This isn't a diesel, but I figued there's some knowledgeable people here, so maybe I can get some help...

I bought my truck and stopped daily driving the blazer. It had developed a leak from behind the water pump so I replaced both the pump and gasket. I drove it occasianaly with no event. One day, no heat would come out and the temp needle slowly creeped passed 220. I sut it off, and went to get a new t-stat. Popped it in and the same thing was happening.
Fast forward to this morning. Start it up, same symptoms. I did note that there was no pressure/heat in the upper radiator hose untill after I shut the truck off. I don't want to randomly change parts, and I'm not sure how to trouble shoot from here

 

Sounds like blocked 1 of 3 possibilities:
1) T-Stat installed in the wrong direction. Some are sensitive to flow direction.
2) Impeller is loose on the water pump
3) Blockage in the radiator, u/l hose, or in the block

I'd go with these in order...They are arranged from cheap to expensive.

For #3, a radiator shop power flush might help.

HTH

Tony

 

Is the blazer a 4.3 liter with more than 60k? If so it more than likely needs intake gaskets. They will leak out or in to the oil. If you can not see a leak externally check the oil and see if it looks like milk, either way it will need intake gaskets.

 

It is a 4.3, and it has nearly 165K on it. It's not loosing coolant and the oil looks good.

I'm gonna take of the t-stat tommrow night and test it/verify in properley.

I really hope it's not the water pump, it was a P.I.A. to take out the radiator. I'm guessing that's not it since it worked fine for ~1000 miles then had the problem aftr sitting.

 

something I am going to throw out there. Does the unit use DEX-Cool extended life antifreeze. That year should have dex-cool in it. orange in colour, tastes great, and less filling.
If someone has mixed standard (green) glycol coolant with dex cool, your overheating problem comes from the mixing of the two. They emulsify and form "mud" for lack of a better term. Looks almost like the bottom of a coffee cup, with way too much sugar in it.
This mud plugs up the cooling passages, and causes overheating, no coolant flow, etc. Water pump and fan can be working just fine, but there still won't be enough cooling to go around.
If you flush the system completely, sometimes you can get most of the mud out, but the rad should still be boiled out, and I have seen that "mud" outlast a 4 hour gig in a hot tank at the machine shop.

Just a thought.

 

No, it's never had dex-cool in it. At least since I have had it (I picked it up in 2000).

 

Took out the t-stat last night, it only goes in one way and started to open at 190F, by 210F (full boil) it was pretty far open. When I take the waterpump out, what should I look for?

 

Double check the dex-cool. Previous owner could have had dex-cool, then flushed and changed to green. Probably not an issue, since you have had the blazer running fine since then.

The impeller should be tightly fixed to the pump shaft; they should turn as one unit.

Would it also be possible to tow the blazer to a radiator shop and have them power flush the entire cooling system while it's torn down?

HTH / let us know how it goes.

Tony