Poor starting

My son’s 2004 Golf Mk4 1.6 petrol (BCB engine code) is occasionally difficult to start on cold, wet mornings. It repeatedly coughs and throws the starter out, but will not start. Then, suddenly, it’s okay. There’s a puff of smoke from the exhaust and it runs and starts fine. I took off the engine cover and the puff of smoke is coming out of the hole at the top of the exhaust cover. I have tried fitting another battery, but it made no difference. David Braithwaite

The BCB engine on your son’s Golf uses plug-on coil packs that are not as susceptible to damp as HT leads. Normally, when these coil packs fail, the engine develops a misfire. Similarly, poor or worn spark plugs may also cause starting problems, although when worn to this extent they would usually also cause a misfire.

The engine cover on the BCB engine incorporates the air filter, so when starting the vehicle with this removed, a puff of smoke from the engine via one of the exposed pipes would not be unusual. The puff of smoke suggests a possible mixture problem and the engine may be over-fueling when cold. One of the prime causes of this is the engine temperature sensor, so this would be the first component to check. If the engine is overfuelling when cold, removing the spark plugs after a failed start should find them in a wet condition and covered in fuel.