Diesel engines are capable of very high mileage with good maintenance, but after three or four years they can become frustratingly reluctant to start. The problem is very often quite simple and the solution is cheap, provided the mechanic is competent. Sadly, I often hear from people who have been told they need all sorts of very expensive repairs from a new injector pump to a new engine, only to find they still have a car that obstinately refuses to start on a cold morning. A little knowledge and observation can point you in the right direction before the expert in the overalls leads you astray.

Diesel engines are easily pleased and need just a few basic supplies to get them going and keep them clattering away happily. They need fuel to burn, preferably in a nice fine spray, air to burn it in and enough heat to start the fire. When the engine won’t start, lack of air is rarely the problem. Unless there is a major restriction the engine will suck it in. So, that leaves lack of fuel or not enough heat as the only possibilities.

If fuel is being squirted in but there’s not enough heat to ignite it, the unburned fuel will pass straight into the exhaust and out of the tailpipe. This gives a simple and easy check. When the engine won’t start, ask someone to crank the engine for about fifteen seconds with their foot pressed down on the accelerator. If fuel is being injected you will see it coming from the tailpipe after five to ten seconds as a dense white smoke. If you get the white smoke, the entire fuel system is working and there’s simply not enough heat to ignite it. More importantly, the very expensive injector pump is working correctly, so question any advice that suggests you need a new one. If there is no smoke when you crank the engine, make sure there is some fuel in the tank, if the car’s parked on a steep gradient and you can get it onto level ground safely, try again, or get the car recovered to a garage for further investigation.

Heat is normally supplied by the very high compression pressure, but for cold starting, glow plugs are small electric heaters that give an extra boost to the temperature. Problems fall into three categories: glow plugs, a worn engine with poor compression or a starter that’s not turning the engine quickly enough. A further problem can be worn or dirty injectors, giving a coarse, uneven dribble of fuel rather than the fine spray the engine expects.

If the white smoke was coming from the exhaust, you will usually get the engine started eventually and once it’s fully warmed up you can take another stroll round to the back of the car and study the smoke. A very worn engine will betray itself by burning oil, giving a bluish tinge and acrid smell to the exhaust. High oil consumption is confirmation of this. A major overhaul is the only permanent cure.

Worn or dirty injectors show themselves by black smoke on acceleration or under very heavy load. An injector service by a diesel specialist will often transform a lacklustre engine – even though the manufacturers claim that it’s not necessary. An alternative is to use a premium diesel fuel such as BP Ultimate or Shell V-Power for a couple of tankfulls. The extra strong detergent in these fuels cleans all the deposits off the injectors and the effect is the same as de-scaling your showerhead. You get a fine, accurately aimed spray rather than a weak dribble, and the fine spray is what the engine relies to burn the fuel evenly and efficiently. If the engine is below about 80,000 miles or so, this may be enough to restore good starting performance.

If there’s no evidence of blue or black smoke and the engine is still hard to start, have the glow plugs and their relay checked by a competent auto electrician, but get him or her to check the starter motor circuit as well. I have known many diesel cars and vans that were cured of an unwillingness to get up and go in the morning by simply cleaning and tightening the heavy starter cable connectors. A high resistance anywhere in the starter circuit starves the starter motor of electricity and it can’t turn the engine quickly enough to get maximum compression and heat. It is a very common, cheap to repair and often overlooked cause of difficult starting.

So, a little understanding and a smoke break can help you only to pay for the repairs you need. And if you have taken the car to a garage to cure a cold starting problem, arrange to collect the car first thing in the morning and make it clear to the garage that the engine must not be started until you arrive. Check this if necessary by feeling the engine when you get there. Then make sure it starts quickly and reliably before you pay up.

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