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Old 08-05-2006, 09:31 AM
ninjaneer ninjaneer is offline
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Oil Pressure Troubleshooting

The previous owner installed a Chevy 305 in my 77 Cherokee. Since I didn't really trust the electrical oil pressure gage, I installed a mechanical one and drove a 70 mile loop around town. Startup pressure was 30 psi, and after it warmed up, operating pressure was between 12 and 18 psi. At idle, the pressure dropped to 0.

A week later I drained the 10w30 oil (discovering valve seal remnants in the oil filter) and installed some 10w40. I then drove the same loop. Startup pressure was the same, but I gained some pressure while driving, putting it at 18 - 22 psi, and I have a few psi at idle.

Since the startup pressure didn't change, does this point to cam bearings, oil pump, or something else? What's my next step?

All help is appreciated since I'm learning as I go.
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Old 08-05-2006, 10:34 AM
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Walt Walt is offline
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I would think the startup pressure did not change because the cold rating of the oil is the same. 10/30 -vs- 10/40 should have the same cold viscosity.
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Old 08-05-2006, 12:31 PM
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bigblack'74 bigblack'74 is offline
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it could just be worn out..cam bearings, mains rods, oil pump aswell...no real way of knowing expcet pull the pan and some caps and see if the bearings are shot...good luck
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Old 08-05-2006, 11:08 PM
Junk Yard Genius Junk Yard Genius is offline
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The other poster is close, you are using nearly the same oil...
It's never going to be thicker than a 40 weight no matter how cold,
and it's never going to be thinner than 10 weight no matter how hot...

The differance between 30 and 40 weight oil at start up is negible.
When fresh oil is clean and new it's going to hold better pressure.
With a carb it's going to get thinned out pretty quickly by fuel getting past the rings so you will see the hot pressure reading drop off pretty quickly...

The reason your pressure drops off is when the engine heats up, the tolerances in the engine increase...
Rod ends get larger and let more oil get out, same with cam journals, ect.
The gaps increase as the engine heat soaks, and if the engine is old enough or been hot enough to have totaled the vavle seals... It's seen better days.

Try a quality 15w40 in the summer. A good diesel truck oil like Shell's Rotella T or Valvoline are good choices in the hot summer months, and drop back to a 10w30 or 10w40 in the winter...

If you still can't hold a good hot pressure, you can go all the way up to a 20w50 oil with out hurting anything.
I'd try the 15w40 first to see how things turn out, but there are plenty of old, high wear engine running just fine on 20w50, so don't be afrade to use it...
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Old 08-06-2006, 03:55 AM
475thFG 475thFG is offline
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What these guys said...engine worn out, either rebuild it or use thicker oil.
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Old 08-06-2006, 07:14 AM
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Crazy_Jeepman Crazy_Jeepman is offline
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Gees.............Its a 305, not worth the time to figure it out. Throw it away, find a good running 350 and drop it in
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Old 08-06-2006, 03:26 PM
ninjaneer ninjaneer is offline
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Thanks for setting me straight on the oil guys, things are making sense now. I'll try some good 15w40 next time around.

And no 305 love? It's a good thing for me to learn on and play around with, but there'll be a new engine and transmission in the next year.
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