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misfire at/around 3000 rpm


The Miracle

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I posed this on a different ranger forum, fixed some other issues based on the replies(Thx, RonD) but not the main issue, so perhaps I can get some fresh eyes on the problem

It is a misfire or intermittent lack of fire around 3000 rpm. The tach drops when it happens.
I have replaced Coil, Fuel pump, FPR, plugs, wires, cap, rotor, TFI, pickup coil, and TPS
I have not replaced the MAP or O2 sensor.
No check engine light or codes present.

It ran ok last year, with the above problem every so often, but after sitting over the winter it does it constantly.
 


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Roert42

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Compression test? Vacuum Leak?
 

The Miracle

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Compression test? Vacuum Leak?
I fixed all the Vac leaks, but it ran fine with a bunch of vacuum leaks last year.
Cant test compression but I think its probably fine, not usually something that suddenly changes I would think. Truck ran strong last year, no oil burning or blow by, and got 21mpg on the highway on a long trip despite being loaded down with a bunch of cargo and it being the 4WD model. Only has 120k on it even though it looks like total garbage.
 

Roert42

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Does the fuel pressure drop under load?

does the issue occur if you rev the engine to higher rpm or only while you Are driving?
 

The Miracle

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Does the fuel pressure drop under load?

does the issue occur if you rev the engine to higher rpm or only while you Are driving?
Fuel pressure is where it should be according to the gauge I have, just in case I made sure to replace the fuel delivery system and the numbers really didn't change. It will misfire under no load as well. Its harder to tell but definitely does it. I can entertain the idea that this maybe a computer issue.
I have verified continuity from every pin of the computer harness to each input and output module(sensors, injectors)
 

Roert42

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I would hook up vacuum gauge and see if there is a dip in the vacuum at rpm. This could indicate an issue with the timing or valve train.
 

The Miracle

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I would hook up vacuum gauge and see if there is a dip in the vacuum at rpm. This could indicate an issue with the timing or valve train.
I can try that tomorrow. I wasn't thinking it was valvetrain as the tach quickly jumps around when it screws up, which made me think the spark was dropping out as the tach is run directly off the coil.

EDIT my apologies, the tach is run off of the TFI, the tach and coil trigger share the same signal
 
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