Oil Consumption Towards End of Interval

[SilverGGA]

My WRX (turbo) consumes ~0.5 quart and my OCI is 5k. Consumption only starts when I am 3500 miles into the OCI though. I have tried many types of oil (Redline 10w-40, Mobil 5w-40, Mobil 0w-40, and Chevron 10w-40) and they all behave the same way. I have also done UOAs and they came back normal.

So my question is, is non-linear consumption common/normal? Why does it behave that way?

 

[addyguy]

My car does the same thing - more consumption after about 5-6k.

A reason would be that further into the interval, oil has thinned from shearing/fuel, so it is able to get past the rings/valves easier.

 

[Pablo]

Originally Posted By: addyguy
My car does the same thing - more consumption after about 5-6k.

A reason would be that further into the interval, oil has thinned from shearing/fuel, so it is able to get past the rings/valves easier.


That's part of it, but sometimes oils that have not sheared do the same thing. My theory is a combo of shearing, increased volatility and the dirty oil, or rather whatever "dirt" the oil is carrying (be it oxidized parts of the oil itself, combustion byproducts, or residual cleaning) - this junk can more easily make it past the rings for consumption.

The Honda doesn't do it. That engine just consumes no oil. My older Volvo will go along fine for 10-12K with zero consumption, then start consuming a bit of oil.

 

[Gary Allan]

Quote:
So my question is, is non-linear consumption common/normal? Why does it behave that way?


Both can be normal. One points to a design/mechanical origin ..the other oil fatigue.

I tried to find your UOA's by those lacking the Re: and the only one I could find was T&SUV 5w-40 where it was down from 14.8 to 12.7 over 5k (which you noted) I could have missed others.

Odd thing on that UOA. If it was Blackstone, they determine fuel % by flashpoint. Yet your UOA shows the baseline
Insolubles were higher than the typical UOA.

 

[JAG]

My one theory is that the piston and ring deposits somewhat suddenly increase to the point of allowing more oil to get past the rings.

 

[Gary Allan]

Interesting thought. I would think that would lead to linear consumption carried over to the next OCI ..but I've learned not to dismiss anything just based on my version of how things work. I'm always on a long (and hard) road to enlightenment.

 

[Audi Junkie]

I dunno, the guys who tell us volitility is the single greatest factor of consumption don't seem to account for the fact that the lighter componets would tend to burn off sooner rather than later.

 

[ADFD1]

My thinking is the oil is worn, has some fuel in it, and is not as good as fresh oil.

AD

 

[fraso]

In the Corvair Flat Tappet article, there is a diagram (Effect of Shear and Oxidation on Viscosity) that shows the viscosity of a Group I oil first decreasing with shear and then increasing with soot and oxidation. In earlier topics, 440Magnum has suggested that overly thick oils tend to overwhelm the oil rings which then increases oil consumption.

Is is possible that, in some cases, the viscosity of a high mileage oil has increased to the point where it is starting to overwhelm the oil rings?

 

[Loobed]

Maybe it's your oil pan shape.

if it looks something like this:

|~~~~~~~~~~~~|
|~~~~~___________|
|*******|
|_______|

(i don't know if the above picture will format correctly on your computer)

you wouldn't notice a big drop in oil level until it got past the step.

 

[SpitfireS]

I've noticed the same with 9.6 cSt Shell GRP IV/V oil.
UOA after 19k km showed it actually was 10 cSt. and yes.. had fuel dilution.
Oil consumption really started at 4-5 k km or so.
And I kept the level high so oil pan desing is no factor (in this case)

I guess it's fuel dilution.
It probably takes a while for the fuel to make oil more volatile.

PCV systems remove some oil too.

 

[StevieC]

Originally Posted By: Loobed

Maybe it's your oil pan shape.

if it looks something like this:

|~~~~~~~~~~~~|
|~~~~~___________|
|*******|
|_______|

(i don't know if the above picture will format correctly on your computer)

you wouldn't notice a big drop in oil level until it got past the step.



What do you see now?

Sorry just had to....

 

[tig1]

No difference at all with the Merc at 10,000 mile OCI. This engine always used a qt. at 6000 miles, still does. The Focus doesn't use any oil even with the dredded iron rebar, anvil, horse shoe, log chained, steel plated iron filled M1 oil.
They say a qt. of M1 weights 3 lbs. more than any other oil.

 

[Gary Allan]

Originally Posted By: tig1

They say a qt. of M1 weights 3 lbs. more than any other oil.


It's enriched with iron.

 

[StevieC]

Originally Posted By: Gary Allan
Originally Posted By: tig1

They say a qt. of M1 weights 3 lbs. more than any other oil.


It's enriched with iron.

 

[Ben99GT]

Originally Posted By: StevieC

Sorry just had to....


An alien from Men in Black after being shot by the noisy cricket.

 

[tig1]

Originally Posted By: Ben99GT
Originally Posted By: StevieC

Sorry just had to....


An alien from Men in Black after being shot by the noisy cricket.


After reading BITOG for a while I see this kind of stuff a lot.

 

[John_K]

My wife's Hyundai never uses a drop. My old Dakota is like this, the dirtier the oil the more it uses. Maybe it's trying to get rid of the old oil to get me to put in new?

John

 

[addyguy]

Originally Posted By: StevieC
Originally Posted By: Loobed

Maybe it's your oil pan shape.

if it looks something like this:

|~~~~~~~~~~~~|
|~~~~~___________|
|*******|
|_______|

(i don't know if the above picture will format correctly on your computer)

you wouldn't notice a big drop in oil level until it got past the step.



What do you see now?

Sorry just had to....


The Walrus from the Beatle's 'I am the Walrus' in Magical Mystery Tour...

 

[lanteau]

It is a boosted engine, is it possible under boost oil is going through the PCV system and burning that way? Many PCV valves let oil get through them under boost, I know thats where my Thunderbird Super Coupe is burning oil, my intercooler tubes all had a coating of oil from the PCV valve.