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I have a 1989 Chevy S10 Blazer with the 4.3 Liter engine and my problem is that when I bought the truck had no heat and it turns out their was no thermostat and when i put one in the truck started overheating very quickly. The thermostat does work and is the original equimement(bought new at auto zone), but when the thermostat is taken out, the truck runs like it should. What could possibly be the problem other than the thermostat?

Clearly the cooling system is unable to disspate the heat generated by the engine unless it’s flowing totally free. The common causes are a clogged up radiator, an inoperative cooling fan, and, on a 19 year old vehicle, a poorly circulating water pump (the impellars can get eroded). My money is on the water pump.

I can only add that it will not be long before it gets worse and you are still going to need to make the repair.

I agree (as I usually do) with mountainbike. I am fairly confident that you will find very little of the impellers remaining when you remove the water pump. This leads to the pump having almost no ability to move coolant through the radiator, thus leading to high operating temperatures.

Well I replaced the water pump, still overheated, same with the radiator, and the cooling fan works fine. The weird thing is that when it overheats and i shut it down and feel the radiator, it is cold. As i said the radiator is brand new as of today and i also bench-tested the thermostat and it works, and so does the heater core. Why is the radiator cold and what else could be the problem?

did you replace both the heater hoses from the top and the bottom? sounds like one may be collapsed. sometimes, although it LOOKS fine, the inside collapses and you cant see it from the outside. try grabbing the hoses (when cold) and squeeze them. are they firm or soft and squishy?

and you mention the radiator. did you replace it? you said; “replaced the water pump, still overheated, same with radiator.” what did you do to the radiator new new, or take it out and flush it?

I bought the radiator new today. I had already tried flushing the old radiator, and since it was cold I decided to buy a new one but that same problem still exists. This does sound like it could be the answer though because the hose is hot near the thermostat and cold near the radiator, but it is only like this with the thermostat in. It runs cold without it (the thermostat) and the hose gets warm, but still not hot like it should be.

Is the thermostat in backwards? Sounds like it.

Nope that was the first thing i thought. The copper plug goes in toward the engine block and I know it is in correctly.

The engine cooling system seems to be operating correctly for the most part, unless the thermostat is installed and completely blocks coolant flow.

Has this engine been modified? It sounds like the pulleys may have been replaced with chrome or aluminum aftermarket versions, and the water pump is running backwards. That would make the thermostat unable to react, since it would be reacting to COLD coolant from the radiator rather than hot coolant from the engine. Make sure that the water pump is rotating in the correct direction.

Well you have eliminated just about everything, but one. How about the hoses? I suspect one is collapsing. Likely it is internally collapsing so you don’t see it, nor feel it.

Well, the water pump is rotating in the correct direction (counterclockwise) and I replaced all the hoses so they are all brand new. Their was a new alternator put on could the people that put it on possibly made the engine run backward?

The engine I think is running backward compared to the water pump (engine is running clockwise, water pump is running counterclockwise, engine is supposed to run counterclockwise) and therefore it is not cooling the engine effictly. How can you reverse the belts or does anyone know where i can find a clockwise water pump for the 89 4.3 Liter?

This is one for the record books. Hee, hee.

The reverse pump is for sale at the place that sells left handed pipe wrenches.

If the fan clutch is slipping excessively, the water pump won’t develop sufficient pressure to push enough water through the thermostat opening. If the water pump passages are partially blocked with a displaced gasket, or sealant, that will reduce flow. Use these checks: You may have to take it to a radiator shop for troubleshooting.

Well, I doubt your engine is running backwards, or the truck would go forward when in reverse and vice-versa.

Standing in front of the truck and looking into the engine compartment I’ll assume the engine runs clockwise. Check the old water pump, the one you took out, and see which direction the vanes point to see which direction it needs to rotate. Then look at the pump on the truck and see which way it turns. Simple.

So have the pulleys been replaced / modified?

No, as a matter of fact the reverse direction water pumps are sold at pretty much the same place as the standard direction water pumps.

They really do exist, not all water pumps are supposed to rotate in the same direction.

So let’s re-cap . . . new water pump . . . new radiator . . . new hoses all around . . . new thermostat . . . new radiator cap . . . fan running OK . . . gotta be either a blockage or air bubble somewhere. How’s the heater core? Do you have heat now? What if you run without a thermostat now? Still overheating? Can you see coolant flow in the radiator when the engine is running? This is a tough one. Was this engine “done” (built-up) ? Rocketman

If the fan clutch skips excessively, or fails entirely and falls off, the water pump will continue to drive all the same. The clutch drives the fan, and is driven by the same pulley that drives the water pump. The FAN clutch does not drive the water pump.

In addition to others’ questions about modification, I’m wondering now if the head gaskets have been replaced. You seem to have covered all the other bases. I have heard of headgaskets improperly installed that restricted coolant flow.

Just a thought.