Idle oil pressure (my logic vs reality?)

[Medical_Bartender]

So hear me out and tell me why my logic is incorrect. In my JK the idle oil pressure is usually around 20psi, give or take a few. When cruising around it's between 32-42psi. When the high pressure oil pump kicks on I see 60s-70s PSI.

The gladiator idle oil pressure is 30-31 PSI, kicks up to 70s-80s PSI when the high pressure pump kicks on.

In theory, the idling gladiator engine receives the same oil pressure as the engine putting around town under 3k RPMs so long(er) idle times won't cause any issues to the internals of the engine correct?

As the title states, I have a feeling my logic doesn't equate to reality. FWIW I don't idle my engine for long periods, but I am curious what would happen considering the oil pressure remains at normal values.

Will

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[TheRealStreetcommander]

I think I agree. Extended idling will not hurt the Pentastar. Pentastar don’t generally have fuel dilution problems and don’t suffer from blow-by or carbon stacking like some DI engines.

There’s a danger in thinking of oil pressure as an inflating ballon or tire, etc. The pressure required to provide an adequate oil film between moving parts may only in reality be 2-3 psi, maybe even less. The pressure is really just a proxy for available volume of oil in reserve with a tendency to flow towards lower pressure. Stated differently, the oil pressure doesn’t inflate the bearings or anything else, it just resupplies oil and the oil film barrier to contact surfaces at a greater rate than what is shed during operation.

There’s a contributor on this forum who works for a taxi firm that has 100s of penstastar taxis. Taxis pack lots of miles on and pack even more idling hours on. He’s indicated that the pentastar are bulletproof. The few pentastars that do have cam problems, simply have their worn cam parts replaced and then continue to pack miles on them. The cam part failures never hurt the rest of the engine.

 

[toddarama]

My limited understanding is that idle time itself doesn't impact the engine directly. I think the concern is more related to scheduling oil changes based on mileage. Theoretically if you have twice as much idle time as you do driving time you would need to factor this into determining when the next oil change required. You could have 3500 miles of driving time and twice as much additional idle time and you'd effectively be overdue for a 10000 mile oil change.

 

[Hootbro]

My limited understanding is that idle time itself doesn't impact the engine directly. I think the concern is more related to scheduling oil changes based on mileage. Theoretically if you have twice as much idle time as you do driving time you would need to factor this into determining when the next oil change required. You could have 3500 miles of driving time and twice as much additional idle time and you'd effectively be overdue for a 10000 mile oil change.

Been a few moons ago but worked a couple summers in a county fleet garage and was always drilled into my head to treat every engine idle hour as roughly 30 miles of equivalent driving wear when factory servicing.

 

[ShadowsPapa]

So hear me out and tell me why my logic is incorrect. In my JK the idle oil pressure is usually around 20psi, give or take a few. When cruising around it's between 32-42psi. When the high pressure oil pump kicks on I see 60s-70s PSI.

The gladiator idle oil pressure is 30-31 PSI, kicks up to 70s-80s PSI when the high pressure pump kicks on.

In theory, the idling gladiator engine receives the same oil pressure as the engine putting around town under 3k RPMs so long(er) idle times won't cause any issues to the internals of the engine correct?

As the title states, I have a feeling my logic doesn't equate to reality. FWIW I don't idle my engine for long periods, but I am curious what would happen considering the oil pressure remains at normal values.

Will

Don't equate oil pressure with protection and life. Pressure is the result of a volume against a resistance. So if you have pressure, there's oil getting through the hydraulic system and meeting resistance. Idling is also less load on bearings which is where you need the liquid oil.
As long as there is adequate pressure to get oil to all things that require oil, it's good. Higher pressure of the JT could be for performance of the valve lift/lash adjusters, 2 step intake followers and so on. They are using oil and pressure for more than getting oil to the bearings and valve train for lube.
20 psi to 30 psi is nothing that's going to change protection at idle. 20 is fine in itself just for protecting parts.
Your JK is a different engine than the JT engine even if they are both 3.6 - very different in oil needs for other functions.

