Ight, so, I do know as thing or two about cars, I've seen a good few people on here who don't. My question is this, here recently my Blazer, a 2001 LT trim level, started to run hot during this crazy heart wave. We got up to 104 one day. Driving it more than 5.5 miles, dang thing started rising during the heat of the day. 95° and higher, it'd start rising from 210 (which is where it likes to run during summer days... has it's own thing going), to halfway between 210 and 260. No overheating yet, but definitely running hot. At night, atm it's 74 degree, runs fine, 195°, and I don't have a reason to question the gauges accuracy. Important to note here, I let it sit for a while, it dropped down to a quarter of the way on the gauge during the day, started it up, turned on the heat, blows hot. Once it warmer up, heat quit working, so obviously I added coolant (it isn't sucking from the overflow, so I added directly to the radiator). Over a gallon, actually, but it did spew a lot out from the cap. It had a coolant flush from a gravity fed machine, basic fluid exchange, when I bought it almost a year ago at this point. Now, I added the coolant a week ago, and it's still doing it during stupid hot days, during the heat of the days, but I drove forty miles tonight, a bunch of red lights and a few miles of interstate, with the outside air at 78°, thing ran like a top and never surpassed 200. Y'all have any ideas? Because the cooling system should be able to keep up even when it's stupid hot.

 

Oh and as far as I'm aware there's no stop leak in the rad.

 

Suggest getting a coolant pressure tester from O'Reilly's or AutoZone and finding the leak. Having to add coolant means there is leak, same as not pulling coolant from the reservoir.
I know my '02 has a small radiator leak, left side plastic tank seam; I have a new radiator sitting on the garage floor, just have not been motivated enough to change it out yet.
113°F here yesterday...makes working in the garage not fun.

 

Of course there are people here that don’t know as much about cars, it’s a forum where people come and ask for help, just like you are doing. Then the people that do have knowledge help those in need like yourself. That’s what we do here.

Like Lanny said, you need to find the leak. You can pressure test as he suggests, use a leak dye tracer and light or inspect carefully. The pressure testing of the system will not check the radiator cap So either test separately or replace. Have the LIM gaskets been replaced? Is there coolant in the oil? Is the carpet wet?if you run the defrost does the windshield fog up? Can you smell coolant inside?

Adding coolant to the overflow tank does not go directly into the radiator. When a hot engine cools down and the radiator pressure drops then coolant gets pulled in if needed. Open the cap on a hot radiator very carefully, that’s 15psi at 200F.

George

 

So I am not aware of the Lower Intakes being replaced, and I know those are an issue on the Vortec motors, however I'm leaning away from those, there is no cross contamination between the oil and coolant, and there is no smoke from the exhaust, however... Think a UV dye might survive the extreme environment of the combustion chamber and cat? If so you'd see it in the exhaust. And I'll throw a pressure tester on it here in a few.

 

Oh, also yeah, nah there's no leaks from the heater core, I can rule that one out, no wetness, no coolant smell and extreme prejudice against fog. Whatever leak it has is small, well.... Coolant leak, anyways. Thing leaks oil like a sive, but I'm not ripping out the motor to replace the rear main, oil pan, valve cover, gaskets, etc, when I paid $400 bucks for it. Runs great and has been taken care of, I know the previous owner personally, just knew the situation I was going through with my '94 K1500 that I tried to daily... I'm the idiot who tried to daily a mudtoy so eh.

 

If the coolant in the overflow tank is not returning to the radiator when it cools down, the first thing I would check is the radiator cap. Make sure it's the correct one and that the upper gasket is good.

 

Here's the crazy part, before it got this hot, I've driven it fifty miles at "70" miles an hour on I-270/I-255, thing took it like a champ. Though I did notice some coolant running down the side of the rad when it first started doing it, so I'm thinking possibly a faulty rad cap.

 

The dye is to inspect under the hood. It will only help with LIM gaskets if they are leaking externally at the motor.

A loss of 1 gallon of coolant is a massive leak, that's 1/3 of the cooling system capacity. Be careful, you can do serious engine damage running at highway speeds in 100F weather this low on coolant. Constant loss and refill at that level and your probably have chronic air purge problems with overheating and poor heater core operation. Even in a healthy motor and cooling system, bleeding air can be a little bit of work on these systems. During all of this, weird fluctuations in ECT readings and heater core problems don't mean much. Get this system diagnosed, sealed up, filled, properly purged and then see if you still have overheating problems.

"It did spew a lot from the cap". Start there, a new cap is cheap. Then get that engine up to temp and shut it down. Get under the hood right away and see if you can hear it. That's a big leak.

If you are leaking oil faster that a one gallon loss of coolant then check that dipstick every time your under there.


George

 

Here is an example of what Lanny is talking about:




Here is what I am talking about:


Lanny's suggestion can do one additional thing. With the pressure tester pumped up and the engine running, a leak between the coolant system and one of the cylinders can be detected with a fluctuating gauge. There are other methods for this like testing the coolant and looking for bubbles etc. I digress. As we have said, it might just be just this, cheap experiment ($7):



George