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, coil, all filters, and verified the timing was correct. I have tried Seafoam 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 (at about 60mph), 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'm not sure, but Idon't think I had the vibration before the balancing. I replaced the clutch, engine and tranny mounts 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 accelerating or cruising. Could it be the tire(s) or the driveshaft/u-joints? (It's too cold and wet outside to swap the suspect tire to the frontso I thought I would ask for ideas first)

 

ok as for you're knock I'm not really sure but you could look into yourpick up coil it may be sending false messages to your ecm.The vibration however,I believe to be one of 2 options A-you purchased a tire that was flawed ie broken belt or out ofround.Or B-your carrier bearing is either loose or needs lubed these are my guess good luck.the carrier bearing is the joint in the middle of the drive shaft if you don't have one check the yoke at the rear end for loose bolts