Is it possible to do this from underneath without removing heads? 1999

 

1999 what?

 

Dakota 2wd

 

Moving to second gen dak section.

I don't think so. Need to drop the crank, and the torque converter sticks into it a bit. You *might* be able to.... but, pulling the motor is gonna be your best bet.

 

you only have to drop crank if you remove pistons from below correct?

 

to remove and install pistons, you have to remove the oil pan to remove the rod caps, then you have to remove the heads and push them out the top. There is no way to pull pistons out the bottom.
The crank can stay in though.
Why do you need pistons? Assuming one is damaged, how is the cylinder wall?
It probably is going to be better to pull the motor and do a total rebuild. JMO

 

I bought the truck from an idiot. It has a leaking heater core so he must of cooked the motor and the oil was black as could be. It still runs good but it has a light knocking sound I'm guessing is the wrist pins and I can smell it burning oil and it seems low on power and prabably hurting the gas mileage and I'm afraid the oil burning might plug the Cadillac converter. I figure I might as well fix it or i won't be happy with it. The body is decent. It only has 138k so I'm hoping the cylinder walls and everything else is OK and I just put in new pistons and rings. Oil pressure is pretty good.

 

To be honest, there are a few other things I would try first. FWIW, even if the motor was over heated several times, I dont think that would cause piston slap.
What I suspect is a bad intake plenum gasket being your main problem.
Below is what I would do if its my truck.
1. Take the air cleaner hat off so you can look in through the throttle body. With the engine off, open the throttle and shine a light in there. If you see oil standing the plenum gasket is blown. This over time will ruin your cats. This also, can cause engine knock.
2 The next thing I would do, warm the engine up, pull the sparkplugs and run a compression test. If its all within specs, and you are holding good oil pressure, I would not tear the motor down for pistons.
3. The last thing I would check is timing chain slack. The way I check, is to remove the distributor cap, put a socket on the harmonic balancer bolt, then slowly turn the engine until the distributor rotor starts to turn. Go back the other way and see how far it moves before the rotor starts turnng again. The dead spot, is the slack in the timing chain. On a new chain, there will not be a dead spot. But as the motor gets miles on it, the chain will stretch, and this throws the cam out of timing. This is a fairly common thing on the 3.9 V6 with high miles on the motor.

But again, I suspect the plenum gasket is your main problem.