Ok. The truck is a 1989 Dodge Power Ram 50 (D50), it belongs to a friend of mine. He has full maintenance records (Every oil change at 3k miles, EVERY time the guy put gas in the truck, how much he spent on gas, how many miles he got on that purchase, Rebuilt engine at 126k miles).
As I stated above the engine was rebuild at 126k miles. The truck now has 186k. So this new engine only has about 60k on it.

Starting last week, the truck would get hotter at a stop light, or when stopped. When driving it would be at normal temp, but would approach 3/4 of the gauge when stopped. He thought something was up, so he took it to a mechanic. Mechanic said it either had a blown headgasket, or a cracked head. He said "from his experience it's most likely a cracked head", but he may just be trying to get more money out of him...

It's starts fine, runs on all cylinders, and sounds great. You wouldn't even be able to tell there was something wrong with it by listening to it. When he started it I was looking for white smoke, but there was none! But it was not at operating temp, either. Would that matter? I'm wondering if its not the HG, or a crack in the head.

His Dad pulled the plugs, and supposedly the #2 cyl plug was wet (I assume it is water, but it may have been gas also, I wasn't there to whitness this)...

I was doubting it was a head problem, and was looking for other excuses, like the thermostat, but it is a brand new 180 degree stat...

What could be wrong with this truck guys?

Could be as simple as a slipping fan clutch, or a plugged radiator core. If you were burning antifreeze you would see it out the tail pipe and it would be draining the radiator also. Is it losing antifreeze? Oil level staying normal and not overfull? Stick with the simple stuff first.

Jason,
Normally, when an engine runs ok at speed, but not at a standstill, the problem is airflow related. If you think about it, when you are driving down the highway, the engine is probably developing the most heat, but the airflow through the radiator is at its greatest. You probably dont really even need the fan at highway speeds. At idle however, the fan is needed most. If it runs cool at speed, the radiator and water pump are probably ok. I would be looking at the fan clutch. A quick test is to get the engine good and hot at idle, then shut it off while watching the fan. It should only turn a couple of revs at best if the clutch is locked up. Once it stops turning try to turn the fan by hand. If it is hot, there should be quite a bit of drag. If it spins freely, you have found the problem.
Hope this helps,

Thanks for the replies guys! Everything said so far pretty much confirms that I am right thinking that there is nothing wrong with the HG, or the head.

The oil level is not raising, and you can not see evidence of water in the oil. There was no white smoke when he started it. Would it have to be at operating temp for the smoke to be white? I wouldn't think so, but I've never delt with anything like this before. I will try your fan test tomorrow after work Bill! I didn't even think of that.

That doesn't explain the #2 plug being wet though. What could that be? I'll pull the plug tomorrow and see what it looks like when I get the truck home! Any other ideas out there? Thanks a lot for your replies guys, I really appreciate it!

I did the Fan test today, and it seems fine. There is restance... Maybe it's the HG after all? I drove it, and it blew white smoke for about 1 minute, but I assume that was just condensation from sitting all night, after than it cleared up and you couldn't see the exhaust. Didn't seem to want to idle very well.

Fill the rad, then check the level after it's at op temp with engine running, check for telltale bubbles indicating the HG.
Aluminum head on iron block?

I'd do a compression test on it.

Jason,
One other thing to try is to get the truck up to operating temp, then try feeling different areas of the radiator to make sure it isn't clogged up, especially near the bottom. Any blocked up areas will be cooler than the rest of the radiator. I dont think this will be the problem, but it's worth a try. I still think it is an air flow problem if its only heating up at idle. If it was a head gasket, it would be getting worse under load. Is it loosing any coolant ?

the 2.6's all crack heads.

Most of the ones running have cracks that have just not goteen big enough yet.

Only solution is to pull it, throw it away, buy a "marnal" head, and put it back together.

The bottom ends of those things are badass. hold 4-500 HP stock. Largest 4 cylinder ever put in a passenger car too. Kinda cool... CRAP head design.

If the head was not cracked before the cooling problem, it is now. Likely a complete cooling system flush, new radiator, neww thermostat, waterpump, all of it.

www.4x4wire.com/mitsubishi (http://www.4x4wire.com/mitsubishi) Check it out there. LOTS of 2.6 info.

I just read that they had a tendency to crack... From the time I left the guys house, and drove it home (about 20 minutes) it lost about an inch of coolant... I checked the oil later, and there is water in it. I'd sat it's a safe bet that the head is cracked... I can get a new "non-jet valve" (one of the main reasons for the heads cracking so easily) for $334 with gasket set, and new head bolts... I'll probably buy that and put the new head on it.