What makes the oil pressure gage.......

[redkneck]

Jump around a lot on older trucks? Mine hovers around the 20 mark at idle and steadies around the 40 mark at highway speeds with 253K on the odo. Should I up my oil viscosity to account for the engine wear and get the pressure up? Still using 5w-30 most all the time in it, though have used 10w-30 when it was on sale

[LETMGROW]

First thing I would try would be a new oil pressure gauge switch. [ sensor ] After this many miles you may have some sludge buildup in the small port inside the switch.

I wouldn't change to a heavier oil. Especially with cooler weather coming on.

Lynn

[MUDRUNNER]

I wouldn't worry about it too much. With that many miles 20 psi at idle is not too aweful bad. I wouldn't get concerned until it goes below 15 or so. My '98 Chevy is about 25 at idle and 50 at cruising speed and it has 150,000. I believe when the pressure starts dropping too much at idle it is usually a result of worn main and rod bearings.

[redkneck]

Sorta the same things I was thinking. Maybe run a higher viscosity in the summer? I'll check the sending unit. If I'm not mistaken it's a bugger to get to right on the back of the engine next to the firewall.

[92xj]

There is absolutely nothing wrong with running a 10-30 in the winter time. I run it in mine as well as a quart of Lucas. If your motor cant create enough heat or power to move a higher viscosity oil, then you have way bigger problems. I personally would never run anything lower than 10-30 in any of my vehicles. I would rather the oil be more viscous and stick to the top end of the motor while it is not running instead of dripping off and down to the pan faster like a lower viscous oil would and avoid having a dry start the next time I crank. To each their own.

[Buckhunter76]

There is absolutely nothing wrong with running a 10-30 in the winter time. I run it in mine as well as a quart of Lucas. If your motor cant create enough heat or power to move a higher viscosity oil, then you have way bigger problems. I personally would never run anything lower than 10-30 in any of my vehicles. I would rather the oil be more viscous and stick to the top end of the motor while it is not running instead of dripping off and down to the pan faster like a lower viscous oil would and avoid having a dry start the next time I crank. To each their own.

I agree, use light oil during the winter or cooler time of the year and heavier in the warm and summer months, helps lubrication. Think about this, if you take oil and put out in the cold what does it do? There for when you start your vehicle it would take allot longer to get where it needs to go causing dry crank, thats definitely not good on any motor, lol.

[Shaun_300]

Remember that the second number is the viscosity, not the first number. The first number is a rating on how much the viscosity changes with temperature. 5w30 and 10w30 have the same viscosity at normal temperature and operating temperature, only difference is the 5w30 flows faster in the cold temperatures. 10w20 is a "thinner" oil, 10w40 is a "thicker" oil.

John I wouldn't get worried about it yet, still lots of oil pressure for that engine. Like Lynn said there could be a bit of crud on the screen of the oil sender. It's at the back of the engine near behind the intake. You'll need an oil pressure sender socket to get it out. Clean it out and put it back in with some pipe sealant on the threads and see if it's any different. Remember that the oil pressure sensor is at the top end of the engine, so chances are the cam bearings are getting a little worn and bleeding the pressure off before the sender. The bottom end would still have plenty. And honestly, even if the bottom end only had 20psi it'd still be lots. I've seen trucks like yours with way less miles on it with lower oil pressure than that with no worries.

The minimum specs for that engine are:

6psi @ 1000rpm

18psi @ 2000rpm

24psi @ 4000rpm

[redkneck]

Cool, that makes me feel better!

This weekend I gotta replace the U-joints in the front shaft. Also looking to replace the seals (if I can, not going to take off a yoke and screw up the crush washer on the front diff).

[Shaun_300]

There's no seals you need to worry about replacing. The pinion seal on the front is the same as the rear. If it's not leaking don't worry about it. Basically the only seals that like to leak on these trucks is the front axle seals. But again, if they're not leaking, don't replace them.

[redkneck]

Front axle seals leak some. Small leak it looks like on the pinion seal of the front diff.

Will check it out more when I replace the u-joints.

[Shaun_300]

If it just looks damp or seepage don't worry about it, when you look under and you see actual oil drips on the housing, that's the time to replace them.

[Orion_70]

A lot of the older trucks had "real" pressure sensors that would fluctuate with the oil pump as they are supposed to. My jeep idle was at 13 psi, which was normal idle pressure and would fluctuate to 42 psi or so running at 65. It's completely normal

[LETMGROW]

There are probably more misnomers about oil viscosity than anyone could imagine. The most popular being "thickness"

Multigrade oils are formulated to flow at very specific rates at a multitude of temperature ranges. This requires very precise blending of additives to acheive this.

When I first started driving the norm was to use a 10w oil in the winter, 20w in the spring and fall and 30w in the summer.

Along came multi-vis and we could run the same oil year 'round.

I am not an engineer or scientist, so I can't tell you how this is acheived ,

but I do know the flow rate of a cold oil in a 5-30 oil is equivalent to a 5w rating, however when the engine warms up it maintains a viscosity of 30w oil.

In the 90's many manufacturers tightened up clearances in their motors, Hence the popular 10-30w oil was replaced by 5-30. In some rare cases a 5-40w oil was recommended. Along came overhead cam engines and 5-20w became the manufacturers choice. This also holds true with many roller lifter equipped engines.

The heavy oil theory doesn't really work when someone says it coats engine parts to prevent "dry starts". The reason being, as the warm engine cools, oil runs off parts and nearly leaves them dry anyway. This is when the 5w flow rate kicks in. With the lighter oil 'in theory" oil is pumped to parts in the upper portion of the engine quicker.

Race engines use heavier oils because they tend to run larger clearances on bearings and piston to cylinder walls. It is not uncommon to find many race engines using 5-50w oil.

In the real world a bearing should NEVER touch a rotating part. IE: crankshaft, camshaft, etc. The higher 2nd. # will fill in the clearance. Let a bearing touch a crank at 7500 RPM and you have a junk motor right now!

Oil pressure is important because you need to have enough pressure to lubricate the entire engine. Low pressure might lube the mains and rod bearings but the oil may not be getting up to the cam and lifters fast enough to have proper lubrication. This could mean the cam is running too warmly, due to lack of lube, lifters are wearing prematurely, and an eventual failure is waiting in the wings.

Lynn