Diagnose Overheating Problems in the Mercedes-Benz 300SL

The issue of overheating is not an uncommon problem when first starting and test driving a newly restored/rebuilt 300SL Gullwing or Roadster. Since there are a number of possible causes and solutions to the condition, we thought we'd list a comprehensive approach that can be taken to diagnose and correct overheating.

First, check the obvious items common to all cars.

• Is the radiator full of anti-freeze/water?
• Is the thermostat stuck? If it is not new and has been sitting around for a number of years, this is a common problem.
• Has the radiator been cleaned and pressure tested? Any leaks?
• What's the condition of the radiator cap? Does it hold pressure?
• Is the fan belt slipping? It should have 1/4” to 1/2” of play....any more and it is probably slipping.
• Has the fan been installed correctly? We have seen occasions where electric fans are installed backwards....which doesn't help the cooling at all. Is there a shroud around the fan?
• Is the water pump new? Has it been rebuilt?
• Have you checked the timing? Retarded timing will cause the car to run poorly and run hot. Certainly worth a double check.
• Is the engine running too lean....a lean running engine will also run hot.
  

The next possibility is a little more serious

• Is there any oil or exhaust gas bubbles present in the coolant? This would indicate a blown head gasket (best case) or possibly a crack cylinder head or engine block (worst case). Often the case with a rebuilt engine; no one checked the mating surface of the cylinder head. Aluminum heads quite often warp when engines over heat. Steel cylinder heads will also warp to a much lesser degree, but enough to cause problems.

The following items are more specific to the 300SL

• Has the water pump been topped off with 90 weight gear oil?
• Has the fuel injection pump been recalibrated? You cannot rebuild the engine on these cars without recalibrating the fuel injection pump.....it's like putting old wine in a new wine sack. The pump has wear on it and has, in a way, recalibrated itself to the old motor. It especially may be running lean/hot.
• Check to make sure there is oil in the injection pump.
• Check the spark plugs. We recommend NGK BP6ES or possibly one range cooler.
• Try one of the many “Water Wetter” products that are on the market.
• Did you use “break in “ oil to break in the engine? What grade of oil are you using now? Is there enough oil in the crankcase? (these take 15 Quarts of oil).

As you can seen, there are quite a number of conditions that can lead to your engine running hot. Start with the simplest, easiest to implement (changing one thing at a time)....and see if you get any improvement. As always...at your service!