Pressurized cooling system.

[Rob Gaskin]

Guys I have an issue with my standard 240Z and I've had it for at least a year.

It builds pressure in the cooling system and my first thought was the head gasket. Remember the thread last year posted by Mike B about bleeding the system and I said that it was too dangerous with the rad cap off, well that's because with my car it is.

If I run it with a thermostat in it will blow water out of the rad overflow when the 'stat opens. If I run it without a 'stat it's much better but it still builds pressure. So you might think that the gasket has blown but it will retain pressure for weeks! So if the gasket had gone wouldn't the pressure leak away?

Compressions are good at about 165psi and no water in the oil. Car drives well and goes as fast as I'd expect a good standard 240Z to go (quite quick).

I know I should take the head off but this engine has never been apart and I'm reluctant to do that and also I might break manifold studs etc.

Any ideas?

 

[jonbills]

Rob I think it must be the HG - but to double check, commercial garages have a gas analyser thing that will confirm there's combustion by product in the coolant.

 

[tel240z]

Rob now this is a bit of me, it seems quite clear to me that combustion pressure (many thousands of psi) is finding its way into the water jacket via cracked bore, cracked head or more than likely head gasket, an easy test to do is run the overflow pipe into a clear bottle filled with water and seal the radiator top ring with a rubber gloved hand and start the engine, if a constant stream of bubbles can be seen entering the bottle then you have your answer, finding the source of the leak a little more difficult

TT

 

[Rob Gaskin]

Terry, I already have the overflow fed into a bottle (to catch any water) and today when I got back I checked and the level was ok. I then switched off and bubbles started coming through for a while - strange. Could that be expansion in the already pressurized system due to localised heat build-up because the water pump isn't running? If I have a leak into the combustion area why doesn't it vent that way?

As I say it holds pressure because today before I started the car I opened the cap and it blew a bit into the bottle - after about 6 weeks of standing!

It runs really nicely and ticks over like a new car.

With the 'stat out it runs cold - as you would expect

I know I have an issue but I'm in denial.

 

[status]

Mine does the same but oil in the rad,defo head gasket,been like it for years since I had the engine really,so it's going in to be fixed this week,at last

 

[jonbills]

The cooling system is meant to be a closed system - nowhere for pressure to leak off to.
I guess you could hope it would leak past the failed gasket into the combustion chamber but it seems your seal is good enough to hold coolant pressure, but not good enough to hold combustion pressure.

 

[Alphabettispaghetti]

I like Terry's way of testing, never thought of doing it that way.

You can also do a chemical test can't you?

 

[Rob Gaskin]

I like Terry's way of testing, never thought of doing it that way.

You can also do a chemical test can't you?

Yes both good ideas.

I can drive it like this, last year I went to Japfest, the Samuri Gathering and the Gold Cup knowing the problem.

If only I had Terry's workshop, I can't even get around my car in my garage but I've manged head jobs before.

 

[status]

You will have to get a bigger house rob,I know a few women that have managed head jobs but the Zed was still broken,bloody annoying

 

[Rob Gaskin]

Well thanks to Jon and Terry I've understood the situation better and particularly why the excess pressure is not leaking away. That was baffling me but if I only have a minute failure of the gasket (or hairline crack in the cylinder head ) I can understand it more i.e. it's acting like a one-way valve.

Cylinder pressure is far greater than the coolant pressure (10psi max).

Glad I posted the Thread and I will try your rubber glove test Terry. (waiting for Sean or Status to comment here ).

 

[status]

I thought I had rob with a least one sensible answer

 

[Russell]

Just a back to basics thought on it that I'm sure has been covered but is your rad cap ok?

If the spring is worn and not holding pressure then it will release water out of the overflow prematurely thus making it look like the system is under excessive pressure?

 

[Rob Gaskin]

Just a back to basics thought on it that I'm sure has been covered but is your rad cap ok?

If the spring is worn and not holding pressure then it will release water out of the overflow prematurely thus making it look like the system is under excessive pressure?

Yes Russell it's good, there is no way I dare undo it until the car is cold.

I also did put a new thermostat in and I've tested it in a saucepan of hot water.

 

[Russell]

Yes Russell it's good, there is no way I dare undo it until the car is cold.

I also did put a new thermostat in and I've tested it in a saucepan of hot water.

In that case the spring must still be fine if it holds that kind of pressure.

I will get back in my box...

 

[Farmer42]

Hi Rob

Did you ever solve your problem?

I have developed a similar issue with mine. It seems to be ok on idle but as soon as I start driving it, I lose all the water out of the radiator cap and overflow. I have tried a new cap and have now removed the 'stat which runs the engine cooler and has reduced the water loss but I don't like running an engine without a 'stat.

I have rebuilt the engine and have put in a new head gasket. It runs sweet with no misfire & good compression but there is pressure in the cooling system. The water is not oily and there is no water in the oil so I don't think it is a fluid leak either way.

I noted earlier comments about compression leak into the water system pressurizing it. Would re-torquing the Head bolts solve it? I suppose there could be a possibility that my torque wrench was not working properly which meant that the bolts were not tight enough

The other thought is whether gunk in the system would cause it to pressurize if it blocks channels. The block had been sat for a few years before I had it. The water is a horrible rust brown colour so I will flush that out a few times and see if that makes a difference.

Need to sort this otherwise my trip to Donington at the end of next week is in serious jeopardy!!

 

[Moriarty]

Gas analyser kits (litmus dip test or indicator fluid) to test it yourself can be bought for as little as £30

Re cylinder pressure, whilst almost every engine is different, a starting point for something like an NA L series could be 1000 psi at medium to heavy load......add higher compression ratios better fuel (modified spark advance etc) and this could get to 1500 psi on a race engine....forced induction is a whole new ball game

And diesels...start at 2000 and go up

 

[jonbills]

If the pressure is overcoming a new rad cap, it does sound like combustion gas is getting into the coolant.
Moriarty's test would confirm, but re-torquing the head can't hurt. Borrow or buy a new torque wrench.

 

[forest racer]

I put Evans waterless coolant in mine Rob, you have to use there flush liquid first, I couldn't believe how much crud came out of the system.
Filled it up with the coolant and no more pressure problems , I got fed up with the cap nearly taking my head off

 

[Rob Gaskin]

No I didn't solve it and I've sold the car now. The new owner was fully aware of the problem and he had a well respected professional look over the car and go for a test-drive etc. He looked in the cam cover (no mayonnaise) looked in the rad and smelled the water etc etc.

The car ran well but I wouldn't have gone on an Alpine tour with it.

If he gets back to me with the reason I'll post-up.

 

[Farmer42]

I think I managed to sort it but it has taken most of the weekend and a lot of water.

It seems as though it had the wrong thermostat. It appears there are 2 different 'stats for n L-series - 1 that opens at 88c and one that opens at 82c. I had one that was 88c opening but it seems as though that was too hot and it was overheating causing massive expansion which gave the appearance of gases in the cooling system.

I had previously tried it without a stat and it was fine once all the air was bled out but it barely registered on the temp gauge and the heater was dire. I had noticed the brown scum in the water so flushed it through several times using Fortes System flush and heaps of gunk came out.

Ordered a lower temp stat and tried that and again, after the air was expelled, it seems as though it has worked. No overheating (the temp gauge sits halfway & stays there) and no loss of water.

I did recheck the head bolt torques with a new torque wrench to be safe and there were a couple that needed a bit of tweaking so I think my old torque wrench was dodgy.

I think the moral of this is if you get similar issues, try a lower rating thermostat which you can pick up quite easily.