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Cold oil takes longer to reach the valves - but why?


Topic starter

I am a huge fan of Scotty and his videos. He is an endless source of useful infos.
But there are some minor things that I not quite understand, and I would appreciate some additional explanations, if possible.

For example, in various Scotty´s videos I remember him saying multiple times, with different words, that most wear happening to the engine happens when the engine is being started cold, one (or maybe even the) reason for that intensive wear being that cold oil takes longer to get pumped into the upper part of the engine, to the valves (hope I picked the main idea correctly).

At first glance, this sounds fairly logical: cold oil is more dense, hence it takes longer to get it pumped up high.

I know that some believe that I tend to overthink things - but still: when I somehow thought longer about it, I figured out that I do not quite understand the physics behind this process. So maybe someone could be so kind to lighten the darkness of my ignorance.

Just imagine the oil pump of the engine. It has steel gears submerged in oil and getting rotation from the engine shaft. The volume of oil these gears pump per one rotation of the pump is firmly pre-defined by the pump hardware. The pump rpms are engine rpm-dependent, hence are controlled by the ECU. At startup these gears "grasp" the predefined volume of oil and forcifully - having the whole huge power of the whole engine available to them - squeeze this volume into the oil channel leading (also) to the motor valves. The power of the engine is so huge in comparison to these small steel gears, that fluctuations in oil viscosity appear to not really make much difference to these gears - they will still push the designed oil volume into the oil channel at the designed speed, no matter what - no matter how high or low the oil viscosity. Feeling is, that even if one would pour fluid honey in, these gears would still pump it through and up high with no viscosity-related problems at all. Since, once the oil is already inside this oil channel, it has no other choice than to move up high exactly at the required speed, pushed up by new portions of oil delivered by the pump - hence, oil viscosity will / can not really be a significant limiting factor for the oil upward pumping speed. Not even the RPM will drop due to higher oil viscosity - the ECU will compensate for that immediately (actually, at lower temperatures the ECU is programmed to inject more fuel and to keep higher idle RPMs for a couple of minutes after a cold start).

So when thinking along these lines, cold oil should actually be reaching the valves not sooner and not later, than hot oil - for the oil pump, oil viscosity makes too little difference. So could the extra engine wear at cold startup be attributable not to the speed at which oil is being pumped upwards, but, maybe, to worse lubricating properties cold oil has, or to larger play cold unoiled engine parts have, or to smth else? 

Thanks a lot for explanations, and for the patience in reading the many letters I typed 🙂

4 Answers

Oil simply does not flow as well when cold. GMC pushrod V6 engines used to address this issue by having the camshaft run in a trough that would be filled with oil when the engine was shut down, thus keeping the cam fully lubricated and preventing scuffing on cold starts (see paragraph #6 in the link.)


Hot oil flows better.

Maybe it pumps at the same speed ... I don't know, but you are overthinking the pump part. The oil has a long journey to make through your engine, through various narrow passages and channels, in order to reach all the parts to coat them, and lubricate them. For example pushrods can have oil opening with a diameter of around 1mm. To illustrate the effect, you can try sucking some cold honey through a drinking straw, versus sucking up water. Or warm up the honey.

The pump is only part of the equation. Don't forget that once the oil reaches the upper heads, it relies on gravity for the remainder of the circuit. A lot of oil is distributed by the timing chain (some models), the oil is also pushed/splashed around by the various moving parts of the engine.

oil viscosity makes too little difference

viscosity makes a huge difference. This is why oil makers are constantly trying to push down the cold temp viscosity index with synthetic oils, like 0w20



Watch the video here @ 5:43, as he takes the oil out of the freezer and pours it.


Here are some videos of transparent auto parts showing cold oil tests:


I've always thought that with a cold engine oil has drained of the parts in question by gravity thus leaving very little lubrication on the surfaces. Starting a cold engine also has the problem of the oil being more viscous until it warms up and takes a few seconds to circulate properly. When you start a warm, recently driven engine, there is still a little oil on the surfaces that hadn't drained off and the oil in the crankcase is also going to flow better.

Does any of this make sense?

All of what you have written makes perfect sense. Frankly, everything you have written was known to me. It is just that nothing you have written has anything to do with the speed at which cold (or hot, or whatever) oil gets pumped towards the valves. However it might well be that I simply misunderstood Scotty´s idea / words. With my learnt English, this might well be the root problem 🙁

I thought the whole point of multi-viscosity oils tend to be at the lowest viscosity when cold. So 5W30 oil should flow better when cold (i.e. 5) and as the engine heats up it gets to a higher viscosity (i.e. 30)?

It will flow better when cold than if it were single-weight oil. That does not mean that it will flow as well as it will when hot. See the video I posted below, although crude it gets the point across. You can question it as much as you like but that does not change the objective reality that warm or hot oil will flow more easily than cold oil.


If I understand correctly what you are asking, I think your theory about the same level of mechanical force being applied to the oil whether it is hot or cold is correct.

However, according to the testing in the video below the same force being applied to cold oil does not mean it will circulate as fast as warm or hot oil.