away a 3000?

Did this truck sit for awhile before you bought it?
 


G8orFord

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:(

So idles smooth and no issues under 2,400rpm?
Does it stumble when REVed and only start missing above 2,400?

I assume cold or warm has no effect on the 2,500rpm?
Yes, smooth idle and smooth acceleration up to about 2500 rpm. Then BAM! It was doing the same thing with the old distributor and the current module, but at about 3200 rpm. Though, at one time before that, it was about 2500 rpm. Temp doesn't seem to have much of an effect, if any.

If you get it to rev up to about 4000 rpm, it doesn't seem to be stumbling or missing.


Is it a Motorcraft module?

I'm not so sure I'd trust the aftermarket to get the color of the module correct.

So does it miss at 2500 on up... or does it go away a 3000?

Did this truck sit for awhile before you bought it?
Couldn't find a Motocraft module when I replaced it, but the one I originally replaced was fairly new as well. I believe they both came from Advance/Carquest. I cannot be certain how long it sat before I bought it. The guy I bought it from, bought it to flip and had it checked over by a shop. They said it just needed driving and it would clear up. I laughed when he told me that. I believe most of what I've replaced had already been replaced by the shop and they were as stumped as I am. They did drain and clean the fuel tank, supposedly. I've checked fuel pressure once but I'm going to do it again, even though I'm mostly convinced this is a spark/ignition problem.
 

alwaysFlOoReD

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Sparkplugs? Porcelain cracked? Loose porcelain? Correct gap?
Maybe try closing up the gap a bit so the coil doesn't have to work so hard to get the spark to jump.
 

G8orFord

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Sparkplugs? Porcelain cracked? Loose porcelain? Correct gap?
Maybe try closing up the gap a bit so the coil doesn't have to work so hard to get the spark to jump.
Newish Autolites. I've only pulled 4 of the 6, but they all looked great, with about 0.060" gap. Pretty sure the shop changed them along with the wires and cap which I am using. Didn't use the cap from the new distributor. I've got a brand new cap, rotor and wires, but after looking at what was on it, saw no reason to replace.
 

Uncle Gump

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I just remember what my tank looked like. I replaced everything!

So why haven't you checked the last two spark plugs? They can be pulled pretty easy with a long extention and a swivel spark plug socket through the wheel well.
 

G8orFord

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I just remember what my tank looked like. I replaced everything!

So why haven't you checked the last two spark plugs? They can be pulled pretty easy with a long extention and a swivel spark plug socket through the wheel well.
Laziness, I suppose. After the other 4 looked good and the fact that it runs fine up to the miss (and beyond if you try hard enough), I decided to look down other paths and never got back to that.
 

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Do you have a vacuum gauge?

Test for blocked exhaust
Idle 17" to 21"
REV to about 2,000 and hold steady
If vacuum is slowing dropping at steady RPM then exhaust is blocked
If vacuum is slowing going up then intake air is blocked
 

G8orFord

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I've got a vacuum gauge but it's probably going to be the weekend before I can mess with it anymore. Also picked up an OBD1 code reader, though I don't expect it to tell me much.
 

G8orFord

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Pulled the codes.
KOEO Self-test: Codes 11 & 31
Continuous Memory codes: 54, 63 & 31

I guess I'll change the TPS and the air temp sensor and see how it goes.
 

rusty ol ranger

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Pulled the codes.
KOEO Self-test: Codes 11 & 31
Continuous Memory codes: 54, 63 & 31

I guess I'll change the TPS and the air temp sensor and see how it goes.
It could be a bad spot in the TPS. I wouldnt think the air temp sensor would do it though
 

G8orFord

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It could be a bad spot in the TPS. I wouldnt think the air temp sensor would do it though
Me either, but WTH. I'm not too sure about the TPS either since it's a consistent RPM and you'd think that would vary with load, but I've ordered one. Hard to believe out of 4 different parts stores, none of them had one in stock. Not even within 50 miles on other stores.
 

Uncle Gump

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Times like these make ya miss OBDII.

Live data would be priceless.

You can see it on a DMM but pulling up a parameter is so much simpler.
 

G8orFord

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Just the right size to touch the ground.
Do you have a vacuum gauge?

Test for blocked exhaust
Idle 17" to 21"
REV to about 2,000 and hold steady
If vacuum is slowing dropping at steady RPM then exhaust is blocked
If vacuum is slowing going up then intake air is blocked
Vacuum held steady at 2k RPM.
 

G8orFord

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4.0 V6
Engine Size
4.0 SOHC S/C
Transmission
Automatic
2WD / 4WD
4WD
Total Lift
0
Total Drop
0
Tire Size
Just the right size to touch the ground.
Times like these make ya miss OBDII.

Live data would be priceless.

You can see it on a DMM but pulling up a parameter is so much simpler.
Yes, I much prefer OBDII systems. They fall right after carbureted engines with electronic ignitions as far as troubleshooting.
 

rusty ol ranger

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Me either, but WTH. I'm not too sure about the TPS either since it's a consistent RPM and you'd think that would vary with load, but I've ordered one. Hard to believe out of 4 different parts stores, none of them had one in stock. Not even within 50 miles on other stores.
Good point.

No one had a 2.9 tps? When i bought one a year or 2 ago i had no issue lol
 

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