Missfires at 40 mph

[SWE_lollo03]

Hi, I have a 1988 ranger with the 2.9 that runs great until i continually drive 40 +- 3-4 mph then it starts to missfire but if i speed up to lets say 50 or slow down to 30 mph the missfire just dissaperes, could something be wrong with the cruise control or something else?

 

[rusty ol ranger]

Hi, I have a 1988 ranger with the 2.9 that runs great until i continually drive 40 +- 3-4 mph then it starts to missfire but if i speed up to lets say 50 or slow down to 30 mph the missfire just dissaperes, could something be wrong with the cruise control or something else?

Pull codes.

Whens the last time it had a full tune up? Plugs, wires, cap and rotor?

 

[SWE_lollo03]

I swaped the whole ignition system last summer to try and get rid of it but it was still there

 

[alwaysFlOoReD]

Do a compression test, dry then wet.
Check your plugs for cracked porcelain or large gaps.

 

[SWE_lollo03]

I did that last summer too and the plugs looked fine and had the right gap, i can't remember the compression numbers but they were plenty good