I'm looking at a 98 Mazda 626 2 liter with an unknown coolant leak from the area in the back of the engine. Seller says a mechanic that quickly looked it over said he thinks it's either the head or the freeze plugs.

A quick history, my exam of the car and then a pic. He was driving and noticed the car was starting to heat up, pulled over and saw he was loosing coolant. Immediately took it into a shop and they saw one of the heater core hoses was leaking and replaced it. He went back out and started driving, but saw he was loosing coolant again ( quite quickly ) and it was starting to heat up again. Had it towed to a shop and they gave him the above report.

I came to look at it and started filling the rad. Noticed that it started immediately giving a light drip near the back of the engine ( I could just barely reach the spot and it was clean water ). Ran the engine a few minutes and the speed of the leak increased significantly.

Not sure what his mechanic meant by that it could be the head or plugs. There doesn't appear to be any water getting into the oil ( dipstick and under the oil fill cap didn't have anything frothy ).

That pipe is the exhaust pipe. The leak is presenting slightly left of engine rear center towards the drivers side.

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IF it's the plugs, I have a theory that it's due to a sticking thermostat. If it was sticking closed that would explain why one leak developed right after another, and it sticking open would explain why I saw water start leaking way back there as soon as I started filling the rad? Could it stick closed then stick open like that?

What do you guys think?

2 Answers 2


In the end I found two problems. The first was a unusually placed coolant hose which on this engine tee's off from the heater core hose and goes behind the engine under the intake manifold and connects to the base of the oil filter. This hose had sprung a serious leak.

As I suspected the culprit seems to have been a thermostat that was stuck closed. You can see the test / comparison I on it to check it in the linked video.

I also did a vinegar flush on the cooling system because it was really full of rust.

So far I haven't noticed any leaks.


The thermostat can certainly stick open or closed, but I don't think it'd be the cause of any of your problems. The thermostat only bypasses the the upper radiator hose through which hot coolant from the engine flows to the radiator to be cooled so your engine reaches normal operating temp faster. The lower radiator hose, through which the cooled coolant flows to the engine, is always open to the system. When filling the radiator, coolant will be gravity fed through the lower rad hose, through the water pump, and into the block whether the thermostat is open or closed.

The plugs are metal discs hammered into holes in the bottom of the block. They're fairly weak, designed to blow out if your coolant freezes for whatever reason rather than the expanding ice cracking your block, but sometimes they also just rust out. There's also usually a coolant drain plug or two on the block - a bolt just like the oil plug. If there is, make sure that sucker's tight.

Your head may be cracked, or you could have a blown head gasket. Sometimes catastrophic head failures don't contaminate the oil (which does indeed produce a frothy substance as you describe); the head could be cracked from the outside into a coolant passage, or, rarely and usually only in poorly manufactured or improperly installed gaskets, the head gasket can blow outward from a coolant passage.

Good luck, and I hope it turns out to be a cheap fix for you!

  • The thermostat and a blown coolant hose running under the intake manifold turned out to be the problem. Sep 23, 2015 at 19:18
  • Excellent! Glad it indeed ended up being a cheap fix.
    – user1103
    Sep 24, 2015 at 11:21

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