oil pressure versus water temps towing?

[JStieger]

I have a mechanical oil pressure gauge in my cab since I don't trust the factory oil pressure "gauge".



On a normal fall day when the engine is fully warm @ ~190* and an empty truck, it will read 60 psi at +2000 rpm.



On a 90* day when the engine is fully warm @ ~190* and an empty truck it will read 55 psi at +2000 rpm.



This weekend I was towing a friend's beater Chevy 4x4 with associated parts junk and was pulling this hill outside of town. I had the Smarty TNT Revo on #5 with a gross weight ~17,000 lbs. I was @ 2600 rpm, about 25 psi total boost (10 psi out of primary turbo), 900* EGT, and about 2/3 the way up this hill when I noticed my water temps hit ~225* or so on the stock gauge, but what really surprised me was that the oil pressure was down to ~45 psi. I then let off a bit because of the oil pressure. Soon after, I crested the hill and the water temps quickly dropped below the center mark and oil pressure increased to 55 psi.



I researched when I got home and found that sustained 225* water temps while working the engine is really nothing to be worried about. I also found that the Cummins service manual states a minimum oil pressure spec of 30 psi @ 2500 rpm. Regardless, it still surprised me how much the oil pressure dropped when the engine was working!



Is this normal? Does the engine oil soak up so much heat that the viscosity decreases to lower oil pressure that much?



I run Delo 15W40 and change at 7500-8000 mile intervals. Oil analyses always state I can go longer on intervals, but I just change at this interval due to my truck being modified just for peace of mind. I have never considered synthetic oil due to cost, but would synthetic oil cause the same oil pressure observations under similar conditions?



Before anyone scolds me, I know towing with 100 HP injectors + Smarty TNT #5 is a little "much" fuel and timing for really heavy towing :-laf, but I did take it relatively easy on the pedal.

 

[JCasper]

I don't think that's too much power to tow with, but your transmission may think so.



Sorry I have no insight as to the oil pressure.

 

[sweeda]

Are you complaining or just bragging! No disrespect here but come on. This sounds like what some of us dream of. And I was just feeling good about doing 100mph up hill while being followed by a toyota that didn't want to follow a truck up hill. When I slowed to make my exit he passed by with a BLUE cloud of smoke from his tail pipe. (I don't think it was a diesel. oh my GOD I'm choking HA!HA! HA! ). NO load just the EMPTY weight of 7500 lbs. (QC with tools). I love my 04. 5 but NO where in your trucks ID leauge. I like the #5 setting because of this trip. very little smoke noticed ( and I was watching). Didn't get over 1100 on the egt (pedalling of coarse. It was just a toyota!) My wife said "Why won't he move over" (just before we passed at 90 + mph up hill) I replied as I step into it again, " He can't beleive that he is being passed at 100 mph by a truck" "UP HILL!" She had no response. I thought for sure I would Hear "Than slow your *** down!" But no reaction, till I stupidly brought it up later and confirmed what had happened. WELL 25years and... . STILL counting but, I'm not to do that again! sorry it reads like a story just wanted to share. THANKS

 

[KLockliear]

Living in Vegas, I get to see some really nice temp variations. Here's what I have found:



Cold mornings (<50*F Brrr) 80+psi idle - 40psi warm, 70psi @ 2000rpm

Warm days (<95*F Ahhh) 80+psi idle - 32-35psi warm, 65-68psi @ 2000rpm

Hot days (95-120*F Whew) 75-80psi idle - 30psi warm, 60-65psi @ 2000rpm.



Also, as the mileage racks up, the pressure falls across the board. I usually change oil when the hot, idle oil pressure drops to 25psi. That's usually around 3500-4000miles for me with Delo 400.

 

[JStieger]

I usually change oil when the hot, idle oil pressure drops to 25psi.

My hot idle oil pressure has always been 20-22 psi, even before doing all these mods. I wonder if the 3rd gen engines have different tolerances or oil pumps than the 2nd gen engines?

Are you complaining or just bragging!

Well, I was pulling away quite easily from my friend with a empty 2003 Hemi 4x4 behind me. :-laf



Seriously, though, a member here emailed me and said that the stock truck's cooling system is designed to cool about sustained 425-450 HP continuously. With the Smarty TNT program and based on previous dynos I was probably at ~550 rwHP. The hill had lots of traffic with curves and stoplights so there were undoubtedly times where the truck was commanding this level of fueling so I can understand why things got hot - just surprised at how much this dropped the oil pressure.

 

[steved]

Although my guage pulls from the top of the oil filter boss...



I never see a change in pressure... hot pulling unloaded or hot pulling loaded, 60 to 62psi always. I have a 25psi hot idle... and that's even with a bypass filter.



