SVX oil pump worn out?

[eddycat2000]

Originally posted by svxxx26
Eddy, please give me your learned opinion on this, because I am now (still?) totally confused on the effects of engine oil weight.

I "seem" to have cured, or at the very least improved my Legacy's early-morning cold start knocking by going to a heavier weight oil - yet, Subaru advises that the fix to a similar-sounding knock problem with the Outback is to go to a lighter weight.

At the time I switched to the heavier weight, my reasoning was that the heavier weight oil wouldn't drain out of the hydraulic lifters as easily.

Am I way off base, or what?

No you're not off base at all. There are different types of engine knocks. Subaru went to the lighter oil to address the piston slap type of noise. They shortened the skirts on the pistons so if you had that type of knock, more towards the center of the engine then the lighter oil would be best. However, if your noise seems to come from the head/valve area, then a heavier weight oil might help the problem. You don't mention what year legacy, or what engine, but remember only the 2.5 engines had the knock that I mentioned in the other post. The 2.2 engine never had a knocking problem. G.M. products also have a cold start knocking problem, and many customers we had used a heavier oil and it helped, but the problem with the GM vehicles seems to come from the upper end (head/valve area).

A heavier weight oil can be pumped at a higher pressure than a lighter oil because the molcules have a higher shear potential. It means they don't slip away from each other and sort of stack up. But the down side is that it moves through the oil journals slower, meaning it gets to your clacking lifters later. At least while it is cold. There is also a spring loaded bypass located near the oil pump preventing overpressuring the system and blowing seals (there aredub). Lighter oil on the other hand moves quicker through the journals and reaches your lifters sooner. But, it does so at a potentially lower pressure because of the lighter molecular shear. Light oil doesn't stack up like heavy oil for lack of a better analogy.

Hope this helps.

JP

[svxxx26]

Thanks, Eddy and JP for the info. I "think" I have a better understanding now.
BTW, the Legacy is a '93 with the 2.2, 4EAT and 155,000. It's probably the best all-around car I've owned.

<<BTW, the Legacy is a '93 with the 2.2, 4EAT and 155,000. It's probably the best all-around car I've owned.>>

The best car I ever owned was an 89 GL hatch back that eddy doesn't believe really existed. It was the last year they offered it. I still might get my hands on the engine from the thing to put in an airplane. (after rebuild of course, it has 250,000+ miles on it).

JP