Police cruisers, taxis, heck, around here, the county pickups, sit around idling a lot (as do the county employees for that matter)

This is a topic that comes up in other places over and over and has for years - related to performance engines, classic cars and more.

 

[Stan H]

I think I agree. Extended idling will not hurt the Pentastar. Pentastar don’t generally have fuel dilution problems and don’t suffer from blow-by or carbon stacking like some DI engines.

There’s a danger in thinking of oil pressure as an inflating ballon or tire, etc. The pressure required to provide an adequate oil film between moving parts may only in reality be 2-3 psi, maybe even less. The pressure is really just a proxy for available volume of oil in reserve with a tendency to flow towards lower pressure. Stated differently, the oil pressure doesn’t inflate the bearings or anything else, it just resupplies oil and the oil film barrier to contact surfaces at a greater rate than what is shed during operation.

There’s a contributor on this forum who works for a taxi firm that has 100s of penstastar taxis. Taxis pack lots of miles on and pack even more idling hours on. He’s indicated that the pentastar are bulletproof. The few pentastars that do have cam problems, simply have their worn cam parts replaced and then continue to pack miles on them. The cam part failures never hurt the rest of the engine.

I can confirm that long term idling does not hurt the pentastar 3.6 I have a 2021 JT Rubicon I use every day at work in the winter it idles on work sites. It drives between sites and in the summer it idles on hot days all day long every day . My shifts are 12 hours long . My engine oil pressure at idle is 29 Psi according to the electric gauge read out. It idles at about 600 or so . I often in spring time mud have to really get on it heavily. I use the 4 wheel drive daily as many places I drive are dirt or mud and are in excess of 15% grade. It has 71,300 some odd miles on it . I don't cut this thing any slack. With the exception of the very first break in miles . The very first oil was the factory stuff after that it has been amsoil 0W-20 . I also use amsoil in the front and rear end and in the transfer case. I have driven the crap outta this thing. As it goes up from idle oil pressure increases topping out at 79 Psi.
When I first got the truck it's idle Psi was 30si when hot but somewhere around 40,000 it went to 29psi. When hot I also noticed that directly after an oil change it will be 30psi. This will generally last for maybe 500 miles. If I have a major climb and am on it to the point the fan kicks in then the Psi will drop to 28psi. Then after it idles for awhile it will go back to 29psi sometimes after a small tap I mean small like a hundred rpm tap on the throttle it will resettle on 29 Psi.
If you ask me so far it has earned the title of King of the off road If you would see what I have done you would melt in terror possibly thinking about. I definitely have put it through its paces.

 

[ShadowsPapa]

Still trying to figure out why anyone is connecting oil pressure to idling. There's no logic to it. So what if it has the same oil pressure at idle as it does putzing around town. Doesn't matter. You could idle at 20 psi and be fine as long as the followers and phasers have enough pressure to operate. The bearings and other parts would still have more than enough.
There's no connection.
These shouldn't have an idle problem regardless of oil pressure. The two aren't related. Oil pressure isn't why they can idle fine without long-term damage. Lack of fuel contamination is one of the biggest factors, other design elements make up the rest.

 

[Escape.idiocracy]

My limited understanding is that idle time itself doesn't impact the engine directly. I think the concern is more related to scheduling oil changes based on mileage. Theoretically if you have twice as much idle time as you do driving time you would need to factor this into determining when the next oil change required. You could have 3500 miles of driving time and twice as much additional idle time and you'd effectively be overdue for a 10000 mile oil change.

I think you are bringing the exact reasoning behind pg 329 oil change should not exceed 350 hours or 10,000 miles.

The oil life Guage seems to be relatively accurate. When we spend the majority of our time running highway we push 7500-8k miles before exhausting the “oil life” and when it runs predominantly city it’s around 4,500 miles.

