I test drove a '84 K20 4x4 6.2, 700r4, 4x4. It started right up after sitting all night, seemed to run good. What else shoud I be looking for? I let it sit for a few mins and it started right up again. When I started it the second time, the water in coolent light came on with the glow plug light, then fadded away, is that normal? Thanks for any help as this is my first diesel I have looked at and I don't know anything about them.

Marc

Welcome to the forum!

The water in fuel and low coolant level lights are supposed to illuminate for a few seconds at startup to tell you the bulbs are good.

Check the cooling system out thoroughly for leaks. This can really turn into an expensive nightmare.

Make sure the transmission is good... Check the fluid and make sure it shifts firm at high throttle settings... Slipping is bad, burning smells are bad, dark fluid is bad, soft shifts are bad, late/early shifts are bad, shifting at a certain speed not dependent on throttle is bad, excessively hard shifts are bad... You get the point. Also check the transfer case... Make sure it works and doesn't make a bunch of noises.

Take off the radiator cap and look for bubbling coolant with the engine idleing. Bubbles are bad.

Look for smoke... Black is okay, blue is not great, white is bad.

Good look!

Went and checked fluids sand stuff today. I couldn't even see water in the radiator, I could see all the way down to the trans cooler baffle. Also the oil was black as night ( I think this is normal for diesels isn't it?). I Couldn't see any water in it, but is wasn't as dense as oil (unless diesel oil is different than regular engine oil). Should I run away rom it?

Black oil is normal. Diesel engine oil is thicker than what you use in a gas engine.

The low radiator may indeed be a concern. Sounds like it needs some work somewhere. See if there is excessive pressure building in the radiator as soon as you start the truck. That would indicate a bad head or head gasket. The low coolant level might be as simple as a loose hose clamp too. You had better spend some more time checking this out before you get serious about writing a cheque.

Does the box of the truck have a bunch of holes in it from a 5th wheel mount? That might be an indication of what the truck's past life involved.

The 6.2 and the 700r4 transmission was not a great combination for heavy towing use. If the truck is to be used for daily driving the combination seemed to last but if it was used for heavy towing often the transmission failed due to heat. It can be upgraded with a cooler and towing shift kit which greatly increased the reliability.

I would have the guy refill the radiator to the top, then go look again in a couple days. Look for leaks, these cooling systems are a nightmare if you have to replace anything more than hoses. Radiators run in the neighborhood of $260-$300 for a junky chinese one, and $400-$500 for an American one...

Fire it up again when you go back and look for white smoke. Remove the radiator cap again and look for bubbles. Just remember, white smoke and bubbles in the radiator are bad things. Should you see either of them, tell the guy you'll think about it and then run the other way...

Oil will be black even right after an oil change. Just so long as there is plenty in there and you keep it changed, no big deal. It might be thin if the guy let it idle for a long time... That can cause the oil to thin out because there is a little diesel fuel in it. The bad part about idleing like that is that it can cause some cylinder scoring... So long as it fires up good and doesn't burn oil, don't worry about it.

The tranny is the only other sticking point... That 700 wasn't very good behind the 6.2, particularly in a 3/4 ton.