Hello all! I have been lurking on this forum for a while and have learned more than I could possibly have imagined. The wealth of knowledge and the desire to share and help on the part of the members here is amazing.
My pickup: 2000 Ranger XLT 4.0 OHV. 4wd. automatic transmission.
I overheated it. Ended up cracking both heads. I recognized the problem as soon as it happened as I had the same thing happen to my 1993 ranger 3.0. I replaced the heads on that pickup with no problems and it is still running today. So I ordered parts for the 4.0 and spent hours on this forum to get any pointers or advice for this, very common job. I spent some time and preparation for the whole torque down procedure. Really did not want to have to buy those bolts again. Got it back together in one evening. To my slight amazement it started up the first try, and ran as good as or better than I have ever known it to run. (I bought it used a few years ago with 90K mi. on it) I left the thermostat out to help in flushing the cooling system after the job because I had put HG repair crap into the coolant in order to get it home. Anyway, warmed it up to about 1/3 on the gauge. Shut it down, no problems or concerns so far and I began to dream of a cold stream in The Sierras again. Let it cool a bit and drained the coolant I could from the rad. petcock, about 3.5 quarts. Replaced with water. Started it back up, beautiful, no problems. Warmed it up again, this time all the way to 1/2 way on the gauge. Idle RPM sitting right at 1100 in neutral. Time for a drive. Put it in D and it sputters and dies. I already moved it once from the street around to the back of the house with no problems. Start it back up and put it in gear again. It sputters and wants to die but a little feathering of the throttle saved it and idle stabilized in D. First concern; although fully warm the idle in D stabilized about 850-900 RPM not the 750 RPM idle it had before the problem. Oh well, it's running and I took it for a drive. Three blocks from the house I went to leave a stop sign and it dies. No sputter, no hesitation, just died, like someone shut the key off. Coasted to a stop and hit the key again, it started, but was now running poorly and lacked power. Went on for about a block (heading back toward the house 'cause this pickup is friggin heavy to push) and dies again. This time it will not restart. Finally got it home and tried to start again. Nothing. The thing is, it now sounds, on cranking, like low/no compression. Pulled the code P0320 with my economy AZ OBDII tool. Research indicated to me the crank pos. sensor malfunction. Didn't think to check for spark first, just replaced it, but, due to reading on this forum I looked for short in the wires. Found there to be continuity from the plug to pin 21 &22 on the PCM plug. Still nothing.
As of this posting here is the situation:
Makes strong spark on all six wires.
Spark plugs cleaned and gap checked.
All injectors have pulse.
I replaced the plenum gaskets.
Compression test is as follows (after adding a squirt of oil into the cyls. and cyl. #1 being pass. side front of vehicle): #1=80psi. #2=85, #3=70, #4=110, #5=100, #6=80. Did a compression test when I bought it and again about 8-10 months ago and all six were between 120 and 130 except for #6 being about 100psi.
Cranking fuel pressure bounces from 15 to 65psi. Mostly stays in the low end of that range with bounces to the upper end. Needle never stops bouncing though. I didn't think to check the pressure with key on engine off while I had the FP tester though.
Manifold vacuum at cranking (taken from the booster end of the power brake booster hose): 1-2 inches of bouncy vacuum, but, at one point in the cranking cycle, the needle actually bounces "backward" indicating a bump of positive intake manifold pressure.
No codes stored (took the battery out to charge a couple times.)

Here are my thoughts from here.
Timing chain should be okay as both injector pulse and spark are occurring. Shouldn't it?
Possible Cyl. wall or deck damage not detected on visual inspection?
Pistons and block visually inspected when heads were off.
Possible head gasket not sealed on right side of motor?
Reman. Cyl. head not flat?
Is it possible I screwed up the torque on the heads? The lower intake manifold?
If anyone has any thoughts, observations or criticisms could you please share them with me?
Am I reading the cranking vacuum correctly?
Please help if you can. Thank you all for your time. Peace.