1985 745t

Getting medium to slow speed coolant loss and am unable to figure out where the coolant is going. The coolant will be filled to the max level and will then drain down over the course of 2-3 weeks. Long trips at high speed tend to accelerate the loss, but it seems to happen regardless of driving conditions. However, it will never drain down to nothing and seems to stop at a particular level just below the expansion tanks "reservoir."

Some of the things I've looked at include the following:

1) Head Gasket/Oil
Coolant shows no sign of oil or contamination and oil is clear of any condensation, sop I've ruled out a headgasket between those two items. Spark plugs check out fine with perfect color and no condensation.

2) Expansion Tank Cap
Coolant cap is new 75kpa cap and shows no sign of any leak or failure (no trademark residue on exp. tank).

3) Head Gasket/Exhaust
Exhaust sometimes shows a little steam at start-up (and more when cold) but it goes away quickly as it warms up. Exhaust smells "slightly" sweet, but I don't really have a baseline to compare it with. No coolant or excess water at the exhaust though, so I'm not sure about a leak from the coolant to the exhaust.

4) Coolant System Leak
Coolant system has been fairly closely inspected and shows no sign of a leak at the obvious points like drain cock, temp. sensor/sender, water pump, radiator, heater valve, core (no steam, no liquid on floor).

Basically I've done everything imagineable to find this leak and can't figure out where the hell teh coolant is going. I do know that pressure seems to make it drain faster; it was fine with an old leaking cap I had, but once i replaced it I started seeing this bottoming out effect.

Any help is greatly appreciated.

Thanks,
rt

I just finished up a head gasket replacement on my 87 745t, the normal symptoms of a bad head gasket (except for the slow coolant loss) were almost non-existent right up until the end, after I ran the engine a bit warm upon blowing the heater control valve. The coolant had always dropped slowly since I bought the car, even after a water pump and hoses change, with no leaks detectable. I think a bad reservoir cap helped things last as long as they did, the cooling system had quite low pressure this summer; after I replaced the heater valve, tank and cap things really let loose under the higher pressures.
At no time, however, did coolant enter the oil or vice-versa, once it gave up the ghost for real and started dumping coolant in earnest it was all leaving the engine directly through the cylinders. The post mortem on the old gasket confirmed this, the oil passages seemed intact but a couple of the coolant passages, and the area between cylinders 2 and 3, were completely destroyed.
I hope you get away without doing the headgasket, however it really is not an impossible task; on an 85 you must be just about ready for one anyhow... It is also a great chance to go over everything on the block to check for any leaks, oil box, engine mounts, whatever-- with the head and intake out of the way everything is really easy to get to. Parts are cheap and if you can afford a little time it is effort well spent; my brick has never ran so well or been so smooth.
Best of luck with it,

Shifted
brickflogger@aol.com
--
'87 745t 205,000

Perhaps you don't have a leak.
If your thermostat is faulty, or cooling system plugged inside or outside,
with debris, coolant temperature will raise quickly and escape via overflow hose.
This will happen when driving, rarely when idling.

Also, if you are checking for leaks, don't forget the inside of the car, around
heater core, and associated hoses.

Good luck !

I guess, but regardless isn't the coolant system a closed system? So unless it overflows out the cap (which I've already eliminated) ther eis no actual "overflow" for it to blow out of. I think it would have to be a closed system since its pressurized.

-rt

Mine is a 85 740 Turbo too, and I have the same problem right now. I am planning on changing the water pump today.

In my case the leak is visable if I turn the rpm up to about 2000 when then engine is cold. I suppose the fact that the thermostat is closed when the engine is cold increases the pressure especially at higher rpms. My leak is just above the pump and the water evaporates when the engine is hot so there are no puddles. You might want to give that a quick check because it's an easy test, just keep your hands away from those belts.

Randy

Sounds similar to a coupole other posters ideas about evaporating leaks around the pump. I'll have to take a look in there asap and see what I can uncover.

Thanks!

-rt

Instead of agonizing over this one, I would spring the $25 or so to do a cooling system pressure test and find out where the leak is. Either it's an odd one, such as the heater pipe leak noted, or it's the head gasket.

Yes, that's probably a good idea. Just figured I'd do some nosing around before I give my next trip past my indy guys garage. Figured it might be one of those aha items I could fix quickly with some new hose, etc.

-rt

Hi. I had a similar leak on my Volvo 940 GLE Turbo a while ago.
It was exactly as you are finding.It was driving me MAD!!!!!
In the end it turned out to be a leak from the junction where the solid red heater pipe goes into the rear of the water pump. The pipe has a shoulder on it which goes into the pump with a rubber sealing washer. The pipe had corroded away, and there was no shoulder on it anymore.
It took ages to find this leak because of course when the engine was running the water was evaporating away in this hot area, so there was no visible leak. When it cooled there was no pressure so no leak!!
We put in some Wynns radiator sealant and it did the trick for about 4000 miles until eventually the leak got worse, and that's when I found it.

Hope this MIGHT help.


KS/oilydad/ Torquay/UK.

That sounds like a pretty good call. I've been wondering about the water pump et al. since I couldn't remember when it was last replaced and figured it miught be something I couldn't see on it. I just figured I would see evidence, but the heat from the manifold and turbo would cause evaporationa dn would probably eliminate anything that might get out before it could leave a mark.

I'll take a look at that today.

Thanks,
rt

I think your best bet is to pressure test the cooling system.
Many hard to spot leaks become visible if you put some pressure on the system when its cold.

Cheers,
Mikael Larsson

Thanks, I'll probably put that on the list next time i have it in.

-rt

You might check the turbo to see if coolant from the water-cooled turbocharger is getting into the exhaust.

Good call. Unfortunately for me the '85 uses the Garrett oil cooled turbo. The water cooled models were introduced later... I think the first year was 87 or 89?

Thanks,
rt