Thursday, November 15, 2007

VP -- Overheating

You'll find the original version of this thread on the Usenet Newsgroup devoted to air cooled Volkswagens. It began with a complaint of overheating by someone who done a lot of bolt-on modifications to their engine. They asked the Newsgroup if adding an after-market oil cooler would solve the problem. The general opinion was that it would not, since the engine had been heavily modified even through it still had the stock displacement, which is where we pick up the tale...

>Given that the items are external, I claim it as stock.


Dear D.....,

You've managed to miss the point.

Normally, only about 15% of the engine's waste heat will appear in the oil. If the engine is fitted with the dog-house type oil cooler and all other cooling system components are in place and functional, the oil cooling system provides about 120% of worse-case capacity.

The point you've missed is that elevated oil temperatures are a symptom rather than a problem. Normally, elevated oil temperature simply means the engine is being operated outside of its designed envelope -- the load is too heavy for the speed or the speed is too high for the ambient air temp, or both. And yes, on a hot day this can occur with just you on-board. The cure is to simply take your foot out. Alas, on a southern California freeway that can earn you a ticket for obstructing traffic, which is why a lot of VW bus owners have learned to fly by night :-)

The root problem is whatever is causing those elevated oil temperatures -- and that could be anything from low tire pressure to a loose nut on the steering wheel :-)

Ever had the measles? Your body temp can hit 105. Dunking you in ice water is a sure-fire treatment of the symptom -- it will bring your temperature down. Of course, the shock usually killed the patient :-)

Audit your cooling system. Every part is critical. Check to see that the deflector plate is installed on the underside of the heads. If running after-market dual-port tin the odds are the gull-wing deflector above the manifold hole is missing. Check your spark plug seals -- any loss of air pressure guarantees a loss of air flow.

Check your fan belt. And the diameter of your pulley. (Those wunnerful 'power pulleys' guarantee the engine will run hot.) Reach around behind the blower housing and feel every blade in the fan to insure you haven't picked up leaves or other debris.

Check your oil pressure. Elevated oil temp is one symptom of a worn-out oil pump.

Check your brakes, tire pressure and alignment. Low tires, draggy brakes and improper alignment all demand more work from the engine. The car may be doing 65 but the engine could be doing 90.

Ditto for the clutch & tranny. When was the last time you replaced your tranny lube? (Recommended interval is 2 yrs or 24,000 miles.)

What kind of shape is your engine in? Yeah, I know -- perfect :-) But a lot of 'perfect' engines show up here at the shop with complaints of over-heating. It's not uncommon to find excessive blow-by, indicating worn rings or valves. And excessive blow-by dumps a huge amount of excess heat into the oil. After-market carbs are one the main reasons for accelerated wear of pistons & valves, mostly because of their inadeqaute air cleaners. It only takes about a teaspoon of dust to trash an engine and anyone who has cleaned an oil-bath air filter knows the engine sucks in ten times that much between oil changes. The other failing of after-market carbs is running too rich a mixture, either because of inadequate manifold heating or running a mechanical advance distributor.


Conventional Wisdom as espoused by the magazines and after-market retailers say that if your oil is running hot you gotta spend some more money by adding one of their sooper-geewhiz oil coolers, with a fan yet, and mebbe one of those ohsokewl thermostatic valves that are guaranteed to leak. And lotsa hose. And fittings. And brackets. And other neat stuff.

Now the oil stays so cool it's kewl. Of course, the root problem -- whatever it is -- is still there, ticking like a time bomb. And when it goes off those wunnerful folks will have some more really kewl fixes you can buy. In fact, the experts have an answer for everything! The only question is, can you afford it?


Hot oil is a symptom. What you need to do is deal with the problem that produced that symptom. Adding an oil cooler is like trying to cure cancer with aspirin; the pain may go away but you're still gonna die.

-Bob Hoover

...then John writes:
>what I've found
>is that most air-cooled VW engines are filthy on their exterior.

(ie, as a major contributor to overheating)


Dear John (and the Group),

Good point.

