Hi,

today I installed a mechanical oil pressure gauge instead of the electrical sending unit. I`m getting 40 PSI at idle cold, between 30 and 40 PSI driving, 33 PSI at about 70 mph, 10 to 12 at idle warm. Does that seem about right with Rotella 15W40, 180F thermostat but (still) no oil cooler?

That sounds about right for a 6.2 with a lot of miles running 15w-40.

Thanks! Next thing is to get an oil cooler. The oil must be getting really hot since there`s no way for it to get rid of the heat.

Hm, is there such a thing as an oil pressure curve for these engines, showing RPMs and oil pressures? Maybe we could make a chart if everyone posts the oil pressures for a given set of RPMs, like 650, 1200, 1600, 1800, 2000, 2200, 2500. (I don`t have a tachometer yet.)

There is a general curve for all piston engines.
You have to pick the right oil and it will be different for all engines.
You want to pick an oil that will give you no less then 10 to 15 psi hot at idel. Then for every 1000 rpms you add to idel speed you should gain around +10psi.
So at 650 you have 10 to 15 psi, at 1650 you have 20 to 25psi then add another 1000 rpm and you have another 10psi now you have 30 to 35 psi at 2650rpm.
This way every time you add 1000 rpms you proportionaly increase your flow of oil through all you moving parts.
That way at 2650 rpms you are flowing 3 time the oil you had at idel and at 3650 you will have 4 times the oil flow.
In an engine pressure equals flow.
This is what you don't want to do:
Say you add 15w-50 oil. Now you have 40psi of oil at idel. then you go up to 1650rpm and have 80psi, now you go to 2650 rpms and still have 80psi???
Your oil pressure releaf valve is bleading off all your oil pressure and flow. So no matter how fast you spin you engine you can only flow 2x the oil you had at idle, that is bad.
You want your oil pressure to peak just under your max use able RPM's.
For a 6.2 you would want to have no more then 80psi at just over 4000rpm's that way the faster your engine turns the more oil flow it gets.
Turning the engine over 4000/min with that 15w-50 oil that maxed out the oil pressure releaf at 2650 would still only be flowing 2x what it does at idel. So that engine would be screwed.

For me I have about 20psi at hot idel with synthetic 5w-40, normal driving it stays around 40psi. When I haul ass it gets over 60psi.
That is what you want to see.

Hm, the tire size/speed calculator tells me that I would have 2000 RPM at 72.2 mph in 4th gear. If the idle speed is 650 RPM with 10 PSI oil pressure, I should have about 30 PSI at 70 mph at about 2000 RPM (not 2650 RPM).

The 6.5 was gouverned at 3740 (or 3745) RPM. What`s the gouverned speed for the 6.2?

Now if we let the oil pressure relief valve out of consideration --- which we can do because the oil pressure is not as high as to open it --- and assume I would switch from 15W40 to 5W40, what oil pressure would I see?

Since 5W40 is thinner than 15W40, would that mean an increased oil flow and thus a higher oil pressure? (Oil pressure on the Tahoe didn`t change much, if at all, when I switched it from 10W40 to 5W40.)

I could try that, but I`m somewhat scared of switching to 5W40. Some people claim that things can start leaking when all the dirt is being washed out, others claim that anything thinner than 10W40 is a bad idea for a 6.2.

Is anyone else running 5W40 in a 6.2?

Hm, I must be missing something here (besides an oil cooler ;) ). Oil pressure goes down with higher temperature, higher temperature means reduced viscosity. Let`s assume the oil in our engines is at about the same temperature. As long as that temperature is in the temperature range covered by our oil, the viscosity of both types of oil should be about the same, shouldn`t it? If it`s about the same, why do you have higher oil pressure? Same viscosity should mean the same amount of flow (letting aside wear).

