Hello all, I'm trying to figure out what is causing my sisters 98 blazer (my old car) to over heat. It's currently sitting at 140k miles and been well cared for.

It seems it all started a few months back when her heater quit blowing warm air. The heater core was gunked up and clogged so I flushed it out and ended up flushing the entire system using a bottle of radiator flush from my local auto part store. I also switched to "green" coolant which I know can cause a ton of headaches. Once finished it had seemed to flush everything out pretty well. It was never over heating at that time, just the heater core clog. It ran great with the heater blowing hot for a couple months up until now...

It's began running very hot, around 225. When I checked the coolant it was a nasty brown/rust color and the heater core was completely clogged again. I flushed the entire system a few times making sure all the "gunk" was out. I also changed the thermostat during the process and made sure to burp all air bubbles out.

It now seems to be circulating through the entire system with hot air through the heater and all lines warm to touch. But the engine temp is still running hot. It now seems to just take longer for that to happen.

The fan is working properly as well. Could this possibly be a faulty guage? I know the water pumps tend to leak when they go bad and that isn't the case. Could it still be faulty? Whats the best way to check this? Could it possibly have a blown head gasket?

I'm pretty stuck at this point. Any help or input of what to try/do next, would be much appreciated. The summer heat is beginning to roll in, which can reach up to 110 degrees here in Boise. Would like to get this fixed asap.

 

Check your oil if you find choclate milk you have blown head gasket.Change yuour coolant sensor and see if that makes any difference to tell if its running hot or not.Dont drive alot till you figure this out or you will blow gaskets either intake or head.

 

The oil looks good. I'll look into the sensor next. Is there a way to tell the actual temp. without replacing the sensor first? Not sure if an OBDII scanner would give me this info or not?

 

What kind of shape is your radiator in? That may be where the particulates are coming from. The water pump is another possibility. And if you have particulates in your coolant, it could be affecting your thermostat, as well. Look for the simple things first.

 

It's probably all clogged up..
Just my thought..

 

OK yea a live obdii will give you the supposed temp but thats if the sensor is good. If it's bad you will just get a false live read from the scanner.
The temp sensor I believe is a resistance sensor. You will need to see what the ohms are across it and compare it to a gm chart temp vs ohms, I have seen them. I do not have one at my finger tips. Its not hard to find.
That said more then likely your still gunked up and need to keep flushing. The head gaskets don't go that often on these and I don't see that you said your blowing coolant.
Just keep flushing a couple more times and see what happens.


Are you sure your fan clutch is working.
Whats it read when your doing 65 for like ten min's straight?

 

The dash gauge gets its signal from the sensor in driver side head. There is another sensor by the thermostat housing, I believe, that gives the computer its temperature reading. A live scanner should give you the reading from the latter of the two sensors. Compare what the scanner is telling you and what the gauge shows. Should at least rule out a faulty gauge.

Also, does it overheat at idle, or driving down the road? If at highway speed, it's probably a coolant issue, because there is plenty of airflow at 60 mph. At idle the only airflow is from the fan, so if it's not working it will get hot while stopped and in stop and go type traffic, but cool down after sustained higher speeds.

 

The rusty brown sludge in your cooling system could be the result of the GM Dex Cool coolant (google that phrase and you will see all the problems and law suits) that GM brought out in about 1996. There were lots of problems with the Dex Cool turning to a nasty rusty brown sludge throughout the cooling system. The first symptom was it would clog up the heater core. Then the GM owner would notice the vehicle's cooling system was gradually starting to overheat. The Cause? Dex Cool would not mix with other coolants, so if you went from Dex Cool back to the traditional green coolant , it would sludge up. The Solution? Repeated flushes of the entire cooling system. I first heard about this on the 1994-96 Impala SS forum. So if you have a 96> Blazer and it has been on the GM Dex Cool coolant, be very careful about the change to the green coolant. Another tip----if you stay with Dex Cool, don't run it for 5 years or 100,000 miles as it was first promoted. Change it every two years , at least.

 

Thanks for all the input guys.

When I get off work I'll be doing some more diagnostics on it. First I wanna see what the coolant looks like now after the many flushes. If it's brown still I have a feeling a new radiator might be in store. I've flushed the crap out of it multiple times now. I hate Dex-cool. Went through it with my zr2. Ended up replacing the radiator because of a crack, but kind of glad that happened because it was gunked up from dex-crap too.

Next will be making sure it is in fact reading the correct temp.

 

Not to turn this into a dex-cool vs. green debate, but I don't think dex-cool is as bad as everyone makes it out to be. Mine had dex-cool in it when I bought it, and it ran cool, and wasn't gunked up. And knowing the previous owners, I highly doubt it had ever been changed out. When I replaced my radiator, I was planning on flushing and going with green,based on the horror stories I've heard. After some research, and talking with some very knowledgeable members on here, I went with dex-cool.

Also, the dex-cool has some additive, or lubrication properties that is needed for the longevity of the water pump. I've heard that green will dry the seals out, and the pump will leak.

Again, I really don't want another coolant debate, there are a billion on this site already, and if this turns into one, a mod will shut the thread down! Just putting in my $0.02, and sharing my dex-cool experience.