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Dropping Oil Pressure Common w/ 73’s?
#1
Hey again everyone. I had engine failure and had to rebuild the motor. 351C and after the rebuild has been great. It had low to no oil pressure before the rebuild. After the rebuild oil pressure was normal and doing what it’s suppose to. Having interior work done the gentleman noticed the oil pressure was at half and thought there’s a problem and of course my dad freaks out the rebuild is bad. I told Gunn at idle the pressure will drop burn when accelerating it should go back up thus meaning its fine. I work for a mechanic shop and that’s why and thats what they said too but the gentleman knows classic cars well and thought that shouldnt be occurring. So dad wanted me to post to see if the half pressure during idle is ok.

1973 Mach 1 Big Grin
Away
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#2
The short answer is yes, it is fine. However, the factory gauges are notorious for not being accurate or linear. The aftermarket sending units are all over the place in terms of their resistance. If you become truly concerned, temporarily install a mechanical pressure gauge in place of the sending unit. Chuck
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#3
OK ! .. I have had a lot of experience with No and Low on a 351C.  .... chose from following options...
  • The easy ... oil pressure sender is giving erroneous readings 
  • The oil is screwed ... damp has got in if car left for decades doing nothing. (this is what happened to mine after doing nothing for 5 years) 
  • The weight of oil is not right for the rebuild ... (if standard oil pump or high vol, if additive required or not)  at first I used Penrite 5w40  engine... found better results with Lucas 10w40 with additive was more stable.
  • Some complete arse has overfilled the engine with oil way way way above the "safe" line ...oddly this lowers oil pressure as the crank whips the oil into a watery smoothie. (Seen this before by someone filling the engine up whilst still high on front axel stands... the  dingbat

The more difficult. 
  • Oil catcher blocked/damaged...(My case - the bottom of my sump looked like someone hit it with a base ball bat) 
  • If sump replaced and someone has over used gasket sealant and it blocks entry to oil catcher (my second case due to overzealous use of gasket sealant because of an engineer who thought that lathering it on was good... no it isn't. 
  • Oil pump pressure release valve = permanently open
  • Oil pump shaft to distributor is out of place/twisted/disconnected
  • Oil pump is toast
  • Someone changed oil pump to some garbage after market junk and it doesn't suit the volume required for hydraulic lifters and demand from journals through crank ( a friend had this one)
     
The worst 
  • Journal or other galleries are blocked due to junk in oil 
  • main bearings worn to bacofoil and big end gapping is so huge you could have sex between them and the crank
  • Someone ground the crank to the size of a pencil and the bearings are now over sized... you get the point
     
I'm sure the Xperts in here will come up with other options.  thumb

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#4
(04-16-2019, 10:59 AM)c9zx Wrote: The short answer is yes, it is fine. However, the factory gauges are notorious for not being accurate or linear. The aftermarket sending units are all over the place in terms of their resistance. If you become truly concerned, temporarily install a mechanical pressure gauge in place of the sending unit. Chuck

+1, normal for cars before the oil pressure gauges started being controlled by the engine computer.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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