Warming up diesel engines

[Dave Potts]

I've received conflicting points of view on this and am hoping someone at the AB can come up with a definitive answer.

I have a 2.5 litre Turbo Diesel, which sounds not unlike a Massey-Ferguson at the best of times, and I'm in the habit of letting it 'warm-up' for 5 minutes prior to setting off on any journey. A friend of mine who is a diesel mechanic advised me that this was good practice to reduce wear and tear on the engine as it allows the diesel to heat up. He also suggested that when I have reached my destination I let the engine run for a minute or two before switching off. The reason for this escapes me but I assume it something to do with avoiding the need to help unload all the shopping. I invariably get lambasted by She Who Must Be Obeyed and all my colleagues think I'm a loon. They may be right. Am I the only one doing this?

[sddsddean]

You should let the engine run for about 30 seconds or so, just to let the oil get round the various nooks and crannies(true of petrol engines also), but you don't need to actually warm the engine up. On most modern diesels, the best way to warm them up (as descibed in the handbook) is usually to drive the vehicle. If you are starting the engine at below 0C, the engine will start to warm the fuel, the excess being sent back to the tank via the bypass pipe. This will then start to heat the fuel in the tank. However, most diesel now comes with 'antifreeze' in it, so this is no longer a real problem (not in Europe anyway). The main reason for leaving the engine to 'run down' for a minute or so is to let the turbo cool down. They get very hot and have been known to seize if the engine is just switched off. They are expensive! If it is a non turbo engine, there is no real point in leaving it running.

[thekraut]

Don't know about letting the engine run down after driving, heating it up first is commom practice. as diesel engines don't have spark plugs the cylinders are designed to reach self-ignition temperature at exactly the right moment under driving conditions (e.i. hot engine). if the cylinders are still cold, this ignition temp is reached at a different point, which is not good. on the other hand, the engine is probably the part of your car that will die last, so I'm not sure if it makes sense after all, to burn all that fuel just to make it live even longer.

[Postie]

It's only necessary to let an engine warm up if the ambient temperature is really low (around freezing or below) then drive away gently, making normal progress (no boy racer stuff until the engine has reached optimum operating temperature). However, with a turbo diesel it is ESSENTIAL that you let the engine idle for at least 30 seconds before switching off. This is to protect the Turbo when you next start up. The turbo is lubricated by engine oil but only receives a smidgeon when the engine is running normally, by letting the engine idle you allow the turbo to become fully lubricated so that it is not "dry" the next time you start. Dry = damage and a short life.

[Dave Potts]

Thanks for all the great answers everyone. Very informative stuff - the mists have cleared at last!

[smudge]

My hubby has a Landrover Discovery TDi, you put the key in the ignition, but have to wait a few seconds until the yellow light goes out before actually firing up. Surely a few seconds would be sufficient, but then again I'm not a mechanic!

[sddsddean]

Thats for the glow plug to pre heat the cylinder smudge. My Escort van is the same. Of course, most of the year it isn't really necessary as a modern diesel will fire up even when its quite cold. My truck has a switch on the dash, so I have to decide whether to pre heat or not. In the 3 1/2 years I've had the truck, I think I've used it twice, when it was about -5C in the morning.

[Janetex]

But, it is only necessary to allow the engine to idle if you have been working it hard - not just locally at normal speeds.