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Checking the engine oil level...


Maintaining the correct engine oil level is extremely important because it ensures that there is enough oil in the engine to properly lubricate all of the moving parts and keep engine wear to a minimum.  Engine oil is also a primary means, in conjunction with the cooling system, by which heat is dissipated from the engine.  Heat is transferred away from the moving parts inside the engine to the oil.  If there’s not enough oil in the engine, this transfer of heat can’t take place effectively.


Don’t let the engine oil fall below the ‘Low’ mark on the dipstick…

The engine oil level should never be allowed to fall below the ‘Low’ mark on the dipstick.  This will result in a lack of lubrication inside the engine and excessive wear on the moving parts, as well as an engine that runs hotter.  Ideally, the engine oil level should be maintained at just below the ‘Full’ mark on the dipstick, however it’s very important to be aware of how easy it is to incorrectly over-fill the engine oil, as I’ll explain below.  Over-filling the engine oil can easily result in your engine developing oil leaks which are expensive and inconvenient to fix.


Figuring out how to read your engine’s dipstick…

You can consult the owner’s manual for your vehicle to determine the volume of oil required to raise the oil level from the ‘Low’ mark on the oil dipstick to the ‘Full’ mark, which then enables you to interpret the reading on the dipstick to determine how much oil needs to be added.


Always wipe the engine oil dipstick clean before taking a reading…

After you pull the engine oil dipstick out of the engine, always wipe it clean with a clean rag before returning it to the engine and then pulling it out again to read the oil level on it.  Because the dipstick sits in the sump of the engine where oil sloshes around, it’s simply not possible to get an accurate dipstick reading by simply pulling the dipstick out of the engine without wiping it clean first.


Procedure for checking the engine oil…

Check the engine oil level weekly and top up the engine oil level to just below the ‘Full’ mark on the dipstick if necessary.  Because it’s critically importance not to over-fill the oil above the ‘Full’ mark on the dipstick, it’s best to leave a buffer of 100 millilitres (1/5 pint) below the ‘Full’ mark.  Over-filling the oil invariably leads to oil leaks arising from excess oil pressure and should be avoided at all costs.  There are four types of errors that can lead to over-filling the oil:

  1. Checking the oil level when the vehicle is on ground that isn’t level which results in an incorrect oil level reading.


  2. Not correctly interpreting the reading on the dipstick to ascertain what volume of oil needs to be added, possibly as a result of not knowing what volume of oil corresponds to the difference between the ‘Low’ and ‘Full’ marks on the dipstick, as indicated in the owner’s manual for the vehicle.


  3. Not accurately measuring out the correct volume of oil to be added in a clear measuring container with a graduated volume reading on the side, but rather guessing as to whether the correct volume is being added and adding an incorrect volume.


  4. Checking the oil level soon after the vehicle has been driven which results in a false oil level reading because the engine oil hasn’t fully drained down into the sump yet.  It’s the oil level in the sump at the bottom of the engine that the dipstick is measuring.

How to avoid a false engine oil level reading…

It’s probably this fourth error which is the most insidious and the easiest error to make.  When the engine has been running and is then switched off, a significant volume of oil remains in the cylinder head of the engine and in the oil passages inside the engine.  It takes at least an hour to fully drain back down into the sump.

If the engine oil level is checked soon after the engine is switched off, this oil which is yet to drain into the sump won’t show up on the dipstick, which results in an incorrect engine oil level reading.  Only oil in the sump shows up on the reading on the dipstick because the dipstick sits in the sump and measures the engine oil level in the sump.

You can measure how much oil remains in the top of the engine after the engine is switched off in your vehicle by going through the following steps:

  1. Measure the oil level at the dipstick when the engine hasn’t been running since the previous day and record the reading.


  2. Run the engine at idle speed until the engine reaches operating temperature.


  3. Switch the engine off.


  4. Wait two minutes to allow the bulk of the oil in the engine to drain down into the sump.


  5. Measure the oil level at the dipstick again and record the reading.


  6. Determine the difference between the first reading from step 1. and the second reading from step 5. as a volume in millilitres/fluid ounces.


  7. This volume is the amount of oil that remains in the cylinder head of the engine and the oil passages inside the engine two minutes after the engine has been switched off after previously running.

For my vehicle, this volume of oil is equal to 450mL (1 pint).  (I also took engine oil level readings at three minutes after the engine was switched off, five minutes after and twenty minutes after.  The corresponding volumes of oil still yet to drain down into the sump were 400mL, 350mL and 250mL.)  What this means is that if I was to rely on the oil level reading two minutes after the engine was switched off and top the oil up to the full mark on the dipstick, I would end up over-filling the engine oil by 450mL (1 pint)!


The danger of over-filling the engine oil…

Over-filling the engine oil leads to excess pressure being placed on the oil seals at either end of the crankshaft (also known as the front main seal and rear main seal or the front crankshaft seal and the rear crankshaft seal).  If this excess pressure persists because I don’t realise that I’ve over-filled the engine oil and I continue to operate the engine while the oil is over-filled, in all likelihood, these seals will be damaged and leak oil.

These seals are inexpensive parts in and of themselves, however it’s very time-consuming to replace them.  In the case of the rear seal, the automatic or manual transmission needs to be unbolted from the engine and removed (as does the flywheel), not to mention re-installed after the seal has been replaced.  This is a huge and time-consuming job.  If a mechanic does this work, the bill for all the time involved will be very large.

Taking care to avoid over-filling the engine oil and damaging the front and rear seals of the crankshaft probably best epitomises the correct approach to auto maintenance, as well as the kind of pitfalls that are waiting for the unwary.  The massive cost and inconvenience involved in replacing these seals as a result of excessive oil pressure from over-filled oil highlights just how much trouble can be avoided by being knowledgable and careful about important issues such as this one.  It’s for this reason that it’s always best to adopt standard procedures for auto maintenance and follow them habitually to prevent problems such as leaking seals from developing.

In the case of checking the engine oil level, the procedure that should be adopted is to check the engine oil level in the morning when the vehicle hasn’t been driven since the day before, so that all of the oil in the engine has had ample time to drain back down into the sump, resulting in a correct engine oil level reading at the dipstick.

If, for some reason, you need to check the engine oil level soon after the engine has been running, it’s vital to already know how much oil is yet to drain into the sump as measured by the seven steps listed above.  By knowing this information, it’s then possible to adjust the engine oil level reading at the dipstick to obtain a more accurate reading.

Note that it’s also necessary to use your watch to measure how much time has elapsed between the time at which you turned off the engine and the time at which you measure the oil level on the dipstick, so that you can use the correct adjustment volume, as measured in the seven steps listed above.  If you’re using the adjustment volume that relates to two minutes elapsing after the engine is turned off, then it’s important that you wait exactly two minutes before checking the engine oil level at the dipstick.

Because it’s not possible to ever make a perfectly accurate engine oil level reading when the oil level is checked soon after the vehicle has been driven, it’s important to exercise the utmost care to ensure that the oil is not over-filled.  By holding some oil back to accommodate this uncertainty, you can avoid over-filling the oil.  You can check the oil level again the next morning before you turn the engine on to get an accurate reading that you can rely on.



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Copyright 2016 Andrew Mackinnon.  All rights reserved.