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  #1  
Old 09-30-2013, 08:13 AM
ayrtonsenna's Avatar
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1984 300SD coolant pressure

Recently my 300SD began to overheat. After changing the radiator/hoses/tstat/coolant, the car now runs at 80*. When I parked it, I noticed the bypass hose (short hose between the tstat and water pump housings) was leaking pretty profusely from the top. I also noticed the top radiator hose was leaking. The hose clamp had loosened and the hose was about to fall off the radiator nipple. I went to pull it back onto the nipple and it fell off the tstat housing on the other end due to the pressure. Is this normal? Are the hose clamps just shot? (They look to be factory originals.) Is it likely I put too much coolant in? I thought it might be the head gasket but there haven't been any other symptoms of that with the car.

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Old 09-30-2013, 10:20 AM
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The Coolant System Pressure is controlled by the Cap on the Coolant Tank on My year and model.

However, the Cap is not the cause of the pressure it only vents off pressure higher than the Cap is set to hold.

I have never had the Coolant Hose slip off of Vehicle were the Hose was good, the Clamp was good and the surface below/Nipple the Hose went on was good. So to me that is extremely unusual.
Change at least one of the Clamps that that is happening to and see what happens.

Examine the surfaces of the Nipples and look for deep corroded pits.

If you are getting a lot of pressure in the System while the Engine is not overheated it very well could be a Head Gasket problem.
But, Head Gasket Leaks can also show up only when the Engine is Hot.

I think there is only a few causes of Coolant System Pressure; heat expansion; Steam due to overheating, compression/combustion gasses getting into the Coolant System.

Also there is a procedure for burping the Air out of the System; it should be in the DIY Section.
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When I change the Overflow Hose I found the Nipple Pitted and cleaned the pits out down to the bare Metal and filled the pits with JB Weld and wrapped the Nipple with Tape so the JB Weld would not flow out as the pits were all around the Nipples. After that I filed and sanded those areas smooth.

I Filled the Coolant System and went for a drive watching the Temp Gauge. Sure enough My temp spiked upwards and I pulled over and let is sit. When I checked I found the Coolant level low.
The System had burped itself of what ever Air Pocket was in there and after adding Coolant all was well.
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Old 09-30-2013, 03:45 PM
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Running a 7# cap will reduce stress on all the cooling system components- and reduce the likelihood of an untimely hose failure.
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Old 09-30-2013, 07:36 PM
ayrtonsenna's Avatar
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I did switch out the cap, to whatever was available on peachparts or ********az (can't remember which.) the new radiator is a nissens, the hoses are uro, the coolant is zerex g05 mixed 50/50 with distiller water. Could too much coolant in the system cause this? I'm really hoping it's not the head gasket.
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Old 09-30-2013, 08:53 PM
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I hate to say it, but I have seen that symptom on more than one car and it always turns out to be the head gasket.

"Too much coolant" isn't really physically possible. It would puke it out of the overflow after the system pressurized but then everything would be fine. It wouldn't cause coolant to leak from other places.

Do the hoses get very hard/swollen? - this can be an indicator of combustion gas leaking into the coolant passageways
Does the system pressurize like this even when the car is not warmed up yet? - same as above
Does the pressure go away if you let the car sit overnight? - if not, it can be indicative of head gasket or head issues
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Old 09-30-2013, 09:25 PM
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Time for the pinch test

If the hose is hard after the engine is cold = combustion gas in the coolant via head breach.

To be sure pick up a coolant test kit checking for combustion gas in the coolant.

And for the love of Pete, get rid of those URO hoses

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