Oil Pressure Drops?

[MN Rooky]

When the truck is cold the oil pressure gauge reads straight up at a idle. After it is warmed up (10 miles down the road) it will show next to nothing on the gauge at a idle. As I accelerate the oil pressure will also go up and I take my foot of the gas, the oil pressure will follow. Is this normal or what should I be looking into? Sorry the truck is a 1971 3/4ton w/350 manual.

[leddzepp]

Both of my trucks do the same thing...exept the gauge is pegged all the way to the right when I first start-up.

[vectorit]

Mechanical or electric style oil pressure gauge?

Mine is mechanical, and it is always showing a steady oil pressure cold or hot.

Though my mill is low mileage...

What kind of oil do you have in there?

Totally different comparison here but here goes:

My 97 evo Harley did not like running synthetic oil, it's oil pressure gauge would do the same thing as yours. On really hot days, and driving in stop and go traffic the oil light would actually come on! Plus it would burn up a quart after doing a hundred miles or so.

I went back to regular multi grade oil, and all is well again with the oil pressure and oil consumption.

[MN Rooky]

I have a Mechanical gauge and running 5w-30.

[highperf4x4]

There's alot of things that can affect your oil pressure but the biggest factor is worn main bearings. If you have alot of wear then you lose a ton of oil around your mains. Your oil pump could also be weak.

If your oil pressure gauge is stock then it should read about 3/4 or higher when it's cold assuming it's accurate.

You could pick up a new high pressure pump and stick it in to see if that's the problem.

I'd also toss in some slick 50 and some Lucas Oil Stabilizer to help with lubrication when the pressure drops.

[highperf4x4]

Originally Posted by MN Rooky

I have a Mechanical gauge and running 5w-30.

5w30?????? That's for newer trucks that have much tighter tolerances!!!

[MN Rooky]

Thanks Oil change slated for tomorrow....

[highperf4x4]

You'd be alot better off with a straight 30w or a good 20w50.

Valvoline VR1 20w50 already has a zddp additive in it for engines like ours with flat tappet cams.

[hchad18]

Originally Posted by highperf4x4

Valvoline VR1 20w50 already has a zddp additive in it for engines like ours with flat tappet cams.

Good info, Thanks

[mikajo39]

Originally Posted by highperf4x4

You'd be alot better off with a straight 30w or a good 20w50.

Valvoline VR1 20w50 already has a zddp additive in it for engines like ours with flat tappet cams.

I used that on my old flat tappet engine. doesn't have as many detergents though so it will get gunky quicker.

[highperf4x4]

Originally Posted by mikajo39

I used that on my old flat tappet engine. doesn't have as many detergents though so it will get gunky quicker.

Yeah I wouldn't go beyond the 3000 mark for oil changes with it but I prefer the added protection for my cam.

[highperf4x4]

Hey Rooky, one quick test you might run before you change your oil....... if you can, put the front of your truck into your garage or pull it up next a wall somewhere and let the engine warm up for a few minutes and then bring it up to about 3000 rpm and hold it there for a few seconds and then let up on the accelerator just slightly a few times and see if the engine makes a distinct clattering noise. If it does then your mains have excessive wear on them. However, if it just makes a hammering noise then ... don't do it again..... that would be a rod bearing!!

Originally Posted by hchad18

Good info, Thanks

Anytime!

[unclewatts]

I wouldn't rule out a worn oil pump if you can't find the problem anywhere else.

[mclairmo]

Switch to 20W50 and see if that helps. I think what you describe is normal. My rebuilt 396 has pretty low oil pressure when fully warmed up but it's much higher when cold when the oil is cold and thick. Remember, you don't need a lot of pressure to be in the normal range. Also, a high volume or high pressure oil pump is not really recommended for stock engines. A good new stock type one like a Melling is all you need. Be sure to run ZDDPlus or an oil with the requisite amount of zinc for flat tappets.

[mrein3]

I wouldn't run 20w50 in MN much past next month. When it gets cold that stuff will be like molasses in there and you'll be asking for other troubles.

I hate to say it by my old, clanky 350 of unknown origins behaved the same way when looking at the oil pressure gauge. I corrected the problem with a rebuilt 383.

I would run 10w30 in that thing year round and start saving for a rebuild. You can pick up a core 350 for next to nothing and slowly work on it while you run what you have.

[highperf4x4]

Originally Posted by mclairmo

Also, a high volume or high pressure oil pump is not really recommended for stock engines.

???? You're talking about 2 different pumps. A high volume pump is made for high performance engines with at least a 7 qt oil pan. They flow alot more oil and could pump a 5qt pan dry at higher rpms. A high pressure pump is for use in stock to mild performance engines. They don't flow any more oil than your stock pump. They have a stiffer spring on the relief valve to keep the oil pressure up so that when oil loss occurrs around bearings, there is still enough "pressure" to push the oil to the top of the engine. It's HIGHLY recommended for worn or high mileage engines.

Originally Posted by mrein3

I wouldn't run 20w50 in MN much past next month. When it gets cold that stuff will be like molasses in there and you'll be asking for other troubles.

