Ron_H

Now the leak is narrowed down to almost bearable during normal driving with the AC off and the cap on one click (not tightly). I can make it several hundred miles without incident, though occasionally the side tank dribbles a bit of coolant. I have a replacement side tank ready for installation, so that shouldn't be a problem.

However, one recent HOT day in San Francisco, we decided to venture into the city and I keep the doors and windows locked and closed on such trips, so I turned on the AC. Within a couple of miles, I had lost almost all of my coolant and the temp was in the red, so I shut it down and waited for the engine to cool down. Once cooled, I added three gallons of coolant and started the engine. It ran well all the way back to Sunnyvale 50 miles and held steady all the way......without the AC on.

My question: Sharkskin advised bypassing the heater core by joining the supply line and return line under the cowl that houses the heater core. That way, if the problem disappeared the heater core would be suspect. Anyone know if the AC system will be damaged at all by doing that? Seems like a reasonable and simple test to me.

Mrmerlin

Ron please replace the coolant bottle and remove the metal heater control valve and put the correct part in and fix the side tank then report back.
My guess is one if not all three of these parts are suspect and your eventually going to damage the heads if you havent already by letting them get hot with no coolant in the system..........
In the mean time dont keep driving the car "to test it" till all of these parts are replaced.
Whats going to happen is the valve stem seals are going to start leaking more oil than they normally would due to the overheating and then you will have an oil burner

Ron_H

update:

I replaced the Audi heater valve with a new factory heater valve. I noticed that the bottle is not leaking; it holds pressure and water level, at least while parked. Of course it could only show up under way and I wouldn't know it. But one thing at a time please. After the valve replacement, I drove off and actuated the AC. It held steady for a few miles, and the level didn't drop in the bottle. The temperature did not change at all. Hard to believe that the valve could have been the problem, but we use the heater so infrequently in this part of California I would only notice the leak during AC use. So for now, it seems to be solved. I still need to replace the passenger side tank on the radiator which occasionally drips for a few seconds. That's next if no further problems arise. I also haven't tried to isolate the heater core, but the heater valve seemed to stop the problem. And the temperature stays put and only rises a small amount in traffic or at a light, then drops when air is flowing over the radiator. The last thing I would have suspected was the heater valve, but it seems to have been the bad guy. Oh yeah. The level sensor sometimes erroneously triggers the red warning light even though the level hasn't changed. It happens when a sudden stop or acceleration takes place for no apparent other reason. So that also needs to be replaced soon.

JHowell37

I don't know if I've already mentioned it, but a cooling system pressure tester is a great tool for quickly diagnosing coolant leaks.

Ron_H

As has been suggested, first I need to replace the leaking side tank. Then, I think the pressure test is the next order of business. I am certain the side tank is seeping now and then and will ultimately fail big time. And I have a new side tank and now only need some time to do it. I'd better hurry because the cold weather is around the corner. Another suggestion was to use colored coolant that will aid in locating the specific point of the leak and that's also sitting on the shelf. I wanted to get the obvious parts replaced first.

Ron_H

So now that I have exhausted all other avenues to a solution, save tearing the beast apart, I called my friend Paul Lang who probably knows more than anyone about all things Porsche. He tells me there is a check valve in line before the heater core that is actuated when the AC is on to divert water from the heating system to adjust the temperature of the cabin air. This is consistent with Sharkskin's original suggestion that somewhere the hot water mixes with the cooled air to achieve the desired cabin air temperature. This valve is the culprit and is defective Paul advises. I have not noticed any such valve in the PET catalog diagrams. I am also advised that I must remove the entire dash to access the valve and the heater core, which I will change WIAIT.
Anyone have any experience with this valve? Evidently, that's what is causing the entire coolant system to drain when I use the AC, but not when I use only the heater.

dr bob

Ron, that 'valve' is the "blend door", actuated by the "blend door motor". That blend door only diverts air around the heater core- no coolant passes through it. The whole assy includes a couple switches though, and one of them opens and closes the heater control valve via the solenoid valve in the little rack in the area in front of ('front' is front of car) the radio.

'Coolant flow through the heater core' description in process flow narrative lingo:

Coolant flows from the back of the pass side head, via a fitting with a hose nipple on it. A very short section of hose connects that nipple to the front of heater valve itself. From the heater valve, coolant is circulated through a hose to the heater core, where heat is extracted from the coolant into air directed through the fins on the core by the blend door and circulated by the HVAC blower.. That heater core has two exposed nipples for hose connections directly onto the ends of the core, so coolant passes into and out of the core right where you can see the connections at the firewall, below/next to the AC expansion valve. Returning coolant goes through that hose across the firewall to connect to a steel tube that goes down the pass side of the engine bay towards the front of the car, and the steel tube also has a Y towards the rear to a short hose that connects to the bottom of the coolant reservoir. The front of the steel tube connects to the water bridge at the front of the engine with a section of rubber hose.

-- Any of the hose connections can leak, including the large hose connections to the radaitor and the engine.

-- Any of the smaller heater circuit hose connections can leak.

-- The reservoir itself will eventually leak

-- The hoses themselves may fail and leak.

-- There's a level switch on the top of the reservoir, sealed with an o-ring that can leak.

-- There's a cooling system pressure switch in one of the vent hoses that connect the top of the reservoir with the water bridge on the engine, and the top of the reservoir with the top of the cold side of the radiator. The switches are subject to cracking with age and physical abuse.

