Dipstick question (oil, not person!)

[reedbr]

I've always been trained that to check the oil level, you pull the dipstick (ignore the reading on that first pull), wipe it, insert and remove it again for an accurate reading.  After years of working on cars old and new, I've always seen the first (inaccurate pull) has oil all over the dipstick (a high or fuller reading).  The second pull after wiping is lower and accurate.

On my C34mkII M35B, the variation is reversed.  The first pull always shows the oil level LOW.  The second pull after wiping shows it higher (FULL). The first time this happened after an oil change I was concerned since the engine drained and took ~4 quarts instead of 5 per the manual.  Well, I see that question was answered in a previous post (the manual is wrong, the capacity is 4 quarts).  I also have made sure the dipstick was all the way inserted before each pull (even the first one).  If I do it a third time or more, it always matches the second one.  This happens every time I check the engine.  The engine is usually cold when I check it.

Does anybody else get similar readings?  It's not a big deal, I use the second pull to gauge oil levels.  It's just a curiosity that it would be great to get an explanation on.  Throw in a lot of words like multi-viscosity, adhesion, and fitzer valve and I'll believe you.

[Gene Regan]

 

     The same thing happens to me every time I check oil after boat has sat a while . Days not hours. Dont understand why but it does. I remember the first time it occoured then I reinserted and checked it 6 more time sto be sure. Oh well I guess its just one of lifes mysteries.
      Perhaps one our resident gurus can offer some explanation for it ,or maybe not but suface tension on the stick may be as such where it will not allow it to remain stuck to the stick for a long time . Hmmmmm ???

[Ray & Sandy Erps]

I too have noticed the same thing and figured it had something to do with the dipstick/oil getting hot when the engine was run last.  Might put a film on the dipstick that the oil doesn't stick too until it's wiped off.  Interesting mystery.  Anyone have a car with a diesel engine that does the same thing?

[Steve S.]

Whew!  And I thought it was only me.  I do the same as above, check it about 3 times to make sure my eyes are OK.

[Stu Jackson]

The dipstick has a rubber boot on the end of it.  It creates a vacuum when the engine cools off.  Car engines do NOT have these rubber boots.

It will ALWAYS read low on the first pull.

Take it out partially when you get to the boat.

Do something else (useful).  Or, sit there a few seconds till the oil levels out inside the engine.

Put the stick back in, pull it out, and read it.

That's it.

I rarely pull mine all the way out, I can catch the line at an angle and not drip oil.  I also use this as an opportunity to scan the engine and bed with a flashlight, every time.

I believe that the reasons were covered in the old FAQs, but it's always good to discuss these things as reminders for all of us and our new members.

A website search on dipstick gets:

[cat34discuss/_borders/disc1_ahdr.htm]
Re: OIL IN BILGE
C34 Chat
From: Stu Jackson 1986  #224  Aquavite


Comments
Steve - a few things for you to check: the oil drain fitting on the bottom, and the seal on the dipstick. Recent List posts or maybe an earlier chat item said that an older dipstick seal can go. Inexpensive fix is to get a new dipstick. Other thoughts: check both the connection of the air filter housing to the engine air intake and the oil breather line. I've run that black hose breather line into a small cup to avoid it leaking oil into the pan. Stu

[cat34discuss/_borders/disc1_aftr.htm]

I'm guessing that there are other references out there from searches.

Aren't ya glad that was so easy!?!  :)

One more thing, don't forget to "up" the dipstick when you're changing oil.  It relieves the vacuum and helps to suck the oil out if you use the pre-installed hose to the bottom of the oil pan.

[Ron Hill]

Guys : What Stu failed to mention is - that if the first pull of the dip stick reads correctly you need a new dipstick because the rubber seal is leaking.  There should be a complete seal in the dipstick tube after the engine is shut down.  This been written up in the Mainsheet many times and in the FAQs.
Always double check that your dipstick is completely seated.  If it isn't you'll find one "heck of a mess" from dirty/filthy oil coming out of the dipstick hole!!!  Believe me!

[Ray & Sandy Erps]

It occurred to me what's going on.  I'm not convinced that the initial low dipstick reading is the result of the engine creating a vacuum, because my motor has a crankcase breather tube on the valve cover with no check valves and if anything the engine creates pressure in the crankcase while it is running.  I don't doubt that the seal around the dipstick has something to do with it though and it occurred to me that the dipstick tube probably extends all the way down into the pan.  When the motor is running and the crankcase gets some pressure, the tube is also slightly pressurized.  When the engine is shut off, the pressure in the tube keeps the oil from filling up the dipstick tube, so the tube is empty, even though it's below the oil level in the pan.  Next time I pull the dipstick, I'm just going to crack the seal like Stu does and see if the dipstick tube fills with oil.  That should confirm the pressurize tube theory.

[Stu Jackson]

Ray

It's the weekend and I've stopped being a weekday engineer and maybe used the wrong word.  Your idea makes sense and I may have misused the word.

Whatever it is, the seal's still important, coming or going.

[APACHE]

Ray : Yes, the breather prevents a vacuum in the oil pan and it remains at normal atmospheric pressure.  
What's happening is when you start the engine the oil is circulated thruout the engine and the oil level in the pan goes down.  When the engine stops the oil from the upper engine flows back into the oil pan.  Because the rubber rings on the dipstick seal the top, air is trapped in the top of the dipstick tube and will not let the oil flow up into the dipstick tube.
Hope this makes sense.