Scott:

You've had some good advice already - for general overheating I think a 
clogged radiator is very common on wasserboxers - especially if the heads have 
corroded and been replaced. Once the heads start leaking people commonly add 
generic anti-freeze, which causes the corrosion products to precipitate and 
clog the radiator. (Phosphate-free AF doesn't really prevent corrosion - just 
keeps it from clogging the system. Add phosphate and -- mush!) Not much 
success getting them cleaned after that.

A radiator clog causes the system to generally run hotter - it increases the 
back-pressure to the pump so the thermostat must open wider to get enough flow 
to stabilize. The gauge will usually climb quickly at long stoplights, then 
drop VERY slowly once you start moving at speed. (That's a good clue - the 
radiator's not pulling heat from the system.) And finally, opening a heater 
valve causes the gauge to drop quickly, because the system can provide good 
coolant flow if there is a functioning heat exchanger. 

If this is your problem get a new radiator. They run $250 to $320 (Bus Depot, 
Bus Boys, Parts Place - all seem to be the same OEM model, so shop for price). 
Replacing it means also emptying the A/C, since the condenser is mounted to 
the front, but it's worth it, and isn't a bad job.

However from what you describe I think you have a different problem, 
especially if it happens most on freeways or driving fast. The temperature 
gauge light normally means that the sensor in the expansion tank is dry. If 
coolant isn't leaking, then air is getting into the system. Next time this 
happens pull over and immediately lift the engine cover and look to see if 
there is air in the top of the expansion tank. (If you can still see though 
the discolored plastic.) If so, air is being pumped in and lowering the 
coolant level. (Lots of "bulge" in all the hoses to allow the level to drop as 
gasses are added.)

How does it get in a pressurized system? A combustion chamber leak into the 
water jacket. The top of the wasserboxer cylinders seal to the head with metal 
gaskets (like the air-cooled engines) and with silicone o-rings. The o-rings 
must fit into counterbores in the heads as the head is installed compressing 
the push-rod tubes. The silicone o-rings are very soft and fragile, and with a 
little cocking or misalignment the edge of the head-bore can cut a section out 
of the o-ring. Or if the heads have be "de-worked" the counterbore may have 
been enlarged, losing the seal. Either can allow combustion pressure to bleed 
into the cooling system, usually under high-speed driving, with symptoms like 
you describe.

Since your van "already had the head job" you could have either of these 
situations - see which symptoms match best, and let us know what you find out.

Tom Brunson
'87 Westy
'91 Carat
'93 EV
etc (to ex '57 Borgward Isabella Kombi)

>When I bought my 1991 Carat with 125,000 miles on it, the VW dealer
>who did a prepurchase inspection said it  "already had the head job".

>A couple of months later, the red coolant light would go on, and shortly
>thereafter the temperature guage would climb.

>I've replaced the coolant and bled the system per the Bentley, twice.
>There does not seem to be any coolant loss, still, we've looked all over for 
leaks,
>including the heads. My mechanic insists it can't be air in the system, but
>has no other suggestions. Can anyone give me some help?

>I have limited e-mail and cannot get to the web to search the archives.
>Please send cc: any replies to me at the address above.

>Thanks, 

>Scott Davis

>sdavis@smithgroup.com