M20 Coolant Loss

Sat Sep 01, 2012 2:42 pm

My M20-engined Touring lost enough coolant in 300 miles to put the light on in the check panel. It doesn't normally lose coolant.

I've done all the usual visual checks - hoses, radiator, water pump, damp passenger footwell, bubbles in header tank - no evidence. The engine isn't running any hotter than normal.

The only thing I have noticed is there's only a very minimal pressure release when taking the header tank cap off. This is not usual - it's normally a noticable hiss. This leads me to think the cap has run-up the white flag. I'm not sure how old it is but I have replaced it at sometime between 3-8 years ago/30-110k miles (I could check, but I can't be ar53d to plough through all my parts invoices).

How difficult should it be to blow through a properly working cap? I can blow through mine with no more effort than a quiet wolf-whistle.

TIA
Stuart.

Sat Sep 01, 2012 3:35 pm

I would say it would be seriously impressive to overcome a pressure relief valve with a pair of lungs

Sat Sep 01, 2012 3:59 pm

johnl320 wrote:I would say it would be seriously impressive to overcome a pressure relief valve with a pair of lungs

Good starting point,do make sure that you buy the correct '140' E30 cap,lesser BMW's seem to use a '200' cap.

My next suspect would be a silted up radiator.

Sat Sep 01, 2012 7:19 pm

Not sure how a silted up Rad is going to cause gradual loss of coolant. The OP has stated that there is no change in normal operating temp. My next step if the cap change doesn't help would be a coolant system pressure test.

Sat Sep 01, 2012 8:31 pm

johnl320 wrote:Not sure how a silted up Rad is going to cause gradual loss of coolant. The OP has stated that there is no change in normal operating temp. My next step if the cap change doesn't help would be a coolant system pressure test.

Fair point,but,old radiators do cause odd cooling problems and really should be regarded as 100k service items.

I skoffed too,but have just cured an odd cooling issue with a new radiator.

Sat Sep 01, 2012 8:45 pm

I'm not scoffing at anyone. He hasn't got a cooling problem, he has a coolant loss problem.

Sat Sep 01, 2012 8:54 pm

johnl320 wrote:...... he has a coolant loss problem.

In my admitted limited experience compared to yourself,cooling system problems are limited to three places;

Thermostat
Waterpump
Radiator.

Sat Sep 01, 2012 9:03 pm

daimlerman wrote:
johnl320 wrote:...... he has a coolant loss problem.
In my admitted limited experience compared to yourself,cooling system problems are limited to three places;

Thermostat
Waterpump
Radiator.

Not sure if that is a sarcastic dig, I don't know what you do/did for a living and I'm fairly sure you don't know what I do, I'm not prepared to have a pissing contest on Stu's thread.

All I'm suggesting is that a pressure test could show a drop in pressure and help pinpoint a tiny leak from, say a core plug for example.

Sun Sep 02, 2012 3:09 pm

daimlerman wrote:In my admitted limited experience cooling system problems are limited to three places;

Thermostat
Waterpump
Radiator.

Thermostat Either causes overheating, if it's stuck shut, or over-cooling, if it's stuck open. Neither are the issue.
Waterpump Either leaks, which mine doesn't as per my OP, or the bearings go, which mine haven't. Extreme cases can have the impeller detatch which causes overheating. Again no issue here.
Radiator Either leaks, which mine doesn't as per my OP, or blocks causing overheating. Neither are applicable to my problem.

You left out hoses, heater valve and heater matrix from your list. You could also have included the viscous coupling - which is another non-applicable item here.

All I asked was a simple question. I have already ruled out everything else. Why complicate the thread? This is precisely why I rarely post here anymore - people either don't read posts properly or they choose to ignore the content of the post and reply with totally irrelevent shite.

If I don't have a dodgy pressure cap (which I'm going to replace tomorrow anyway, got to be worth it for the sake of a few sovs) then I'll get a pressure test. I would have replaced the cap yesterday but by the time I'd ruled everything else out the dealer was shut.

I'll get my coat.

Sun Sep 02, 2012 5:08 pm

I know you've said hoses, but have you checked the metal pipe that runs across the front of the block?
It can rust and develop small pin prick holes that can be hard to spot.

Sun Sep 02, 2012 6:02 pm

You say that neither the water pump or the radiator leaks. In my experience either could be leaking at the kind of rate that puts the light on in 300 miles without the leak being apparent. I've also come across leaking hoses where the stream emitted is so fine that I've only discovered it when my skin got in the way.
Get yourself a cheap ebay pressure testing kit and get it up to 1.4 bar cold. I find that this usually shows up leaks on cars that don't even have a coolant loss problem.
Pressure caps usually maintain pressure OK as long as the lower O ring is present and correct. The problem with pre recall caps was the PRV not releasing, giving excess pressure. Check that the vacuum release valve on the base of the cap is seating properly without any debris trapped in the seat.

Sun Sep 02, 2012 7:56 pm

^^ Ditto ^^
Last year I was losing coolant slowly and pressure tested it at work, 1 hour and not a drop I was beginning to fear the worst maybe a porous cyl head but upped the pressure and the rad went bang at the top corner.
Obviously just enough to lose slow pressure gradually.

Get it pressure tested

Sat Nov 03, 2012 1:12 pm

While I've been on the forum with another, unrelated, problem I thought I'd update this one.

A new pressure cap did nothing to resolve the problem. I decided to replace the water pump, as the timing belt was due in about 6 months anyway. The pump shaft had a little play in and it looked like there may have been a very minor weep of coolant along the casting. That didn't cure it, either.

I couldn't be bothered to go any further with my investigations, so I decided to take the cheat route and chuck a bottle of K-Seal in. That didn't have any effect, until the first time I had the heater on............................

Sat Nov 03, 2012 2:19 pm

Watching this with interest as been having a few cooling probs myself.

what happened the first time you put the heaters on after using the K-seal??

Sat Nov 03, 2012 4:06 pm

steve_k wrote:what happened the first time you put the heaters on after using the K-seal??

Nothing happened, as such. I had been checking the coolant level every day and topping-up every 3-4 days, depending on mileage. The day after I used the heater, the coolant level hadn't dropped, hasn't since, and the system now takes a lot longer to de-pressurise(sp?).

I'm not 100% happy, though, as I'd rather have found out exactly where the problem was and cured it properly than just mask it. I can only assume that it's the heater matrix at fault.

I'll make a note to replace it the next time I change the timing belt/water pump/coolant - if it lasts that long.