Ron_H

Some very revealing posts on this topic, and I thank all who contributed because I too have an elusive coolant leak. In fact I can't even begin to understand what is actually happening. At the risk of irritating others, I'll present my baffling findings and seek some advice too.

Normally, with the cap only one click tight, and with a slight passenger side tank seeping once in awhile but not significantly, I can last indefinitely without needing more coolant. But with the AC on, in about ten to twenty miles, almost all the water is gone, and if I park the car after about a ten mile drive, LOTS of coolant gushes from under the firewall around the cats. Hoses are changed, and heater valve is changed. So we reasoned that the heater core is leaking. BUT.......wait!! I can turn on the heater to full heat and drive all over the place with no apparently significant loss of coolant nor any dripping or leaking behind the firewall or visible on the ground while idling, as when the AC is on. So the replacement heater core remains on my dest awaiting installation, but why? It doesn't appear to be the heater core or it would leak when the heat is on too.......wouldn't it?? Before you ask, the amount of coolant pouring from above the cats at the firewall is far more than AC condensation. (walks off scratching head in bewilderment.....) For the last week, I have run the heater and have suffered no loss of coolant that's significant. Of course, I don't touch the AC for fear of being stranded and watching the needle soar into the red zone. And no I haven't been smoking anything and don't smoke anyway.

Nicole

So, in a nutshell:

Heater on, AC off = no significant loss
Heater on, AC on = significant loss
Heater off, AC on = significant loss

Correct?

That would raise the question how the AC can cause coolant loss, given that it is a separate system.

Ron_H

Nicole, I either have the AC on or the heat on, but not both simultaneously. If the AC is on, the top lever is in the "cool" position to the left; with the heat on, the AC button is off and the heat lever is in the extreme right or "hot" position (of close to it). As Dave has pointed out, the AC must rely on the heater core to mix hot and cold at some point to achieve the desired cabin temperature, so it is likely the heater core has hot water in it at that time. What causes the coolant loss when using the AC is the mystery. The last time in San Francisco, I drove from 15th and Mission to Haight-Ashbury (approximately 3 miles) with Jane in the car and thus the AC on, windows closed and doors locked. And on lower Haight St. the needle approached the red zone and water was pouring out (I added at least two quarts after it cooled off). I made it back to Sunnyvale without incident so long as I left the AC off. Doesn't make sense to me, but there must be something I don't know about the system.

pcar928fan

I have a heater core leak in my '94, but it is SO LITTLE that you never see a drop. You only smell it burning off the exhaust, especially if you USE the heater. Real bitch to fix too as the heater core is a PITA to get to...SUCK! Since I don't lose a noticeable amount of water I have not bothered to worry with it at this point...

Good luck with your problem. I hope you get it resolved very soon!

Ron_H

Thanks. I don't look forward to removing the dash and all the rest of the interior, but I suspect I must sometime soon. But I still don't know why it only leaks with the AC on.

Nicole

Ron: If you use the climate control, you almost always have a combination of AC and heat. It uses the heat to adjust the temperature.

Ron_H

That's exactly Dave's point: the heat is mixed with the cool air to achieve the desired cabin temperature. Thus the heat is on during the AC use. Suspect the heater core until I turn the AC off but keep the heat on.

dr bob

My car selects the heater valve when outside air is cooler than setpoint .and. inside air is cooler than setpoint. Otherwise no heater valve. The heater valve is operated by the mixing motor when it moves to something like 2/3 position. This is regardless of AC on or off

Ron, the AC typically adds noticeable heat load to the system. So running AC operation is more likely to raise cooling system pressure and help you discover weak spots in the system. From your description, I'd be looking at the heater valve and the little connecting hose, the heater core hoses between the heater valve and the core and the heater valve to the steel Y, the short hose from the Y to the reservoir, and at the reservoir itself. While heat alone may cause leaks to happen, it's virtually always the increase in pressure from that heat that really speeds up leakage. So the diagnostic step is pressurizing the system with everything still cold, and start looking for the leak while the engine bay is safe to crawl around in. Most parts places will loan you a cooling system pressure tester if you leave a deposit.

In my colllection of things I have a little home made tester for the 928 that uses a mini pressure regulator and a couple short hose sections with a T. Pull the vent hose off the top of the radiator, plug that onto the T, and the spare hose from the T goes back to the radiator vent. This is 8mm (5/16+0.003") hose and fittings so it's common parts store fuel hose fittings. Anyway, air compressor through the regulator, slowly bring the pressure up as you look for leaks. The cap should relieve at 1 bar, slightly less than 15 PSIG on the regulator. But stay below that reliefpoint so you don't bleed coolant from the cap and that too-short drain hose on it. You can do this with a bicycle tire pump or one of those mini tire compressors too, just don't go any higher than you need to start the coolant leaking.

Ron_H

Thanks for the advice Dr. Bob. I think that is an excellent approach. I tried to borrow a pressure tester from PepBoys once, but they no longer loan them out. Your method should at least reveal weak points and I have some blue antifreeze from VW which should allow me to visualize where these leaks occur. It is expensive stuff but should save considerable time.

The heater valve and all the hoses have been changed, so I am assuming they are not malfunctioning.

dprantl

+1 what Dr. Bob said. The A/C condenser dumps a large amount of heat right into the radiator raising its temperature and therefore its pressure (since volume is constant). Are you sure the stuff dropping on the cats from behind the firewall is not in fact water condensate? Or maybe you have a heater core leak and the condensate mixes with the leaked coolant and gives the impression it is only coolant leaking down there...

Dan
'91 928GT S/C 475hp/460lb.ft

928SS

never thought I'd ever recommend this but - try a can of stop leak. seems to be great for minor issues.

iirc, some OEM's use some kind of stop leak in every new car before it goes out the door. seems to work pretty well too.

I fretted for months over chronic dribble from my AL C&R radiator and a headgasket. then as a last ditch effort after doing a bunch of research, I tried some Barrs Radiator and Headgasket Stop Leak.

I'm still amazed all is perfect after 2 months, 1K "spirited" miles, on a track, cold and hot weather.

several brands are available, and I think 928 intl even sells one that they recommend.

sure beats a lot wrenching just to fix a minor issue, imo. and you've got no downside if it doesn't work.

Ron_H

I might try stop leak. Once I used it on a VW Rabbit and it seemed to work for awhile but I fear it clogged up the system so badly that eventually it overheated and I lost the head = end of that Rabbit. I'll check out 928's recommendation. Thanks.

Dan, I know this is not condensate. It dumps steadily as the car is moving and when I stop, tons of fluid escapes, up to two quarts. There is no way this is condensate.

I have another heater core. I may simply have to allocate a weekend to tearing out the dash and replacing that core. I'll perform Bob's check first though.

the flyin' scotsman

Ron............you may want to check RW; we might have the answer.

https://rennlist.com/forums/928-foru...ar-engine.html

Ron_H

Thanks Malcolm. I'll check that part this weekend.

UncleMaz

Struggling to find a coolant leak which left coolant in the valley of my 85, I came across this thread and Bill Ball's post about the damper/FPR mounts leading to coolant passages.

Anyway, a loose fuel damper mount bolt was squirting coolant into the valley under the intake. I had the air box off to tighten and check the heater valve plumbing, but did not look there, never thinking there was any link to the cooling system.

Three weeks ago I resealed the intake and reverted back to stock on my FPR/chips and must have left the bolt loose.

I used paper towel to soak up the coolant and used a push button type prong picker-upper to remove the wet paper towel.

Thanks OP. Thanks Bill. Thanks Rennlist.