96 Blazer overheating and then....

[7T9Shark]

Lately when I drive off, I get 4 -5 miles and my temp guage starts to climb off the scale. When this happens the heater simultaneously blows cold air. If I bring things back to idle, the temp drops back to normal and the heater once again blows warm. I have replaced the thermostat with no change.
I have also noticed that I lose lots of coolant through the pressure relief hose when this happens.

With the engine warmed up, I have noticed that the upper radiator hose is not firm, like I would expect with a 15 psi cap. Both upper and lower hoses are original. The lower hose is quite firm, so I wouldn't suspet it of collapsing, but maybe the upper one is at higher rpms.

Any thoughts on this???

AJ

[Jerry80871852]

I don't know if this will happen on our Blazer engines or not. But back in my truck driving days if something like you speak of happened it could be because the radiator was low of coolant. In fact when this happens the temp gage very well may show a normal temperature when in reality your melting the engine down due to high heat. For with the low coolant level the sending unit will not send the proper temperature to the temp gage. I know of a few truck drivers that burnt up their diesel engine because of their lack of knowledge of this happenming. Of course a collapsed hose can do this, stuck thermostat, stopped up cooling system, or bad water pump.

Maybe some of the others might be a bit more helpful than I.

[ricco88]

honestly I know u just replaced your thermostate but I recently replaced mine from lordco and they gave me the wrong one. might be similar issues. they only cost 10 bucks so u might consider replaceing it again. just a thought I could be wrong but I never trust the minimum wage want to be mechanics that work there.

[TCGrove]

I had a couple of thoughts reading your problem. I had trouble with a plugged heating core on my Blazer. I just unhooked it from the firewall and flushed water through it. This doesn't explain the high temperature though. Another thought....is the thermostat installed in the correct direction? That may sound crazy, but it could definitely cause your problem.

[Jerry80871852]

Originally Posted by ricco88

honestly I know u just replaced your thermostate but I recently replaced mine from lordco and they gave me the wrong one. might be similar issues. they only cost 10 bucks so u might consider replaceing it again. just a thought I could be wrong but I never trust the minimum wage want to be mechanics that work there.

You can check a thermostat with a pan of water on the stove and a thermometer and tell exactly what temperature it opens.

[7T9Shark]

I top the radiator off every time it does the over heat thing, so being low shouldn't be an issue. Also, the temp does cool back down to normal and then back up again, so if it were a stuck thermostat, it should get hot and stay hot.

I might replace both hoses tomorrow, and see what that brings. Not a big expense and they are 12 years old.

The water pump is working, because with the radiator cap off, you can see coolant rise and drop in the tank when the engine is revved.

[viggy58]

sounds like a plug somewhere...

i'd drain and flush the whole system, then see what happens. temp cycling from the heat is def a sign of low coolant level. a blockage is the only thing that i can think of that would cause the coolant to overflow when hot.

[7T9Shark]

My next approach will be to do a pressure test of the system to see if there is a leak.

If that's OK, then I"ll replace the upper and lower hoses. It's 20 degrees outside, so I'll have to forego the flush.

If the above steps fail, it's off to the repair shop...

[viggy58]

good luck! please let us know how it goes...

[7T9Shark]

I haven't pressure tested it yet, but two days ago, I topped it off with coolant and let it idle in the garage for 30 min. Temp guage came up to normal and stayed there. So, I got brave and took it out for a drive for a few miles - temp continued to remain normal. Then yesterday I drove it 3miles into town for a hair cut. After cooling down for 20 min. I drove it a few more miles on an errand and the temp guage rose to 260 with no heat coming from the heater.

While this was going on, I would rev the engine off and on and the temp would come back down and the heat would come back. In the mean time the system puked a gallon of coolant out through the overflow tank.

According to troubleshooting info on ALLDATA, this points to a plugged radiator. OK, but if so why isn't it consistent? The tubes look real clean, but I don't see any flow coming through them. That doesn't seem right.

Looks like my next move is to remove the radiator and have it flow/pressure tested.

Anyone else have any ideas?

[Jerry80871852]

A few years back I replace my radiator on my 92 S-10 Blazer, about $150.00 at the parts store for a new one. Mine had a crack in the plastic on the drivers side and was leaking.

On some models its nearly as cheap to replace the radiator as it is to get them worked on if you do your own work, plus you've got a new radiator. On some models it isn't. I thought mine would be the expensive one, for it had the large radiator with towing package, oil cooler, and automatic transmission.

Of course you would only want to do this if you was certain the radiator is the problem. And I'm just telling you this to help you make the best decision.

Sometimes at a wrecking yard you can get a good radiator cheap, yet sometimes it may be worse than your old one.

Hope you get this solved soon.

[7T9Shark]

I apologize for dragging this thread out, but I've never had such a mysterious problem like this before. Plus, it's 12 degrees F. outside (and in my garage) so I am not working on the thing much.

In doing some additional research, one source suggested removing the thermostat and running the engine without it to see if coolant is circulating through the radiator tubes. That's a cheap test, so I think I'll do that one next. If nothing is moving, it might be a non-functioning water pump or dead thermostat.

I plan to save the radiator replacement until I have thoroughly ruled everything else out. Why? because my philosophy is that when troubleshooting problems without known causes, start with the cheapest part (thermostat, which I have already replaced) and move up the cost curve from there.

Thanks for bearing with me. I'll post my "education" results when they become apparent.

AJ

[danielsatur]

Shortage of coolant can cause a false indication of temp to the guage, and cause the thermostat not to open correctly.

[534BC]

It sounds like you have vapor in the system. If it's combustion gas then it can be tested and repaired. If it's just air then it can be a leak somewhere to atmosphere.

[danielsatur]

1) Use 50/50 Mix of water + coolant.
2) Get engine to normal operating Temp with Heat on max.
3) Be careful and use a garden hose with water pressure to force out any air-pockets.

The trick is to find out if the shortage is caused by a leak in the coolant system, or is the shortage caused by air-pockets, causing the over heat condition to loose more coolant.