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Rebuild motor low oil pressure

Old 08-21-2015, 05:07 AM
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Default Rebuild motor low oil pressure

I have a 95 cherokee and it needed a motor over haul. It was Rebuilt from the previous owner but after the motor tear down it was a very poor quality rebuild so another one was under way.
I decided to use a different block that came from a running 99 that a good friend of mine pulled out of his jeep to drop in a stroker motor he build.
Everything that was done
99 block
Magflux block
Hot tank block
Bored .20 over
Line hoaned
99 crank
Resurface crank
All new bearings (including cam)
New pistons
Isky cam and lifters
Rebuilt head from existing 95 to keep the good 7210 head (think that's cast number)
All new valves, spings, seals, but used existing rods, and rockers.
Also new High Volume oil pump
Now during 20min break in have good oil pressure over the 40 mark
After 20 mins I drained oil and replaced oil filter I was told to put another easy 500 miles on it and change oil and filter again. Now after 400 miles I have seen a dramatic change in oil pressure. When cold oil pressure is 25 at idle and 60 under load at around 2k rpm after about 15 mins of driving oil pressure pressure drops to 5~7psi at idle and 35~40 under load at around 2k rpm
I'm using 10w 30 valvoline with lucus break in additive and mopar oil filters.
After speaking with the machine shop they want to replace my oil pump with another new hv pump should I be looking for something else because my gut doesn't tell me that will be the fix. I will be pulling the pan later this afternoon to pull out the pump. Lastly I have double checked my sender and took measurement again with a mechanical gauge. And see same results.
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Old 08-21-2015, 05:10 AM
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Forgot to add there isn't any engine noice and it does run beautifully! No ticking or knocking
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Old 08-21-2015, 05:30 AM
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No mention of new crank or cam bearings ? That a must on rebuilds...This guy has a lot of helpful videos this just hapen be a jeep.


https://www.youtube.com/watch?v=8f2fcbTh5yw

Last edited by freegdr; 08-21-2015 at 05:53 AM.
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Old 08-21-2015, 06:05 AM
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while you have the pan off look up at the cam bearings and make sure one of them hasn't walked out of its bore.
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Old 08-21-2015, 06:26 AM
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Originally Posted by bigbadon View Post
while you have the pan off look up at the cam bearings and make sure one of them hasn't walked out of its bore.
Crank bearing, and to check clearence you need plastic gauge.
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Old 08-21-2015, 07:02 AM
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Originally Posted by EBowen95 View Post
All new bearings (including cam)
Originally Posted by freegdr View Post
No mention of new crank or cam bearings ?

Yep. He mentioned it.


EBowen, 40 at 2k isn't a problem, but 5-7 at idle would worry me.

But lose that Lucas crap!

A cam bearing walking out might be an issue. I'm with the recommendation to drop the pan and give it the hairy eyeball.

Last edited by BlueRidgeMark; 08-21-2015 at 07:05 AM.
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Old 08-21-2015, 07:31 AM
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Using a manual gauge to verify true oil pressure readings?
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Old 08-21-2015, 07:55 AM
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Excessive main bearing clearance will cause low oil pressure. For each .001" over the oil pressure will drop 10 psi.


You don't need a high volume pump, it will only serve to eat the gear off the distributor drive. Install a standard volume pump.


Some high volume pumps have caused problems at the pump to block surface, where there's interference and a gap is formed at the intersection of the pump flange and the block.


If #1 cam bearing has rotated in its bore it will choke off the oil port from the oil gallery, causing low oil pressure. There is an art to clocking the oil port in the bearing to the oil port in the block gallery at bearing installation.
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Old 08-21-2015, 09:07 AM
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All above posts can apply but a HV Pump is not required in a stock motor.
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Old 08-21-2015, 09:30 AM
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Originally Posted by cruiser54 View Post
Using a manual gauge to verify true oil pressure readings?
This. Before you do anything, verify with a manual gauge. You'd be surprised what the electric senders can do when they're failing.
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Old 08-21-2015, 12:48 PM
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it clearly states in the first post that he verified the oil pressure with a mechanical gauge. I'd be looking for bearing clearances from whoever built the engine and I'd be plastigauging the mains myself.
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Old 08-21-2015, 01:06 PM
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Originally Posted by s346k View Post
it clearly states in the first post that he verified the oil pressure with a mechanical gauge. I'd be looking for bearing clearances from whoever built the engine and I'd be plastigauging the mains myself.

Per the FSM.
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Old 08-21-2015, 02:44 PM
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okay I will give the cam bearings a good look and plastigauge the mains I will try and ask what the machine shop should of set it at and take a measure myself. Just very stressful given how much money I have invested. Now why would this machine shop be assuming its my oil pump tho? They are giving me another new free one but they want me to put another HV pump in. Also a side note they want me to run zinc additive in the motor from now to the rest of its life but I haven't ever done that before or heard of either.
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Old 08-21-2015, 05:09 PM
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Originally Posted by EBowen95 View Post
Now why would this machine shop be assuming its my oil pump tho?
Because otherwise the problem is their workmanship.



Originally Posted by EBowen95 View Post
Also a side note they want me to run zinc additive in the motor from now to the rest of its life but I haven't ever done that before or heard of either.

No need. Read this.
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Old 08-21-2015, 05:36 PM
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Originally Posted by BlueRidgeMark View Post
Because otherwise the problem is their workmanship.






No need. Read this.
If its to honor warranty do it.A machins shop built the motor its ther problem take it back. Ive got 200,000 plus and close to 20 psi at idle . Ive had mine 17 years.never any issue.
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