Ok just a question 91 mustang gt.Did a pressure test on my radiator and coolant system, while cold I put about 16 lbs. Of pressure on it, it would slowly dropabout a pound every five minutes. Then I released all the pressure started the car and pressure slowly built to 15 psi , it took about 15 mins. Of running the car.i turned the car off. Now when it was hot I was losing about a half pound of pressure per minute. So is this normal should it loose pressure so quick.
Coolant System Pressure Check
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Depends on where it's losing pressure. When you did the cold pressure test did the radiator inlet remain perfectly leak-free? How do you know? If coolant leakage is occurring somewhere, you should both see the juice, and experience some pressure loss, but radiator level has to drop either way. Loss of vaporized coolant still = coolant loss.
How did you observe the pressure loss while hot? imp
Both tests done with radiator pressure test kit, I'm honestly only assuming the neck of the radiator was sealed during the test. As far as the hot test also done with pressure test kit I started car it warmed up for 15 mins and had about 15 psi of pressure on the gauge.then I cut the car off and the gauge was dropping about a half pound of pressure per minute.i have never done tests like this before so I'm wondering if it should drop that quick, I'm concerned I may have a blown head gasket is the reason for the test.im gonna do a compression check tomorrow when I get one. Just trying to figure out if this is normal or not thanks for any help
Built-up cooling system pressure varies with the coolant temperature, it's called "vapor pressure". In a cooling system having no "expansion room" in the radiator, the rad. cap, having 2 seals, allows liquid to escape at the cap's set pressure level, typically 16 psi, and the liquid is "caught" in an overflow "bottle" which is vented to the atmosphere. The 2nd. seal on the cap seals the very top of the rad. fill neck, to prevent loss of liquid, AND to allow the shrinking liquid within the cooling system after the engine is shut down, to be drawn back out of the bottle into the radiator, which thus remains full at all times, hot or cold. You likely know all this already; I have no way to tell that.
If you are reasonably sure the pressure loss is not occurring due to a faulty rad. cap., and that the pressure-testing device itself did not leak at the connection to the rad., then, no, pressure loss of any amount is not normal. A failed head gasket, depending on severity, usually causes some degree of overheating, especially if the combustion gas leakage is finding it's way to the water jacket. If the leak is to the outside of the engine, or to an adjacent cylinder, overheating is much less likely, and no coolant will be lost, nor will cooling system pressure drop. This is, however, theoretical hocus-pocus, explaining what CAN or MAY be happening.
Does the engine run hotter than normal? Is coolant being lost? A good test is widely available to detect combustion gas in the coolant vapor either in the top radiator tank, or the catch bottle. I would do that test before tearing the head(s) off, based on answers to the two questions above. A very seriously leaking head gasket can cause a drop in cranking compression pressure, a useful check IF you know one or more cylinders did not already have low compression. imp
Thank you for the help. But to answer the questions, the car does not get over 210 degrees with a 195 thermostat, there has been very little coolant loss but also I just replaced the radiator with a new 3 core aluminum so I was thinking maybe it was the system getting the excess air out, the pressure tester I'm using goes where the radiator cap would go so that rules out the cap, the tester sits down inside the fill neck with a round rubber seal, so I don't think the tester is losing pressure I also poured water in the fill neck after I put the gauge and tester on to see if there were air bubbles coming out but there were none.so all of this is telling me I'm losing pressure somewhere,all the hoses are in good shape clamps are tight no visual sighns of leaking.so like I said I will try a compression check and look into the test kit you described I'm assuming I can get one from advance or auto zone, I'll check the plugs when I do compression test.And again thanks for your time and suggestions.
Coolent leaks can be sneeky, pull the carpet back under the heat/ac box and see if it's wet/sticky. You can also put a large piece of cardboard under the engine, big enought to cover the engine
You seem to have covered the bases pretty well. Sorry if my wording may seem a bit offensive, one never knows the level of background of the listener in this faceless society, so I try to explain things as I see them, not always right, though! The sniff test detects hydrocarbons in the coolant, which normally has only some type of Glycol (an alcohol), water, and soapy additives. I imagine most reputable auto parts suppliers have them either for sale, or rent. Good luck! imp
When juices leak out! Ahhh.......Put new thermostat in my 351W F-150, seems to lose a tiny bit during use, but never enough to see even a damp spot. Suspect very small vapor leak, only at operating pressure, either housing gasket, or bypass hose. Has lost maybe 2 cups in 6 months. Very small leaks occurring at higher temp. than the boiling point of the juice go away as gas, leaving little or no evidence. imp
Appreciate that! I've been called snotty, know it all, "some fu#$ing engineer', etc., when I really am only interested in helping where I can, and occasionally getting some help from others, too! imp
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