I always change my own oil. My Impreza is the only car thats needed the ignition sensor or the plugs taken out so you can dry crank the engine.
Answer me this. Empty oil, no oil pressure. Fill filter, ( i agree) and fill engine. Then dry crank to bring pressure up. Then replace sensor or plugs and start.
If the pressure is lost when the drain plug is removed why isn't the pressure lost when we check oil with dipstick. Doesn't that lead to the sump. If there was always pressure wouldn't it be lost.
Don't get me wrong I am no expert with engines but it seems weird why only the imprezza needs this process.
If I am proved wrong can someone post a picture of the ignition sensor.

Cheers

Andy

 

Andy
The oil is simply held in the sump from where it is drawn by the oil pump and distributed around the engine via the filter. The oil pump is of course driven by the engine, so there is oil flow only when the engine is running. The oil pressure is the resistance as the oil "squeezes" it's way through the very fine clearances at the crankshaft main and big end bearings etc. keeping the bearing surfaces apart.
When you change the oil filter the oil galleries will be partly drained and need to be re-filled before full oil flow is achieved through the filter and hence the bearings. So it is a good idea to crank the engine to get the oil flowing into the galleries to prime the system before firing up (good idea on ANY car-not just Subarus. I used to do this back in the '60s even!)
There have been a few scare stories of bearing failure on here and the reasons are varied, so if you can eliminate one of the possible causes then it's worth doing (if only for your own peace of mind!
JohnD

 

To be honest it's a desirable practice on any engine. The difference with the Subaru is that the flat configuration makes transient oil starvation more of a risk than on a typical inline design of similar displacement.

pmsl, you've got the wrong end of the stick there, in a major way. The oil in the sump isn't under pressure. The only reason it falls out when you remove the drain plug is simple gravity.

Because there's no pressure in the sump to lose. If there was, oil would p*ss out of the dipstick tube if you removed it with the engine running, wouldn't it?

Of course it does, however, that's the unpressurised side of the lubrication system. The pump picks up the oil from the sump and forces it through the oilways into the bearings (among other places) from whence it drops back into the sump, to be picked up and pumped round again.

If you drop the oil and filter out, you will introduce air into the pressurised side of the system. If you immediately restart the engine, there will be a temporary interruption in oil supply while the pump reprimes, the filter refills and oil pressure is re-established throughout the system. During that time there's a risk that the remaining oil film can be overcome by combustion forces, allowing metal to metal contact, from which there's usually only one end.

Doesn't show.

As mentioned, it wouldn't do any harm following a similar practice with other engines. The reason why it's more important on the Impreza is that the engine is horizontally opposed, which, among other things, means the main bearings are a lot narrower than you'd find in a typical two litre inline four engine, and there are two separate heads and four camshafts, two of which are quite a distance away from the pump.

You don't need a pic, everything you need to know is in this thread.

 

You in a good mood GM ?..... that was very restrained!

 

Gee a thread I started got to 5 pages! Am proud!

Hope it helped some with GM and API David's tips!


Any sign on the new SN of a FAQ for these seemingly simple questions/help?

 

Cheers for the replies.

Andy