Hello all, long time lurker, first time poster.

I've got a 2wd 4.3 vortec 1997 blazer with an overheating problem. It started very suddenly and makes the vehicle undriveable for more than about 5-10 minutes. Have replaced thermostat, backflushed the system including heater core (which helped) but it is back to overheating.

When I backflushed the system the coolant was a very nasty brown/mud mixture, but after time came out nice and clear.

If I drive it long enough I hear the heater core making a knocking sound and the reservoir starts to boil. No antifreeze in oil and recently flushed the system using the ethylene instead of Dexcool.

Does not blow hot air, which I know also suggests a clog in the heater core, but when I backflushed the system and ran it to operating temperature the air got nice and warm.

So I'm curious which options to pursue now.
New radiator cap? (Replaced the reservoir cap already)
Chemical flush?
New radiator?
Replace heater core?? (I really hope that isn't it.)

I really enjoy my new to me Blazer, and hope you all can help me fix this issue.
~GeoBlazer

 

More than likely the lower radiator passages are plugged. When the truck gets warmed up turn it off and feel the radiator fins. If it's quite a bit cooler on the bottom part then it's plugged. Although there's a procedure for chemically cleaning the radiator it rarely works on totally plugged cores. I usually just wind up replacing the radiator. In most cases you can unclog the heater core using a garden hose with no more than 50lbs of pressure. There are a few threads here on the procedure. I'm going to assume that at some point someone mixed regular coolant with the Dexcool leading to the brown sludge you saw in the system. Other causes of total cooling failure could be a broken water pump impeller assembly (rare) or your replacement thermostat is bad.

 

Sorry...
but seems to me it bears all the symptoms of a blown head gasket {coolant passage - combustion chamber}

Do a cooling system leakdown test. Even if pressure doesnt drop much, then disable spark and crank engine for 15 seconds. If pressure in rad rises, then you know.
Sometimes, there's little water jacket to cylinder flow, but compression pressures overcome small crack.

 

I appreciate your advice, will check the radiator fins after a warmup and see what I find.

I agree that it sounds like a possible head gasket issue but I sure hope not.

 

I know that one time I was hearing boiling in the resevoir, and it was just the raderater cap but it wasnt overheating. So that could be the problem.

 

That's over-pressured water coming out of your radiator...not the cause. That little spring on your radiator cap is set for about 16lbs of pressure. When your cooling system exceeds that air, water or steam goes into your reservoir. It's usually happens when your vehicle is overheated but it also could be a symptom of a blown head gasket. There's a tester for combustion gases in your coolant..not sure if autozone has one in their loan a tool program or not.

Head Gasket or Combustion Leak Test Procedure (Gasoline Engines Only)

 

Replaced the radiator cap and took it for a drive. 15 minutes later the temp would still not go above 220-230. still no heat. I felt the radiator fins and they were indeed warm up top and progressively cooler the further down I felt. With the radiator cap off it is evident that even the top of the radiator is coated in mud.

Here is my current thought, maybe the radiator us so clogged at the bottom that when I run the truck the coolant picks up some muck, takes it to the heater core and plugs it. That explains why my heat worked fine when I was flushing the system but stopped working after I ran the truck with a closed system.

Any thoughts?

 

That's pretty much identical to my experiences. You can try a chemical flush but I've never had much luck with them and after wasting time and money wind up replacing the radiator. The heater core may or may not be plugged again as an air pocket can cause the same symptoms. Connecting a garden hose to the heater core will tell whether it's clogged or not.

 

Be careful with that garden hose. Apply the pressure to the end that goes to water pump {reverse flush}

I'd just hold the end of garden hose to end of heater hose. Yeah I know it'll be messy and get you wet but better than replacing the heater core.

Other method: use in-line garden hose shutoff and increase flow VERY slowly.
Years ago I was replacing a heater core and the instructions that came with the new part said to test it for leaks before installing.
so I did. It was the 'waffle type, not tube and fin, so when I plugged one end and hooked up garden hose with clamps, it swelled up like a balloon



But it didnt leak.

The guy at NAPA wasnt too happy with me but when I showed him the instruction slip, he agreed it didnt say not to do that.
In a tube and fin, I'd worry about the tank solder cracking under pressure

 

Uhoh. I used the peak brand flush kit. It involved cutting the heater core outlet hose in half, fitting a T inlet to both ends, then attaching a water hose to the t inlet. Open the rad cap, start engine and start garden hose. It seemed to work very well as it remedied all symptoms for a short time, are you saying it may have damaged something?