Overheating

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As we all know, the babies we all love so much are cooled by air. Adequate cooling of the VW engine, and the flip-side, overheating, are concerns.

In any system in which work is being done, the Second Law of Thermodynamics dictates that some of the energy produced is wasted in the sense that it cannot be utilized for work. This waste energy, in the form of heat, must be transferred to a “heat sink.” In a water-cooled car, the water circulating through the engine block and heads carries the waste heat to a radiator, where the waste heat is dumped to the atmosphere. The atmosphere is also the “heat sink” in the aircooled VW engine, but in Volkswagens the waste heat is removed from the engine by flowing air rather than water. Finning on the heads and cylinders facilitates the heat removal, and a cooler is provided in the system to remove heat from the engine oil; heat is also carried from the oil cooler by flowing air.

We are often asked, “What is the best way to keep my Bug cool?” And being the stock enthusiasts that we are, we respond, “You already have (or perhaps, had) everything you need.” Volkswagen engineers recognized from the outset the importance of keeping the engine cool, and they came up with a very efficient design, consisting of a blower (fan), a set of air vanes, thermostatically controlled flaps, air dams and deflector plates. This system is designed to efficiently direct the flow of cooling air toward the cylinders and heads, paying particular attention to the exhaust valve areas of the heads.

The smart VW owner will watch carefully for the tell-tale signs of overheating. In his technical manual(1), Gene Berg states that the safe upper oil temperature is on the order of 235F (113C) so. If the temperature is higher than 235F, the heat can cause case studs to pull out, head and case sealing surfaces to warp, cases to be internally distorted, permanent case metal fatigue and engine bearings to wear prematurely. Sounds like a horror story, but it is easily prevented if a few precautions are taken and followed religiously.

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How Do You Know If Your Engine Is Running Too Hot?

There are several of ways -

  1. The “Poor Man’s” Dipstick Test -

    2. With the engine completely warmed up, grab the dip stick and pull it out. If you can JUST hold it without it burning your fingers, the engine temperature is OK. If it burns your fingers (and the engine will usually smell hot too), then the temperature is too high -- possibly as high as 285F (140C) -- VERY hot!

  2. Install an Oil Temperature Gauge -

    3. An oil temperature gauge requires an oil temperature sensor/sender. There are two types of oil temperature sensors -

    • Oil Sump Plate Sensor. This sensor will screw into the oil drain plug hole, but the sensor extends upward enough that it impinges on the oil screen assembly. The instructions that come with this sensor say to drill another hole in the plate, to one side of the plug, thread it, and install the sensor there. Some oil sump plates have no drain hole. If yours is one of these, drill and thread a hole to the side of center for the sump plate sender.

    • Oil Pressure Relief Plug Sensor. If your engine is equipped with a dual oil pressure relief system, you can replace one of the plugs (i.e., the front one -- at the flywheel end of the engine) with an oil relief plug sensor and save yourself the hassle of drilling a hole in the oil sump cover plate.

      • Oil Relief Temperature Sensor(2)

      This sensor screws into the forward oil pressure relief port -- assuming your vehicle has two ports. If not, you will have to install the oil temperature sensor in a hole you drill in the oil sump plate.

       

      Oil Relief Temperature Sensor Installed.

      Our oil temperature sensor installed in the forward oil pressure relief port. Note the wire running forward -- be sure to attach it securely with clips or cable ties so that it doesn’t interfere with the workings of the heater boxes, etc.

       

      Wiring instructions come with the gauge (e.g., VDO). Generally speaking, you run a wire from the sensor up to where you install the gauge in the cabin. You will need to drill a small hole in the floor pan behind the rear seat so you can route the wire into the area under the rear seat, then forward from there). To wire the gauge you simply connect the gauge to power (we used the #11 fuse) and to ground, and you’re all set. In our Superbug we found a nice place in the dash to install the gauge, just to the left of the steering column.

      Oil Temperature Gauge.

      We found the perfect location for our gauge in the dash panel of our Super, immediately to the left of the steering column. Access is a bit difficult unless you remove the steering wheel. Using a right-angle attachment on the drill, we drilled a series of holes around the circumference of the larger hole for the gauge, then connected them with a coping saw blade. A bit tough on the fingers!

       

      A word of caution from Gene Berg: “Practically all (temperature gauges) read low in the 212 to 260 degree range by 40 to 50 degrees ...” So far we haven’t experienced this, but we’ve installed a Gene Berg temperature-sensing dipstick as a back-up -

    • Install a Temperature-Sensing Dipstick -

      • This sensor is sold by Gene Berg Enterprises. It installs in the dipstick hole and connects with a wire the oil pressure switch. When the temperature reaches about 225F, the spring inside of the dipstick turns to make electrical contact, and the oil light in the instrument cluster begins to flicker. As the temperature increases, the oil light will flicker more rapidly until it remains on continuously -- which means your oil temperature is dangerously high.