 

[Medical_Bartender]

Still trying to figure out why anyone is connecting oil pressure to idling. There's no logic to it. So what if it has the same oil pressure at idle as it does putzing around town. Doesn't matter. You could idle at 20 psi and be fine as long as the followers and phasers have enough pressure to operate. The bearings and other parts would still have more than enough.
There's no connection.
These shouldn't have an idle problem regardless of oil pressure. The two aren't related. Oil pressure isn't why they can idle fine without long-term damage. Lack of fuel contamination is one of the biggest factors, other design elements make up the rest.

Not disagreeing with what you said at all Papa, just bringing up a topic and asking a question I've had on my mind for a while since I usually keep my oil pressure gauge displayed while i'm driving. I think part of the reason for thinking pressure equates to wear, at least for me, is because I've been told and read that idling can be bad and more harsh on vehicles because there is potentially less total oil flow at a given RPM. I.e. my JK idling at 800 RPM getting 16 PSI compared to 2k RPM getting 32 PSI vs. Gladiator getting 30 PSI at idle and 2k RPM. My career is in medicine and not mechanics (although I do enjoy wrenching on all my stuff) so the in's and out's of intricate engine details are lost on me sometimes.

For example, blood pressure in the body is based around a set point. It increases and decreases depending on activity/position/resistance to ensure organs are adequately perfused via relatively constant pressure. In a car, position is constant (level with the ground), activity can increase or decrease (RPMs), and I'd imagine resistance is also relatively constant since there are no "vessels" in an engine that can dilate and constrict. With that "logic" one could say that the engine is "over-perfused" at idle considering the pressure is the same at idle RPM and at 3-4x that RPM which would mean that idling causes no more wear than driving around.

Your post regarding Jeep possibly upping the pressure for valve lift/lash, followers, etc is understood. I do suppose they changed quite a bit in this engine compared to the older 3.6 pentastar. Not saying an engine equates to a human body, but this is how my brain connects them and is what sparked the question. I was just curious and wanted some conversation, so thank everyone for their feedback!

 

[Rusty PW]

Here's something to think about. In my Nismo. Cold idle oil pressure is 115 psi. Once warm. It drops to 37 psi. At cruising at 70 mph. It's 90+. At 7000 rpm to 8,200 rpm red line. It 105+ psi. The engine uses oil pressure to operate the variable valve timing, plus the variable valve lift.

 

[ShadowsPapa]

Here's something to think about. In my Nismo. Cold idle oil pressure is 115 psi. Once warm. It drops to 37 psi. At cruising at 70 mph. It's 90+. At 7000 rpm to 8,200 rpm red line. It 105+ psi. The engine uses oil pressure to operate the variable valve timing, plus the variable valve lift.

Yeah, you beat me to part of my explanation.
the general rule has been that you need 10 psi for every 1,000 RPM in a "typical" engine. It's why idiot light sending units close the contacts at about 8 psi.
I've a friend with Mercedes - he gave me some factory information from some years ago telling of how they needed to increase the oil pump VOLUME to get pressure to operate things like the VVL, variable valve timing and the variable displacement intake manifold runners. They use oil for all of that and the result was all of a sudden people were seeing high oil pressure numbers and what the heck is up with that!?
Same for these - they could operate fine on 15 psi idle if they did not have VVL and VVT and other fun features.

