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Thread: Excessive coolant pressure?

  1. #1
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    Excessive coolant pressure?

    1997 318is m44 engine
    I notice that the upper and lower coolant hoses seem excessively hard and the coolant system still has a lot of pressure in it after sitting overnight. The engine runs fine with no overheat, no milkshake in the oil, no white smoke from exhaust, and no loss of coolant. Does anyone else think head gasket?

  2. #2
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    If anything, wouldn't a bad head gasket cause you to lose pressure?


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    It's possible to have exhaust gases go into the coolant system, but what kind of pressure are you talking (in psi or bar)?

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    Maybe you should try replacing the coolant cap.

    So in the morning with the engine cold, you remove the coolant cap and a lot gases are released?? It's normal for a little pressure to be released, just means the system is well sealed.
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    Maybe you should not do that.

    Pressure is a function of heat, and is perfectly normal. No pressure is worse than pressure. If the coolant is pressurized, it will not boil when it gets to 212F. Your car will not build more pressure than it can stand. If there is a leak, then the coolant could boil at 212F, which is well below the normal operating temperature. As the coolant boils off, the temp rises. This is a problem. As the temp rises, the steam in the system is the pressure that vents to the outside that you observe coming out from around the wheel wells. If you have no steam, you have no problem.

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    I don't know an exact pressure reading, but the upper and lower hoses get almost rock solid. If I remove the cap the next day, there appears to be excessive residual pressure retained. I tried opening it 1 time after sitting for 12 hours and the coolant began spewing out before the cap was completely unthreaded. Some other things I noticed, it began to overheat 1 day while idling with the a/c on. When I revved the engine, the temp went right back to normal and didn't give any problem since. I also notice that at idle, there is no heat. And to make matters worse, the heater core started leaking

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    Quote Originally Posted by pauliescff View Post
    I don't know an exact pressure reading, but the upper and lower hoses get almost rock solid. If I remove the cap the next day, there appears to be excessive residual pressure retained. I tried opening it 1 time after sitting for 12 hours and the coolant began spewing out before the cap was completely unthreaded. Some other things I noticed, it began to overheat 1 day while idling with the a/c on. When I revved the engine, the temp went right back to normal and didn't give any problem since. I also notice that at idle, there is no heat. And to make matters worse, the heater core started leaking
    You need to bleed the coolant system. Hopefully there is no damage. The heater core leak should be fixed first though.

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    Quote Originally Posted by Auto Parts Guy View Post
    You need to bleed the coolant system. Hopefully there is no damage. The heater core leak should be fixed first though.
    I have bled it numerous times over the course of a month or so. It didn't overheat to the point of steaming or coolant loss. It approached the red and I revved the engine right away and it went back to normal temp. The fan is working too

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    If your heater core is leaking, then you're losing coolant. If you're losing coolant from the heater core then most likely you have air in the system. You could also, in combination, have a bad water pump (blade(s) broken off the impeller).

    Are you using a 50/50 (or close) mixture of coolant & distilled water?

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    Quote Originally Posted by Auto Parts Guy View Post
    If your heater core is leaking, then you're losing coolant. If you're losing coolant from the heater core then most likely you have air in the system. You could also, in combination, have a bad water pump (blade(s) broken off the impeller).

    Are you using a 50/50 (or close) mixture of coolant & distilled water?
    I am using 50/50 mix. The heater core wasn't leaking before though. I was not losing or having to add any coolant at all. The heater core leak is something new

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    So here's an update. I did a combustion leak test with the lisle tester. It seemed to be ok, until the coolant started puking out and I think contaminating the tester. It was literally spitting out the expansion tank. I pulled the thermostat and there was none, I also pulled the water pump and 2 things: the plastic impeller had separated and the metal surrounding the shaft had broken off and gotten stuck in the block. So, I'm going to try a new thermostat and pump

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    Wait, what? Combustion tester? You mean compression test with a lisle tester. If you had coolant gush from the exspansion tank from a cylinder compression test than your head gasket or and your head is bad in the least. The other things probably caused an overheat that happened while you werent paying attention. The only way for any pressure to get into the cooling system witg a broken water pump is a cracked head and bad head gasket. No buts about it.
    Nobody would recertify these machines after somebody screwed with them without any visibility into what they did.

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  13. #13
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    Quote Originally Posted by spyderg0d View Post
    Wait, what? Combustion tester? You mean compression test with a lisle tester. If you had coolant gush from the exspansion tank from a cylinder compression test than your head gasket or and your head is bad in the least. The other things probably caused an overheat that happened while you werent paying attention. The only way for any pressure to get into the cooling system witg a broken water pump is a cracked head and bad head gasket. No buts about it.
    I think he meant a coolant pressure tester.

    Quote Originally Posted by pauliescff View Post
    So here's an update. I did a combustion leak test with the lisle tester. It seemed to be ok, until the coolant started puking out and I think contaminating the tester. It was literally spitting out the expansion tank. I pulled the thermostat and there was none, I also pulled the water pump and 2 things: the plastic impeller had separated and the metal surrounding the shaft had broken off and gotten stuck in the block. So, I'm going to try a new thermostat and pump
    I've seen the broken water pump impeller before. Glad you found it. Should be an easy fix. The heater core not so much.

  14. #14
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    Quote Originally Posted by spyderg0d View Post
    Wait, what? Combustion tester? You mean compression test with a lisle tester. If you had coolant gush from the exspansion tank from a cylinder compression test than your head gasket or and your head is bad in the least. The other things probably caused an overheat that happened while you werent paying attention. The only way for any pressure to get into the cooling system witg a broken water pump is a cracked head and bad head gasket. No buts about it.
    A combustion leak detector is what I used. Not a compression tester. It detects gases of combustion.

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    IMG_3233.jpgIMG_3234.jpgSo the 1st picture is checking for combustion gasses. The 2nd pic is the old waterpump. I replaced the waterpump and thermostat. And all seems to be good. Combustion check determined no head gasket leak.

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    Thanks for the pics and good luck with the HG
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