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I have a newly rebuilt '82 Chrysler 318 in a 1934 Plymouth 4 dr. sedan. It has about 2000 miles on it. I noticed early on that the electric oil pressure gauge was showing only about 5 psi @ idle after the engine was hot. ( showed 20+ when cold). After engine break-in period I went to a heavier 20W50 oil and it was a little better. By the way, it shows 40+ at speed when hot. I finally put a mechanical gauge on it just to see if the electric gauge was off, and it shows almost zero psi at idle when hot. All bearing clearances are well within factory specs (everything was plasti-gauged on assembly). It has a new Melling oil pump. I pulled the valve covers off thinking maybe the plugs came out of the ends of the rocker arm shafts, thus limiting the restriction of oil flow, but they were fine. Anybody have any ideas before I pull the oil pan off (which isn't too easy of a job in this car) ? Thanx
 

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Welcome to Allpar. If the engine is quiet at idle, I wouldn't worry too much about it.
Minimum oil pressure at hot idle is ~6 psi according to the service manual. The factory low oil pressure light sender is calibrated to come on at ~4 psi. I would like to see around 10 psi at idle personally, but that's just me and if you are getting 40 psi on the road you should be good.
It's up to you.
 

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The other internal leak/preassure drop location is the cam plate and the lifter gallery plug behind it. If the bearings are good and all that, and the rocker arm plugs are good (I haven't ever seen them come out myself), cam plate check would be ahead of the oil pan. Only other thing I could think of would be a broken spring or stuck check ball in the oil pump pressure relief itself, which I have seen be a problem, stock and aftermarket, but yes, Mellings are good pumps.
 

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One other thought.............................. from a long time ago, I found that a rotary style oil pump had excessive clearance between the rotors and end plate. This caused oil pressure to drop drastically from cold oil to hot. A change from 10W-30 to straight 40 brought the same results that you talk about. Even though you have a new pump installed, could it possibly be faulty?
 

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I would suggest that you stop using 20/50, first. Then do the cam plate check. With that viscosity, the bearings are starving at cold start. With proper clearances, 10/30 should do well. You might add some Lucas oil supplement, or ER to your 10/30 to assure surface protection.
 

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Don't use Lucas.

It is well marketed crap. It dilutes the add pack of any motor oil it is added to, as it has no additives itself like ZDDP Moly or Boron. It has no detergents like magnesium or calcium. It has no antifoam agents like Silicon. It causes many motor oils it is added to to foam up. How well of a hydrodynamic film covers the bearings when foamy oil is pumped through them? Don't believe the foam? Pull the dipstick on a running engine with Lucas in it. The foam absorbs noise. and artificially raises oil pressure.

It is the used car salesman's best friend.

If You want better motor oil, use better motor oil.
 
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