94 Blazer Coolant Temperature Fluctuating

[bowtie-72]

I had a ‘94 S-10 Blazer with the 4.3 Vortec that I bought brand new in Colorado (approximately 5,000 ft. elevation.) Mine would do the same thing when cold to highway operation. It would spike up on its way to operating temperature, enough that it would trip the “check gages” light. Then as soon as it did it would come right back down and settle down at operating temperature no matter what. It seemed like it was worse at elevation. It acted like the thermostat was lazy to open, but when it did it was fine. I took it to the dealer under warranty and they wouldn’t do anything about it. I wanted them to replace the thermostat with a 180* unit and they said that the emissions system would not function right. So I just waited until it ran out of warranty and replaced the thermostat myself with a 180* and didn’t have any more problems with the “check gages” light coming on after that. It would still spike, but not as bad. I never had any problems passing emissions inspections and ran that engine to 315,000 miles with no problems.

 

[Schurkey]

1. Given a full radiator, a properly-filled overflow tank, and a radiator cap that works right, THERE IS NO "AIR" IN THE COOLING SYSTEM on any vehicle where the rad cap is also the highest point of the pressurized cooling system. All the "air" was blown out the rad cap, into the overflow tank; and when the engine cooled, liquid coolant was drawn back into the radiator.

2. The first time or two that the thermostat opened after servicing the cooling system, all the "air" in the engine was blown into the radiator. At most, this is a "low coolant level" which should have been taken care of by the tech doing the cooling-system work.

3. Combustion gasses blown into the water jacket via a popped head gasket or a cracked casting is not "air". It's combustion gasses. And small leaks will get blown out of the radiator, into the overflow tank. This may or may not result in low coolant level. Some combustion-gas leaks are so small that it takes a hundred PSI or more to get gas into the water jacket, but there's only 15-ish PSI in the jacket--no liquid can be lost into the cylinder. The leak is one-way: from the cylinder to the water jacket.

4. If a thermostat is sticking, replace it. But inspect the wire harness from temp sending unit to the gauge--if it's grounding anywhere the gauge will be erratically high. If it's open anywhere, the gauge will be erratically low. Yes, water pump, drive belt, or gauge problems are a possibility.

5. Vehicles new enough to have "stepper-motor" gauges tend to have the gauges driven by the computer. A proper scan tool can communicate with the instrument cluster to test the steppers. Stepper-motors can be erratic; but usually they just fail and quit moving.

 

[bigfutz]

My ‘95 Tahoe with close to 200k started cycling, like the t-stat was opening and closing fully. Aren’t t-stats supposed to be analog, i.e. open just enough, or should it cycle like this?

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[Schurkey]

My ‘95 Tahoe with close to 200k started cycling, like the t-stat was opening and closing fully. Aren’t t-stats supposed to be analog, i.e. open just enough, or should it cycle like this?

IF that's a thermostat problem, and not a sending unit problem, a wiring problem, a gauge problem, or an instrument-cluster grounding problem...yes, the thermostat is faulty. They'll sometimes cycle a little bit as the engine warms up.

It's unlikely to be a radiator/fan problem, low-coolant problem, or a water-pump/drive belt problem. The "temperature" is cycling too quickly. [EDIT] Never mind, that's a time-lapse video. Therefore, any diagnosis is difficult.[/EDIT]

 

[bigfutz]

Sorry [mention]Schurkey [/mention] I should have mentioned the time-lapse thing. I guess it lapsed my mind.

 

[Dirty Bird]

I had a ‘94 S-10 Blazer with the 4.3 Vortec that I bought brand new in Colorado (approximately 5,000 ft. elevation.) Mine would do the same thing when cold to highway operation. It would spike up on its way to operating temperature, enough that it would trip the “check gages” light. Then as soon as it did it would come right back down and settle down at operating temperature no matter what. It seemed like it was worse at elevation. It acted like the thermostat was lazy to open, but when it did it was fine. I took it to the dealer under warranty and they wouldn’t do anything about it. I wanted them to replace the thermostat with a 180* unit and they said that the emissions system would not function right. So I just waited until it ran out of warranty and replaced the thermostat myself with a 180* and didn’t have any more problems with the “check gages” light coming on after that. It would still spike, but not as bad. I never had any problems passing emissions inspections and ran that engine to 315,000 miles with no problems.

 

[Dirty Bird]

Next time you change the thermostat drill a 1/8" hole on either side to let some coolant get to the thermostatic spring on a top side and that will help alleviate the stickiness. If you live in northern climates I would suggest the 195 thermostat as it will make the engine more efficient and produce better heat in the winter, if in the south then I would generally stick with e180 if you were having overheating problems.

 

[Schurkey]

Next time you change the thermostat drill a 1/8" hole on either side to let some coolant get to the thermostatic spring on a top side and that will help alleviate the stickiness.

No thermostat engineer pops bigass stupid holes in their product.

There's no "thermostatic spring" on top. All the thermostat magic is in the bulb on the underside.


Any thermostat with a "hole" is either so tiny that it hardly flows any water (but will pass air after cooling system service) or they put a jiggle valve in it that opens when the engine is off, again to bleed air after cooling system service; but then closes again when the water pump starts building pressure 'n' flow.

The only time a thermostat should be drilled for bypass is when the OEM bypass has been deleted; and there's no full-flow heater system.