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#2863795 - 01/09/13 07:27 AM Oil Pressure Problem
kilkm68 Offline


Registered: 01/06/13
Posts: 6
Loc: Illinois
A friend of mine has an unusual oil pressure problem I never heard of. It's a new rebuild of an AMC 390 in a '69 S/C Rambler. When he accelerates his oil pressure goes DOWN. We checked with a different gauge and same result. Ran it a quart over full, still same result. A friend suggested maybe the screen came off the oil pump sump. Any thoughts on this?9

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#2863827 - 01/09/13 07:56 AM Re: Oil Pressure Problem [Re: kilkm68]
Jim Allen Offline


Registered: 08/12/05
Posts: 4477
Loc: NW Ohio
The screen is a good suggestion. Drop the pan ASAP. DOuble check gauge operation with a mechanical (unless that was how it was/is tested).
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#2863830 - 01/09/13 07:59 AM Re: Oil Pressure Problem [Re: kilkm68]
Trav Offline


Registered: 11/20/06
Posts: 8444
Loc: MA, Mittelfranken.de
Could be a bad/broken bypass spring, slightly loose oil pump cover, bad pump to block gasket or the bolts might be loose.
Just a few things of the top of my head.


Edited by Trav (01/09/13 07:59 AM)
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#2863908 - 01/09/13 09:08 AM Re: Oil Pressure Problem [Re: kilkm68]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
Just worked on my buddy's AMX with a 390.
I see fresh rebuild ,and a bad problem here.
Yank the pan and be anal about checking things. Of course, let the builder know NOW.

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#2863917 - 01/09/13 09:15 AM Re: Oil Pressure Problem [Re: kilkm68]
440Magnum Offline


Registered: 02/01/09
Posts: 5746
Loc: Texas
Originally Posted By: kilkm68
A friend of mine has an unusual oil pressure problem I never heard of. It's a new rebuild of an AMC 390 in a '69 S/C Rambler. When he accelerates his oil pressure goes DOWN. We checked with a different gauge and same result. Ran it a quart over full, still same result. A friend suggested maybe the screen came off the oil pump sump. Any thoughts on this?9


Excessive bearing clearances will do that, was this engine set up for racing with loose tolerances? Could also just be a measurement error and someone got the clearances too big.

When the engine is under load, there's a much larger net downward (away from the heads) force on the crankshaft than at idle or light load, which opens up the clearances on the oil feed side of all the main bearings and lets the pressure drop a bit. I've seen that on Mopar, GM, and Ford engines on several occasions. Normally increasing RPM offsets that effect and you never notice it, but if you're cruising with a more or less constant RPM and then apply throttle, before the RPM come up, the oil pressure may drop a few pounds. The real fix is to fit the main bearings better. May require undersize bearings... SHOULD require undersize bearings if the crank has been turned.
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#2863945 - 01/09/13 09:38 AM Re: Oil Pressure Problem [Re: 440Magnum]
Brule Offline


Registered: 11/12/12
Posts: 400
Loc: St. Louis, Mo
Originally Posted By: 440Magnum

Could also just be a measurement error and someone got the clearances too big.


That's what came to my mind, too. Because I did it myself on my first engine rebuild. Mine was accompanied by a knocking sound as well.
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#2863992 - 01/09/13 10:01 AM Re: Oil Pressure Problem [Re: kilkm68]
TFB1 Offline


Registered: 09/03/10
Posts: 2622
Loc: VA
My '69 428CJ Fairlane has that issue but it's because the oil will run away from the pickup upon hard acceleration, keeping it at least half quart over full solves the problem... No doubt needs a deeper sump pan, but I want to keep it original... It's made 44 Years and 107K mi, so I ain't messin' with it...

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#2864033 - 01/09/13 10:31 AM Re: Oil Pressure Problem [Re: kilkm68]
OB4x4 Offline


Registered: 01/06/13
Posts: 107
Loc: sc, usa
If you have oil pressure at idle and no pressure once you accelerate you may want to check the pickup to pan clearance. Too little clearance will cause the pump to cavitate due to the oil not being able to flow in between the two rapidly enough.
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#2864097 - 01/09/13 11:22 AM Re: Oil Pressure Problem [Re: kilkm68]
TallPaul Offline


Registered: 10/22/03
Posts: 12897
Loc: By Detroit
If the rod big end bearings are too lose, they will leak oil. When you increase rpm they will leak more. Maybe that's it. Maybe something else as mentioned above.
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#2865224 - 01/10/13 09:25 AM Re: Oil Pressure Problem [Re: kilkm68]
SS1970chrysler Offline


Registered: 05/01/11
Posts: 1265
Loc: PA
My '79 Thunderbird w/351 Windsor did that around 102k miles. The oil pump was shot and rear main bearing was out of tolerance.
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