Might be where you're pulling the pressure from? Although I don't remember my 2nd gen changing pressures either (and that was at the factory location).

 

[cerberusiam]

Is this normal? Does the engine oil soak up so much heat that the viscosity decreases to lower oil pressure that much?

I think your example pretty much established the answer to that question.



The oil is flowing thru every heating surface in the engine and being sprayed on the pistons to keep them cool. It tends to pull a lot of heat away from the areas that need it. The more heat it holds the thinner it gets.



As you said, about 450 hp is sustained output then you over run the cooling AND oiling system to dump the acucmulated heat.



This is why when I hear the statement "Yeah, I tow and daily drive at 600 hp all the time" I just

 

[JHardwick]

10# of oil pressure for every 1,000 rpm is a rule of thumb. If you drop below that, then I'd be concerned.

 

[JStieger]

The oil is flowing thru every heating surface in the engine and being sprayed on the pistons to keep them cool. It tends to pull a lot of heat away from the areas that need it. The more heat it holds the thinner it gets.

10# of oil pressure for every 1,000 rpm is a rule of thumb. If you drop below that, then I'd be concerned.

Is there such a thing as the oil becoming too thin, but oil pressure is still within the acceptable guidelines indicated above?



How much lower/higher does coolant temps run compared to oil temps? Is this relationship fairly consistent or does it diverge?

Might be where you're pulling the pressure from?

My oil pressure is tapped off the top of the filter housing. I run -3 AN stainless steel braided tubing into the cab to a Isspro mechanical gauge on the steering column. It's been this way for about 2. 5 yrs. I originally wanted to tap off of the oil gallery on the driver's side engine by the ECM, but was pressed for time and went the other route.

 

[steved]

Oil can shear and become thinner... it can also "boil off" and become thicker. The latter is the problem I seem to be currently having.

The CTDs are fairly easy on oil in regards to shearing...

It can also thicken from addition of soot...

 

[rbattelle]

How fresh was the oil during this experiment?

Ryan

 

[JStieger]

How fresh was the oil during this experiment?



Ryan

6,000 miles

 

[rbattelle]

6,000 miles

As someone who changes oil every 3k, that's "old" oil to me. :-laf



I'd try the same experiment on some fresh oil.



Ryan

 

[cerberusiam]

Is there such a thing as the oil becoming too thin, but oil pressure is still within the acceptable guidelines indicated above?

Yes, that is entirely possible. Its not unusual for an oil to hold enough viscosity to maintain oil pressure yet not pass a wear test. Its not so much about how thin the oil gets as it is about the ability to lubricate at lower viscosities.

Good question. Since the oil does not have a cooler where the coolant does I would guess the difference will get greater the higher the temps. That is also going to be influenced byt the ambient conditions also. I think it is safe to assume that if you see spiking coolant temps its a good bet the oil temps are doing the same but ata greater rate.



Oil temp is another gauge that is probably a good choice if one is consistently pushing the limits of the cooling system.

 

[rbattelle]

Since the oil does not have a cooler where the coolant does I would guess the difference will get greater the higher the temps.

There is an oil-to-water cooler that sits right next to the oil filter. As a result, oil temperature pretty much tracks water temperature. Or so I've heard. (I have an oil temp gauge but it reads very low, presumably due to the way I've got the sensor mounted).



Ryan

 

[cerberusiam]

There is an oil-to-water cooler that sits right next to the oil filter. As a result, oil temperature pretty much tracks water temperature. Or so I've heard. (I have an oil temp gauge but it reads very low, presumably due to the way I've got the sensor mounted).



Ryan

Forgot about that one. However, its relatively small so when things start to get really heated up it does really cool enough. Thats where the temps start to diverge quite a bit.





Interesting it would read that low if you have the coolant up to temp. One would think it would be close to the water temp unless it has a thermostat bypass like the trans cooler.

 

[SBeretz]

My oil readings are always at least 10* warmer than my water temp. I do use the X Monitor and take my oil temp from on top of the filter housing. Water temp is from the stock location. I also run a very mild box on low power settings. I do track the oil and water temps, but since I am so close to stock I don't really sweat running them too hot. If I were at your power levels I may try to keep them closer to what a stock truck would run at Full throttle with a load. I also agree the trucks seem to be very easy on oil and I don't see much advantage to synthetic oil if you are keeping it with 7500 mile intervals. I run Rotella 5/40 but only because it starts a little quicker in the winter time. Scotty

 

[steved]

As someone who changes oil every 3k, that's "old" oil to me. :-laf



I'd try the same experiment on some fresh oil.



Ryan

Some of us run 10k on dino...



I ran a guage at one point... I remember it being really close to water temps...