How Adding Too Much Oil Can Be Detrimental To An Engine

Long time ago I overfilled, it was okay according to the instructions on the container. The engine developed a leak not long after. I ended up getting rid of that car a couple of months later even though it was only about three years old, just didn't want to repair it or live with the leak. Now I'm very mindful of the oil level and always keep it just a tad under full.
I miscounted litre bottles once when I had a Citroen AX with TU 1.1 l engine. I added 1 litre or more over max. That crankshaft hit the oil and I knew it, shook the whole car like the engine was going to drop out, and lots of blue smoke out of the exhaust until the level dropped sufficiently.

No offence, but according to the Picture of the TU engine i have found in the Intertent this can not be in my opinion. There was something different happening inside the engine. The crankshaft is several inches away from the oil level. 1 liter more will rise the oil level... a 3/8 inch, 1 cm?
Thats the the kind of wrong assumptiom that create the myth of the "Crankshaft hit the oil" myth.

But yes, it would be interesting to know what really happend inside the engine. The oil cap on My golf MK III also says "Dont overfill or the catalyst can be damaged". To me, that´s a warning that to much oil can get into the exhaust. You also report about blue smoke. The oil get into the cylinders and was burnt, that would explain the rough running engine that shakes the car.
The only way i could imagine is that the oil found its way through the crankcase ventilation into the Air inlet of the engine. But how could this happen?


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I've been overfilling my 2.4l 2AZ-FE in my 2005 Scion tC by 1.2 quarts for over the past 4 years (50k miles). Capacity is IIRC 3.8, possibly 4qts, I just dump the whole 5 quart jug in. No leaks, no problems that I can tell. These engines burn oil in higher mileages (190k) and though mine is not burning very much, *(maybe .5 qts after 5000 miles) I add it as insurance. I guess some engine sumps have more room for extra oil than others, maybe I'm lucky, IDK, not an engineer.

For the experts: Is 1 qt extra too much for the 2.4 liter?
I wouldn’t, but it seems like you have proven that it is fine. If it wasn’t, you’d have known by now.
I wouldn’t, but it seems like you have proven that it is fine. If it wasn’t, you’d have known by now.
I agree, I think I'll cut back by .5 qt, just enough to offset burning over 5K.
I don't understand how overfilling can cause a leak. The PCV valve should let pressure out, even if that pressure is reached quicker correct? And if pressure wasn't being relieved, the engine would leak regardless of oil level right? I'm assuming for a leak to be caused by overfilling, there is somehow greater pressure inside the engine pushing out on the seals. The oil level itself shouldn't cause a leak unless you've got seals barely working?
I’m curious too of how much overfilling is considered over full…in order to create damage?
I overfilled an engine by 1 full qt…forrreverr without any problem.
After only having vehicles the required 5 qt OCI and buying my first car that only took 4qts,
I was putting in 5 qts out of habit…and I did this forrrreverrr!
But, is just one quart overfilled too much, or is 2 qts or more needed to create issues?
How many qts. of oil does your Corvette's engine hold? Do you add extra oil just to get it up to the full mark?
My Corvette takes 6 quarts with a filter. I never over-fill it and always check the oil level after changing the oil and filter. I change it at least every year or 5,000 miles. I seldom go over 5,000 miles in one year however I am retired now and might rack up a few more miles. I really want the Z-06 C7 style with 650 HP however they are over valued right now on the used car market IMO.
One I don't get is the 5.3 on 2014+ GM trucks. We have one oil changer who insists on 8 qts, one who insists on 8.2 qts, and most of the mechanics want 9 qts.
I have two and have owned three. 8 with filter and 7 if running filter 2 x 5k etc …
An outstanding source here on this very forum has said that FCA (Stellantis) engines are certified to meet all performance requirements from 1 quart low to 1 quart overfilled. I would suspect most manufacturers have a similar margin of error built-in.
I would suggest that that's an unwarranted assumption.
In most engines the crankshaft is several Inches away from the maximum oil level when the engine is not running.
I think it is one of the myths of the internet: "The crankshaft splashes into the oil". It is simply not possible.
At one time, Chevrolet had an engine with a crankshaft that had little "scooping buckets" on the throws. The purpose of these scoops was to grab some oil from the sump while the engine was running and throw it up onto the cylinder walls to aid in lubrication. So, not only is it possible, but some engines were designed that way.

While most contemporary ICEs don't use such a system today, it is untrue to say "it is simply not possible."
It is not an assumption. It is straight from an engineer with FCA/Stellantis. It is a fact.
You said: I would suspect most manufacturers have a similar margin of error built-in.

That's what I was addressing, or intended to address ... sorry if I wasn't clear.
You said: I would suspect most manufacturers have a similar margin of error built-in.

That's what I was addressing, or intended to address ... sorry if I wasn't clear.
Exactly. That's not an assumption.

As far as FCA/Stellantis, that is a fact. Their internal standard is that their engines pass all of their performance tests a quart low or a quart high.

I think it would be risky and silly for a manufacturer not to have a safety margin either way, overfilled or underfilled.
Unless testing of data were made case by case, and engine specific, then there is no “one size fits all.” The big takeaway here is that eventually bad things begin to happen with an engine that is ACTUALLY overfilled. Maybe the Toyota can take on an entire extra quart, but the Ford pickup can’t. Who really knows unless somebody conducts a study on that engine model?

But the truth is, some engines can be safely overfilled. For the average user, best to just use common sense and listen to the manufacturer. It does amaze me though that some engine makers can mess up something as simple as dipstick accuracy after all the other complicated R&D was done for that engine.