Not too hot and not too cold...

QI have a Mercedes R107 380SL fitted with Bosch K-jetronic fuel injection. Cold starting is always excellent. When driven, then left for an hour or more, it's difficult and often starts on four or five cylinders for a few seconds until all eight join in. Four years (and very few miles) ago, the injectors, fuel pump, filter and pressure accumulator were changed. Any advice?

David Houlgrave, Upholland

Nigel says

AStarting a car that's cooled is tricky: with a carburettor, you juggle the choke (enrich) and throttle (weaken). So, this may be a case of over- or under-enrichment. First check: stop the hot engine. Remove the oil filler cap and smell the oil. If it smells sharply of petrol, change it. Vapours are passed to the inlet manifold to be burned and this can upset the correct mixture strength on warm/hot starting. On K-jetronic, the part-warm starting and running phase is dealt with by the warm-up regulator, which is bolted to the engine and has a heated bimetallic strip inside it. It is connected to the main fuel distribution unit by two pipes and diverts fuel pressure to bias a fuel-regulating valve in the main unit, enriching the mixture. The warm-up regulator has a spring-loaded rubber diaphragm valve. When the engine is cold-started, the valve is open to allow biasing for enrichment. The valve is then closed down over a short time period, returning the mixture strength to normal. This is done by an electricallyheated bimetallic strip which bends as it heats, acting on the spring-loaded valve and gradually closing it.

The regulator is bolted to the engine, so engine heat also acts on the bimetallic strip, keeping the enrichment off during immediate hot-restarting. If the engine cools somewhat, the valve might open – causing enrichment until the bimetallic strip heats up and closes it again. Try turning on the ignition and waiting ten seconds for the bimetallic strip to warm up a bit. This might eliminate overfuelling.

Make sure the regulator is firmly attached to the engine, to allow good heat transfer, and that the two-pin electric plug is attached. Some are fitted with a vacuum pipe, so make sure, if fitted, that this is in good order and correctly connected at the other end (to the inlet manifold or throttle body). Sometimes these regulators are modified to increase enrichment suggesting underfuelling may be an issue in some cases, but this would be felt as poor driveability when cold.