[/B]My AC was fighting with heat coming through the vents.
Narrowed it down to an ineffective heater valve and/or neglible vacuum leak.

So far I've tried two metal units:
First a NAPA "Balkamp" unit for the Audi 100 like the one pictured above that Jim describes as "Napa P/N 6601410".
Immediately after install, noticeable heat mixing with vent or AC.
A cheap part, so I figured that I'd try something else.

Tried one identical to the unit that Jim refers to as "Napa P/N 6601410 used to be this:". This one is currently installed and performs the same way as the basic metal unit: allows warm air to mix with vent or AC. Removed and tested it. Unpressurized water dribbled through with the arm fully closing the valve. However after applying significant pressure to it in the closed position, it sealed completely. I used a zip tie to close the valve completely and no warm air mix. But that defeats the purpose of even having a valve in the first place. Either these units are designed for more vacuum than the 928 system provides, or there's a very minor vacuum leak in my system (more likely).

So, I'll be removing the levered vacuum valve in the next week or so, and will be replacing it with this:


(Four Seasons #84707)
I had initially referred to the wrong part number (#84706) which is for a 5/8" hose. #84707 is for the correct size hose - 3/4".


..which I can say with certainty will shut off my heater core completely for summer driving, and will likely last well into the next millenium.
Kind of like putting snow tires on in the winter & going back to the Michelins in the summer. A pain in the ***, but effective. My guess is that most water cooled track cars that still have heater cores use these primitive petcocks even though it weighs as much as a handful of lugnuts.

A notable characteristic of the stock system that was pointed out by MrMerlin (I believe) is that in summer driving with a properly operating stock heater valve, every time you turn off the engine, the vacuum is relieved allowing the valve to open & allowing hot water/coolant to flow into the core. When you start the engine, the AC has to work extra hard to cool the now-heated cabin.
These engines put out enough heat as it is for me to want to tolerate sub standard AC performance.
..so the petcock goes in every summer & the Audi unit goes in in the winter.

 

I need to go dig up the research I did a few years ago. There's a British Ford (Fiesta?) valve that fits in the line between the factory valve and the heater core. Vacuum to open. Tie it to one of the lines for the FPR, so it sees vacuum only with the engine actually running. Stops the heat flow to the heater core when the engine is off, maintains normal operation with engine running except no heat under hard acceleration I guess.

 

The choices are limited since the big 3/4" hose seems to be used less these days.
Bob, now that I think of it, you may have been the one who pointed out this engine-off heat-on phenom.

 

the top right coolant tank is the next weakest part,
then the coolant bottle,
then the top hoses that connect to the water bridge where the little nubs wear through the inner layers of rubber.
the other concern about adding more metal to the cooling system is that corrosion can also cause deposits to fall out in the system, usually found when the radiator is drained or the coolant plugs are pulled on the block and you need a rod to poke into the holes

 

JP, I like the petcock!

Stan, doesn't the expansion tank have a pressure relief valve on the cap?

 

I remember a series of posts by Mason where he was chastising radiator buyers who pressure-tested the used radiators to 1 bar and caused them to leak. Might lead me to believe that the pressure cap isn't the weak spot on tired cars with under-maintained radiators. Still, I like theidea of having everything in the system bulletproof to at least anything up to a 1-bar bullet. Then let the cap do the dirty work if needed.

 

the hard plastic of the 25 yo coolant bottle turns into the 15yo cap,
and thus when its a dark tan it will crack and leak

 

JP, love your Four Seasons #84706 idea ... OE plastic sucks, my '07 vintage Porsche plastic valve broke when I went to un-wire it for winter driving last fall.

Hot coolant will definitely displace whatever's in the heater core when you turn the engine off and vacuum holding the valve closed is released.

 

..yea under $10 @ Amazon.
You Texas guys have it worse than us DC guys when it comes to hot, humid Summers.
Keep in mind that this petcock isn't really usable in the colder weather since you'll have no way of regulating the heat in your cabin - either heat or no heat regardless of cabin temperature.

I may end up putting both in as a permanent install: the petcock would be up-stream of the vacuum actuated unit. This way the petcock would still be used to completely shut off the flow in the summer (particularly beneficial during short trips during dog days of Summer). The petcock would need to be located so that it's easily accessible w/out removing the air filter assembly.
In the winter it would be fully opened (manually) & assuming that the temp sensor/vacuum actuator is working, the downstream vacuum unit would work well enough to meter the flow based on cabin temperature without roasting the occupants. That unit wouldn't need to be as accessible.

 

Agree, a two-valve solution is prob the best way to go ... petcock handles the summertime-off requirement and then the vacuum valve can handle the heating season.

 

Interesting - never had a failure of the factory valve - plenty of them fitted to my cars here in TX.
I change them as part of a major service along with the short hose.
No record of anyone telling me one of the factory units has failed either and I sell lots and lots of them.
Probably should be changed every 5 years anyway.

 

Roger,
Admittedly I never replaced the failed oem valve with another oem valve.
I had assumed that the aftermarket unit with the actuating arm (as shown in Jim's pic) would do the same job as the factory unit and looked to be better built. But considering that the factory unit has a unique guillotine valve that takes very little effort to fully close - as opposed to a plunge valve that likely requires more vacuum to fully close, maybe I replaced an oem apple with an aftermarket orange.

 

The valve that was on my car when I bought it was bad, so I replaced it with a new Porsche one. That new one was bad in around a year. It still held vacuum and moved, but the inside had warped so that t didn't seal off completely. I replaced that one with one of the metal ones like the p/n 6601410 pictured at the beginning of the thread. That one's been in for around 10 years now.

For anybody installing one of the valves, remember that at least some of the nes being used by people are directional. Put them in backwards and they may not work as well as they should.

 

I would say they are telling you something happened when they order a new one.

 

I had heat all the time and replaced the the valve with the plastic unit in Jim's post. Also replaced the short hose from Roger. No mixing and Heat and AC work OK.