It's Broke!

My wife loves her 96 S-15 Blazer and isn't yet ready to part with it, but I'm at my wits end. Call me cheap, but I hate spending money on it and it's too darn cold in Michigan to do any serious work on it. Lemme describe the issues...

She came home one night complaining of not having heat in the truck for two days and a "crackling noise" coming from behind the dash. I knew immediately that this was the sound of coolant boiling in the heater core. She said she hadn't noticed a change in her Temperature Gauge though. I took it for a spin, and sure enough, the temp would rise to the standard 195 and there WAS heat for several minutes, until the temp gauge would climb up to about 230, and I wold hear the boiling coolant in the heater core. After a few seconds the temp gauge would plummet to under 120 and the heat would start to flow from the vents again. The temp would then rise again but stop at it's normal temp of 195. Coolant was low in the radiator, but not in the reservoir. After topping it off I drove it to work the next day with no issues and steady heat. When I left for lunch I checked for obvious signs of a coolant leak but didn't find anything of note, but the previous symptoms returned. Again, I checked the coolant, and it was low about 1/4 gallon. I decided to throw down my 8 bucks for a new Thermostat and pop that in, but the symptoms continued.

This led me to check the obvious possible problems...

1. Coolant leak? Possible.... The coolant is boiling from a pressure loss and I am losing fluid, but can't find it, and why would it re-pressurize?
2. Head Gasket? Oil looks good. No burning coolant smell from exhaust, which is clean and not white. No water from the tailpipe.
3. Water pump? Need to check the weep hole more closely but don't know if this would explain the erratic engine temperature.
4. Heater Core? Would explain crappy heat, but not the engine temp.

For a while I wanted it to be the head gasket, just so we could finally get rid of the poor thing, but She still loves it and isn't a fan of the TrailBlazer.
I appreciate any help and advice you gents (and the occasional lady) are willing to offer.

-Wayne

 

Start by replacing the radiator cap with a Stant Safety Cap - 16psi rating. It is possible that the cap is allowing it to boil over due to lack of cooling system pressure (which increases the boiling point if properly maintained).

If you were low on coolant, you may want to try bleeding the system more thoroughly as an air pocket can cause the system to boil over as well.

If head gaskets may be suspected, then pull the plugs. A strong leak will steam clean the plugs, leaving the ceramic white. A compression test can be used for further evidence to an head gasket leak.

 

I had that same problem and I replaced the Temperature Switch and it solved the problem, and Coolent Teperature Sensor. I'm not sure which solved the problem but I know it was solved!!