I recently bought an '02 D2 with an overheat problem. Runs fine, just seems to be a drinker. They guy I bought it from stated "the head gasket was smoke checked and was fine." That's a new one on me, perhaps he meant an exhaust gas check in the overflow bottle but I never could get to the root of that statement. The system appeared to be empty so I disconnected the lower hose from the bottom of the thermostat, stuffed a garden hose in the overflow and ran the truck for a while to flush the system. Eventually the water came out clear. I put in a known good thermostat and filled the system with water then bled it and drove it around a few times. I only went a couple miles each time but the engine sounded great, didn't overheat and there were no misfire codes. In fact, it ran in my driveway for 15-20 minutes and it would cycle between heating up to 201° and then the thermostat would open and it would drop back down to 196°. It did this cycle a few times and did lose a little level in the overflow bottle.

I pressure tested the cooling system and after sealing up all the obvious minor external leaks (hoses) it dropped a half a PSI in about 15 minutes. I figured something was up but it was running so well I considered moving on to redoing the brakes and figured I would just drive it until I could recreate the overheat. It almost seemed like it was simply improperly bled all this time. However, when I pulled the dipstick the level of chocolate milk was 2" above the full mark and draining the oil revealed copious amounts of water/coolant in the oil. I'm not sure if the PO ever changed the oil since the first overheat, but he said that it would go through coolant and eventually overheat so I presume the coolant had been getting into the oil for some time previous to me buying it. I have no history on the severity of the overheats or for how long he had been refilling and driving it.

Here are the results of the compression test.



I pulled the heads and there were no obvious signs of a combustion chamber breach in the headgaskets, and there is no evidence of steam cleaning in any of the cylinders. The carbon deposits are distributed evenly across all pistons and combustion chambers. Where to go next? Do I reassemble with new headgaskets and hardware and see what happens? Or does this sound like a strong candidate for a cracked block and I should go the extra mile(s) to remove it and have it checked?

 

@ahab My understanding is a cracked block is almost always an overheat, but 120 is a significantly lower compression than the rest. A bad head gasket can put water in the with oil almost no overheat depending on the failure. My failure was a hairline crack to the water jacket, overpressure but no overheat literally 10 Deg F hotter than normal.

 

I'd change/check front oil pump cover gasket, betwen block and cover. Yeah, this means pulling the oil pan...but it wouldn't hurt to properly clean it after water/anti-freeze entering cavity.

Yeah, low cylinder compression is a concern...but should have done a cylinder leak down test on that cylinder prior to removing head. Could have verified cause. Now, you're basically looking at cylinder bore condition, inspecting valves, wondering about condition of rings/piston and head gasket. If you do drop pan, could just pop that piston out and inspect rings and bore.

Probably should have done exhaust gas check on coooling system, too, before disassembly...could have given more insight on condition of block. Anyhow, since it wasn't overheating, no visible steam-cleaning of pistons and it's a 4.0...me...like l say...l'd check front cover gasket and see if this is intially where the anti-freeze was entering oil pan.

 

I'm considering gambling on the low compression translating to a simple headgasket problem, which then dumped all the coolant into the oil. Visually everything looks fine WRT valves, cylinders, pistons, etc. Yes, leakdown would have been a good idea and I have a regret there but I was already bellying up to the poker table on the headgasket once I saw the compression numbers and the next closest piston was second lowest so that helped push me towards that conclusion. The truck ran fine in all other respects, power was normal, the temps were what they should be and there was no smoke of any color out the tailpipe. If I drop the pan to pull the front cover, etc, which is not a bad idea, I may as well pull the block. The hard part is done and now all that's left is a few electrical connections, the bellhousing/torque converter and a couple motor mounts. At which point I may as well put new bearings in along with a 4.6 rotating assembly... What the hell, maybe I'll redo the interior and paint the truck for good measure!!

@Ricard Gallant How did you verify the block was cracked?

 

@ahab Sorry badly worded my block was fine the head gasket had a hairline crack. The only way to test the block it to pressure test it, you can do it in place by by fabing up some blanking plates for teh water inlets then pressure with water and a air compressor. Put dish washing soap around the top of each sleeve look for bubbles dropping the helps you see stuff coming out that end.