      Temperature-Sensing Dipstick.

      This ingenious device was invented by Gene Berg and is marketed by Gene Berg Enterprises. The dipstick is calibrated to signal a warning at 225F.

       

      Connection to the Oil Pressure Sensor.

      The wire from the dipstick connects to the wire running up to the oil pressure warning light in the instrument cluster. It’s easiest to make this connection right at the oil pressure switch, using a three way connector as shown here.

       

      With the temperature-sensing dipstick, you don’t get any indication that the temperature is rising until the oil light flickers, telling you the oil is becoming too hot; but at least you can do something before any damage occurs. The dipstick is very easy to install in the car, since it uses the existing dash oil pressure light as an indicator.

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How Do You Keep the Operating Temperature Within Safe Limits?

Below are some things that you can do (short of pulling the heads and fussing with the compression ratio. Rob Boardman: “Dropping the compression ratio is counterproductive; more compression means a more efficient engine.")

  • Put a larger main jet in the carburetor. A larger main jet makes for more evaporative cooling of the inlet, so the engine runs cooler. This works for an engine which is running too lean (running lean makes any engine run hotter), but it doesn't do as much good for a car which has the right mixture already.

  • Make sure that the air correction jet is the right size. A feature of all open-throat type carburetors is that they tend to run over-rich as the airflow increases through them. The air correction jet is used to adjust this mixture. This jet adds air to the fuel delivery nozzle, so larger numbers mean leaner at higher speeds. With the stock Solex carburetors (or their modern Brosol and Bocar replacements), the air correction jet is usually in the 110-130 size range. If it's up around 140-170 it's running the car lean at higher speeds (it does very little at low speeds).

  • Make sure the power jet (in the right side of the carburetor near the idle jet) is the right size. From the 30PICT/2 carburetor onwards (and the modern replacement Brosol and Bocar models), a power jet was added which re-enrichens the mixture at full throttle and high rpm, because the air correction jet works too well at the carburetor’s maximum airflow. Without the power jet (or with one which is too small), the engine will run lean at very high rpm and high throttle, and as noted before, running lean makes the engine run hot. The most common power jet size is 65. The power jet feeds fuel, so larger means richer.

  • Make sure the cooling vanes and thermostat are working properly and that the connecting rod is shaped properly to prevent binding between the cylinder head fins. When you have the engine out, make sure the flaps are opening easy and all the way -- you may have to unscrew the push rod from the thermostat, but you should then be able to push it up and down easy enough and check the flaps.

    • Cooling Vanes(3)

    This picture is looking up into the fan shroud on either side, where you can see the moveable cooling flaps. The connecting rod is also visible; this rod is moved upward by the expanding thermostat to gradually open the cooling flaps. Fully open, the flaps properly direct the flow of cooling air over the engine cylinders and heads. Please don’t remove them -- they provide a very important function.

     

    Old-Style Thermostat)(3)

    This is the stock “bellows” style thermostat, shown installed. These are not longer made and are becoming increasingly rare.

     

    The original style thermostat is no longer made and is becoming more and more difficult to find. A very good option is to use the new-style thermostat pictured below. This thermostat is the original replacement style for Mexican fuel-injected Beetles made from 1975 to 1979 and is a suitable replacement for the old bellows-style thermostat. The thermostat provides enough upward movement of the connecting rod to fully open the flaps on both Type 1 and Type 4 applications.

    Note: The fan associated with the "doghouse" oil cooler provides plenty enough air to cool up to about 80hp (a warmed up Type 4 engine in a Bug for example), but to get best effects you have to ensure every bit of cooling air goes where it's most needed. That means you must have the cooling flaps above the cylinders -- with these fully open more air is directed to the hotter heads and less to the cooler cylinders. The cooling flaps also work with the thermostat to provide a fast, even warm-up on those cold mornings. The importance of cooling flaps used to be misunderstood and many have been removed by engine builders as “not needed”, but they are essential in getting more air where it’s needed most – the cylinder heads.

    New-Style Thermostat(2)

    The “Mexican-style” thermostat, used in ’74 - ’79 Mexican Bugs.

     

    New-Style Thermostat Installed

    Fortunately the “Mexican-style” thermostat perfectly replaces the older bellows style and is very easy to install.

