Oil pressure drops as engine gets warm

[hilltop150]

Hello, I am looking for a little advice here please. My son and his grandfather just rebuilt a 1994 Chevy 1/2 ton 4 x 4. Everything is new but the motor(350), which ran fine. The problem is when the motor temp hits 160 degrees the oil pressure begins to drop from 40 pounds all the way to 15/20 pounds. The engine temp is fine and everything else seems ok, just the oil pressure. I am wondering if it could be the factory gauge, or maybe the oil (10w/40) is too heavy/light? Or is it the worst case scenario, a failing oil pump? I should mention, I just replaced the oil pressure sending unit and the same scenario exists.

[Blue Bowtie]

Welcome Aboard!

Even if the gauge is correct, that oil pressure is just fine. The general rule for a SBC is that it requires 10 PSI for every 1,000 RPM. At an 800 RPM idle, 15 is just fine. If you are really concerned, drain the oil and replace the filter. Install any oil filter you want so long as it reads "Wix" or "Hastings" or "Delco" (more specifically, anything that doesn't read "Fram") and add the appropriate volume of 10W30 Mobil 1 or Amsoil. The viscosity index of a PAO synthetic is a lot better than that of a mineral oil with the same base viscosity range.

[j cAT]

Originally Posted by hilltop150

Hello, I am looking for a little advice here please. My son and his grandfather just rebuilt a 1994 Chevy 1/2 ton 4 x 4. Everything is new but the motor(350), which ran fine. The problem is when the motor temp hits 160 degrees the oil pressure begins to drop from 40 pounds all the way to 15/20 pounds. The engine temp is fine and everything else seems ok, just the oil pressure. I am wondering if it could be the factory gauge, or maybe the oil (10w/40) is too heavy/light? Or is it the worst case scenario, a failing oil pump? I should mention, I just replaced the oil pressure sending unit and the same scenario exists.

GM recommends 10-30wt oil and actually states do not use 10-40wt...when the weather gets hotter , if it falls below this level you may find that the oil will not be able to cool properly.....at this pressure it should be ok... how will it look in summer heat ???? that will determine if your ok

[hilltop150]

The strange thing is that when you start off on the road it reads 40 and as your driving you can actually watch the pressure drop. What could cause this??? And thank you in advance for the posts, this site is incredible.

[silicon212]

Originally Posted by hilltop150

The strange thing is that when you start off on the road it reads 40 and as your driving you can actually watch the pressure drop. What could cause this??? And thank you in advance for the posts, this site is incredible.

The oil gets thinner as it heats up. Increased clearances due to wear can cause pressure to bleed off faster with lower viscosity oil, leading to lower pressure on the gauge. More than anything, it could be some wear. It could also be some sludging or carboning of the oil pressure relief valve in the oil pump. This latter cause was the reason why my pressure slowly came up after I switched to Mobil 1 - a PAO synthetic. I, myself, am running 15w50 due to the fact that I have 300k miles on mine, there has to be some increase of clearance after that many miles. I had been running 20w50 dino oil (Castrol GTX) in it however, since I built it (except for a couple of years when I was running 15w40 Rotella).

My suggestion is to run 15w50 Mobil 1 or Amsoil in it, change your filter to the non-Fram of your choice, run it for a bit and see what happens. You're likely to see the oil turn black pretty quick as PAO synthetic is highly detergent and works well at removing sludge.

[j cAT]

Originally Posted by hilltop150

The strange thing is that when you start off on the road it reads 40 and as your driving you can actually watch the pressure drop. What could cause this??? And thank you in advance for the posts, this site is incredible.

after thinking about this I now would get a gauge and check the oil pressure ... when cold should be 60-80psi then at normal operating temp 30-45psi....possible dash reading is off by 20psi.....this could be just oil sender off......

[hilltop150]

Thanks to all that replied, your help was immeasurable!!! What I did was change the oil and put in an additive (Lucas engine oil treatment) I upped the oil from 10-30 to 10-40 Castrol GTX for high mileage engines with a special anti-sludge formula. Then I changed the filter to a high mileage filter(not fram). Immediately the oil pressure went up to 45 psi. When it reached 160 it never dropped below 38 psi and has been steady since then. I told my son to keep an eye on it and if it drops down again, change the oil by repeating what we had just done. Its cheaper to do a 25 dollar oil change than it is to swap out a block. Thanks again to all!!! You guys are great!!


Tony N.

[silicon212]

Good deal, but please keep in mind that a 10w40 is about the worst oil grade you can put in it. If you want to run Castrol, 20w50 is not a bad oil to run, but keep in mind that if that engine has a flat tappet cam (which were still used even in '94 on trucks) that you will need to put in an additive which contains ZDDP (zinc dialkyl dithio phosphate) to keep the lobes on the cam from going away.