I have a 91 Dakota 4x4 with a 3.9 liter, 5spd, and 158,000 miles. The truck idles smooth when the engine is cold, but once warmed up, it has a rough idle and what sounds like intermittent spark knock at idle and sustained spark knock when lugging the engine. I have cleaned the EGR valve (the tubes were completely blocked by carbon), replaced the PCV valve, oil breather, spark plugs, wires, distributor cap and rotor, and verified the timing was correct. I have even tried a top end engine cleaner poured through the throttle body. No luck. The spark knock only seems to come from the right front side of the engine. The engine seems fine on power but has a slight lag/dead spot when first accelerating when cold or hot. I checked for codes but the computer showed none. Could it be carbon in the intake/throttle body, the notorious stretched timing chain, or just a vacuum leak?
I also have a vibration that I feel through the body/floor/seat when decelerating from highway speed. If I push in the clutch and/or put it in neutral, the vibration is worse. With it in gear, the vibration goes away fairly quickly (with in 5 mph), with the clutch in and/or transmission in neutral, the vibration goes away around 45 mph. I have just recently had the tires balanced and the technician told me that he had to put a lot of weight on one tire and suggested I put it on the back (though I did notice all the tires had a lot of weight on them). I don't think I had the vibration before the balancing. I replaced the clutch right before that so I did have the driveshaft out but am pretty sure I aligned it properly when I put it back on.
It does not vibrate when accerating or cruising. Could it be the tire(s) or the driveline?