Just bought it, not familiar with twin spark 4bangers. 96 Ranger, engine has probably 90k miles on it. It seems to run rough at idle, get it up to about 3k RPM and it feels like it smoothes out. A/C running does not affect the drag on the engine. Has good pickup, I dont have it road legal yet so have driven it approx 60miles total up to 75. Feels like it run smooth at 75. Also, mechanic told me no compression in the 4th cyl... I somehow doubt that with it running smooth at higher RPM. Plus the engine does not shake at idle, just runs rough (but never stalls even at idle)

BTW: Checked firing order and it was correct, cat was removed by previous owner. Someone mentioned something about O2 sens could be causing this?

Any starters? Mechanically inclined, know more than enough to keep myself out of trouble, just not much of a gearhead.

 

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Is the service engine light on?
There's 2 plugs that are nasty to change on the drivers side. How old are the plugs?
The throttle body could be dirty. This usually shows at idle.
AL.

 

With what Dealford brought up, I BET the last plug or two was not changed and that's how the rougness is noticeable until you get up to highway speeds.

The cat should not have an effect. An oversized exhaust pipe would slow the flow down enough to "plug" things up in a sense which would be trouble.....but that must not be the case here.

 

Well odd thing, replaced the Air Control modual and cleaned it up abit cept under A/C was about as bad. Compression test comes back with 80lbs instead of the norm. So i'm thinking burned valve. Sound bout right?

 

Well, that puppy won't be barkin' on all 4.
Did you squirt some oil in the cylinder and check the compression again to confirm valves as the problem?
This may be your chance to re-ring & bearing the bottom end cheap after you freshen up the head.
AL.