Tom,
Thanks for the reply. Yeah, my model doesn't come with a cabin air filter and the vehicle isn't exposed to any harsh elements such as pine needles. I may have some dust and dirt in there though. Who knows what could get in those diverter doors? I wonder if I've still got some air in the system? I thought for sure I found the problem when the reservoir was blocked but soon after the cold/lukewarm returned. There is a threaded bolt in the thermostat housing that has me wondering. Is that for bleeding the system or what is it for? I took the reservoir out the other day to clean it out. Would that constitute bleeding the system again to rid the system of air?

That plug in t-stat housing could be for a sensor you don't have. It's after the t-stat so really shouldn't be the spot to let air out but if you want to try, go for it. If you take radiator cap off it should be full to spilling if it's been a while since filled properly. If there's still room to add coolant there fill it up as it should purge out to recovery tank and only be able to draw liquid back in - self bleeding in time.

What engine is in this? I have a 5.7 myself and it's not fussy about an air lock. Do check the cap and level again. The cap should maintain pressure when warmed and you can feel that in upper hose right there. When it cools the little dime flapper under cap opens and allows fluid back in. I'm guilty as all hell for not replacing caps as much as I should and they do cause problems. I clean them all the time though and use silicone grease on the rubber parts (two) so they stay working a long time.

This could be the trouble and shame on me for not thinking of it!!!!

T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/

My engine is the 4.3 litre V6 Vortec. 231 cu. in. I think. I just replaced the cap when I replaced the water pump and heater hoses. Seems to have plenty of pressure on upper hose.

Ok: That engine is a 5.7 with 2 cyl cut off - really! If rad is staying full that isn't it. It still could hold pressure but not seal at the unsprung gasket and purge out air or coolant there and not pull back just coolant. I'm still thinking

T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/

A leak at the thermostat gasket on the housing? Or a bad t-stat?