J.L. Classics

TR 6 Bosh Pump stops car in warm weather.

TR6 & TR5 cars were originally fitted with a Lucas fuel pump. History now shows that this type of pump was very prone to overheating. The resolve to this problem is widely known. The majority of TR6`s now have Bosh style pumps fitted as a replacement. These pumps are very reliable and not as susceptible to hot weather failure.

There is however a warm weather problem associated with Bosh pumps, little reported. Bosh pumps are pushers and not pullers. They run at the same speed all the time, moving a similar volume of fuel whether you are doing 100 mph or just 20 mph in traffic. The fuel not required by the metering unit gets sent back to the petrol tank being a recirculating system. In warm weather, when the total volume of fuel in the tank is low, the engine speed not demanding a large volume of fuel, the fuel gets sent round and round. As the same small volume of fuel continues to be recirculated it gets progressively warmer and warmer. The composition of modern fuel has a lower boiling point than that of the fuel it replaced. In warm weather, with ¼ a tank of fuel or less, a Bosh pump can cause the fuel to boil within the pipes.
The symptoms are – the engine stops, the fuel pump wines like its out of fuel and air bubbles can be seen within the in-line glass fuel filter (should you have fitted). Resolve – wait for it to cool down & add fuel. Prevention – run with more than ¼ of a tank full of petrol in warm conditions.

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