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The problem: when i start her, pressure is good. After i drive her for a while it drops. all the way down when im at a complete stop, the faster i go, the higher it gets. Im quite sure it has the stock oil pump ( '66 289 Ccode). I used castrol gtx and pennzoil 10w 40. Now im using Castrol gtx 20w 50. Any suggestions? Does anyone know whats wrong or what i can do to fix this? Thanks to all.

@jay
even god doesn't know everything about mustangs
 
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I had something similar. It could be that your engine bearings are worn; that was my problem.

Oil viscosity, when cold, keeps pressure high, so when it warms up, more oil "leaks" out the side of worn bearings, causing a loss of pressure.
I tried changing the pump to a high volume one and this helped for about a year or so.

Thanks,

Mike
'66 Springtime Yellow Coupe
289, slightly warmed over
driven daily
 
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dude my car did the same thing and it blew the engine dont mess with it you have to get a new pump a.s.a.p. its not worth it it happened to me
 
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Like Mike says, worn bearings. Cam, rods, and mains. You
could try a couple cans of stp or the like as a bandaid while
you pick out your parts for a rebuild.

66 Coupe, 289/2v, C4, CA Red
 
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Yeah, my '66 289 A code does the same despite the oil pump change but, only at idle-100x worse if the air is running-It's just fine at 1500+ rpm. think I'm looking for a crate motor too?

Raven Black 66 Fastback GT, 289 A Code, PS, C-4, AC
Pony w/Rally-Pack, TTD's
Cool Huh?
 

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Check or verify the operation of your oil pressure sending unit. If the pressure drops to zero and really is at zero, the motor won't last long. Same thing occurred on my Suburban tow vehicle, new sender - no problem.

Jim Gates
72 Mach 1 - 460, C6
65 Olds 4-4-2, 400 ci, 4-sp
71 Dart Swinger 340 - Project in the queue
 
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