Question about Chevrolet Blazer

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My exe's 98 blazer overheated on the way home last week, replaced thermostat on the side of the road, no help. She took it to a repair shop they told her they did a CO check of the coolant and she had a bad head gasket, our son changed the gaskets, refilled the system and it still overheats. It appears there is not enough circulation, the heater hoses do not get hot like there is no coolant going through them, there is circulation through the upper hose but it seems like it's boiling and bubbling by the time the thermostat opens. Is it possible the thermostat is stuck open? Our son replaced the water pump about 3 weeks ago. Is it possible we just didn't get all the air out of the system? Thanks

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  • bolwin7 Oct 05, 2010

    Thanks for all the help, believe it or not the new water pump was the culprit. Took off the water pump and found the impeller not turning with the shaft, needless to say my son took it back to O'rielly and got another new one. Not too sure about the quality of some of these discount parts.

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Assuming the following;
The head gaskets were replaced properly
The Water Pump was replaced properly
The Thermostat was replaced properly

There are only a few other things to check.
1) if you you remove one of the heater hoses from the engine bib during a cold filling from the radiator - it will release trapped air until you see the coolant coming out the motor side - this works great to ensure the air is out. Hook hose back up before running.(or you could leave the hose off for a second and start it to see if water gushes out - water pump check - kinda messy though)

Remember that heater water will circulate even with the thermostat is closed.

if that does not work, then you need to ensure the top hose going to the radiator warms up indicating the thermostat is opening and hot engine water is starting to dump into the radiator.

2) the radiator more than likely is clogged and needs cleaning by a radiator shop, or replacement if it is not one that can be cleaned.

Ask your son if he remembers looking into the water ports where he took off the old water pump, sometimes of the system is not flushed at proper intervals, it will sludge up the water jackets and keep water from circulating through the motor.

one last thing (gulp) That CO check is pretty accurate - and cheap. You might want to have it checked again - because the heads should have been inspected for cracks as well as pressure checked before putting them back on - especially since the motor overheated. That alone will crack the heads beyond repair - more so than not.

Let me know if you have any other questions.

Good Luck.

Posted on Oct 03, 2010

  • 4 more comments 
  • bolwin7 Oct 03, 2010

    I verified the head gaskets and water pump were put on correctly, the heads were boiled out and checked for cracks as well as planed. I'm wondering if the water pump even though new is defective somehow. Funny thing is there is water circulating through the upper hose, and intermittently water will circulate through the heater core. Is it possible like you say to have a passage behind the water pump clogged and if so can it be cleaned out using a flush tee and garden hose?

  • R M
    R M Oct 04, 2010

    Yes and no - you would need good volume and pressure. You haven't mentioned anything about the radiator - or the second CO check. Remember they found CO in the coolant. Yet you replaced everything that would cause hot combustion gases to get into the water jackets. I'll wager CO is still present.

    It is highly unlikely that the water pump is not functioning. They have the impeller pressed onto the other side of the driven shaft - where the pump fails is a bad shaft seal, causing it to leak. The impeller coming off I haven't ever seen except in a Honda prelude, and it was the drive pulley that came off.

    I have an idea - only if you are sure the radiator is fine. You could do a leak-down test on every cylinder if you have an air compressor and the spark plug fitting to air hose fitting - Napa would have that.
    If you pressurize each cylinder (about 50psi) on the combustion stroke(make sure the radiator cap is OFF before doing this!). Once the air pushes the cylinder down, both valves will be closed - see if any air makes it into the cooling system - you can tell allot from this, like if your intake valve is bent or not seating, you will have air rushing out of the intake, and if the exhaust is bent or not seating, air will come out the tail pipe - same thing for a cracked block, head, or bad gasket. Remember there will be SOME leakage around the piston rings and even the valves, but after you do a couple cylinders, you will get know whats normal and whats not.

  • bolwin7 Oct 05, 2010

    Thanks for the help, believe it or not the 3 week old water pump was the problem, my son removed it and found the impeller not turning with the shaft. He took it back to O'Rielly and they gave him a new one. Not too sure about some of these discount parts.

  • R M
    R M Oct 05, 2010

    Well, I just learned something new, in 30 years, I've never seen a water pump impeller come loose like that. I'll wager Chinese junk is the culprit?
    Great diagnostics skills there. :)

    Take Care

  • R M
    R M Oct 06, 2010

    You might want to make sure there are no obstructions or interference with the new pump when you go to install it. Wow, amazing.

  • bolwin7 Oct 06, 2010

    Nope no obstructions works as advertised now.

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