Pumps generate VOLUME, not pressure.
Pressure is the result of a volume against a resistance.
Oil pressure does not lubricate or protect an engine. Oil pressure only is an indication that there's enough volume of oil to overcome any leakage out the rod's piston spray holes, the lash adjusters' rocker spray holes and so on.
As long as there's enough volume to get oil into those places, it's fine.
For most parts you are relying on the oil's film strength to lubricate - valve followers, valve stems, etc.
For rod and main bearings, you just need enough volume to keep liquid oil in the spaces between bearings and journals to keep an uncompressible oil layer between the parts.
A friend, Ken Parkman - a guy who once sent people into a tizzy with his carburetor flow bench testing showing that the stock Motorcraft 4300 was superior and flowed better than a similar Edelbrock carb, did a dyno test years ago when people were claiming that AMC engines were losing their rear main or rear rod bearings due to lack of oil pressure at the rear of the engine because of bleed-off in the lifter valley. People who have never been trained in hydraulics will believe a lot of stuff, I guess. He showed that idle or under the stress of full HP output on a dyno, the oil pressure was the SAME at the source - the filter/pump area, as it was in the rear oil gallery of the block. Makes sense - hydraulic pressure is the same throughout a system unless a leak at the end is larger than the passage that supplies oil to that area. Even at the rear it will be the same.
Supporting that here's what a small auto company in Kenosha did years ago - from what I heard they were the only ones doing this? They put engines on a stand, connected a drive system to the crankshaft so they could spin the engine, and put a very sensitive pressure monitoring system in the oil pressure sending unit area and connected it to a computer. Knowing the exact position of every single moving part of that engine in relationship to the crankshaft position, they were able to detect and miniscule deviation in oil pressure and determine where the loss of pressure may be coming from. They knew when the oil holes in #4 rod journal lined up with the piston spray hole in the rod, where each lifter was, and more. They could find a problem with that device. Proof that the pressure is consistent through a system front to back in the case of a leak or problem. If it drops, it drops everywhere, but that a good pump will still have the volume to get oil out to where it needs to be. (A former employee of the company told of these tests and that device - I have what remained of their distributor machine and electronic ignition testing system, too bad it was raped and pillaged before I got it - pieces and parts missing)

I.e. my JK idling at 800 RPM getting 16 PSI compared to 2k RPM getting 32 PSI vs. Gladiator getting 30 PSI at idle and 2k RPM. My career is in medicine and not mechanics (although I do enjoy wrenching on all my stuff) so the in's and out's of intricate engine details are lost on me sometimes.

And I'm thankful for those who have a career in medicine. (some of the real unsung heroes, IMO)
My HS chemistry classes were enough to show me I didn't belong there. I've enough chemistry to aid me in my current restoration and plating business, and removing rust via electrolysis, but you won't find me working at SpaceX.
I had extreme encouragement from family/parents, friends and teachers to go another direction so my training/college is automotive.
Hydraulics as well as I farmed for a few years, and repaired and operated ag equipment as well as trenchers, back hoes and that fun stuff.

Anyway, these engines have other reasons for their long life and ability to idle. Not that they are unique in that way - look at the FoMoCo products used for years as taxis in the big cities - people buy those cars and still get many thousands more miles out of them.
They've lightened up the valve train a lot in these - smaller, lighter valve stems means they can operate at lower spring rates, lowering the pressure on the followers. The timing chain system - anything like that used 20 years ago would have died in half the time but their newer chain designs mean a lot less friction (less wear, less drag) and better oiling of the chain means longer life. The fuel injection being NOT direct injection (non-DI) means less fuel contamination of the oil. Fuel in the oil is really bad.
As much as I appreciate and use ethanol, it's an oil killer. If these could use E85 you'd need to be changing the oil in half as many miles so their decision to keep these U88 max may be helping their life as well.
The fact they run lean - close to or at stoich as much as any engine, maybe more than others, also means less fuel contamination of the oil.
Getting an engine up to temperature for an extended period is a big key to longer life. Burn out the moisture and contaminations that cause harmful acid build-up. Those acids etch and eat at the soft bearing materials - copper, tin, etc. are soft and easily etched by acids (there, your chemistry will come into play)
So these guys who buy a Jeep and drive it 10 miles to work and shut it off will end up needing engines more quickly than the guy who runs it a lot even if at idle speeds because they are generating the heat that removes moisture and fuel contamination of the oil. Short commutes are engine killers. Trust me, over the 50 years I've worked on engines of all types - a short drive will kill any engine more quickly than taking it to the track on the weekend and trying to get .01 shaved off your quarter mile time. I've seen bearings that looked like carpenter ants had been into them, or babbet termites or whatever. Little areas of recess in the bearing material. I've seen sludge so bad the valve covers leaked badly and the oil light came on because the oil was pumped up into the valve train but the return passages were plugged with ash and sludge and it took chisels to get it out. That's short drives.
Oh, but my coolant gets up to 190 so I'm fine, right?
LOL - no. Not at all. That just means the coolant is there, not the oil, not the engine parts themselves. What's water boil at - and they think getting it up to 190 will boil the water out in a 5 or 10 minute commute.
If we were talking 1990s or earlier I'd say idling is bad, not a good thing.....for many engines.

