Nanni engine - strange dipstick query.

[Loggo]

Sorry for what is probably a numptyish question but this is rather outside my past experience of car, lorry and van engines and you guys generally have the answer.

Checks carried out with a cold engine (maybe a few hours or weeks) same result.

The scene - Cold engine - check engine oil dipstick is fully pushed into hole - remove - just a quarter of an inch showing on the dipstick !!

Replace dipstick - check again that it's fully inserted - remove - oil shows full to the upper marker.

It seemed to me that something, maybe crankcase pressure was lifting the dipstick an inch or so but, getting wise to the situation, I always ensure that it is fully in place before the first check.

Has anyone experience something similar ?

 

[lynall]

My volvo penta 4.3 lies to me sometimes,shows low, check it another day and it shows fine! have come across same on landrovers, the auto boxes pushed the stick out slightly and the older tdi engines need to be left standing for hours otherwise they give a false low reading.


Lynall

 

[Bandit]

To take oil level pull out dipstick, wipe on rag, re insert , withdraw and read level.

 

[Loggo]

I hear what you say Bandit,and it is good practice. But I guess I was looking for more. Why would a metal rod - inserted into a pool of oil - left standing for a few weeks - give two different levels when checked ?

 

[Bandit]

Probably because when the engine is running there are several litres of oil say equivalent to 20 to 25mm of oil in the sump that are in the upper part of the engine, rocker cover head etc.

You have a column of air held in by the seal on the dip stick, when the engine is running level of oil drops an when the engine stops the column of air holds the oil out of that column by air pressure

When you remove the dipstick you break the seal allowing the oil to find its own level which is higher.

 

[Mistroma]

My Yanmar engine does exactly the same. I was worried that I'd lost oil by burning it off but that made no sense as it was brand new. I was about to top up and re-checked first only to find that the level was already at maximum. Thought I'd not read the dipstick correctly until same thing happened when I checked it again.

I realised that oil was rising up the tube after the dipstick had been removed and that the dipstick had a really good rubber seal at the top. Only thing that made sense was that air pressure in the upper part of the tube was depressing the oil level. I came to the same conclusion as Bandit that oil settled back into the sump after stopping the engine but the dipstick seal caused enough pressure to prevent it rising up the dipstick tube. I expect that oil does get pushed up into the tube as the engine cools and volume of air in the tube reduces. However, that effect isn't large enough to have much effect.

I tested the theory by lifting the dipstick out immediately after stopping the engine, wiping it and replacing it very loosely. I pushed the dipstick fully home when I needed to use the engine a few days later and found the level was correct when it was removed.

I've never seen this mentioned in any Yanmar manual and would have expected it to be displayed prominently. I wonder how many owners have topped up new engines and found they'd overfilled with oil. So not such a silly question at all if it is fairly common and not mentioned by manufacturers.

 

[Loggo]

Thanks Bandit and Mistroma for the detailed explanation. I will try your test Mistroma but I am sure you have hit the nail on the head.