     

  • Test the oil cooler and replace it if it is plugged. Make sure the "Hoover bit"(4) around the bottom of the oil cooler is there. This is a roughly L-shaped bracket which seals off the underside of the oil cooler on doghouse engines, preventing spillage of cooling air back into the engine compartment. A lot of folks don't realize just how important this piece is and remove it.

    • The “Hoover Bit”

    This piece, which wraps around the bottom of the oil cooler and prevents spillage of heated air back into the fan, is often removed and discarded. The piece should be left in place to assure that hot air is not sucked back into the fan.

     

    Note: Be careful not to over-torque the nuts on the oil cooler studs – you might squash the small rubber seals too much and reduce the oil flow through them.

  • Make sure the drive belt is properly adjusted, but not TOO tight -- you don't want to ruin the alternator bearing. A half-inch deflection in the center of the belt, when pressed with your thumb, is about right. You'd normally start off a new belt with 3-4 shims and work up to 5-6 shims as the belt wears. Replace the belt as soon as it shows signs of wear – cracks or fraying. Always carry a spare fan belt – without it your engine will fry in minutes.

  • Use the stock black valve covers on the heads -- black covers radiate heat better. We've heard it said that using the stock black valve covers will reduce the oil temperature by as much as 30F (17C). Fancy valve covers might look good, but the stock ones are better for correct engine temperatures. (Years ago VW used to supply an arctic kit for engines to be used under very cold conditions. This kit included chrome valve covers to keep heat IN the engine.)

    • Stock Black Valve Cover

    We removed our “cool” shiny aluminium valve covers and went back to the stock black ones. We’ve noticed a significant decrease in the engine operating temperature as a result.

     

  • Make sure the tin is properly fitted around the oil cooler to assure heated air from the oil cooler is being dumped to the outside, not back into the area in front (front means front of the car) of the fan where it can be sucked back into the system.

  • Make sure all of the engine tin is fitted properly and all of the seals are in place to assure that only fresh air is being sucked into the engine compartment and thus into the fan -- not hot air from under the engine. This includes the seal around the engine, the engine lid seal, and any holes in the tinware. Doing this will reduce smell in the cabin when the heaters are on, too.

    • Engine Tin(5)

    This great exploded view of the engine tin, courtesy of the Old Beetle Home(5), shows all of the tinware pieces, each specifically design to maximize the flow of cooling air. Please leave them intact!

     

    New Engine Seal

    An excellent replacement for the stock rubber seal, especially if you’re having trouble with the seal falling down onto the muffler around the rear. Sold by Aircooled.Net.

     

    Note: Dave installed a fiberglass rear apron on his '73 SB. This apron does not have the slot for the stock rubber engine compartment seal, so the stock seal would not stay in place but kept falling down onto the muffler. Dave discussed this situation with John Connolly of Aircooled.Netand decided to go with an alternate seal that John recommended, shown in the picture above. This seal is used in the 1972 and newer Type 2's and the Type 4 VW. The seal is pressed down into the space between the engine tin and the body of the car.

  • Make sure the rubber air dams around the spark plug leads are in place. We’ve heard it reported that there is a 25F (14C) rise in head temperature with these left off. They harden with age and can then slide a little way up the plug lead, resulting in a lot of leaking air. If they are not soft and flexible, replace them.

  • Make sure the deflector plates under the cylinders are in place. There is a small flat plate on each side, between and under the cylinders, and an even smaller one under the center of each cylinder head (on the right side the under-head deflector has a slot in it for the thermostat pushrod). These are vital for good cooling. They force the cooling air through the underside finning -- without them the cooling air spills straight down and does not provide much cooling for the underside of the heads or cylinders. Some folks leave them off, not realizing how important they are.

  • While you are under the engine, make sure the underside plenum pieces of tinware are in place (a large curved piece of tinware under the left side and two pieces on the right so the thermostat can be easily changed). These turn the used cooling air backwards to the rear of the car. If they are left off, the underside engine finning is exposed to the turbulent high pressure air under the car and poor cooling will result. On the stock engine the cabin heat exchangers are used to dam off the outer sides of this plenum, and if you have replaced the heat exchangers with J tubes you still need to seal the outer edges of the plenum with some cut-to-measure pieces of flat metal.

  • "Cool tins" are used on Type 3 (pancake) engines to improve underside cooling and can be fitted to the Beetle engine too. They replace the cylinder deflector plates (the head deflectors stay in place) and cup the underside of the cylinders so all the cooling air is forced through the underside and out through the exit holes in the tinware. They also have "ears" which cup the outer corners of the head near the exhaust valves so that those valves get a better flow of air through the fins there. They work by actually reducing the cylinder air flow a little (but using it better), and this results in a higher airflow through the heads, so cooling is improved there too.