Anyway, bottom line, the oil pressure of today isn't about lubricating parts as much as it is operating the added engine technology, VVL, VVT, in the case of Mercedes and others - the variable intake runner volume and more.
Pumps supply volume. Pressure is the result of volume against resistance. As long as there is pressure, oil is getting to there it's needed. The pressure is the same throughout the hydraulic system. Pressure doesn't lubricate or protect, it's just an indication you have enough flow to get oil everywhere. Oil film protects, in the case of bearings, a layer of incompressible oil keeps the metal parts apart - and that's because of the volume of oil being pushed in, not the oil pressure.

 

[Medical_Bartender]

Yeah, you beat me to part of my explanation.
the general rule has been that you need 10 psi for every 1,000 RPM in a "typical" engine. It's why idiot light sending units close the contacts at about 8 psi.
I've a friend with Mercedes - he gave me some factory information from some years ago telling of how they needed to increase the oil pump VOLUME to get pressure to operate things like the VVL, variable valve timing and the variable displacement intake manifold runners. They use oil for all of that and the result was all of a sudden people were seeing high oil pressure numbers and what the heck is up with that!?
Same for these - they could operate fine on 15 psi idle if they did not have VVL and VVT and other fun features.

Pumps generate VOLUME, not pressure.
Pressure is the result of a volume against a resistance.
Oil pressure does not lubricate or protect an engine. Oil pressure only is an indication that there's enough volume of oil to overcome any leakage out the rod's piston spray holes, the lash adjusters' rocker spray holes and so on.
As long as there's enough volume to get oil into those places, it's fine.
For most parts you are relying on the oil's film strength to lubricate - valve followers, valve stems, etc.
For rod and main bearings, you just need enough volume to keep liquid oil in the spaces between bearings and journals to keep an uncompressible oil layer between the parts.
A friend, Ken Parkman - a guy who once sent people into a tizzy with his carburetor flow bench testing showing that the stock Motorcraft 4300 was superior and flowed better than a similar Edelbrock carb, did a dyno test years ago when people were claiming that AMC engines were losing their rear main or rear rod bearings due to lack of oil pressure at the rear of the engine because of bleed-off in the lifter valley. People who have never been trained in hydraulics will believe a lot of stuff, I guess. He showed that idle or under the stress of full HP output on a dyno, the oil pressure was the SAME at the source - the filter/pump area, as it was in the rear oil gallery of the block. Makes sense - hydraulic pressure is the same throughout a system unless a leak at the end is larger than the passage that supplies oil to that area. Even at the rear it will be the same.
Supporting that here's what a small auto company in Kenosha did years ago - from what I heard they were the only ones doing this? They put engines on a stand, connected a drive system to the crankshaft so they could spin the engine, and put a very sensitive pressure monitoring system in the oil pressure sending unit area and connected it to a computer. Knowing the exact position of every single moving part of that engine in relationship to the crankshaft position, they were able to detect and miniscule deviation in oil pressure and determine where the loss of pressure may be coming from. They knew when the oil holes in #4 rod journal lined up with the piston spray hole in the rod, where each lifter was, and more. They could find a problem with that device. Proof that the pressure is consistent through a system front to back in the case of a leak or problem. If it drops, it drops everywhere, but that a good pump will still have the volume to get oil out to where it needs to be. (A former employee of the company told of these tests and that device - I have what remained of their distributor machine and electronic ignition testing system, too bad it was raped and pillaged before I got it - pieces and parts missing)



