expansion bottle. pressure cap queery?

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al-one-arm

al-one-arm

Well-Known Member
Posts
409
Location
burnham-on-sea,somerset
hi all
seeing as you are all well experienced in water problems, i thought that you would be the boys to ask.
my friend, has a nissan 300zx, temperature gauges stay at 82 c, top hose pressurized, no water ever in the expansion bottle?
changed pressure cap, checked hole in pipe , in top of radiator, clear.
checked pipe to expansion bottle, clear.
checked expansion bottle for leaks. ok.none.
touch , pressure cap, water sprays out from around top of cap.(same as it did with old cap on)
surely the water should be going into the expansion bottle, if its getting past the pressure seal?
any ideas?
he has pressure tested for head gasket failure, test liquid remained blue
 
al-one-arm said:
hi all
seeing as you are all well experienced in water problems, i thought that you would be the boys to ask.
my friend, has a nissan 300zx, temperature gauges stay at 82 c, top hose pressurized, no water ever in the expansion bottle?
changed pressure cap, checked hole in pipe , in top of radiator, clear.
checked pipe to expansion bottle, clear.
checked expansion bottle for leaks. ok.none.
touch , pressure cap, water sprays out from around top of cap.(same as it did with old cap on)
surely the water should be going into the expansion bottle, if its getting past the pressure seal?
any ideas?
he has pressure tested for head gasket failure, test liquid remained blue
Click to expand...

Are you sure the expansion bottle is pressurised? Most slitty shave a non pressurised expansion bottle which is fed from an overflow pipe on the top of the rad (just below the radiator cap). The rad has a conventional spring type rad cap. If you take the metal rad cap of, yes it will be under pressure even if the expansion tank is empty. If the pressure in the rad gets too high, the spring is compressed allowing the coolant to flow down the overflow pipe.

Secondly, how did he test the pressure by conducting a sniff test? A pressure test is done by pressurising the coolant system, A compression test is done by testing the available compression on each cylinder.

They are all different tests.
 
ratty:-Are you sure the expansion bottle is pressurised?
never said that "top hose pressurized"
but, however, your description of the system is correct,
radiator has the pressure cap, pipe coming out from the side of the radiator neck, goes to the expansion bottle, expansion bottle , has a dipstick in, water level never moves, whether hot or cold?
 
al-one-arm said:
ratty:-Are you sure the expansion bottle is pressurised?
never said that "top hose pressurized"
but, however, your description of the system is correct,
radiator has the pressure cap, pipe coming out from the side of the radiator neck, goes to the expansion bottle, expansion bottle , has a dipstick in, water level never moves, whether hot or cold?
Click to expand...


Sorry. It was this bit that made me think you were saying it was pressurised.:eek:

no water ever in the expansion bottle? changed pressure cap,
Click to expand...

The level in the bottle shouldn't move unless there is a problem.

If the vehicle overpressurises the pressure cap opens the valve and allows the excess pressure/coolant to vent down the pipe and into the bottle. It is a one way system. THe coolant can only flow to the bottle. It cannot come back.

Is he losing coolant when he's driving and if so do I take it that it's not making it to collecting bottle?
 
occassionally, it gets very hot, taking the gauge up to 120c.
he can drive it, for a long time at full, chipped,twin turbo, boost, and it stays at the correct temp 88c, but he occassionally gets white water coming out, from the pressure cap, and the gauge goes up to 120c, the expansion bottle is still on max. the top hose is only about 4" long and at least an 1 1/2 " is onto the ally pipes in and out, you can still squeeze it, but i am guessing that it is more than the 16psi pressure of the cap?

ratty;- Is he losing coolant when he's driving and if so do I take it that it's not making it to collecting bottle?
yes you are correct
 
Last edited:
al-one-arm said:
occassionally, it gets very hot, taking the gauge up to 120c.
he can drive it, for a long time at full, chipped,twin turbo, boost, and it stays at the correct temp 88c, but he occassionally gets white water coming out, from the pressure cap, and the gauge goes up to 120c, the expansion bottle is still on max. the top hose is only about 4" long and at least an 1 1/2 " is onto the ally pipes in and out, you can still squeeze it, but i am guessing that it is more than the 16psi pressure of the cap?
Click to expand...

try a new thermostat first. If that doesn't do it I'd be looking at HG or cracked head. Judging by the temperature reached it's not sounding good. Anything above 100 is BAD. If it's 120c at the coolant temp sensor, it would be a good deal hotter at the engine core.

I had overpressurising on my TD5 when I got it. It was due to a cracked head. The turbo was pressurising my coolant.:eek::eek::eek: It still never overheated though. It just kept overpressurising and blowing hoses, expansion caps etc.
 
ratty;- It is a one way system. THe coolant can only flow to the bottle. It cannot come back.
you are slightly wrong with this part, the radiator pressure cap , has a small, little, toggle valve, which seals when the pressure pushes on to it, but opens when the coolant cools, allowing the water to be syphoned back into the radiator,
have found that it takes an awful lot of pressure to get air/water into to expansion bottle. so am going to remove and clean bottle and pipe, to see if it changes anything
 
al-one-arm said:
ratty;- It is a one way system. THe coolant can only flow to the bottle. It cannot come back.
you are slightly wrong with this part, the radiator pressure cap , has a small, little, toggle valve, which seals when the pressure pushes on to it, but opens when the coolant cools, allowing the water to be syphoned back into the radiator,
have found that it takes an awful lot of pressure to get air/water into to expansion bottle. so am going to remove and clean bottle and pipe, to see if it changes anything
Click to expand...

If you use the quote tab in the bottom right of the post you would like to quote, it'll make life easier for you. Just delete the bits that you don't want.;)

As you say, Some coolant systems can pull coolant back from the overflow tank. These tanks generally have a min and max mark on them. As you have stated that' there is no coolant in the expansion tank EVER', it would lead me to say that the tank is a catchment tank only.

Assuming the coolant can flow both directions on that vehicle, if the overflow is always empty it can't pull anything back. Have you tried partially filling it to prevent the coolant system sucking in air?

Reading back through your posts I also see that if you simply touch the radiator cap when it's hot it expells coolant. Is this correct? Have you checked the filler neck below the radiator cap for signs of distortion or a build up of scale? If the rubber cannot seal properly it will allow the pressure out.
 
revised qoute;-water level never moves(in expansion bottle), dipstick has min and max on it, always stays on max,whether hot or cold?
trying, today, to flush out expansion bottle,. will also check the radiator filler neck for distortion or scale, cheers
 
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