Part 3 in the ongoing saga of my 98 Mazda 626 2L ATX's cooling system. Here are links to Part 1 and Part 2.

A quick recap. My coolant tends to overflow my expansion reservoir. I've flushed the cooling system and got quite a bit sediment out, some of which was rust and bit's of metal, some of which was black aluminum oxide. I've replaced the water pump because all it's fins rusted off and even pulled one of those fins out of the block's main water passage.

What I've noticed is that if I turn on the A/C I don't have this problem. I just took a three day trip through the mountains and didn't loose a drop of coolant because I was running the A/C the whole time.

So I hooked it up to ForScan to gather some data. The lines are RPM's, Temp and Fan On / Off ( X axis is time in seconds ):

enter image description here

Basically, when the ECU is reading about 97-98 C* is when I start to see the coolant in the expansion container expand. By the time it reaches about 100C* ( 212F* ) the coolant is bubbling and overflowing the container. About the time the fan turns off the coolant recedes rapidly. About 20 seconds after the fan turns off the ECU reads about 92-93C*.

The fan is on for about 35 seconds each time, and about 190 seconds passes before it comes on again.

Now as to why this is all happening. I can think of a few possibilities.

  1. Maybe I've got a partial clog in the engine block water jacket? I've flushed everything backwards and forwards multiple times and water seems to flow everywhere without any problem. I installed a tee on the heater core outlet and pinch off the top radiator hose to force the water backwards through the block with thermostat and lower hose removed.
  2. Slightly off Temp sensor? I've taken out the ECT and tested it and it seems accurate, although I guess it could be off by 5 or 10 degrees and I may not have noticed.
  3. Radiator Cap could be bad, it's old. 1.1 is written on it. I assume that's probably bar. I would think the temperature would need to be much more than 97-98C* before there would be sufficient pressure for it to start opening and letting coolant out.

Two side notes. I'm currently running distilled water, but it was also happening with the green Prestone coolant which is rated up to 126C*.

The dash temperature gauge has gone wonky recently and is usually pegged low or bouncing around randomly, even while the ECU reports a steady, normal temperature.

As a reminder, this doesn't happen when the A/C is on.

I'd really appreciate everyone's input.

PostScript 18/7/2017

So it was in fact the radiator cap. Thanks to zipzit and SolarMike for their great answers!

Here's a new graph with the new radiator cap installed:

enter image description here

Even when the temperature hits 101C* there is no evidence of any movement at all in the expansion container. Also, some other things have changed. The fan stays on for only 24 seconds and 195 seconds pass before it comes back on, which is about a 1:8 On/Off ratio now. And it only takes 13 seconds after fan shutoff for the temperature to get down to 93C*.

  • I guess it won't happen when the A/C is on because on an active A/C the fan is always (AFAIK) turned on. Perhaps you want to change/check the temperature sensor
    – Martin
    Jul 17, 2017 at 12:26
  • @Myself As you'll notice in the body of the question, I took the ECT out and tested it and it seems to be good. The A/C forces the radiator fan to be on all the time, so that's why I don't get the problem when the A/C is on. Jul 17, 2017 at 13:45
  • About the ECT: Doh! Apparently speed reading has drawbacks..
    – Martin
    Jul 17, 2017 at 14:32
  • I suspect restricted coolant flow through the radiator due to plugged tubes. As long as air flow is constant with the fans always on heat transfer can keep up, but if it gets behind it starts to overheat. Jul 17, 2017 at 18:09
  • @FredWilson It seems like flow through the radiator is OK. I've back flushed it with a water hose both via the out hose and the fill cap and it doesn't give the impression of being blocked. Plus, the on / off ratio is almost 1:6, which seems pretty reasonable, isn't it? Jul 17, 2017 at 18:34

1 Answer 1


My guess is bad radiator cap. The cap has two sealing systems. One that holds high pressure (13 to 15 psi). Water at 13 psi doesn't boil until 245 Deg F it so. The other sealing system is the overflow return. As the engine cools down this allows coolant from the overflow to return to the radiator. My guess is that little valve is stuck open.

I'm also guessing you are located at a high altitude causing you to boil off coolant well below 212 Deg F. ( boiling point of water at sea level)

Inexpensive fix.

Edit. At 900 meters of elevation the boiling point of water is 206.2 Deg F / 96.2 Deg C. That's in the normal temp range for your engine. So yes replace cap immediately.

You can probably see the defect in the radiator cap visually. Do note: The way that works is after vehicle cool down, a slight vacuum is formed in the cooling system. That vacuum "sucks" extra coolant out of the overflow bottle. Note: While it takes 15 or 20 minutes to heat up the cooling system, it takes a LONG time for this cool down vacuum return thing to work, generally eight to ten HOURS after vehicle shutoff.

radiator cap with flowback sealing plate

  • Live in the mountains ( Jerusalem, Israel ) about 900 meters above sea level. Jul 17, 2017 at 13:43
  • very nice I think you are right. I'm going to buy a new radiator cap tomorrow and see what happens Jul 17, 2017 at 15:00
  • You were correct in your deduction, and again, great reasoning! Jul 18, 2017 at 16:18

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