Oil pressure question

[Money_Pit_Incoming]

I plan on switching to a higher viscosity next oil change, I just had the 10w30 on hand from when I bought everything to replace the head. Was hoping the oil pressure would come up again being it suddenly dropped when the coolant leak got worse, maybe had a max of 200 miles on it at that point since the pressure dropped (2k miles on the oil/filter at that point)... I changed the oil, put dye in and drove another 30 miles when I verified the crack. I was really hoping it was just thinned out oil, but apparently not lol.

I'm planning on pulling the oil pan still, just because there was a lot of sludge still in the block. Seafoam treatment didn't seam to pull anywhere near what I expected out, although I didn't expect much to begin with. Rear main seal does appear to be seeping, even though it was replaced maybe a few hundred miles before I got it and I've got maybe 2500 miles on it since then. I don't trust any of the work that was done to it by the guy my uncle had do it for him... most of the oil leak that ever other gasket was replaced to fix, seems to have been the head gasket seeping oil and the oil pressure sender. The gaskets that guy replaced, including the valve cover were still leaking when I got to it. Engine is basically bone dry now after I got done and 200 miles later.

I wouldn't be surprised to find sludge in the pan and pickup, as well as a damaged bearing/cap at the rear of the crank.

I'll post back after I get to that point

 

[Ol Blue]

I've always wondered what you actually gain from switching from 10W-30 to 10W-40 (or any thinner oil to a thicker oil). The extra pressure on your gauge is just extra friction pressure since the 10W-40 is harder to pump. The actual oil flow rate would be close to the same or probably slightly less due to the thicker oil. Imagine trying to pump cold honey through an engine. The pressure will be astronomical but your flow rate will be next to nothing, given the same pump. In theory the 10W-30 should be better at dissipating heat and run more efficiently but the 10W-40 should provide better protection and film on surfaces.

So I guess I'm not convinced the 10W-40 is actually doing much...besides raising your pressure gauge and therefore making you feel better. If you want a thicker oil for more protection then that's fine...why not keep going to a 15W-50? I guess my thought is the engine was designed to run on 10W-30, why not keep it at 10W-30?

Also if you think about the pressure that you should see...for example: "no less than 13 psi at hot idle". This number was intended for 10w-30. If you put 10w-40 in then "no less than 13 psi at hot idle" is completely irrelevant because its a thicker oil and it would need a different (higher) threshold of what you should see.

 

[Money_Pit_Incoming]

I'm not so much worried about idle, as I am max oil pressure being so low at higher rpm. Basically the 10psi at idle and 10psi per 1k rpm gets surpassed before 3k rpm (more like 2500 in my case). At this point I feel my options are, run it as is and risk not enough lubrication under load, or try a little bit thicker oil to get some pressure through the worn bearings which now have more clearance than stock. Either way is a gamble, but I've seen and heard many people getting more mileage from the motor using thicker oil (although I haven't heard of anyone not switching to compare).

Again, I'm not looking to get like new pressures... just a few more psi at higher rpm.

 

[maxbraketorque]

I've always wondered what you actually gain from switching from 10W-30 to 10W-40 (or any thinner oil to a thicker oil). The extra pressure on your gauge is just extra friction pressure since the 10W-40 is harder to pump. The actual oil flow rate would be close to the same or probably slightly less due to the thicker oil. Imagine trying to pump cold honey through an engine. The pressure will be astronomical but your flow rate will be next to nothing, given the same pump. In theory the 10W-30 should be better at dissipating heat and run more efficiently but the 10W-40 should provide better protection and film on surfaces.

So I guess I'm not convinced the 10W-40 is actually doing much...besides raising your pressure gauge and therefore making you feel better. If you want a thicker oil for more protection then that's fine...why not keep going to a 15W-50? I guess my thought is the engine was designed to run on 10W-30, why not keep it at 10W-30?

Also if you think about the pressure that you should see...for example: "no less than 13 psi at hot idle". This number was intended for 10w-30. If you put 10w-40 in then "no less than 13 psi at hot idle" is completely irrelevant because its a thicker oil and it would need a different (higher) threshold of what you should see.

Interesting points. No doubt that the oil passageways and bearing clearances were designed to promote useful oil flow/pressure through all bearing surfaces. Your concept makes sense, but I propose thinking about it in terms of flow rate. As the oil becomes thicker, the flow rate across bearing surfaces will drop while the flow rate through oil passageways (much lower flow cross section compared to an oil passageway) may be less affected. My thought is that as the viscosity increases, oil pressure should become more uniformly distributed at the bearing surfaces. But I suppose this idea depends greatly on frictional pressure drop through the oil passageways as a function of viscosity compared to through the bearings.

If everything is proportional to viscosity, then your idea about viscosity having little effect on differences in oil pressure across bearings would be correct. A chemical engineer would be able to provide some insight on this.

 

[Money_Pit_Incoming]

I'm not 100% sure I understand everything you said, but I've taken into account that all the bearings are unlikely to have worn evenly (there's a possibility one or more may have very little wear) and the flow there would be greatly impacted. But for the bearings that did wear, I would get more flow there.

My understanding of oil viscosity is that the first number, in my case 10, is the viscosity when cold... the number following the "w" is its resistance to thinning when hot. I would think finding something like a 10w40 would provide similar oiling ability, compared to driving in cooler weather? I used an infrared thermometer at the oil pan while watching pressure, and it seems to drop right when the pan reads around 170-180 degrees F. Anything before that, I've got at least 18psi at idle, 40psi at around 2k rpm. After driving it for a bit, the hottest I've seen it get was around 190-195 degrees F.

I'm even contemplating hood vents to help lower under hood Temps, opening the hood for a few minutes helped bring it up too. And it would be a good way to "fix" the small rust holes that are already there lol!

 

[Rob Mayercik]

Have you considered pulling off the rubber weather strip at the back of the hood? Probably not as effective as actual hood vents, but should help at least a little and is free (not to mention, you can pretty easily undo it if you don't like the results).

 

[Money_Pit_Incoming]

I havent considered that, but im undecided on what I want to do with the hood as of yet. It's got 2 small rust spots, both having maybe a 1/8" hole that have been driving me nuts... but I got the jeep real cheap, and been more concerned with all the mechanical/drive ability issues. Next is a shake on the highway, thinking tires are out of balance but it's not acting typical of that and is intermittent. Maybe axle joints? Got a slight pull to the right as well. Brakes aren't dragging, suspension looks good and seeing the after specs after an alignment it should be pulling left! Lol