Havw a 99 4x4 with a 3.9 auto etc. Had to install used engine-old one locked up and 130,000. Used engine is same year with 90,000 says salvage yard. Have a #4 complete miss-300 counts per min- and #3 is at about 10 counts. New plugs-Champion, wires-Beldon, cap rotor-Napa. Wires run per TSB. Compresion test 130-150 lbs on all cyl. All inj. firing per noid light.Pulled injectors and checked spray pattern-good. Swapped plugs and inj. around. Still a miss. Changed intake gasket and valley pan gasket. What am I missing?????

 

Bump-?anyone?

 

swap the wires on that cylinder... just because they are new doesnt mean they are perfect. give that a try and let us know what you get

how are you getting the 300 counts?

 

You might have the wiring wrong. You can put the wires wrong and get a miss fire on one plug. Look for the little one on the rotor and match it up to the Firing Diagram.

If you need the diagram send me a PM. I tried to upload to post but didn't work.

 

the firing order is stamped on the intake manifold. if it was a miswire on a cylinder, he'd most likely have a poor running motor. his problem sounds like it's running pretty good, just is tossing a code for the missfire

 

Have swapped wires-plugs-injs etc. At a idle there is a lope from the miss-above 800 rpm mis is gone.The only thing I havnt swapped was the crank sensor? Missfire count is from my scanner-an OTC Genisys.

 

it would have to be in the cylinder then, there is nothing left. the crank sensor signal is either there or not, if the crank sensor was failing, you'd have a random misfire and not one tied to 1 cylinder. the only other thing that it could possibly be is the tone ring on the back of the crank, if it is damaged in some way, it could also give you this problem. do you have a scope you can put on the crank sensor? you'd be looking for a steady signal for all cylinders. if you had it, then you could eliminate the tone ring possiblity.

another thing just came to mind... do you still have the dist. from your old motor? if so swap it over, that would eliminate any dist. problem. My son's '91 gmc had a bad coil pickup in it which threw us for a while until we went to go swap the pickup coil under the cap in the dist. one of the pickups was broken off and the rest basiclly fell apart when we touched them. Although the magnum motor doesnt have the pickups like the gmc, it does have a pulse ring inside, have a look at that as well.