Why does my oil pressure drop at idle after the engine gets hot?

Messages
17
Location
Bristol, CT
Why does does my oil pressure drop at idle after the engine temperature starts to reach 225 degrees F? At normal speeds the pressure is about 45-50 psi and at idle when hot its about 25 psi. When cold the pressure is the same as normal speed. The oil pressure at idle is fine if the engine temp did not exceed 210, once it goes past it, the pressure drops. 93 Jeep Grand Cherokee 4.0 I-6 Delo 400 15w-40 this occurs no matter what kind of oil you use, 5w-30 or thicker, dino or syn.
 
Messages
12,385
Location
Northern CA
quote:
Originally posted by rlabutis: Why does does my oil pressure drop at idle after the engine temperature starts to reach 225 degrees F? At normal speeds the pressure is about 45-50 psi and at idle when hot its about 25 psi. When cold the pressure is the same as normal speed.
The pump pumps the same amount of oil per revolution. The resistance of the downstream part of the oiling system is what causes the pressure to develop. When the oil thins, it flows easier so it is leaking out of the bearings and other points at 25 psi fast enough that the pressure doesn't get a chance to build up any higher. As 427 alluded, 25 psi at idle is probably good. It's not a problem and you get a bonus point for being observant and asking a good question [Big Grin]
 
Messages
39,805
Location
Pottstown, PA
Your pressure spec is very broad. Min 13 psi @ 600 rpm 25-35 psi @ 800 rpm @ 1600 rpm or higher 37-75 psi Pressure relief 75 psi This is pretty much the same for the 4.0/2.5 for as far back as I can remember. This was from the 99 FSM.
 
Messages
1,779
Location
Central Iowa
quote:
Originally posted by rlabutis: Why does does my oil pressure drop at idle after the engine temperature starts to reach 225 degrees F? At normal speeds the pressure is about 45-50 psi and at idle when hot its about 25 psi. When cold the pressure is the same as normal speed. The oil pressure at idle is fine if the engine temp did not exceed 210, once it goes past it, the pressure drops. 93 Jeep Grand Cherokee 4.0 I-6 Delo 400 15w-40 this occurs no matter what kind of oil you use, 5w-30 or thicker, dino or syn.
It's about the same range my 96 Cherokee and my former 2000 Wrangler, both with the 4.0 were. The owners manual for the Wrangler stated it was normal. Don't worry about it.
 
Messages
855
Location
India
225F come to 107C apprx. Isn't that a bit hot for any engine to run, 100C would make me start worrying. I use Delo 400 on my diesel OM 616 turbo engine and that too at 45C+ weather, however, my engine temps are at 85C steady and at idle my pressure is at 1.5bar.
 
Messages
1,835
Location
Ky
Think of it as a milkshake being sucked through a straw. When the shake is still really cold,it takes more effort to get it through. As the shake thaws(thins)less effort(Pressure) is needed to move it through the straw. The same principle can be applied to the oil.
 
Messages
4,868
Location
Lakeville, MN
As noted, your pressures are AOK for the 4.0L. This is part of the reason some manufacturers have gone to more idiot style guages - people notice this variation and assume something must be wrong! I agree that 225F is hot, even for the 4.0l, which runs a bit hotter than most, around 210 is normal.
 
Messages
2,917
Location
Georgia/Retired
I guess this is a very elementary answer but one to consider none the less. Viscosity is the measure of resistance to flow. As the temperature increases, the viscosity decreases, hence the decrease in resistance to flow. Less resistance results in less pressure needed to flow the same amount of fluid. Shearing forces also act on non-newtonian fluids by reducing their viscosity which we see in reduced viscosity and flow pressures with certian oils.
 
Messages
700
Location
USA
My 95 Grand Cher. runs 170-185 all summer. Yours sounds hot. I have found the my pressures are the highest with a thin (5w-30) oil (maybe it flows faster)?. At speed my pressures are 60 psi hot and at hot idle, 35 psi.
 
Messages
1,835
Location
Ky
Sometimes the simple(elementary)answer is the most admired and most wanted. Techno stuff is fine and dandy,in its place. Every day explanations are easier to understand and get the point across so it is easier to understand.
 
Messages
783
Location
Austin Texas
The "time honored" rule of thumb is that you want 10 PSI per 1000 RPMs for <blah blah blah>. I think most manufactures shoot for 2X this number when the engine is new, so that when the engine is almost completly worn out it actually fits the 10 PSI/ 1000 RPMs rule.
 
Top