-- There's a plastic fitting on the top of the radiator where the vent hose connects. The fitting itself cracks, and the rubber sealing grommet that it passes through can also leak.

-- The heater core itself can leak, as described by Nicole.

-- The radiator end tanks can leak at the o-ringed connections where they mount on the radiator core.

-- There's a plastic drain plug at the bottom of the hot-side radiator tank. The plug will give way if overtightened, plus there's an o-ring seal aound it where it enters the radiator tank.

-- There's a coolant temperature switch (pre-S4 cars) on the front of the cold-side radiator tank, bottom. On S4+ cars, that switch is replaced with a temperatre transducer. The connection to the tank for either can leak.

-- The pressure cap on the reservoir has a flat rubber washer seal where it fits into the top of the reservoir. That seal will leak if it's deformed, or if the spring is tired in the cap.

-- The water bridge can leak at the o-ringed connections to the engine, or the thermostat cover connection
where the radiator hose attaches.

-- There are two galley drain bolts on the lower rear of the block, one on either side. They originally seal with an aluminum washer.


Those are the places I'd be looking.

WallyP

The entire cooling system is pressurized to the same level, whether the heater is on or off, or whether the A/C is on or off. No relationship to leakage...

If you want to learn more about your system, go to our web site, click on the photos to enter, click on Tips & Links, Wally's World, HVAC. Download, print, read.

Ask as many questions as you want - be VERY verbose, describe what is happening, include every symptom possible. Someone on here can help you.

Let us know if we can help.

SharkSkin

Ron called me and I clarified that I was talking about the air mixing flap as the "mixer of hot & cold". We talked a bit more and he said he was going to put in a u-pipe to bypass the heater core and try again ensuring the cap is tight, also to take off some of the plastic intake parts so he could see -- with no coolant the evidence of leakage disappears pretty fast. For this reason I also suggested wrapping rags in some strategic places to maybe catch some water and help find the leak.

Pressure test offer is still open, just get ahold of me so we can set up a time.

jcorenman

Ron,

What kind of coolant are you using? One nice thing about G-05, besides making the Napa guy smile, is that it leaves a very clear trail when there is even a tiny leak.

If you've had a chronic leak and the coolant is mostly water, then leaks can be very hard to spot.

Just a thought...

Cheers, Jim

dr bob

Seems like an awful lt of headscratching and speculation, when the pressure test is pretty darn foolproof.

Ron_H

Thanks to all who have posted. Wally, I will review your site's material and Jim I have bought and used some VW coolant that is specifically for that purpose: blue and viscous to facilitate detection of leaks. Of course, it all was lost when I switched on the AC and it is quite expensive. Dave has warned me to resume using antifreeze to lubricate the water pump seal, so I will immediately resume mixing it with the water. I will perform the tests Dave suggested and then perhaps we can perform a pressure test as Bob correctly points out is so basic. But before I do the pressure test, it seems I should correct any known points of leakage like my side tank which seeps if the cap is screwed on all the way. I will do more homework and return, but the basic problem, Wally, is as stated: use of the heater, regardless if the cap is tight or not, doesn't result in the loss of significant amounts of coolant; use of the AC, regardless if the cap is tight or not, does result in highly significant amounts of coolant in very little time, meaning drain the system in a few miles on a hot day, and it all dumps by or behind the cats as if it was coming from someplace around the firewall. The heater core could not be leaking, or it would also be dumping coolant when the heater was being used. Dave and others on this forum has suggested that the amount of heat the AC dumps in front of the radiator affects the heat in the engine, and thus the pressure in the system, and if it is large enough the coolant will be forced out by steam in the engine if somewhere in the system there is a compromise (as Bob has pointed out) in the component parts. I must simply find the errant component part(s) and I have seen that they are around the firewall area.

Fogey1

Lucky you, you can buy another tool!!

You can use it to measure the heat on the driver side of the radiator with the a/c on and the a/c off. That'll give you a read on the theory above.

Non-Contact Laser Thermometer $59.99
http://www.harborfreight.com/cpi/cta...emnumber=96451

Infrared Thermometer $19.99
http://www.harborfreight.com/cpi/cta...emnumber=93984

You could also buy or make the tool you need even more.

Radiator Pressure Tester Kit $79.99
http://www.harborfreight.com/cpi/cta...emnumber=65053

WallyP

The extra heat theory is the only thing that makes any sense - but there should be serious overheat warnings before it dumps the coolant.

Here is another vote for a pressure test. My vote is for a leak (heater valve or small hose comes to mind) that only appears when the pressure rises in the system. I would suspect that the pressure cap is wrong/faulty - the system pressure should rise to one bar as the engine warms, never get any higher.

Doing a pressure test would probably solve the puzzle instantly. I would not be willing to repeatedly overheat my engine...

SharkSkin

One more thing to add -- Redline Water Wetter will also serve to lubricate the WP seal, and is cheaper/less messy/nicer to the environment when it ends up on the road.

Note that I also have an IR thermometer, no need to buy one just for this(though they are very nice to have). If you do buy one, make sure it has a high D:S(distance to spot ratio), i.e. a narrow angle of view. The cheapest ones have such a low D:S that it can be difficult to pinpoint some areas for temp measurement. For example, the $20 unit linked above will measure a 1" circle at 6" distance.