Oil filter bypass on cold starts?

Messages
428
Location
Brittany, France
I never really cared about the oil filter bypass valve and thought it would only open if the filter really gets clogged up but recently read a few things about it opening on cold starts and when taking off with a cold engine when the oil is really thick and this sounds scary to me. Maybe i'm overthinking it, especially if owner's manual chart says 5w30/40 is ok down to -30c, 10w40 down to -20c and 15w40 down to -15c and says not to warm up the engine before driving. I let engines idle for at least 30 seconds up to a minute before taking off and try to shift at 2000 rpm max until coolant temp gauge starts to rise above 40c. I use 5w40 or 10w40 and temps are almost never below freezing. Could it justify 5w40 over 10w40 at freezing as well as short OCI to keep the oil really clean and reducing cold start/driving wear in the long run?
 
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16,531
Location
Upper Midwest
Explain "cold start wear" please. I see that thrown around a lot here but I don't understand how it happens.

Wear is caused by metal-to-metal contact and that's prevented by having an adequate MOFT. Is the oil too thin at cold start to provide an adequate MOFT?

And even if the filter goes into bypass how would that hinder or impede flow through the engine?
 
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1,680
Location
South Carolina
Contrary to popular belief, small particles in the oil aren't a leading cause of wear. Excessive heat, torsional vibrations, corrosion, and of course loss of lubrication are primary causes of wear. Small particles circulating in the oil don't tend to contribute a whole lot. The main reason for a filter in the oiling system to ensure you those particles don't collect somewhere else where they might restrict oil flow to a critical part such as a lifter bleed or pushrod hole. This is why there's largely no significant difference in wear numbers in UOAs regardless of the efficiency of the filter used. A temporary opening of the bypass valve on initial startup or at wide open throttle is a good thing. I'd much rather have a little dirty oil circulating than no oil circulating.
 
Messages
319
I meant wear caused by unfiltered oil lubricating the engine (if it is the case to begin with).
If the filter was filtering and not in bypass mode the whole time it was running, then shut down over night, where is all this new stuff coming from? Your oil in a cold engine is already as clean is it will be at that point in the OCI. The filter just catches stuff as it is created and circulated.

I guess it could be argued that filters going into bypass mode will wash contaminants off the pleats, but I don't know enough about oil and filters to say one way or the other. I would imagine a filter would have to be super clogged to have large deposits outside of the pleats waiting to get flushed into the engine. I would also assume that because a filter goes into bypass, doesn't mean no oil is going through the filter media. There will always be a positive pressure and flow of oil going through the media if it isn't clogged to death, which probably limits the effect of anything washing off of it and into the bypass valve.
 
Messages
1,967
Location
USA
Contrary to popular belief, small particles in the oil aren't a leading cause of wear. Excessive heat, torsional vibrations, corrosion, and of course loss of lubrication are primary causes of wear. Small particles circulating in the oil don't tend to contribute a whole lot. The main reason for a filter in the oiling system to ensure you those particles don't collect somewhere else where they might restrict oil flow to a critical part such as a lifter bleed or pushrod hole. This is why there's largely no significant difference in wear numbers in UOAs regardless of the efficiency of the filter used. A temporary opening of the bypass valve on initial startup or at wide open throttle is a good thing. I'd much rather have a little dirty oil circulating than no oil circulating.

For anyone who may have missed this the first time.
 
Messages
1,967
Location
USA
I never really cared about the oil filter bypass valve and thought it would only open if the filter really gets clogged up but recently read a few things about it opening on cold starts and when taking off with a cold engine when the oil is really thick and this sounds scary to me. Maybe i'm overthinking it,
This is BITOG but worry not- if it were to happen ( it go into bypass for a brief period) nothing bad will happen. ( it obviously hasn't happened yet has it?)
 
Messages
2,085
Location
missouri
neighbor had a 61 (I think) Rambler that did not have an oil filter. Rambler owners were "Frugal" and that filter was a needless expense. Has a oil bath air filter. No power brakes, disk brakes, power steering and or even FM radio. It ran well over 100K in 61 that was exceptional.
 
