Re: oil pressure gauge (REALLY LONG)
:::VERY LONG POST ALERT:::
Viscosity is certainly a factor, as is the condition of the oiling system but keep in mind that high pressure does not mean that all is well! You could partially block a passage and get high pressure but be f-ing your engine post-haste!
Cold oil is 'thicker' than warm oil which accounts for the main difference between the readings warm and and cold (the viscosity changes as temp changes). My manual has a few things to say about it all but basically it boils down to this:
Cold idle - gauge can be in between the two white marks
Warm idle - gauge can be between the bottom and the lower mark.
Cold running - the important part here is that the gauge doesn't show oil pressure too high, can be inbetween the higher mark and the top.
Warm running - the gauge can be in between the two middle marks.
(I'll try to remember to look at the manual tonight to see what the real verbage is)
Anyway, there are two important things going on and pressure is only one of them. Flow is very important as you want as fresh a supply of oil as possible getting to the friction surfaces. So if you put in 90w gear oil, you'll have great pressure but the oil will be sitting on the friction surfaces for waaaaay too long, burning up and breaking down and leaving crud...bad!
If you use oil with too low a viscosity (too thin) it will squirt out the biggest 'holes' and pressure will drop to the rest of the system. This leaves some friction surfaces with insufficent oil supply...also bad!
So like most things in the automotive world, it's a balance. As we don't have oil flow gauges in our rigs we rely on pressure readings. In the case of automotive engines/gauges, we also need the manufacturer to tell us what pressure relates to proper flow. When the pressure gauge reads within manufacturers specs., we have to assume flow is ok as well as long as (and this is important) everything else in the lubrication system in clean and operating correctly. We already said that a blockage can give us artificially high readings (well, the readings are accurate technically but the relationship to flow has been thrown out the window by the blockage).
Because oil viscosity is a fairly level standard on which to base the calculations and as most automotive engines use said oil as their primary source of lubrication there is a pretty acceptable range of pressures for general automotive use, but even if you do have a numeric gauge installed you are working with generalizations about how the pressure relates to flow (unless the FSM has actual pressure information, which it certainly may!).
Anyway, if you're still awake (again after the airbag diatribe)...
The net result is that oil pressure in the context of factory specs. gives you some important information about the health of your lubrication system. With the correct viscosity oil at the correct temp. your gauge should read within factory specs. (assuming the gauge and sender are working correctly, ALWAYS CHECK THIS FIRST before assuming you have an oiling problem!)
If the gauge reads higher than it should, you most likely have a blockage or colluded passages (crud build-up in the oil passages making a smaller orifice for oil to get through), or even a blocked filter/filter bypass.
If the gauge reads lower than it should, you either have a blocked or colluded passage on the INTAKE side of the pump (scanvenge side, something blocking the pickup [gasket material anyone?]) or you have 'bigger' openings or clearances between friction surfaces or a bona-fide leak in the internal lubrication system. The most common on our higher-mileage engines is that normal wear has enlargened the clearances on moving parts and more oil is escaping in various areas. This means higher flow to these higher wear parts (good) but less flow to the other areas of the engine (uhh, that would be BAD).
The trick on an engine with this type of wear is to restore the balance. If we want the correct flow, and we have to determine flow with pressure, we strive for the correct pressure. We want the gauges to be where they should, right?
Here's where personal preference comes into play. As my cars get older and the pressure drops from wear, I use a higher viscosity oil. When my LC hit 100k and the pressure was a bit lower than it was at say 30k when I bought it I started to add 10w-30 and 10w-40 mix to bring the pressure closer to spec. What does that do? It REDUCES the acual flow of oil through the system(!), but it allows the areas that would not be getting as much oil as they would with thinner oil to be properly lubricated. That is my preference for managing wear in my engine.
The net result is this: as an engine wears, it does so exponentially. The more wear in your motor, the more loose the clearances, the worse the lubrication system works as the pressure/flow begin to deviate and drop. Most manufacturers account for this in their designs but you can't change physics! The engine that is seriously over-lubed to begin with will have a longer life (as it pertains to pressure lubricated friction surfaces eg: not piston rings etc.). A prime example is the well maintained Porsche 911 which has a serious oiling system as it's basically oil cooled. If the engine is maintained it generally will go over 150k miles, and even then the bottom end is usually intact! This isn't necessarily fantastic when compared to 'regular' cars but in the land of 'real' sports cars it's unheard of (compared to Ferrari, Lambo, etc.)...
However, should my oil pressure drop enough to start requiring 50w or higher, I would seriously think about rebuilding. Compensating for lower pressure with higher viscosity oil is managing the demise of your engine, not fixing the flow problem. The longer this goes on, the lower the more accelerated the wear, the lower the pressure, the less efficient the lubrication system, the higher the clearance until something breaks. Here's where it gets ugly:
If your oil pressure is getting really low and you decide to rebuild, you decide the following:
Time
Place
Components to be replaced (to an extent)
Cost (based on the above, again to an extent)
If you try to compensate for too long, something will self-destruct, usually in spectacular fashion taking out A LOT of very expensive parts with it (remember, were talking about the pressure lubrication system here). Something like a rod or crankshaft at 3000RPM... while you're 100 miles from anywhere... on your way to an important work meeting... with the only local garage being a Jeep shop... that doesn't take credit cards... in a town with no hotel... You get the idea. You don't get to manage much at all!
As your motor runs towards it's inevitable demise, it does so faster the longer it goes. Not to say the LC bottom end won't go 650k or longer, but eventually it will die and the longer it goes, the sooner it will happen (well DUH!).
Ok, enough already. Having said all that, I drove my 40 knowingly as long as it would go. But I had a backup vehicle and didn't stray too far from home with it. It threw a rod right through the case and took out the top end with it. Spectacular destruction! But I had to have a tow, was very late to a wedding, and havent' had the cash or time to fix it yet. If I depended on it I would have rebuilt when I knew things were heading downhill based on a number of factors, the most important being oil pressure.
Wow, and I could have probably just said 'adding higher viscosity oil doesn't really solve the solve the problem of low oil pressure, it just helps manage the flow changes due to wear'...
But I didnt! :-X :-X :-X
Keep in mind that the oil sending unit on these is a piece of dung.
idn't you run through several shortblocks in a row?
