Cooling system pressurising

[Chris_C]

Any ideas why this could be? My 1.7 was pressurising and blowing hoses and expansion tanks a while back, then it all seemed to fix it self for a while. I've recently changed the radiator (the old one had blocked cores and fins weren't touching the cores) as well as the stat and waterpump (stat was sticking, and waterpump leaking out the front shaft seal after I flushed everything through) and it's gone back to doing it, it blew the seam on the expansion tank yesterday which I've stuck back together with a bit of localised heating. The system lifted the cap again later on though, and the tank forms itself a nice bulbous shape.

It's as though there is too much coolant in there, but there isn't. Any ideas? Only thing I've come up with is head gasket between cyls and waterways, but it doesn't pump air into the expansion, just solid water, and a compression test is good.

[SteveP]

I would've thought HG or maybe a bad air lock in the system seeing as you mentioned changing the rad?

[Chris_C]

But surely headgasket would show in a compression check and blow bubbles on the recirc pipe? Bad airlock I could agree with, but it swallowed a fair chunk of water and all hoses are hot... hmmm, confused Chris!

[SteveP]

Yeah if you did a compression test recently and it's all in check I'm doubting HG too. No mayo or anything anywhere? I was erring towards bad air lock, but then if all hoses are hot... dunno How's the water temp looking?

[SteveP]

Borrowed from a Toyota Celica forum

The symptoms of air-lock are that the engine will idle, then suddenly appear to boil dramatically. This action is due to the overheating of the higher parts of the engine, where there is no coolant in the system, then as the coolant at lower levels rises, it immediately boils, creating pressures more than the rad cap pressure rating.

[Chris_C]

Hmm, it does seem more and more like it, water temp is showing perfect, if a millimetre hot (it'll wander just over the the rhs of LED, but it is summer). The expansion tank really does bulge when it does it though

When the expansion tank burst, it lost a fair quantity of water, but I'd put that down to steam coming out the puncture over time. Might not have been though, so best mention it

[SteveP]

I'd try bleeding the system, see how that goes. No idea of the procedure on the 1.7 though

[shimon340]

Hi

Having recently replaced two radiators on two 1.7s. Block, rad and heater matrix were all drained in the process.

During the first rad replacement; Once the expansion tank stopped emptying as water was added, the engine was started to let it continue to ‘self bleed’ and wait for the thermostat to open (as this usually results in the water level dropping in the expansion tank as the full cooling system comes online). The 1.7s use a return line to the expansion tank from the carb heater plate etc so a lot of air comes into the coolant via these lines and also out of the radiator as it bleeds air. (1.4s don’t use this ‘other’ connection on the expansion tank). 400 series have a similar small coolant return line or ‘feed’ to their expansion tank. For this first rad replacement, the coolant started to froth excessively via A LOT of aerated coolant coming from the smaller coolant lines to the base of the carb etc. Never seen this before but it settled down eventually. Once thermostat opened coolant topped up to half way between min and max. Expansion tank cap placed on but not tightened. Coolant left to calm down. All unfoamy and ok after an hour.

Second rad replacement. Again, rad, block and heater matrix drained. No foaming whatsoever. However, even though I checked again and again for leaks on the new radiator and hoses etc I could find no leak. However, for the remainder of the afternoon (expansion cap loose but covering aperture), the coolant level continued to drop. Even 6 hours later is was again back down below minimum. NB the car had only ‘taken’ 5.5 litres of coolant including the topping up once the thermostat opened. System capacity is 7 litres. In the evening, coolant was again at minimum. Grrr. Was concerned that either my fresh 50:50 antifreeze was now over diluted with the very weak antifreeze that was originally in the car (long story) or indeed I had a leak somewhere which I couldn’t see!

After taking the car for a ½ drive, again even when warm the coolant was still only just above min (dropped from a mm below max -topped up when cold).

The next morning the coolant was topped up to 80% level, below max. Fortunately, it hasn’t budged since (checking cold car). 

It seemed to me the second rad replacement (same car by the way, had a bit of a saga with replacement radiator) was self bleeding rather slowly slowly. In the end, the car ate it full requirement of 7 litres of antifreeze as needed.

Curious how the first time around, the same car had foamed its new coolant as it bleed air so fast!

So, either your car if overpressurising the coolant system due to trapped air (cap should lift before the expansion tank bows though!) or sorry to say the general symptom of overpressurising the cooling system does point to a failed head gasket. However, this rarely shows up on a dial gauge compression test as the loss of compression from an initially failing gasket isn’t big enough. Very accurate compression test might show it but the loss is still small enough to not be affected combustion behaviour yet.

