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Old 03-15-2019, 08:24 AM   #1
TempleUWS6
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Default Coolant Over-pressurizing

'79 B21 with an AW55, sustained high rpm on highway running the car hard at 75-80 cause thanks 3speed, coolant will push out from under the cap and makes the whole area wet, even shoots it onto the cooling fan. I flushed it out with distilled and used Zerex G-05 maybe a month or two back. What could be going on here? Hopefully not headgasket related, could it be related to the water pump? Appreciate any input.
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Old 03-15-2019, 09:38 AM   #2
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It's head gasket related most likely.
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Old 03-15-2019, 10:13 AM   #3
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Yep. You could diagnose with a combustion gas checker (free-rental at Autozone, but you have to buy the fluid).

But really, given the symptoms, I'd just go ahead and buy a head gasket kit and reserve a weekend day to swap it.

And consider putting a TB/WP on it at the same time unless that stuff's pretty new.
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Old 03-15-2019, 11:28 AM   #4
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Not sure if this is a valid pre-test, but the way I've done it before is to feel how fast the radiator hose gets turgid from cold start-up. Pressure from exhaust into coolant sometimes develops faster than from an overheating that might occur from restricted passages in the head, thermostat, or radiator. I think... Admit to having little experience here.

Once I drove around with a turbo gauge teed into the coolant expansion tank overflow hose; that trouble was coolant loss past a dissolved water pump gasket.
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Old 03-15-2019, 12:30 PM   #5
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OP you don't mention if it is overheating.

Decades ago the radiators would plug up.

Modern aluminum core radiator gets more heat out then copper and brass.

I use a leak down tester to find which hole is burning coolant. One can usually see it on the spark plug for that hole. Can't miss it.
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Old 03-15-2019, 12:39 PM   #6
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You could just have a bad cap. It's normal for the coolant to expand with temp. If the cap doesn't hold pressure it will eventually expand enough to overflow the reservoir.
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Old 03-15-2019, 02:24 PM   #7
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A proper coolant recovery bottle and a good sealing cap, it is best to not allow any air into the cooling system.
Foamy coolant will not transfer heat well, any air bubbles in the head will make hot spots which make steam pockets which will push out more water and overheat.

If it pushes out water without overheating a combustion leak is likely.

A rotted off or fallen off water pump propeller will make it overheat first.
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Old 03-15-2019, 02:38 PM   #8
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Quote:
Originally Posted by VB242 View Post
It's head gasket related most likely.

Rent the test kit from AutoZone to confirm.
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Old 03-15-2019, 02:54 PM   #9
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Try a new cap first. If the old cap smells like exhaust then it's probably a hg and possibly a corroded out bcp head. My friend and I bother have 79 b21 cars, well mine was before I swapped it. Both his and mine had screwed up bcp heads. Is the coolant tank swollen up?
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Old 03-15-2019, 05:33 PM   #10
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Quote:
Originally Posted by JohnLane View Post
OP you don't mention if it is overheating.

Decades ago the radiators would plug up.

Modern aluminum core radiator gets more heat out then copper and brass.
False
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Old 03-15-2019, 07:26 PM   #11
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Quote:
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False
Depends. A two core copper wouldn't do as well as the same thickness aluminum. A three core copper would likely do about the same.

Copper has a higher heat transfer coefficient than aluminum, but the modern design of the aluminum radiators usually render the better heat transfer coefficient moot.

Modern microchannel heat exchanger aluminum radiators are hard to beat. They have a lot more surface area inside the tubes in contact with the fluid. The old school copper radiators have much larger passages - and so much less fluid in contact with the copper itself. That's why when you wanted more cooling, you would have the radiator re-cored to a three or four core. More cores = more fluid in contact with the heat exchanger = higher heat transfer.

So like most things, it depends.

In this case, it really sounds like either a bad cap or a bad head gasket. Like others have said, you can rent (for free usually) a test kit. Sometimes they give false positives or negatives - its not a perfect science. If it comes up negative (or even if its positive), a cap is a cheap thing to try. If its barfing out of the cap (and doesn't show that its overheating), it either isn't holding pressure or is venting excess.
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Old 03-15-2019, 08:21 PM   #12
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With the exact same symptoms my problem was simply that the radiator was plugged up to the point that it was not adequately cooled.

Showed up on the temp gauge.

A $10 infrared temp gauge is your friend.

https://www.ebay.com/itm/Nice-Non-Co...4383.l4275.c10
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Old 03-15-2019, 11:50 PM   #13
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Just add a 150 shot and send it.
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Old 03-16-2019, 10:01 PM   #14
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You could also pull plugs and see if one is shiny clean from water getting in there. Any oil in the coolant? Water coming out the exhaust?
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Old 03-20-2019, 06:59 AM   #15
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and further to all this, when the HG is completely blown, when cranking the engine you can feel the pressure pulses on the top (hot) radiator hose. When that happens, she's dead Jim........
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Old 03-20-2019, 08:57 PM   #16
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If your cooling system is working correctly, freeway driving will not cause it to overheat. We have run my track car for 24 hours straight at high RPM without any overheating.

At a race a few weeks ago we blew a head gasket. The driver was aware enough to cut power and limp it in. There was just a slight bend in the gasket on the number 4 cylinder which was enough to let coolant into the cylinder.

With the coolant cap off, water would shoot out of the expansion tank. Steam was coming out the exhaust.

On other race cars, a bad cap will not let the system pressurize correctly and the car would overheat and lose fluid. We would sometimes put the overflow line pointed at the windshield so our drivers would notice if the car was burping coolant.

Check the cap first. See if there is bubbling in the coolant tank. Check compression if you can.
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Old 03-21-2019, 12:26 AM   #17
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Quote:
Originally Posted by rb92673 View Post
If your cooling system is working correctly, freeway driving will not cause it to overheat. We have run my track car for 24 hours straight at high RPM without any overheating.

There was just a slight bend in the gasket on the number 4 cylinder which was enough to let coolant into the cylinder.
Holy crap, that is impressive.

I have also seen a redblock HG fail in the same manner, slight bend on #4. It would come and go, sometimes it would misfire, sometimes not. Usually once the thermostat opened, coolant would spurt from the overflow. Oddly enough, the warm motor closed up the fire ring and it ran fine warm, but still overpressurized the system. I replaced it anyway of course.

Compression was perfect all across the board, so you most certainly cannot always tell from a compression test. Leak down is a better idea, or those cooling system combustion chamber tests like others have mentioned.
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Old 03-21-2019, 10:36 AM   #18
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Holy crap, that is impressive.

I have also seen a redblock HG fail in the same manner, slight bend on #4. It would come and go, sometimes it would misfire, sometimes not. Usually once the thermostat opened, coolant would spurt from the overflow. Oddly enough, the warm motor closed up the fire ring and it ran fine warm, but still overpressurized the system. I replaced it anyway of course.

Compression was perfect all across the board, so you most certainly cannot always tell from a compression test. Leak down is a better idea, or those cooling system combustion chamber tests like others have mentioned.
We have run two 24s and two other 14 hour races across two days. We have had other problems, but never a problem with heat. Compression was 90, 140, 140, 140 when the head gasket went.

We have seen lots of overheating on other race cars for bad caps, leaks or air pockets in the cooling system.
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