Burning oil at same rate regardless of oil thickness --- What does it tell us?

Good for another year.

 

Hi Allen. I've heard it said (meaning I don't have 1st hand experience) that bad oil control rings often cause oil consumption that is relatively independent of the oil grade used. The rational is that with thicker oil the rings are wiping a thicker oil film, so that tends to negate the natural tendency for thicker oil to be more resistant to getting past the rings.

So I think the piston soak idea is definitely worth a try. Do some research to find the best product (autorx or kreen or MMO for example) and remove the spark plugs and give it a soak. I'd try gradually adding it in over a period of a few days to give it a real good soak. Remember to turn the engine over without the plugs inserted to let it expel any excess crude before replacing the plugs. You might need to use inspection mode to crank it, I'm not sure (but I'm sure there's other members here who could tell you how to do it).

 

I would suggest that the crankshaft be turned by hand, using a wrench on the center crankshaft bolt. Especially with no plugs installed, that will be a very easy task.

 

I also have come across the theory of a thicker oil possibly causing more oil use, and in my opinion has some merit. Thinner oil does flow more easily, but is also more likely to flow through partly blocked piston drain holes and be wiped more easily from the cylinder walls by the oil control rings. Obviously the opposite affect will take place around the valve stems. The thinner oil flowing down the valve guides more easily particularly on the intake stroke on an inlet valve with negative pressure at the valve head end.

John (Britprius)

 

Remember that you want to really to force any liquid remaining above the rings to literally be blown out the spark plug holes though Patrick. Allowing the engine to crank certainly does this (spark plug holes blow out air like a breaching whale). I'm not sure if hand cranking would achieve this.

Yes, that is exactly what I was getting at John.

 

Good point. In that case the OP can just try starting the engine with the spark plugs removed. Hopefully MG1 will spin the engine sufficiently to achieve this purpose before the engine ECU logs DTC associated with no-start. A gas/air mixture will be introduced in the cylinders via the fuel injectors, while the engine is trying to start but hopefully most of that will also blow out of the spark plug holes.

Then if the 12V battery is disconnected, the DTC will clear.

 

When you first make the Prius READY from a cold resting point, and leave the gear selector in P, after a brief pause the gasoline engine will spin up. This is because the engine ECU wants to heat up the catalytic converter.

 

Just leave the plugs out (though still connected to the igniters if possible). The prius has no sensor to know that the plugs are out, so a few seconds after you turn it on (make ready) it will try to start the engine for the normal warm up cycle. If you're in very warm conditions and it doesn't try to start the engine, you can always force it by holding the brake with one foot while giving the accelerator a tap with the other.

BTW. It will probably only crank for a short time (though plenty long enough for our purpose) before throwing an error code due to engine non start. Disconnecting and reconnecting the battery should fix that. I'd just leave the 12V disconnected while you replace all the spark plugs.

 

Is this correct that thicker oil causes more oil use? I thought older engines usually tend to go to heavy grade oil to reduce oil use? Shouldn't going from 5-20, 5w-30 to a 5w-40 grade reduce oil usage?

 

I wonder if using a 20w-50 grade would cause engine damage, being such a heavy weight oil. Looks like the winning formula for you is 10w-40. So basically you added 15 quarts of oil for half a year without changing anything out?

 

There are to many variables in an older or high millage engine "using" oil. The consensus of opinion says use thicker oil. This probably works well in the area of the valve guides, and maybe on piston ring ware. It will also reduce noise from excess clearances of the valves, bearings, and piston to bore.
However the thicker the oil the thicker the oil film left on cylinder walls the more oil control ring have to remove and pass through the drain holes in the piston skirts that are probably already blocked or partly blocked. Flow is restricted further by the thicker oil. If the rings are stuck in there groves and do not wipe the cylinder walls the thinner oil will flow more readily up into the combustion chamber.

John (Britprius)

 

This is really unfortunate for the Prius, such a great car but many are using excess oil around 140k-150k mile mark. Even my car at just over 125k is starting to burn some oil. I was planning to keep this car for another 10 years but with this oil issue, not sure what will come of it.