When it is safe to use full power on a diesel?

I think it's still wise to wait until an engine gets to running temperature before caning it. Ideally the engine should be hot so the running clearance on moving parts is at an optimum. I've no evidence to support that view, but from an engineering POV it seems sensible. Mike.

Wait until the oil is up to temperature which will mean that the bearing clearances are at the correct amounts. If you have an oil cooler (not uncommon on a diesel) then the oil temperture probably follows coolant fairly closely.

I always allow my diesel to warm up if I can before revving it. However I work next to the motorway so it does not always end up like that on the way home !

Engines are a lot more inefficient when cold so this is a good enough reason for going slowly. The oil is thicker at cold temperatures too so it may not be lubricating so well. I suppose parts like the camshaft at the top of the engine could be at risk.

I think petrol only acts as a solvent in the engine when you (used to ) leave the choke out. The petrol condenses on the bores and goes past the oil scraper ring or dissolves the oil like you say.

Simon

temperature

I wondered if that was the case. But I remember my first car in 1997, it was a really old banger and had knackered main bearings. They sounded fine when cold, but made a terrible din when the engine was hot, and even worse in very hot weather. From that I would conclude that when hot the increased bearing clearance increases the risk of crank vibration (and hence breakage). So high engine speeds when cold would not damage the crank or it's bearings. Perhaps I am missing something here?

I would tend to agree here, from the view point of fuel consumption too. I only asked the question because there are times I would like to overtake on single carriageways whilst the engine is still cool, but am afraid to because I thought requesting the necessary power to do this safely might be bad for the engine.

cheers James

As far as I'm concerned, I only keep a car for a few years, I'm not bothered about how long the engine will last as all engines will go over

That seems quite a slow warmup compared to my Mercedes Truck which hits

When the main bearings are knackered you have a different situation. Good main bearings will have about 1 thou clearance all round. Just enough clearance for a film of oil to fill. If the clearance is much greater, when the engine is cold the oil being thicker can still fill the increased gap. as the oil thins as it gets warm, it allows the big end and main crank journals to rattle about in the bearings themselves. The same occurs in a good bearing, but in the main only causes drag until the oil gets thin enough to reduce the drag to a minimum, and fill the gap nicely.

Multigrade oils are designed to lessen the difference in oil viscosity due to temperature changes, but to a lesser degree it still occurs.

From that I would conclude that when hot the increased

Probably not, but there are so many variations due to temperature, that I'm only really guessing. Don't forget that we not just talking about the effect of temperature on the bearings and journals themselves. The whole engine is affected. Sounds silly but the engine gets bigger, and probably changes shape slightly. Fair enough we're only talking about thou's but bearing clearances are only thou's.

The engine will run in, or wear in according to its shape when hot, so allowing it to reach it's ideal running condition gently, seems the sensible thing to do.

What I say about the affect of temperature on the engine physically, I know to be correct, but as to it's long term affect on the longevity of an engine, that is just my opinion. Seems a logical one to make, but I don't know if it's correct.

I wouldn't have thaught the occasional use of power in that way would have a noticeable affect on engine longevity. Mike.

I let mine warm up for the initial 30 odd seconds it takes to reverse out of the driveway then, I'll go up through the gears only revving to 2k in each (55mph in 6th @ 2k so I'm not speeding when I say this :) ) by around 200 yards down the road there is a speed hump I have to slow down for some mornings I cane go over this and pull away in 4th some mornings I need to drop down to 3rd if it's 4th then it means the engine is fine if it's 3rd then it means I need another 100 yards to the traffic lights to warm it up fully all in all though, by the time I've done the 30 second reverse and the 300 yards down the road, the heater is blowing full hot air out of every vent so I know the engine is warm enough to cane it away from the next set of lights :)

Get an engine preheater unit and you can drive it as hard as you feel like as it will be up to full temp almost immediately., also the heater will work.

mrcheerful

The message from "James" contains these words:

I'd not be so sure about that.

Interesting if you measure the oil temperature, though, which is the true indicator if all the bearings are up to temperature.

They did this on my BMW at MOT time once, and despite having driven a few town miles to the garage, it took a long time running the engine at about

I've got a BMW auto which modifies the gearchanges when cold. It stays in second for much longer than usual - at town speeds - presumably to heat up the cat as quickly as possible. And won't take fifth at all until warmed up.

Personally, I wouln't worry about it too much. As part of my job I regularly give my car (1.9 diesel - non turbo) a thrash (and i mean a thrash) from cold for about a mile (approx 5 times a week). The car is now 7 years old (70k on clock) and runs like a dream. Hasn't been to a garage for a 'service' for the last 4 years, except I change oil etc myself every 2 thousand miles.

Then again I could be lucky.....