Overheating Troubleshooting Guide for Air-Cooled VWs

by Sami Dakhlia

* Why care?

Unlike a watercooled engine, there is no safeguard: watercooled engine will boil over and force car to stop, thereby preventing further damage to engine. On the other hand, unless equiped with an oil temp gauge, the driver of air-cooled automobile will not know if temp suddenly goes up. Consequences of overheating is first damage to gaskets and if it gets really hot, breakdown of oil viscosity hence lubrication failure.

* Is it really overheating?

Gauges are not always accurate; try dipstick test. Can you hold dipstick without burning your fingers? Verify gauge calibration by plunging temp sender in boiling water (212 F, 100 C).

Note that you may not find out if your engine is prone to overheating unless you drive it at cruising speed for at least 1/2 hour. That's the time it takes mine to reach the steady state temp.

* What is a "correct" temperature?

180 F: too cool; is the thermostat still there?

180-210: peachy.

210-250: can live with it; many busses w/ type4 engines get that hot under load in the summer.

250-280: something is wrong and must be fixed; however, viscosity of good synthetic oil should still be fine. If it's after dark and I'm in East St. Louis, I keep on driving.

280 F: better stop; unless the oil pressure light is on (in which case you must shut the engine off immediately!), let the engine run. Glen Buhlman recommends that it not be left idling, but that revving it will allow the fan to do a better job of cooling the engine.

* Troubleshooting

Problems can roughly be classified as either cooling problems or as heat generation problem:

* Cooling *

- hot summer day? (duh)

- driving in the wrong gear? Remember that for a given speed, higher engine rpm => faster cooling fan => cooler engine.

- do you keep your engine lid open to "let heat escape"? Don't. For better cooling and to make work easier for the fan (hence the engine), you need positive air pressure.

- missing or damaged seal between engine and engine compartment? (you don't want to "recycle" hot air.)

- holes in tin: are all seals and covers there? At holes for sparkplug wires, oil pressure switch, CHT sensor (on FI models), adjusting screw for alternator belt (type 4 engines)? Is all the tin there, correctly installed and tight? No fastening screws missing? A listmember found that missing the seals for the spark plug holes increased oil temp by 10-15 F. I've heard that Corvair spark plug seals are superior to VW seals.

- Kim Kariotis says: "Be sure the aux heater fan is properly ducted to the engine. There are flapper valves in the fan housing that prevent your cooling air from escaping through the auxiliary fan via the ducts. No ducts, big leak, engine runs hotter than it should."

- oil cooler unobstructed and clean? Leaves and debris can be sucked in through the fan and cover up the oil cooler; if the cooler is oily, dust will stick to it, impeding air flow and acting as an insulator.

- do you have chrome valve covers? Valve covers are in contact with oil and work as oil coolers, *provided they're black*.

- clean engine? The goop under your engine acts like an insulator; scrape it off.

- type of oil: air-cooled engines are also oil-cooled and some oil brands are better than others at picking up heat in the engine and shedding it in the oil cooler. A reputable source (Ed Hackett) told me that synthetics are better in that regard, although some listmembers did not share that belief based on their personal experience. (Note that synthetics are much better at withstanding high temps, hence provide added protection for overheating engines.)

- amount of oil: too little ain't good; too much ain't good either.

Dennis Haynes says to keep it halfway between the marks on the dipstick. If there's too much, it get's beaten into foam, causing a drop in pressure and less cooling.

* Heat Generation *

- timing: check advance at idle and check total advance, too. Compare with specs in Bentley. (Still not clear to me if heat goes up steadily with total advance, or if the only significant increase in heat is the jump at the detonation threshold.) It is generally said that 29 deg total advance is safe and that 32 is "pushing the envelope". I'm not sure if this applies to type 1 engines only or to all VW air-cooled engines. When I checked up the specs of the dual vacuum dizzy for my type4 in Bentley, I found that 40 deg total advance is normal; unless I misunderstood the figures...

- lean running: adjust carbs or, if you have fuel injection, check for false air. False air is air that sneaks into the intake manifold without first passing through the air-flow meter; hence the computer underestimates the amount of fuel to be injected. Possible sources of false air are an EGR valve that was not disabled, say, after the EGR filter fell off; old, leaky rubber boots at the air intake distributor, disconnected vacuum hoses, etc.

- engine friction: general condition of the short-block; crankshaft endplay is said to be a good indicator (provided it was not reset since the last rebuild). A smooth, well-balanced engine will be more efficient and generate less heat. Retorque the heads. If you suspect a bearing is going bad, have a lab analyze your oil.

- non-engine friction: poor aerodynamics exacerbate the fact that required work is proportional to the square of speed. Hence you need about 40% more hp to go 65 mph instead of 55 mph. Reduce loss to friction in transmission by replacing transmission fluid every 2 (?) years. Do wheels turn easily? Check wheel bearings, look for binding brakes...

- air-intake thermostat: here's an exotic one. I'm talking about the tiny thermostat located before the air filter, which selects whether air destined for combustion should be cool air from the engine bay or preheated air. (Often, on older cars (non VW) there was no such thermostat, just a lever that could be switched either to the "summer" or the "winter" position.) Now, if this thermostat is stuck in the "preheat" position on an already hot day, you'll be burning really hot air. A quick back-o'-the-envelope calculation shows that a 2 liter engine turning at 4,000 rpm will suck in 4,000 liters (that's about 1,000 gallons) of air every minute. That should be a lot of btu's...

- valves adjusted? A tight exhaust valve, for example, will never completely close and will let partially unburned fuel escape into the exhaust system, which in turn gets extremely hot. Moreover the loss of compression in that cylinder translates into loss of power, hence requires more work from the other 3 cylinders to keep up initial speed.