NEED HELP Trailblazer coolant bubbling, no coolant flow?

[JPutnam]

I noticed last week that the overflow tank is bubbling a few minutes after the engine is shut off. It doesn't seem to do it while the truck is running and the gauge shows the temp is a tick past center. I hooked up my gauge and it shows between 210 and 220. It does get to 230 occasionally when pushed hard, but drops back down when let off.

Some history:
Bought the vehicle second hand from father-in-law. Just reached 200k so am doing a full service on the vehicle. The upper hose was replaced prior to my ownership because it was leaking and was topped of with green coolant by mechanic who was old school and said not to use Dexcool. Temp shows fine and heater blow hot when warmed. I decided to flush the old coolant out because it was a mix of green and dex and filled it with water to flush the system. When the vehicle was to temp I noticed the upper hose was hot to the touch and the low hose was cool. There also does not seem to be any flow though the hoses. I disconnected the upper hose and let the engine run for a few seconds and no water flowed, it just seeped up from what I guess was expansion from the heat.

Is there any way to test to see if the pump is working? Shouldn't there be some flow even with the thermostat closed? I know that on my SBC I could tell that the pump was working by disconnecting one of the heater hoses which bypassed the thermostat. I would just take it out all together to eliminate that from the equation but that doesn't seem to be an option on these engines.

Oh, there might also be a problem with the fan. It doesn't seem to kick on full when the coolant is bubbling like my last Trailblazer did. But that might also be because the ECT shows normal temps. Could it actually be showing cool because the water in the hose near the thermostat is cool? The sensor is in the same pocket as thermostat, but I would think the water inside the engine would be hot in there if the water was not flowing.

I do have a spare engine that has a ECT I can use that I know works and have a pump in that engine, but I don't have the tools to get the stupid clutch off. I also have a new thermostat for that engine, but it is a Duralast one and I have read that only the Delco ones should be used.

 

[Mooseman]

Please use the edit button if you need to add more info instead of posting consecutively. Thanks.

You might as well change the t-stat. Delco is preferred but any good brand should work. I found the temperature with the Delco higher (better) than the Stant.

It might be bubbling because you are using just straight water. When stopped, the engine is heat soaking the water and probably creating steam, which has to go somewhere. It creates pressure and the rad cap releases it into the reservoir.

Water pumps just work. They usually fail from the bearing or the seal but will still work.

You can check the fan clutch as per this thread:
How to test the electro-viscous fan clutch

 

[JPutnam]

Will do with the edit, thank for fixing my post.

I did notice it bubbling before, but it did take longer to happen. There could be some air trapped in it though causing the bubbling, but not sure. Shouldn't the water be flowing out of the upper hose if it is disconnected from the radiator? I know I disconnected the one on my Tahoe and it did not flow at all when the pump was dead but worked like a garden hose when I replaced the pump. Even with the thermostat closed there was some flow out the hose.

 

[Mooseman]

Although I haven't seen it happen on our platform, it is possible the impeller on the pump either broke or rusted off. If that would have happened, it would have overheated quite quickly and there would be no heat. I would assume more an issue with the t-stat or possibly the rad is still blocked from the Dex-green coolant mix.

For the bubbles, it could be a leaky rad cap or loose overflow hose allowing air in during the cooldown phase.

 

[Tiggerr]

I'd defiantly get some "flush".... a couple bottles worth...and re flush it... that dexcool does NOT play nice with regular antifreeze...

 

[KNBlazer]

Not to alarm you, but on hot days I would have that issue... after shutting off the truck, I would hear the bubbling....now I no longer have that issue... the repair that was done...Cylinder head gasket blew, and engine got rebuilt.... I think there's a test you can do, is it a leakdown test? Also do a compression test... cheaper if you fix it now rather that allow it to cause additional damage...I had done a compression test about a year prior and the cylinder that ended up eventually triggering a misfire was the low compression bore...

 

[JPutnam]

Might be a gasket, but I hope not. I tried to replace the head gasket in another trailblazer and the front two head bolts broke off deep in the block. Still cant get them out. I am avoiding that route if all possible.

Last night I flushed the system with water and dumped one bottle of the Prestone flush in and ran it. Top hose warn, bottom cool, hot air out the heater, guage at 210, and still seems to be no flow. I took off the top radiator hose while it was warm, cranked the engine, and water just trickled out the top hose again. I also noticed there was a "beer foam: building and coming out the radiator. I was thinking blown head gasket or just from the flush stuff.

Today I replaced the thermostat (real fun time there, but not that bad) because the old one was stuck close. Boiled it in a pot of water and it did not open. Boiled the new one just to be safe and it worked fine. Cranked the engine and I am having the same symptoms as before. Tomorrow I am going to swap the pump with a known good one and see what happens, if I get the fan clutch off. Counter clockwise to take it off right? That is what the threads look like on here. Want to make sure before I start the bloodshed!

 

[Mooseman]

That is correct, counter clockwise looking at the front. If having trouble getting it off, use a chain as per this how-to:
http://gmtnation.com/forums/threads/how-to-replace-the-water-pump.15639/

I also like to use the air hammer (aka: zip gun) one the flats of the hex in the direction to unscrew but it does mar the metal.