Actually that's a misconception. 20w50 is not a "thick" oil at all. 50w is thick, 20w50 is not. A multi-grade oil is never any thicker than that first number whether it's hot or cold. It's 20w oil. That's it. However it has adjuncts and certain fillers that stop it from breaking down as much when it gets hot as all oils thin down when they heat up. Therefore, It's 20w when it's cold and it's "equivalent" to 50w when it's hot. I run straight 30w in my trucks year around and it's actually thicker than 20w50 when it's cold.

The oil that Rooky has been running is 5w oil when it's cold and that's the thickest it can ever get. When it's hot, it's equivalent to 30w. Not much good for older engines.

[QKENUF4U]

Originally Posted by mrein3

I wouldn't run 20w50 in MN much past next month. When it gets cold that stuff will be like molasses in there and you'll be asking for other troubles.

would just mean a bit more warm up..... 20W (W stands for winter) means it will flow like a 20w in the WINTER but protect like a 50w.... (lol i need to start reading the whole thread before i answer)

as for the oil pressure up and down.... try a good/new mechanical guage....that factory guage is old and who knows what the inside of the stock line looks like (gunked up ?)

[mclairmo]

Check out this article. Assuming the engine is not knocking on death's doorstep, a new stock pump should be fine.

http://www.stockcarracing.com/techar...ock/index.html

[mrein3]

Originally Posted by highperf4x4

Actually that's a misconception. 20w50 is not a "thick" oil at all. 50w is thick, 20w50 is not. A multi-grade oil is never any thicker than that first number whether it's hot or cold. It's 20w oil. That's it. However it has adjuncts and certain fillers that stop it from breaking down as much when it gets hot as all oils thin down when they heat up. Therefore, It's 20w when it's cold and it's "equivalent" to 50w when it's hot. I run straight 30w in my trucks year around and it's actually thicker than 20w50 when it's cold.

The oil that Rooky has been running is 5w oil when it's cold and that's the thickest it can ever get. When it's hot, it's equivalent to 30w. Not much good for older engines.

Originally Posted by QKENUF4U

would just mean a bit more warm up..... 20W (W stands for winter) means it will flow like a 20w in the WINTER but protect like a 50w.... (lol i need to start reading the whole thread before i answer)

"In theory, there is no difference between practice and theory, in practice there is."

I've read the same reports, studies, theories. I know what "w" stands for in engine oil weights. I've also seen, back in my service garage days, a lot of previously rattly engines with owners running heavier oils come in on the hook in the winter with a spun bearing or part of a rod sticking out winking at me through the block.

I ain't calling nobody a liar or acusing nobody of nothing. I just know what I've seen - and don't run heavier weight oils in the winter up here. Or summer for that matter.

When I was a kid I ran 10w40 in the summer and 10w30 in the winter. After I worked my way through college in a garage and saw what I saw I now run 10w30 in all my V8s year round. Even the oil pressureless 350 I previously had in my truck survived on a diet of 10w30 until I could put the funds together to create my 383.

[leddzepp]

As long as there is oil in it, you are fine.

[hayhauler71]

rule of thumb on oil pressure 10 psi per 1000 rpm minimum.

[oldblue1968chevy]

lol my mech. oil guage goes up when i rev the motor..10-30w

wtf lol

[highperf4x4]

Originally Posted by hayhauler71

rule of thumb on oil pressure 10 psi per 1000 rpm minimum.

Right on!


Well Rooky, it looks like you have varying opinions to choose from. Bottom line is that the truck belongs to you. I've been building performance and street engines for 20 years. One thing I know is oil. However, there are basically 3 ways of knowing something. 1. Authority figures. (people who tell you what they know) 2. Audacity. (you know something just because you know it and you're not going to change your mind) 3. Collect your own data. (pretty much self explanatory). As you can imagine, I prefer #3.

So, here's what you can do for some research. Buy yourself 3 qts of quality oil. 1qt of 10w30, 20w50, and straight 50w. Stick all 3 in your freezer over night. Take them out the next day open then up immediately, and pour them into another container and note the flow rate of each. 20w50 is not much different than 10w30. It's only a comparison of 20w to 10w. The straight 50w however will be like molasses. The next test you can do requires an outside electric grill. (do not try this inside your house unless you want a divorce ) Take the same 3 qts of oil and put them in seperate pots. Heat them up to about 300 degrees. Again, do a pour test and note the flow rates and thickness. The 10w30 will be the thinnest by far and the 20w50 will be pretty much the same as the straight 50w.

What you'll figure out is that the 20w50 will be not much different than the 10w30 when they're cold but it should be as thick as straight 50w when they're hot.

Or, you could just take the advice of someone who deals with it everyday.

The engine I built for my 4x4 is about 425hp. It has a flat tappet cam and I can get about 6000rpm out of it. I went a little too high on the compression for the stock cooling system so in the summer with the a/c on it generally reaches 210 degrees. My oil pressure runs between 60 and 70 psi under throttle and it only drops to about 50 psi at idle and that's with the engine at 210 degrees. That's excellent. I built this engine several years ago and not only do I use it daily but I like to abuse it in the mud whenever I get a chance. I'll show you a pic and you can see the mud baked onto my headers. I run straight 30w oil in it all year and 30w is thicker than 20w50 when it's cold. When I start the engine in the winter, I have instant oil pressure. But like I said, you figure out what's right for your truck and your climate.