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I have a 2006 Mazda 3 s with 167,000 miles. The engine is overheating and the heater stops blowing warm air soon after starting the car. I replaced the thermostat to no avail. I took it to a radiator shop and they want $2,100 to re-machine the cylinder head and replace the head gasket. I'm not sure that addresses the problem. Can someone here identify what the problem is?

Rough timeline:

2 years ago: Noticed some coolant leakage. Topped up as needed.

1 year ago: Engine temperature spiked several times. Pulled over when I noticed it.

1 year ago: Engine was loosing more coolant. I tightened a hose that seemed to be leaking. Also added a Bars Leaks Radiator Stop Leak and topped off. Leak went away.

Last 1 month: Heater blows warm for about 6 minutes then goes cold. About that time the engine really heats up. I watch the coolant temperature on my OBDII device and pull over if it gets to about 230 degrees F, which happens after about 10 minutes.

2 days ago: I replaced the thermostat, replaced several pipes, flushed water through the system, and added new coolant. Did not fix the problem.

Today: Problem is worse than before. I had to stop every two minutes to let the engine cool down. Heater worked less. I felt the lower radiator pipe with my hand. Near the radiator it was cool but a few inches up near the engine block it was hot. I also confirmed that the radiator fan was running. I took it to a radiator mechanic and they did a test to determine if exhaust gasses were in the radiator fluid. The test came back positive. They told me the head gasket may be broken or the cylinder head could be warped and needs to be machined for lots of $$$.

Possible Causes

Based on my research, here are some things I think are possible causes:

  • Blocked radiator
  • Broken water pump
  • Bad cylinder head leaking gasses?

Questions:

  • Do these facts point to a likely cause of the overheating? Or, is there a test I can do to determine the problem?
  • Does the mechanic's test, which found exhaust gasses in my coolant, really suggest that I need my cylinder head rebuilt?
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    If you trust the mechanic that did the coolant analysis, then I would suggest "yes" - you have a serious problem with a head gasket failure. No amount of coolant system repairs will fix this. Try running the vehicle with the radiator cap off (stationary, idle) and look for bubbles in the neck. It's a sure sign of head gasket failure. – SteveRacer Nov 13 '18 at 8:06
  • I ran the vehicle with the reservoir cap unscrewed and did in fact see bubbles come up. – Calvin Nov 14 '18 at 4:44
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In addition to what SteveRacer noted about the coolant analysis, you have all the symptoms of interrupted coolant flow. Usually its a broken water pump but in your case it could also be a clogged cooling system from driving with a leaky head gasket for a prolonged period of time.

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