LSJR Does the oil filter go into bypass during cold starts?
- Thread starter GW.
- Start date
When I watched it, I thought of @Hohn and his SAE40 grade thread. Their testing showed the starting viscosity at 420 centistokes and 1 psi difference across the filter media. The largest pressure delta across the media wasn't due to viscosity, but flow, regardless of the oil temp. The greater the flow, the greater the delta. But even at the highest flow they tested after the oil was above 220F, the delta was still only ~6psi.
The numbers should be close... IIRC.
The numbers should be close... IIRC.
4wd
$50 site donor 2026
Well … the engine and the coolant was not 
cold …
The Mobil 1 Truck and SUV looked good …
cold …The Mobil 1 Truck and SUV looked good …
They acknowledged this, pointing out it's hard to get the overall temps very cold in southern California, thus putting the filter and jug of oil in the freezer in what looked like a break room. They did monitor coolant and oil temperatures across their testing. I don't believe they ever mentioned this being a scientific test, but they seems to have fun doing it. At least they had a thermometer in the freezer.Well … the engine and the coolant was not
4wd
$50 site donor 2026
Yeah - this was not proving allot. And then he said the PRV MIGHT open in operating conditions? I nail my L83 and it’s at max pressure early on and stays there as RPM’s increase allot. Doesn’t it have to bypass to see that?
4wd
$50 site donor 2026
I’d sooner conduct the test without an engine …When I watched it, I thought of @Hohn and his SAE40 grade thread. Their testing showed the starting viscosity at 420 centistokes and 1 psi difference across the filter media. The largest pressure delta across the media wasn't due to viscosity, but flow, regardless of the oil temp. The greater the flow, the greater the delta. But even at the highest flow they tested after the oil was above 220F, the delta was still only ~6psi.
The numbers should be close... IIRC.
Just a VFD pump so that it holds close to steady state temps …
Patman
Staff member
Would love to see this test done at a proper extreme cold temperature like -20F for instance
I watched it this morning, fwiw I found it to be a worthwhile watch. That said, "ideally" having the engine/vehicle in the same as oil and filter temps would be more results conclusive. As a related aside, many years ago member Jim Allen did a before and after filter pressure gauge test and determined that the oil filter bypass very rarely opens.
The LSJr test makes sense if the oil pump is pretty far into pressure relief. The dP across the filter is a function of oil viscosity and oil flow rate, so cutting the flow in this case looks to be the stronger function on the filter dP. The higher the pump volume per rev and the higher the pump pressure relief setting will affect how much oil flow actually goes to the filter when the oil is cold and thick.
I think Jim did see some filters go into bypass when the oil was cold and engine RPM elevated, and it didn't take near redline revs. His tests were obviously more real world accurate.
Thing to realize in cold starts is if the oil is too thick, even though it will still pump, the oil pump could go into relief so far that it could reduce the oil flow volume to the engine to a critical level and cause engine excessive wear or damage. The whole purpose of the winter rating (xW) is to prevent the pump from cutting back too much and cause a lack of lubrication.
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CleanSump
Site Donor 2026
This confirms what I believe about compensated youtube infomercials. Paid for clicks and likes, not for truly informational content.
dnewton3
Staff member
Somewhere in the annals of BITOG, more than 10 years ago, a real-world test was run by our member Jim Allen. He used real-time datalogging with ISO validated instrumentation on his F150. He drove it and logged the dP from saturated cold start to fully warm. Ran this several times.
Filter BP events are fairly rare in reality. Even when they do happen, they are very short momentary blips and not minutes on end, despite what many theorize.
About the only time he could get the 10w-30 to bypass was a cold start and WOT immediately after firing. And even then it only lasted a second or two.
The MAIN restriction in engines is the oil circuit itself, not the filter media. (presuming the filter has not been neglected)
Filter BP events are fairly rare in reality. Even when they do happen, they are very short momentary blips and not minutes on end, despite what many theorize.
About the only time he could get the 10w-30 to bypass was a cold start and WOT immediately after firing. And even then it only lasted a second or two.
The MAIN restriction in engines is the oil circuit itself, not the filter media. (presuming the filter has not been neglected)
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I think this test was useless.
I'd have to go look for the threads, but I don't think he was revving to near redline with stone cold oil. The revs were elevated quite a bit, but not very close to redline. Could be those events only lasted a few seconds because it took that long for the oil pump pressure relief to react.
What wasn't informational here?
CleanSump
Site Donor 2026
Engine was room temp and the oil was warming the whole time the filter and oil were being installed. Wouldn't float when I tested aircraft.
Since I mentioned member Jim Allen's bypass testing and what I got from it, I'll link to his 'specific post" in his testing summary. In particular I referenced his final thoughts in the 'last two sentences' starting with "Overall". Also note there, Jim primarily used 10w30 oil for his bypass testing.
https://bobistheoilguy.com/forums/threads/differential-pressure-testing-update.195296/post-2981529
https://bobistheoilguy.com/forums/threads/differential-pressure-testing-update.195296/post-2981529
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