I've noticed that my coolant expands quite a bit when I drive around, so much that I'm loosing coolant through the expansion bottle. When I park after a lengthy drive, I'll usually hear the coolant bubbling via the overflow bottle.

When just sitting and idling ( without having driven around ), my engine temperature looks likes this:

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I can sit and idle like this all day without any problem or any coolant overflow. It's only if I drive around for a while that the coolant starts to expand and overflow the bottle, and as long as I keep it topped off there are no overheating problems.

I changed my thermostat not too many months ago, and my heat seems to work OK so I don't think the heater core is clogged ( although I should probably double check that ).

I recently changed the seal on my oil cooler and when I drained the coolant noticed black gritty sediment, but no trace of oil. While I had the oil cooler off, I flushed it and also saw a ton of this black gritty sediment.

I did compression tests a few months ago, and had very good compression on all cylinders, so I don't think it's a blown gasket unless of course I've blown it in the last thousand kilometers or so.

I've blown three different coolant hoses in the past year, and the car is barely driven. It seems to me that excessive pressure is building in the system when driving, but I'm not sure why.

Any suggestions?

98 Mazda 626 2L running premixed Prestone green coolant boilover rated to 259*F/126*C and the engine never gets even close to that temp unless I forget to keep it topped off.

  • Maybe it is the cap?
    – method
    Sep 27, 2016 at 7:51
  • @method Maybe... don't have any way of checking that and I'd like to diagnose the problem if possible before I start throwing parts at it. Sep 27, 2016 at 8:56

1 Answer 1


The first thing I look for when we see overheat at cruise speed but not at idle is poor heat transfer through the radiator. The heat load at idle is much lower at idle than cruise. Fan generated air flow is adequate to cool the radiator at idle. Air flow over the radiator at low cruise speed is not enough to carry away the much higher heat generated at that load. Your temperature graph shows that the fans are on more time than they are off. This also suggests poor radiator heat transfer. At idle expect to see the fans off much longer than they are on in a normal system. The ratio of off to on depends on outside air temperature and humidity. At 75 degrees F and 50% humidity we expect to see 10 to 20 percent on time. On a hot, low humidity day fan on times of up to fifty percent are sometimes seen.

There are two common causes of poor heat transfer. 1) Partially plugged radiator core tubes. 2) poor coolant flow through the radiator. Low air velocity caused by bad fans or blocked air flow are less common.

  • I also noticed that my bottom radiator hose was cool while the top hose was burning hot. So I pulled my thermostat and checked it just in case and it's fine. I stuck the my water hose in the fill neck to back flush it and got allot more of that black sandy sediment coming out from the drain plug. Also noticed that that my heater core hoses were lukwarm so I'm concerned this sediment may be clogging other parts of the cooling system as well. Sep 28, 2016 at 11:29
  • Actually, it turns out that all my impeller blades on the water pump disintegrated. Oct 6, 2016 at 10:04

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