    • “Cool Tin”(6).

    You shouldn’t need the “cool tin” if all of the other parts of your cooling system are in place and functioning. But it is available at minimal cost (e.g,, Aircooled.Net)(6) and is easy to install (with the engine removed from the car).

     

    You have to remove the engine to install the cool tin, and to fit them to the Type 1 Beetle tinware they might need to do a little cutting and bending. They are not really needed for a stock Beetle engine, but if you have a higher performance engine or are experiencing cooling problems, they might help.

  • Correct casting imperfections in the heads. If you look through the finning on VW heads, you'll often find "dags" of metal partially blocking the airways. Removing these with rat-tail files, a Dremel, a chisel, etc. can improve airflow through the heads.

  • Use the standard VW pulley -- not a "power pulley." These spin the fan slower and reduce cooling. A “power pulley really isn’t worth it -- you gain about 1 hp at the wheels for the loss of maybe 20% of your cooling air -- a BAD trade-off).

  • Run higher octane gasoline. This will help if you have been running a too-low octane fuel, but won’t help if you already use the correct octane. But don’t think “higher is better” either. A too-high octane number will not harm the engine in any way, but won’t help it run any better either. The stock 1200 and 1300 needs only 87 RON (84 AKI in the USA) and the stock 1500/1600 needs 91 RON (87 AKI in the USA). If your compression ratio is higher than stock, then you’ll need fuel with a higher octane number.

  • Avoid oxygenates in the fuel. The most common oxygenates added to gasoline are methyl-tertiary butyl ether (MTBE) (used extensively the USA) and ethanol (when added to gasoline the mixture is sometimes called “gasohol”). Because old VWs have no engine computer to adjust the mixture on the run, oxygenates will cause the engine to run leaner than it should, and lean means hot. If you cannot avoid oxygenated fuel, increase the size of the main jet to return the mixture to its proper balance. For example, fuel with 11% MTBE (used in many parts of the USA) runs the engine 2% lean and a main jet one size larger will fix this problem. Ten percent ethanol in the fuel (called E10 in many countries) needs a main jet two sizes larger to bring the mixture back into balance.

  • Some think that installing an oil sump will bring about greater cooling of the oil. All of the advertisements say that adding a sump increases cooling; however, Gene Berg says it just gives you more hot oil and is only advantageous in racing and hard cornering applications (to prevent oil surge -- not to increase cooling). The surface area isn't much more for additional cooling, and you also loose a couple of inches ground clearance. Oil sumps are good for racing, but not for cooling.

  • Driving style can certainly affect the engine temperature. Constant hard acceleration and braking will heat the engine. If you have to brake hard then you’ve just used more engine power than you needed. Always shift down a gear before the engine starts to lug on long hills or when towing a heavy load -- this unloads the engine and increases the fan speed for better cooling.

  • Regarding cooling the engine while driving – we’ve occasionally heard of people driving at low speeds in 4th gear -- low rpm -- to reduce the temperature of an overheated engine. In fact it works just the other way around. Since the oil cooler is in the cooling fan's air stream, the best way to cool a hot engine is to cruise gently in 3rd gear rather than 4th. This way you don't ask for lots of power (heat) from the engine (up to a nice steady 40mph is fine), and 3rd gear keeps the cooling fan’s speed up, which helps both the oil cooler and the cylinder head temperatures.

There you have it. Use all the stock parts of the cooling system, properly fitted and without any exposed holes in the tinware, use the right fuel, set the carburettor mixture right, drive sensibly, and your engine will run in its preferred heat range in summer or winter.

Credits and references -

  1. Courtesy of Gene Berg Enterprises.

  2. Courtesy of California Import Parts, Ltd.

  3. Courtesy of Stephen Gutknecht -- SGutknecht@i405.com.

  4. This piece is referred to as the “Hoover bit” because of Bob Hoover’s discussion of it in one of his “sermons” on the subject of cooling. Unfortunately Bob Hoover’s Sermons are no longer available on the Internet.

  5. Courtesy of The Old Beetle Illustrated Parts Catalog.

  6. Courtesy of Back to Table of Contents

    Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.

    We hope you find this information useful, but we don't take any responsibility for anything which happens to you, other people, your VW or any other property or goods resulting from your use of this material.

    Feel free to print off any of this information for your own use. If you intend to link this material to another site, reprint it, or in any other way redistribute it, please leave the information complete, including this disclaimer section, and provide a link to this Web site.

    Contact us.

    Have fun fixing your VW - just keep them fweeming, OK?

    Last revised 23 June 2004.

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