A friend recently asked me to look at the air-cooled engine on a paving machine (!). I don't know anything about paving machines. Nor those nifty two-cylinder Honda engines. But he'd gone through two engines in two years and it was pretty obvious the Honda guy just wanted to keep selling him new engines at about two grand a pop.

The engine's fins were covered with fur.

Or what looked like fur. It was dust, glued to the aluminum fins with hydraulic fluid.

Actually, it didn't look all that bad... you could brush it off, which they did to the inlet grill every morning. But it would come right back because there was lots of oily vapor getting sucked in by the cooling fan and the machine spends it's life in a dusty environment.

It reminded me of a 40 horse with a leaky oil cooler -- the famous Volkswagen Toaster :-)

The really funny part was the reaction when I pointed out the layer of dust (which makes a dandy insulator). My friend doubted that such a thin layer of crud could be enough to over-heat the engine. But it was easy enough to rig a duct to insure it sucked in only outside air... and without the oily vapor, the aluminum fins stayed clean... and the thing didn't overheat. (Truth is, I think the thing originally came with such a duct... that someone had removed to make it easier to get at the hydrualics.)

Liv & lurn -)

-Bob Hoover

PS -- My personal favorite was the doctor's Porsche and the engine filled with leaves from a Chinese Elm that shaded his reserved parking space. Even when you showed him the cylinders packed solid with leaves he kept insisting they would blow right on through; that SOMEONE must of packed them in there on purpose. Makes you wonder about his diagnostic skills, eh? :-)

Then someone asked:

> What do >mean with the gullwing tins?


Look inside the stock DP cylinder-head tin-ware. There is a gull-wing shaped deflector that directs the blower's output toward the exhaust valves.

Making sure the deflector is a fairly tight fit against the fins immediately adjacent to the two upper-middle studs is a standard procedure when mantling a DP engine. Without a well fitted deflector the cut-out for the DP manifold end-castings is little more than a hole through which your cooling air can escape.

Your OP readings look okay. As a test, change to straight 30W and compare the results. No difference is good, anything else is a clue that needs to be checked out.

It would probably be a good idea to rig some instruments to check the stoichemistry of your mixture. Same for your ignition timing and compression ratio. The fact an engine runs does not mean it is running well.

There IS a reason for this sort of thing. Odds are, it will turn out to be something you've assumed to be okay... and therefore did not check.

-Bob Hoover

Then came several questions, the upshot of which made it pretty clear the person was expecting to find one single problem as the cause of their overheating


Your engine is a SYSTEM. All of its parts interact. Problems seldom have a specific single cause but are the accretion of numerous small problems, usually things deemed unimportant by the 'experts.'

The deflector plates on the underside of the heads serve to maintain adequate air pressure in the plenum space ABOVE the heads; the gull-wing deflectors direct the air to where it is needed most; the thermostatically controlled flaps are designed to align with the central fin on the cylinder head so that the densest slug of air from the blower is directed to the hottest parts of the heads... all 'unimportant' details. The failure of any one of them may go unnoticed until you get a really hot day, or climbing a grade with a heavy load. Then you will have more waste heat than the engine can managed.

You're looking for a smoking gun -- a single cause of all your problems. Odds are, you aren't going to find it. What you are finding are a host of small 'unimportant' problems, the combination of which have lead to an episode of overheating. Now the camel has his nose inside the tent because excessive heat carries a legacy of future problems; it's a genie that can't be stuffed back in its bottle.

Most people simply do not believe that such close attention to detail is necessary. Indeed, the archives of this Group contain dozens of messages from idiots saying exactly that when someone having more experience tries to explain why their failure to devote any attention to those 'unimportant' details is the reason thier ride is a piece of shit.

"All my buds say..."

"I've driven for ten years without..."

"Nobody does it that way!"


The good dope is in the manuals. The distillation of thousands of man-years of engineering excellence backed up by more than twenty million engine's-worth of experience. Build a few engines yourself and perhaps you can add to that sum-store of knowledge but most of what you read & hear about Volkswagens is from an idiot with one engine's-worth of experience expressing his expert opinion, or an after-market retailer preying upon your ignorance.

-Bob Hoover