VW - Normal Oil Temp

What is a normal oil temperature reading for a 74 bus 1800 engine? or any bus for that matter...i’m getting an oil temp gauge and once i install it i would be curious as to what normal is compared to “too hot”.

This is one of the most common questions I hear. The correct answer is rather fuzzy.

The fact is, there’s no one ideal temperature. By their nature, air-cooled engines have a wider envelope of ‘normal’ operating temperature than does their water-cooled cousins. What you’re given to work with in the case of air-cooled engines is a range of safe operation. On simple instruments the range of normal operation is usually marked in green, caution in yellow and waythehelltoohot in red.

I’ve never seen a published figure for the normal oil temperature range for any air-cooled Volkswagen but their Industrial Engine Division showed the green arc as being from 170 to about 220, with a yellow arc above that, apparently up to about 250, and red over the last segment of the dial-face.

You could get the industrial engines with a set of gauges for oil temp, oil pressure and amps. Pressure was picked-off at the gallery where the oil-pressure signal-lamp switch goes in vehicles. Oil temp was picked-off at the inlet to the oil pump, a suitable adaptor replacing the threaded plug found there. (Ed. Note: Only found on early crankcases.) Installing the OT sensor in a different location will give you a different reading, one that is typically lower than you'll see at the inlet to the pump, which probably accounts for the wide variation in Oil Temp figures cited by various VW owners. Then you've got the accuracy & precision of the gauge itself. ( ALWAYS calibrate your gauges. )

Another reason for the fuzziness has to do with that stuff we use as our cooling fluid . . . air.

The operating instructions for the industrial engines listed a range of air temperatures at which you could run with a maximum load on the engine. I think the upper limit was about 85 degrees Fahrenheit. Above that you were cautioned to reduce the load if the oil temperature rose out of the green . . . pretty much common sense, if you’re running a grain-drill or an irrigation pump.

It’s not too surprising to find the Owner’s Manual for Volkswagen vehicles saying about the same thing, albeit with reference to the oil-pressure warning lamp . . . if it’s a hot day and you’re driving fast or carrying a heavy load and the lamp begins to flicker, the manual tells you to slow down... to reduce the load on the engine.

The funny part here is that while a farmer is bright enough to figure this out, most driver’s somehow miss the point. I get a lot of messages from people asking why their bus overheats when they drive seventy miles an hour on a 90-degree day, as if there’s some dark mystery involved. Sadly, telling them they’re driving too fast often gets a rather snippy response. :-)

The truth is, air-cooled engines are more suitable for cold climates. Water-cooled engines do best in the desert. This is one of those grizzly facts that continually bumps heads with Conventional Wisdom... at least, until they bother to sit down and figure it out on paper. Pointing out that Berlin is as far north as Winnepeg sometimes helps but the myth of Kubelwagens in the Sahara usually overpowers any intelligent answer. There were some Bucket-Cars used in north Africa but according to German mechanics who were there, they had a habit of swallowing #3 exhaust valve, a failure-mode woefully familiar to anyone pounding across west Texas in an early beetle. (A lot of Texas is farther south than Cairo. Most of the north African campaign was fought near the shore of the Mediterranean, in Tripolitania and in the Libyan Desert, hundreds of miles north of the Sahara.)

This isn’t to say an air-cooled engine is unsuitable for a hot climate, it’s simply not as suitable as a water-cooled engine under those conditions. You can keep right on using your air-cooled engine in Brawley or Qunianga Kebir (which is in the Sahara Desert), but you’ve got to keep your foot out of it . . . you simply don’t have enough latent cooling capacity to handle maximum output at high ambient air temps.

So what’s the ‘normal’ oil temperature? I don’t know. I know what’s ‘normal’ for my engine and vehicle and instruments and load and climate. But I don’t know what’s ‘normal’ for yours.

If you’ll examine the archives you’ll see that the question of temperatures is a pretty popular theme. You’ll also see a lot of different numbers, the ‘normal’ temps registered by different people in different locations doing different types of driving with their vehicles. About the best you can do is make a note of the range of temperatures they’ve cited and see if your combination gives a number within that range. It ain’t too scientific but you could do a lot worse.

-Bob Hoover