77 Mack 237 Low Oil Pressure

[RDog95]

Hi all, I have a new to me 1977 mack DM585S. I ran the truck for about a month with no issues and started noticing the oil pressure not building to what it used to. When I crank up the truck cold, it takes almost 6 seconds for the oil pressure to build. Then it cold idles around 30PSI. I can rev truck cold to make about 70PSI but after going down the road and heating up oil the pressure stays around 15Psi idle and 35 MAX going down the road. I have swapped gauges to to verify pressure and have changed filters and cut them open with no obvious signs of metal. I do not think I have fuel in oil at this time although we will be sending off a sample this week as we are diving into the engine today. No obvious sounds of rod knock although I am still leaning towards the truck needing some new bearings and possibly an oil pump. We are going to look at the jake brake assembly today as well. Hopefully we can get truck inside soon to drop oil pan. I am not running truck at this time. I took out the relief valve on the filter housing and cleaned the plunger and spring, spring and plunger seemed to be in fine condition and moved freely when I reinstalled. I understand there is a relief valve on the oil pump itself? Im sure we will find out whats going on here this week, I have been waiting for a friend to get his superliner out of his shop so that I can use the indoor bay. (Indoor work spaces are limited where I am from) Anyway, just wondering if you all had any ideas as to what else to look for when we pop valve cover and oil pan, also is it worth rebuilding oil pump? or just placing a new one in.... Thanks in advance!

IMG_3735.HEIC

[fjh]

47 minutes ago, RDog95 said:

Hi all, I have a new to me 1977 mack DM585S. I ran the truck for about a month with no issues and started noticing the oil pressure not building to what it used to. When I crank up the truck cold, it takes almost 6 seconds for the oil pressure to build. Then it cold idles around 30PSI. I can rev truck cold to make about 70PSI but after going down the road and heating up oil the pressure stays around 15Psi idle and 35 MAX going down the road. I have swapped gauges to to verify pressure and have changed filters and cut them open with no obvious signs of metal. I do not think I have fuel in oil at this time although we will be sending off a sample this week as we are diving into the engine today. No obvious sounds of rod knock although I am still leaning towards the truck needing some new bearings and possibly an oil pump. We are going to look at the jake brake assembly today as well. Hopefully we can get truck inside soon to drop oil pan. I am not running truck at this time. I took out the relief valve on the filter housing and cleaned the plunger and spring, spring and plunger seemed to be in fine condition and moved freely when I reinstalled. I understand there is a relief valve on the oil pump itself? Im sure we will find out whats going on here this week, I have been waiting for a friend to get his superliner out of his shop so that I can use the indoor bay. (Indoor work spaces are limited where I am from) Anyway, just wondering if you all had any ideas as to what else to look for when we pop valve cover and oil pan, also is it worth rebuilding oil pump? or just placing a new one in.... Thanks in advance!

IMG_3735.HEIC 2.84 MB · 2 downloads

How is your oil level ? is it rising? have an oil sample done you may have fuel dilusion  !

[RDog95]

33 minutes ago, fjh said:

How is your oil level ? is it rising? have an oil sample done you may have fuel dilusion  !

Oil Level does not seem to be rising, I also did a halo test on cardboard and did not see any indications of fuel in oil. We will send oil off for testing regardless.