No, it's only under pressure when the pump is forcing it through the system.

The crankcase isn't a sealed unit if that's what you mean, that's what the positive crankcase ventilation (PCV) system is for. While much of the oil will return to the sump unless it's being pumped and sprayed around, capillary action and the non-return valve will keep it on the bearing surfaces, and in the oilways.

A small amount of time, yes. That's why Impreza engines crank over a couple of times before they actually fire. However, in this circumstance, all that is happening is the re-establishment of the pressure . After an oil change you have to re-establish the supply before you can get any pressure, and that's where the problems can start. Air being blown through the oil system by the pump on a running engine is a really bad thing, not matter what the design of engine.

Yes. When you drain the sump and remove the filter, air will be introduced into the pick-up pipe, pump and oilways that wouldn't otherwise be there, and of course if you put the new filter on without pre-filling it, you get a load more in.

The pump must be reprimed and the air purged through the system, and it's much better to do this with as little load as possible on the bearings.

As mentioned, the ECU takes care of this under normal circumstances. An oil change though, is a special case.

[Edited by greasemonkey - 11/11/2003 8:45:06 PM]

 

Thanks

 

right i have read most of wot has been said on here and even posted my own notice on it ??? changing oil that is , so at the start of this topic it was detailed how to do a basic oil change?? prefilling the filter etc,,, taking out the plugs and crank sensor?? couldnt i just change the filter prefill it take out the plugs and crank the car over then refitt the plugs take for a smooth drive then re top the oil would that be a good ??? as i dont really want to screw the main bearings up tahnks for any help

 

It's normally the big end bearings that fail rather than the mains.

 

Going back to what Ive has mentioned early in the tread:
Why need to remove the spark plugs? is there another (easier way) to stop the spark when turning the engine

 

Don't think its the spark that's the issue, it's the load on the bearings during compression, which obviously doesn't occur with the plugs out. That said, the load on the bearings must be very small on the compression stroke compared to the load when the engine actually fires.

IMHO, the main issue is pre-filling the filter (which I have always done, even on my mk1 Escort some 20 years ago), and dry cranking (which I have done on all my recent cars - pulling the ECU main relay is another easy way of achieving this).

Pre-filling the filter and dry cranking cost virtually nothing in terms of time and effort, so why risk destroying a few grands worth of engine just to save a few minutes?

For later cars (>MY01?), I though cranking with the crank sensor disconnected would give a temporary (5 ign cycles?) CEL and store an error code in the ECU, is this correct?

As an aside, my old man used to run a garage/MOT station. And I distinctly remember that oil filters always got pre-filled before fitting. Don't think they bothered with dry cranking though.

[Edited by dr_ming - 11/13/2003 11:57:14 AM]