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26,403
Location
PNW
Another thing to consider is if the oil is really cold and thick, the PD pump's pressure relief valve will also open, thereby cutting down some flow to the filter and oiling system. If the flow is cut back some with pump in pressure relief, then that helps reduce flow and delta-p across the filter. If the filter has a really high set bypass valve, then it may not open for very long anyway. If the filter isn't clogged much, then the majority of the flow is still going through the media, and chances of anything washing off and going through the bypass valve are very small.

If cold starting in very cold conditions, let the oil warm up some before driving off and keep the RPM low for awhile.
 

M119

Thread starter
Messages
428
Location
Brittany, France
Thanks, i did not realize that. The filter elements in both my cars are bigger than the tiny spin-on filters used on most cars (much much bigger on the diesel). Does a bigger filter allow for more flow/less resistance when the oil is super thick? As i said, as an example the owner's manual chart says that 15w40 is ok down to -15c, can we assume that at say above -15c and of course being easy on the engine until everything is warmed up, filter bypass or at least oil starvation shouldn't be an issue? By the way, 15w40 used to be the standard in europe for these engines, especially for taxi use and various fleets. 10w40 too.

Edit : The more i think about it, the more i think cold start wear (or at least oil starvation) is a non issue as long as you follow the chart. As scary as words such as oil starvation/cold start wear sound, maybe oil manufacturers marketing teams know how to scare people and push even people living in mild climates into buying 0w or 5w synthetics. My observations on Mercedes engines of the 90s such as the M111 and M104 is that there's still an oil bath around the lifters and camshafts are never operating dry and oil pressure gauge is pegged when the engine starts (if o-rings are still good inside the filter housing on some diesels and holding oil when engine is shut down). I can't believe the thick film of oil left inside the bearings wouldn't be enough even if oil pressure needs a few second to build up. Maybe it is more a matter of if and when oil filter is bypassed if you follow the temperature/grade chart and maintenance schedule.
 
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Messages
26,403
Location
PNW
Thanks, i did not realize that. The filter elements in both my cars are bigger than the tiny spin-on filters used on most cars (much much bigger on the diesel). Does a bigger filter allow for more flow/less resistance when the oil is super thick?
Yes, a bigger filter will have less flow restriction compared to a smaller filter made with the same media. Less flow restriction means less delta-p under the same operating conditions, and therefore a bit more headroom before the bypass valve might open.
 
Messages
2,808
Location
High Tax Illinois
If the filter was filtering and not in bypass mode the whole time it was running, then shut down over night, where is all this new stuff coming from? Your oil in a cold engine is already as clean is it will be at that point in the OCI. The filter just catches stuff as it is created and circulated.

I guess it could be argued that filters going into bypass mode will wash contaminants off the pleats, but I don't know enough about oil and filters to say one way or the other. I would imagine a filter would have to be super clogged to have large deposits outside of the pleats waiting to get flushed into the engine. I would also assume that because a filter goes into bypass, doesn't mean no oil is going through the filter media. There will always be a positive pressure and flow of oil going through the media if it isn't clogged to death, which probably limits the effect of anything washing off of it and into the bypass valve.
 
Messages
2,808
Location
High Tax Illinois
Isn't most dirt down inside the the pleats? Darn near all C&P show the collected "dirt" on the most inner pleat bend. I don't think you get a lot washed dirt off the outside of the pleats.
 
Messages
1,923
Location
Paradise of Florida
No pleat washing since media is still flowing at its limit

Bypass only bypasses excessive flow

Bypass valves open more often than most believe. Its not just when cold or higher rpm. Often!

Why I only recommend synth media since it wont be in bypass as much as cellulose based media:
 
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Messages
463
Location
Cheshire, England
I think the filter bypass will open on every cold start regardless of chosen oil viscosity and any other time the set differential pressure is exceeded It's a good thing that it does, so as not to crush the oil filter. You wouldn't want your filter core to end up looking like this.

Crushed Filter3.jpg
 
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