At full tempature the oils should be about the same.
Remember, I have an oil cooler and a new set of connecting rod and main bearings in my truck and the crank shaft was in A+ shape. I went form 5 to 10 psi hot at idel with 15w-40 before the big over haul to no less then 20psi hot at idel with 5w-40.
When I start the truck on a nice day the oil pressure is pegged to about 80psi.
After busting my ass for a week changing those bearing it wasn't getting any thing less then synthetic. Also its still on its first oil change since the new bearings.
My connecting rod bearings were just about shot and my Main bearings had been to hell and back several times. Thats where your oil pressure is going, out through the bearings.
I don't leak any oil because of my new oil pan seal, new rear main seal and new valve cover seals.
I think I have the only 6.2 that doesn't leak oil, but it burns about 1Qt about every 1200 miles.
The early 6.2L were governed to 4400rpm's so I have been told.

Hm, the tire size/speed calculator tells me that I would have 2000 RPM at 72.2 mph in 4th gear. If the idle speed is 650 RPM with 10 PSI oil pressure, I should have about 30 PSI at 70 mph at about 2000 RPM (not 2650 RPM).

The 6.5 was gouverned at 3740 (or 3745) RPM. What`s the gouverned speed for the 6.2?

Now if we let the oil pressure relief valve out of consideration --- which we can do because the oil pressure is not as high as to open it --- and assume I would switch from 15W40 to 5W40, what oil pressure would I see?

Since 5W40 is thinner than 15W40, would that mean an increased oil flow and thus a higher oil pressure? (Oil pressure on the Tahoe didn`t change much, if at all, when I switched it from 10W40 to 5W40.)

I could try that, but I`m somewhat scared of switching to 5W40. Some people claim that things can start leaking when all the dirt is being washed out, others claim that anything thinner than 10W40 is a bad idea for a 6.2.

Is anyone else running 5W40 in a 6.2?

Hm, I must be missing something here (besides an oil cooler ;) ). Oil pressure goes down with higher temperature, higher temperature means reduced viscosity. Let`s assume the oil in our engines is at about the same temperature. As long as that temperature is in the temperature range covered by our oil, the viscosity of both types of oil should be about the same, shouldn`t it? If it`s about the same, why do you have higher oil pressure? Same viscosity should mean the same amount of flow (letting aside wear).

The viscosity numbers 5w and 15w refer to the viscosity at some, probably SAE specified, low temperature. Same with the 40 except for high temperature. At normal operating engine temp the viscosity for both 5w40 and 15w40, for all practical purposes, is the same. Someone please correct me if I'm wrong.

Just as a reference for you, my hot idle pressure is 18 and hot cruising (65mph @ 2085rpm) is 38. Engine temp is 205. RPM is estimated using measured rolling distance of 94.25 inches for 1 rev of rear tire. I'm using Rotella T 15w40.

Peyton

After busting my ass for a week changing those bearing it wasn't getting any thing less then synthetic.


That makes sense since you know the condition of the engine. Sounds like you did a great job on it. I`d be quite scared of messing it up.


Thats where your oil pressure is going, out through the bearings.
Would that mean both our engines basically flow the same amount of oil, but mine has lower oil pressure because there is less flow resistance due to wear?


I don't leak any oil because of my new oil pan seal, new rear main seal and new valve cover seals.
I think I have the only 6.2 that doesn't leak oil, but it burns about 1Qt about every 1200 miles.
cool :) Mine leaks a little --- I will have to clean the CDR and get an oil cooler and then see what happens. Before the starter bracket arrives, I`m trying to avoid to start it ...


The early 6.2L were governed to 4400rpm's so I have been told.That`s pretty fast! I better get a tachometer that goes to 5000 instead of 4000 then.

At normal operating engine temp the viscosity for both 5w40 and 15w40, for all practical purposes, is the same.


exactly


Just as a reference for you, my hot idle pressure is 18 and hot cruising (65mph @ 2085rpm) is 38. Engine temp is 205.Thanks! I`m seeing coolant temps between 150 and short of 200 on the stock gauge. That should be about right with the 180 degree thermostat. But the oil probably gets really hot since there`s nothing for it to get rid of the heat.

exactly

Thanks! I`m seeing coolant temps between 150 and short of 200 on the stock gauge. That should be about right with the 180 degree thermostat. But the oil probably gets really hot since there`s nothing for it to get rid of the heat.

Perhaps I misunderstood. Is your temp constantly fluctuating between 150 and 200? If so, shouldn't it keep a constant temp, within a few degrees, once the coolant reaches operating temp?

