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Discussion Starter #1
2004 trailblazer with the 4.2 inline engine.

I haven't had a chance to troubleshoot this yet as its my Dad's vehicle and I just got a few minutes to glance at it tonight, but I'm looking for thoughts of what I'm getting into.

Temp gauge reading very hot (not sure exact reading as I haven't driven the thing), doesn't appear to be much if any coolant left in the radiator, doesn't seem to leak or smoke. He mentioned that a few months ago he had to add some coolant because it was also hot then. My folks are semi-observant, but I wouldn't bet on them catching a small leak, so thats definitely a possibility, but I would have thought that if it was actually leaking, it wouldn't make it months. It seems t-stats and sensors are trouble spots, but the fact that there is like no coolant in the radiator and the thing is still hot 3 hours after it was driven lead me towards a leak or consumption issue.

Other than looking at the thing for 5 minutes tonight, I've never even been under the hood of one of these things.

Anyone dealt with one of these before?
 

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I'd suspect a blown head gasket from what you're saying. I can't think of anything else that would cause it to lose coolant, since there's no obvious leaks. Some of the earlier 4.2's had issues with cracked cylinder liners, but if that were the case, you'd hear it - assuming it would even start.

Any visible smoke (whitish) from the tailpipe? If not, first, check the oil and the underside of the filler cap for a milkshake appearance. Then, top off the coolant completely, remove the cap and start the engine. Let it run and observe if coolant is being expelled from the cap - the result of combustion gases being introduced into the cooling system. If any of these signs exist, you got gasket trouble.
 

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The Warbird
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Check the coolant for oil and the oil for coolant, if clean. Could be the water pumps going out and its boiling off when it gets to hot.
 

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Check for a bad radiator cap.
 

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Discussion Starter #5
Update

Talked to mom and dad this morning asking if they'd seen any leaks and they said "well....there was a spot in the back that it had been dripping lately."

Sure enough the rear heater core leaking and dripping out the condensate drain at the back.

Tore it all apart and yes, it does have a small leak and the area all around the rear HVAC unit is wet. Turns out GM revamped how they installed the rear heater core and I have to order several hundred dollar conversion kit to make the updated core work with the older hoses. I've got the parts on order, and they'll be here in a couple hours, but I can't imagine what this conversion kit is going to consist of.

Anyone dealt with this before? I looked for a good way to just bypass the whole thing but didn't find a classy way of doing it.
 

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Just cap it off if you don't want to use it.
 

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Discussion Starter #7
Ok, so the conversion kit is a joke. Two hoses with matching fittings, still had to be spliced into the existing lines with barb fittings and hose clamps. Looking back at it now, we should have just spliced the 2 lines together bypassing the heater and gone on with life, but fixing it right seemed like the correct thing to do. Seems to be working now, but to anyone reading this in the future i'd recommend cutting the ends off the existing lines and connecting them with a barb fitting and hose clamps.
 

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RTCTTFMF
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Blown head gasket.
 

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Discussion Starter #9
Just an update:

This sure appears to have resolved the issue, the trailblazer made it from Ft. Wayne, IN to La Foulette TN and back in 100+ degree outside air temps loaded with a family and pulling a small Uhaul trailer. Temp guage never moved, didn't use any coolant.
 
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