Oil Use and Timing change - only after approx 60min @ 70Mph

[davhaney]

I lurk on this forum quite a bit, but rarely post. However, this has me scratching my head. I've got a 1999 Subaru Legacy outback, 2.5. Manual Transmission with 150k miles on the clock. Maintinance has been kept up, including timing belt, oil changes, etc. Have also replaced the knock sensor, clutch, oil seperator plate, and clutch slave cyl.

 

However, when I drive at 70 mph for more than about an hour, two observable things occur:

 

1) I can feel the timing retard, and the engine lose a bit of power when climing up hills, or gently accelerating for more than a few seconds. The feeling is similar to the performance before replacing the knock sensor -- though there is no CEL this time.

 

2) The car uses approximately 1 quart of oil every 3 hours of highway driving.

 

The car is almost exclusively driven in-town at slower speeds for 20-30 minutes at a time. Under these conditions, it never uses oil, and I never feel the timing change. Temperature is normal, and it idles smoothly.

 

I'm taking a vacation this month, but not sure if I should rent a car, or drive mine. Advice?

[davhaney]

Also - MAF has been cleaned, Spark plugs replaced, and Spark Plug wires replaced (subaru OEM).

[davebugs]

Almost sounds like the cat.

 

Do you know is it's burning the oil or it's leaking somewhere.

 

BTW that sounds like a lot of oil. What is the problem with that?

[davhaney]

I don't know for sure about where the oil goes. If I were gambling, I'd guess it's losing it. If it were burning that much oil, I imagine it would cause the exhaust to be blue.

[davebugs]

Perhaps someone with similar experience will chime in.

 

If I had a car using oil like that it'd already be parked until it was fixed - no matter how broke I was. Because it ain't gonna get any easier or less costly to fix, and is likely to cause more problems, which means more work and money.

[johnceggleston]

when did it start using the oil? soon after the separator plate was done? did you replace it or reseal it?

 

did you torque the knock sensor to spec?

 

did you replace the rear main seal??

 

have you looked under the engine to see where the oily dirt is, front or rear?

 

did you reseal the oil pump, o-ring / backing plate screws, when you did the timing belt?

[davhaney]

I rarely drive the car such distances, so I can't pinpoint when it started (before or after separator plate replacement). I know it occurred the last three times I drove over an hour (which was two years ago, and again this last spring).

 

The plastic oil separator plate was replaced with the metal one, along with screws, and sealant. While I was doing the work, I also replaced the rear main seal. Under normal circumstances it isn't leaking (no spots on the garage floor, no topping off oil between changes). I haven't looked for oil spots under the car recently.

 

When I took off the oil pump during the TB job, the screws holding the back on were tight enough I couldn't remove them. I did however, replace the O-ring (subaru OEM) while I was in there.

 

EDIT: I absolutely used a torque wrench when installing the knock sensor, and tried to orient the device as specified, as well.

Edited December 8, 2010 by davhaney

[OB99W]

I suggest checking the PCV -- the valve for proper operation (or just replace it), and the plumbing to make sure it isn't full of sludge. An engine that's mainly used for short trips at slow speeds can tend to have problems in that respect.

 

Also, you might have worn valve stem seals (not common, but does happen), or stuck rings. Again, the type of driving you've described moves that up on the possibility scale.

 

If you haven't replaced the air filter recently, take a look at it or just replace it. (Yes, I said air filter. )

 

EDIT: Assuming the oil is being burned, enough of it can cause knock, which would of course lead to the timing being retarded.

Edited December 8, 2010 by OB99W

[naru]

Higher oil temps and blowby at highway speed are surely factors.

 

Power loss is probably due to the knock sensor pulling back timing because of detonation caused by oil in the combustion chamber.

 

Conversely,a malfunctioning knock sensor may allow detonation and resultant oil consumption.

 

Replace the PCV valve,but,don`t expect miracles.

Use higher weight oil if possible.

 

I have a non-sube w/similar symptoms.No smoke either.

[johnceggleston]

well it sounds like you did it all right. (you have been lurking here for a while , good work.)

[davhaney]

OB99W -

 

Good idea. A couple years ago, I cleaned the PCV valve with brake-cleaner spray. At that time, it rattled when I shook it. I also removed and cleaned the hose connecting it. I'll check it again (along with the air filter).

[nipper]

Replace the PCV valve, don't clean it as that is not much of a test anymore. Check the PCV hoses. Do a full tuneup, as the loss of power at speed can be bad wires (use OE wires). The wires can fail with heat. Also this is a classic coil failure (weak spark when hot) but lets do a tuneup first.