Peyton

Perhaps I misunderstood. Is your temp constantly fluctuating between 150 and 200? If so, shouldn't it keep a constant temp, within a few degrees, once the coolant reaches operating temp?


It depends on the load. Going 70 on the highway brings it up to short of 200 and it stays there. Driving in the city makes it go down and eventually fluctuate. Driving slow, like on country roads paved with gravel, makes it go down to 150 without much movement from there. --- "Short of 200" is probably between 180 and 190, it`s hard to tell on the gauge.

To keep a steady temperature, the engine must generate enough heat to keep the thermostat open, I guess. If it generates less, the coolant has (more) time to cool in the radiator and gets relatively cold. Then you see it on the gauge when the thermostat opens.

Maybe I should go back to a 195F thermostat, dunno. Maybe the gauge is off and shows 150 when it`s 180 ...

It depends on the load. Going 70 on the highway brings it up to short of 200 and it stays there. Driving in the city makes it go down and eventually fluctuate. Driving slow, like on country roads paved with gravel, makes it go down to 150 without much movement from there. --- "Short of 200" is probably between 180 and 190, it`s hard to tell on the gauge.

To keep a steady temperature, the engine must generate enough heat to keep the thermostat open, I guess. If it generates less, the coolant has (more) time to cool in the radiator and gets relatively cold. Then you see it on the gauge when the thermostat opens.

Maybe I should go back to a 195F thermostat, dunno. Maybe the gauge is off and shows 150 when it`s 180 ...

I think a thermostat stuck open might exhibit the symptoms you describe. FWIW, whenever I drive my suburban, the temp goes to 205 (gauge temp) fairly rapidly and stays there until I turn it off ... whether driving around town or on the highway. I very much doubt that your engine doesn't make enough heat to reach temp/thermostat equilibrium. If everything is working normally, your thermostat should hold the temp steady once warmed up.

Peyton

the 6.2 is governed to around 3000 rpm (i want to say 3200-3400). any more than that and you'll start spitting parts out of your bottom end...like the crank

I think a thermostat stuck open might exhibit the symptoms you describe.


Yes, it can look like that. But I replaced the thermostat not long ago, as one of the first things after I got the truck. Can they get stuck within about two months?


FWIW, whenever I drive my suburban, the temp goes to 205 (gauge temp) fairly rapidly and stays there until I turn it off ... whether driving around town or on the highway. I very much doubt that your engine doesn't make enough heat to reach temp/thermostat equilibrium. If everything is working normally, your thermostat should hold the temp steady once warmed up.


Hm, maybe I should install an aftermarket gauge to get a more accurate reading. If 205 is normal operating temp, I`d be a bit far away from that.

But I guess it would already be higher if the oil coolers built into the radiator were in use since more heat would be delivered into the radiator.

Hm, 150 is really low. I should get a better gauge.

Yes, it can look like that. But I replaced the thermostat not long ago, as one of the first things after I got the truck. Can they get stuck within about two months?

You can always get a bad *anything*.


Hm, maybe I should install an aftermarket gauge to get a more accurate reading. If 205 is normal operating temp, I`d be a bit far away from that.

But I guess it would already be higher if the oil coolers built into the radiator were in use since more heat would be delivered into the radiator.

Hm, 150 is really low. I should get a better gauge.205 is just the gauge reading. I don't know that my gauge is absolutely accurate, I just know that it is steady and consistent which indicates to me that the cooling system is working as designed.

You could look at the thermostat as having 2 functions: 1. to regulate the coolant so that the engine doesn't overheat and 2. regulate the coolant so that the engine doesn't underheat. These engines were designed to run most efficiently, if I remember correctly, somewhere between 190 and 210 and it is the function of the thermostat to maintain that range. Someone please chime in here if these aren't the correct temps.

Peyton

Hm, ok, it could be stuck.

What temps do you guys with 180F thermostats see?

Uhh, I wanted to edit the above, but the time limit has expired. It should be like this:


Hm, ok, the thermostat could be stuck. I`ll see if I can get a mechanical gauge. If it still goes down to 150, I`ll have to check the thermostat because 150 is just too low.

What temps do you guys with 180F thermostats see?