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E9 oil pump pressure relief spring

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Hey all asking for a friend, just wondering if a broken oil pump pressure relief spring would cause low oil pressure, seems like it would. the truck in question is around 40 lbs at governed rpm and about 30 at idle. Book spec says that's low at governed rpm. My truck as around 80-90 at governed rpm and about 40 at idle, but I have a brand new oil pump pressure relief spring in it. Any thoughts? An yes I put a mechanical gauge on it to check.

thanks all

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Rare to see a spring break however we had a unit loose exactly 10 PSI once because the front cam bushing worked its way back and  got pulled out by the counterweight of the crank ! check the pan see if there is a bearing laying in the bottom! rare but it did happen ! THE FIX you ask ?

Lock tight and Install a new bushing! Didn't hurt anything else in our case!

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It is not that uncommon on the Mack V8's to break the pressure relieve spring. Also the springs fatigue and do loose tension over time. I have an 866 with total rebuild and shined the spring because I could not get one at that time. oil pressure is about 55-65 psi at 1500rpm's 15psi off from my E9's. I got a few rebuild kits from Dale Frances for the E9's and he said it is standard practice to replace the oil pump spring in his rebuilds. He has seen more than a few broke springs. I got a few springs off him for the oil pumps on rebuild. or when pressure suddenly drops.10 -15 psi. He even found the spring for the 866, so just have to pull the pan and put it in.

Yes relief springs break and pressure does drop.

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The reason that made me think my friend was having a spring issue was also that when we got all our engine parts from Dale Francis to do the top end of our engine he also highly recommend putting a spring in, my oil pressure was fine really about where it is now, it may be a bit higher now but the spring was worn. The old spring is the one in the back and the new one is in the front, sure looks like a difference to me. Those shiny spots on the old spring are spots where the coil it worn thin. So before I was going to tell my friend to pull his pan and check the spring I figured I'd see what you guys had to say about it.


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14 hours ago, gearhead204 said:

its amazing that the low rpm pressure is that good with a broken spring,

the pressures that your e-9 runs are the same as mine. 

A bad, fatigued spring does have some compression just installing it or a broken spring is only short a coil turn. That little bit of change in length and spring rate does make a larger difference in oil pressure as RPM and oil flow increase. So you will not see a difference in pressure at idle for the most part but pressure will not increase to 75-90PSI at operating speed. Don't forget there is a pressure spring that opens the piston cooling jets that can open early and compound the low pressure problems. Bad bearings will show low oil pressure at idle as well as operating RPM, that is the difference to tell what is wrong. 


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