And I'm thankful for those who have a career in medicine. (some of the real unsung heroes, IMO)
My HS chemistry classes were enough to show me I didn't belong there. I've enough chemistry to aid me in my current restoration and plating business, and removing rust via electrolysis, but you won't find me working at SpaceX.
I had extreme encouragement from family/parents, friends and teachers to go another direction so my training/college is automotive.
Hydraulics as well as I farmed for a few years, and repaired and operated ag equipment as well as trenchers, back hoes and that fun stuff.

Anyway, these engines have other reasons for their long life and ability to idle. Not that they are unique in that way - look at the FoMoCo products used for years as taxis in the big cities - people buy those cars and still get many thousands more miles out of them.
They've lightened up the valve train a lot in these - smaller, lighter valve stems means they can operate at lower spring rates, lowering the pressure on the followers. The timing chain system - anything like that used 20 years ago would have died in half the time but their newer chain designs mean a lot less friction (less wear, less drag) and better oiling of the chain means longer life. The fuel injection being NOT direct injection (non-DI) means less fuel contamination of the oil. Fuel in the oil is really bad.
As much as I appreciate and use ethanol, it's an oil killer. If these could use E85 you'd need to be changing the oil in half as many miles so their decision to keep these U88 max may be helping their life as well.
The fact they run lean - close to or at stoich as much as any engine, maybe more than others, also means less fuel contamination of the oil.
Getting an engine up to temperature for an extended period is a big key to longer life. Burn out the moisture and contaminations that cause harmful acid build-up. Those acids etch and eat at the soft bearing materials - copper, tin, etc. are soft and easily etched by acids (there, your chemistry will come into play)
So these guys who buy a Jeep and drive it 10 miles to work and shut it off will end up needing engines more quickly than the guy who runs it a lot even if at idle speeds because they are generating the heat that removes moisture and fuel contamination of the oil. Short commutes are engine killers. Trust me, over the 50 years I've worked on engines of all types - a short drive will kill any engine more quickly than taking it to the track on the weekend and trying to get .01 shaved off your quarter mile time. I've seen bearings that looked like carpenter ants had been into them, or babbet termites or whatever. Little areas of recess in the bearing material. I've seen sludge so bad the valve covers leaked badly and the oil light came on because the oil was pumped up into the valve train but the return passages were plugged with ash and sludge and it took chisels to get it out. That's short drives.
Oh, but my coolant gets up to 190 so I'm fine, right?
LOL - no. Not at all. That just means the coolant is there, not the oil, not the engine parts themselves. What's water boil at - and they think getting it up to 190 will boil the water out in a 5 or 10 minute commute.
If we were talking 1990s or earlier I'd say idling is bad, not a good thing.....for many engines.

Anyway, bottom line, the oil pressure of today isn't about lubricating parts as much as it is operating the added engine technology, VVL, VVT, in the case of Mercedes and others - the variable intake runner volume and more.
Pumps supply volume. Pressure is the result of volume against resistance. As long as there is pressure, oil is getting to there it's needed. The pressure is the same throughout the hydraulic system. Pressure doesn't lubricate or protect, it's just an indication you have enough flow to get oil everywhere. Oil film protects, in the case of bearings, a layer of incompressible oil keeps the metal parts apart - and that's because of the volume of oil being pushed in, not the oil pressure.

Thanks for taking us to school via an informative post per usual. I didn’t realize how oil dependent all these new systems were.

I am, however, very thankful that these engines use port fuel injection as opposed to DI, purely for longevity and cleanliness of the valves. Unfortunately DI seems to be the way of the future since car manufacturers are milking every HP & torque out of these newer engines. Unless it’s performance oriented give me PFI every time.

cheers ?

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