A 1/3" layer of sludge in the bottom of the oil pan could displace 0.3 qt of oil.
…and those 15k oil change intervals could certainly have led to that amount of sludge.
I am constantly mystified by the number of people who think that their “magical” synthetic oil somehow allows them to ignore the car manufacturer’s specified oil change intervals.
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If its crusted up top, nothing worth doing anything about. If there is sludge in the oil pan though which is a good thought, the only thing that can be done is pull the pan and clean it out. How hard it is depends on the car.
Have the valves ever been adjusted?
The OP’s claimed MPG is above the EPA rating for highway only MPG. And they are sad about a lousy 1 to 2 miles per gallon drop, OK fine.
I recommend they be checked; measured, not just checked audibly for clicking, at least every 60k miles.
Valve clearances out of spec can reduce performance & MPGs, even damage the valves.
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Have you checked the dipstick itself for any changes such as the rubber seal missing or the cap sliding up the stick? Either could be causing a false reading.
As for crusty drain back holes, IF you have that problem, they must be mechanically cleaned, a flush will not do it. If the drain holes were clogged, you would have high oil consumption as the oil would back up around the valve stems and get sucked into the cylinders.
Some oil is retained by the head though. There is a trough under the cam that the cam lobes dip into for lubrication. This oils the cam immediately on start up so the cam does not have to wait for oil pressure to build up. It is possible that the ends of the trough where the oil drains out could have some build up making the trough deeper. This is more likely than plugged drain holes.
If you are having a sludge issue, you should be able to see it when you remove your oil fill cap. There is a baffle that limits what you can see, but if you only see clean, slightly brown metal, you are good.
A small drop in fuel economy along with an increase in oil consumption is normal as a vehicle ages. Yours is doing remarkably well.
Yes, good suggestion on the valves. I forgot that Honda doesn’t use self-adjusting hydraulic lifters. Also, you won’t hear clicking/ticking when the valves get too tight which is more common than them being too loose. Basically they won’t be able to completely close. This will harm your efficiency and eventually lead to burned valves if you wait too long.
You might also have deposits taking up space in the oil pan as suggested. What is your driving style? Is is mostly highway or a mix of highway/city? I wouldn’t go anywhere near 15,000 miles if there is a lot of city driving.
I don’t think I would use an engine flush on this engine yet. It obviously isn’t a total crud monster with sticky rings. I would change the oil and see how long it takes for it to turn dark. If it turns dark in like 100 miles, meaning black on the stick, then you probably have crud the new oil is dissolving.
I am not familiar with this engine but can you see inside the valve cover with the oil fill cap off or not? Some engines have a baffle so that oil won’t be lost if the cap comes loose or is left off. You can’t see much on these. Others have no baffles and you can see right in. You will see the cam lobes all shiny and silver on this type. The bad thing is you will get quite the oil mess under the hood if the cap comes loose. If you have this type, look inside and use a flashlight if that helps. Is the metal pretty clean looking or a solid dark brown color? Remember all the metal in here is an aluminum casting so it should be a dull silver color if perfectly clean. A light tan/brown tinge is OK and normal but a tar like substance or solid opaque brown is not. This is something I always do when looking at a used car for myself or others. Some people claim to have always changed the oil but the appearance under the valve cover says otherwise.
Look up “neglected engine oil change” on Google Images to see what can happen.
think there is any correlation between the engine burning 1/3 quart and now it being 1/3 quart overfull after oil changes?
Could it be you are filling the 3.7 quarts up at oil change, checking the level before starting the engine (so no oil in the filter,) then checking it later and that 1/3 quart is in the filter so now the oil level reads 1/3 quart lower?
When you get the valves adjusted, they’ll have to pull the covers off so that would be a good time to have them check for any deposits.
Wow, thank you all again. Apologies for this delayed response, Monday was a 12 hour day for me. I’ll have detailed responses to these various helpful inquires in a bit. (Or tomorrow; I’ll do my best. First week of school, etc.)
@keith I had not considered the state of the dipstick, but
I’ve never noticed a change of dipstick placement, or any decay that would
affect the degree of insertion. Likely not relevant: The dipstick no longer
twists on insertion/removal. (It rotates freely.) The internal piece that
guides the dipstick twist is out of place, broken, missing … I’m sure it’s in
there somewhere, benignly (?) out of the way of Important Moving Parts. That
predates the overfill issue by years.
Regarding internal appearance visible though fill hole: I don’t know that these images are helpful. Some surfaces are shiny. Some surfaces are light brown/tan. I observe oil residue on various surfaces, but nothing that looks to be any viscosity other than motor oil. Nothing looks tar-like to me, but then, I don’t know what I’m doing. Several images with different angles or focal points to follow:
I don’t know why you’re worried about plugged oil drain-back holes on the head?
If that were the case, some of the oil would be trapped above the head and not drain back into the oil pan. That would cause the oil reading to be low and not high.
Tester
This thread has gone beyond ridiculous IMO . 190,000 miles , uses 1/3 qt of oil in 15,000 miles & fuel mileage has dropped from 42.1 - 41.8 . What a devastating sequence of events .
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I don’t see any sludge in those pics, but I do find the irony of using overpriced oil and not ever changing said oil, funny though.
Looks very clean in there, considering the mileage, just about ideal.
Now get those valves adjusted.
BTW, I’ve owned 4 Hondas ('75-'88) and valve adjustment is one of the most satisfying DIY jobs.
Change the thermostat too. Not expensive and it might help.