Is a long idle killing your diesel engine?


RVD: For how long should you warm-up the engine on your diesel tow rig? Thirty minutes is probably a bit excessive, right?

Tony: Warm-up is a little bit of a personal thing. I feel that you should start the vehicle give it a minute or two then proceed off at a steady pace. Don’t over-rev the vehicle, just take it steady until it warms up to normal on the temp gauge. 30 minutes of warm-up is not necessary and on modern diesels will cause problems with DPF-type vehicles and excessive soot build-up from EGR in the intake, etc.

RVD: Apart from getting oil around the engine, what’s the purpose of warming-up the engine, from an expert’s point of view?

Tony: Warming-up the vehicle is to allow oil to flow around the engine. The oil moves pretty quickly but when the oil is cold it is too thick to lubricate correctly. The other reason for warm-up is to allow all of the moving parts to get up to their correct operating temperatures and expand and contract to the correct clearances. An example of this is if you have a performance engine with forged pistons these may get a rattling noise when they are cold and get a noise called piston slap. It goes away once they are warm and up to temperature as they expand to the correct clearance. It is more about not putting the engine under excessive loads and high RPM while it is cold. If you give a cold engine a hard time you increase the risk of engine wear and/or engine damage.

Tony builds everything from itty-bitty car turbos, right up to monstrous gas-power turbocharged generators

RVD: Does a longer (than recommended) warm-up time help or hurt the longevity of the engine? Or are they just putting hours on the engine and being fuel unnecessarily?

A wastegated turbo (top), vs a newer technology VNT/VGT turbo system

Tony: Longer warm-up times on older diesel vehicles does not really hurt the engine as they don’t have the pollution controls of modern diesels. All this does is put hours on the engine and burn fuel unnecessarily. Modern diesels have DPFs and EGR valves, etc. where excessive idle periods can cause these to operate in manners not recommended by the manufacturer. This routine can cause the intake manifolds to soot up more than normal and the DPFs to choke up faster causing more burns and excessive fuel usage. Modern diesels are all computer-controlled; some limit the amount of fuel delivery until the vehicle is warm enough and reduce the performance. Taking off at a steady pace and taking it easy for the first few minutes of the trip will not hurt the engine. Taking off and giving a cold engine high RPMs and high loads unnecessarily will cause excessive wear and damage. Modern diesel vehicles have better cooling systems than those of old and are designed to warm-up the vehicle quickly. Letting the vehicle start and idle for a minute or two will not hurt it and will only help but much more than that is really unnecessary in my opinion. It just causes excessive noise in the caravan park, unnecessary smells, etc. for no gains.

RVD: On cool-down, is there any great point to letting the tow rig idle for five minutes after pulling into a town/caravan park? Back in oil-cooled only turbo days, it made sense, but with modern turbos with both water and oil cooling, is there really any point?

Tony: Idle-down really depends on the conditions in which you have been driving. If you have been working it hard right up until you pull up to shut it off, five minutes is definitely worthwhile. If you idle through the town then get to the caravan park and back your van into its spot, it would be cool enough to just shut down, as you have basically done the job of the turbo timer anyway. VNT (Variable Nozzle Turbine) turbos spin at idle and at a fairly high speed compared to the older wastegated turbos. Idle time is more about temperature and allowing the temps to reduce before cool-down.

RVD: As a side question from cool-down, we’ve all seen the big haulage trucks sit there idling while the driver runs into a shop, or to the loo, is that more fuel-efficient for them to leave it idle, or is it a cool-down thing, or is it another reason entirely?

Tony: Allowing your vehicle to sit and idle on the side of the road may be helpful if you have been working it hard and you want to stop and let it cool down or to try and keep your batteries charged or the fridge going, air conditioning going for the passengers you’ve left sitting in the car. Running the vehicle for five minutes or so for times like this will not really hurt anything. But if you have already driven through a town and everything has cooled down it is not really necessary to do this like the trucks do. Trucks keep running for a number of reasons: Mechanical sympathy on starter motors; keeping the air up to the system for brakes, etc. Systems on trucks are vastly different to the light automotive vehicles we all drive, and as such there are many different reasons for keeping the trucks running. Just remember, that leaving your car running and being more than six metres away from the vehicle can get you in trouble with the local constabulary!


So there you have it; idling your tow rig for 30 minutes while you hitch up in the morning is not only wasting fuel and a detriment to your engine but will potentially infuriate everyone around you (especially at 5am). Massive thanks to Tony at Motovated Turbo and Mechanical in Toowoomba. I’ve personally had the pleasure of having Tony do all the turbo and fuel system work on my old 80 Series over the years, and made the nine-hour trek up to Toowoomba from Newcastle anytime I’ve needed anything done to the old girl.

If you need any work on your tow rig, from the turbo to the fuel system and all things mechanical, feel free to drop in on Tony and John at Motovated Turbo and Mechanical, Shed 4/398 Taylor Street,  Toowoomba, QLD 4350, or give them a call on 07 4598 0033.

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