An Overheating Mystery

[mromrell]

I present to you, a Mystery. Maybe you can help me solve it.

I just took my '84 waterboxer on a 1300 mile road trip. Midway through it starts to overheat, but only for 3-5 seconds before the light stops flashing and the temperature needle settles back down to its typical 3/4 position. This repeated itself spuratically, usually at least 15 minutes apart until I pulled over. Soon I learn that I don't even have to stop the van, I just lay off the gas and shift to neutral.

This all occurred while on the freeway in outside temperature's ranging from 88-35 degrees F. (Granted it definitely occurred more frequently when it was hotter outside). Additionally if I stopped the car, even for just 5 minutes, the engine wouldn't overheat for a good hour. Lastly, it seemed to only overheat when going downhill or while on flat ground, never while I was pushing it to go uphill.


I suspect 1) air in the coolant lines 2) faulty thermostat that's opening up too late or 3) my radiator fan not working? (I did remove the radiator fan fuse just before the problem started, but replacing it didn't help) I'm not sure if the fan was working or not. But I don't have a heater fan, not sure if that's related.

Any ideas on what the issue might be? Thanks!

[dobryan]

There is a coolant level sensor on top of the pressure tank in the engine compartment. It sounds like it is intermittently misreading the coolant level. The probes into the tank get corroded and do not sense properly. For your year when the level is low (or sensed that way) it also pegs the temp gauge as well as flashing the red light. My guess is you have a faulty level sensor...or your coolant is low enough that it is triggering the sensor....
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Dave O
'87 Westy w/ 2002 Subaru EJ25 and Peloquin TBD

"To travel hopefully is a better thing than to arrive." Robert Louis Stevenson

MD>Canada>AK>WA>OR>CA>AZ>UT>WY>SD
https://www.thesamba.com/vw/forum/viewtopic.php?t=620646

Building a bus for travel in Europe (euroBus)
https://www.thesamba.com/vw/forum/viewtopic.php?t=695371

The Western Syncro build
https://www.thesamba.com/vw/forum/viewtopic.php?t=746794

[Terry Kay]

Remove the sensor from the top of the expansion tank.

With a toothbrush and a little CLR give it a good cleaning.

The fouled sensor usually will function as new once you knock the accumulation of snot off of it.

Ta-Da--

Easy fix.
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T.K.

[mromrell]

Oh, I like easy fixes . I'll try that out
So it doesn't sound like symptoms of air in the line or a bad thermostat?

[dobryan]

Usually when they overheat they don't go back to normal in 3-4 seconds.
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Dave O
'87 Westy w/ 2002 Subaru EJ25 and Peloquin TBD

"To travel hopefully is a better thing than to arrive." Robert Louis Stevenson

MD>Canada>AK>WA>OR>CA>AZ>UT>WY>SD
https://www.thesamba.com/vw/forum/viewtopic.php?t=620646

Building a bus for travel in Europe (euroBus)
https://www.thesamba.com/vw/forum/viewtopic.php?t=695371

The Western Syncro build
https://www.thesamba.com/vw/forum/viewtopic.php?t=746794

[Californio]

Another possibility is the 14-pin connector below the dash, kind of on the right side of the steering wheel. If this is wonky it can send your temp gauge all over the place. Maybe when it happens again, try reaching up in there and fiddling with the connector; you may see the needle go right back down. Yes, welcome to Vanagon land.

[Terry Kay]

83 to 85 have this temp guage pegging & red light blinking problem when the level sensor gets skuzzy.
More common a problem than the wire wiggle mambo.
An inbred issue.
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T.K.

[Ahwahnee]

[atomatom]

x2 to what @ahwahnee said. as you rev the engine, the pump sucks coolant out of this tank to other parts of the engine.

one thing to add; if your coolant tank (behind licence plate) has coolant in it, and the pressurized tank has air in it, then the hose connecting these tanks may not be air tight. when functioning correctly, after running hot the engine will push air out of the pressurized tank and into the other tank. when the engine cools, it will draw in coolant to replace the air.

if the tank behind the plate has no coolant in it; well, that is an easy fix.

my hose, along with most other plastic sitting above the exhaust, was cracked. i replaced it with some commonly available fuel line and put clamps on it for good measure.
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84 Vanagon Westy, 1.9L, California raised but defected to Canada.

[borninabus]

i would recommend a new, Blau pressure cap while your at it.
if you haven't done so yet, they are cheap and it couldn't hurt.
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88 Van WBX, A/T - 13 JSW TDI 6M/T - 2012 Touareg TDI Sport

[mromrell]

You guys are awesome! Thanks for helping me out on this. And @atomatom, thanks for the detail on how the two tanks worked together, I've always wondered that.