I know you’re very experienced but from my recent work on the 1.7 cooling system recently I’d be tempted to open the expansion tank overnight, then start car from cold with the cap off and run at idle then at 1500-2000 bursts once a bit warm to encourage the air out the system. NB heater needs to be on hot as well as this can hold quite a bit of air. Run the car till pretty warm but don’t go so far as the temp gauge at half way as with a non-pressurised cooling system you can get bubbles of vapour from about 80deg. Let car cool again with expansion tank cap off.

No bleed screws etc on the 1.7 cooling system. Some 400s have them though…

A faulty expansion cap can cause the symptom of everything being fine as the car warms but then loosing coolant as it boils and faulty cap can’t contain the pressure. Wouldn’t expect the expansion tank to bow in such a situation though. Cap should lift before pressure got that high.

You have a history of blowing hoses etc so sorry to say but my feeling is on this being head gasket.

PS ‘exhaust gas in coolant’ testers only work on coolant within its service life. Coolant becomes acidic overtime anyway (as anti-corrosion compounds get used up and ethylene glycol decomposition kicks in) so checking for acidic exhaust gas in the coolant which is older than service life is a flawed test. You’ll always get a positive result ie acidic coolant!

Just to make you feel better, a friend’s 480 always always has a ring of crystalline antifreeze around the expansion tank lid. Changing the lid didn’t resolve it and no cracks can be seen in the tank. Could be a little leak from a hairline crack in plastic maybe. Car also looses coolant over the course of a few weeks but no steam ever seen from expansion tank lid etc. Could also be a head gasket on its way out….

[Chris_C]

After taking the car for a ½ drive, again even when warm the coolant was still only just above min (dropped from a mm below max -topped up when cold).

The next morning the coolant was topped up to 80% level, below max. Fortunately, it hasn’t budged since (checking cold car).

I've driven about 1000 miles since the job was done, and it was "all ok" (I didn't look to see if the thing was pressurising as I wasn't expecting any problems)

Curious how the first time around, the same car had foamed its new coolant as it bleed air so fast!

Yes, I agree. I've had the same car do completely different things as it bleeds!!!

So, either your car if overpressurising the coolant system due to trapped air (cap should lift before the expansion tank bows though!) or sorry to say the general symptom of overpressurising the cooling system does point to a failed head gasket. However, this rarely shows up on a dial gauge compression test as the loss of compression from an initially failing gasket isn’t big enough. Very accurate compression test might show it but the loss is still small enough to not be affected combustion behaviour yet.

Agreed again, my dial gauge compression tester can only show me accuracy of a snadgers of a bar (it's the standard gunson one) so would probably miss it

You have a history of blowing hoses etc so sorry to say but my feeling is on this being head gasket.

Yes, very fair point, Fake does pop hoses, and expansion tanks (this is her second...) and waterpump seals, and radiator cores if they are past there best! I've got another 500 miles to get to 210,000 which must be some kind of 1.7 record, then she'll be getting some proper treatment!

[shimon340]

hello

nice to hear from you

yeah..sorry...didnt want to be the bearer of bad news but the symtoms do seem to stack up to a head gasket.....

210,000 miles!!! Nice one mate!!!!

PS I recall you had a handy tip for changing distributor caps on the 1.7. I've got one to do for a friend which is on (what looks like) the original orange/brown cap. Screws of course, not bolts and they are rather inaccessable!!

[Chris_C]

Yup, remove the heater box. Ping the reusable cable tie, then two screws and one nut, slide the heater box under the heater valve bowden cable and you have huge amounts of access to get to the cap Have a nosey at the stat housing whilst you are there, just to make sure it's not weeping whilst you have good access.

[classicswede]

Could well be hg but before you do that 2 things.

Is the fan cutting in on time???

If the fan is working ok then try removing teh thermostat and see if it still builds the pressure.

[Chris_C]

Yes, in traffic I can hear the fan at "just before end of red led", so I've assumed thats right. The stat is pretty much brand new (fortnight ish?) so I've assumed it's good, not a hardship to try without though!

[Ronnie]

Change your expansion tank cap - it should vent and release pressure, and it obviously isn't - that's why stuff is blowing up.

Dontchalove a cheap fix?!

[Chris_C]

Thats the problem mate, the cap does vent, even under normal load Granted I can't test the pressure when it's venting. I'm pretty certain the HG has gone now, it's getting worse rather than better. Ah well... 400 miles to go...