Oil pressure range when stopping
#1
Hi Guys,

My oil pressure dummy light started flickering when coming to a stop then it started getting worse. I tested the pressure with a mechanical oil pressure gauge and was getting about 60 psi after a cold start. I recently replaced the oil sensor and switched over to an electric oil sender and gauge. I get about 60 psi after a cold start and while driving when warm. The driving range is about 40-60 psi when hot but when I come to a full stop, the needle drops to 10 psi. I suspect my idle may be too low because it feels like the car is border line choking. At what psi should i see when coming to a full stop?

BTW - when I put it in neutral, it jumps to 20 psi.
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#2
this is a 69 mach 1 351w with mild cam and auto.
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#3
Is this a fresh, or high mileage engine? If it's a old high mileage engine, Fords were pretty know for low oil pressure at idle. Which with an automatic was supposed to be set around 700 RPM You can easily raise the idle to 800 or 900 and probably see an improvement as noticed by putting the car in neutral. The problem is normal engine wear/exaggerated engine wear. No telling how it was taken care of before you got it, oil changes etc. If the raised idle works (roughly 20 psi in Drive) and keeps the light off. I'd drive it as a little extra clearance in the bearings won't really hurt it as long as you don't "lose oil pressure". Change to a higher viscosity oil, instead of 10w30 or 10w40 go up to 20w50. I would also put a quart of Lucas, in place of 1 quart of oil when changing the oil. The other choice is to rebuild the bottom end, new crank, bearings and an oil pump, will most likely bring the oil pressure back to where it should be. If you're going to all that trouble you might as well rebuild the whole engine. As I said I'd put the heavier oil and Lucas in it and drive it. I gone 10's of thousands of miles in several old Fords with low oil pressure at Idle, and never blew the bottom end no matter how hard I ran it. Just some personal insight and experience. Hope it helps. JTS 71 Mach1
"We built these cars to drive the "HELL" out of them, not to be museum pieces!"  Carroll Shelby

2008 Mustang V6 5 speed "Diablo Sport Predator" tuner, 87 octane tune. WOW!
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#4
From earlier threads on this site I think this motor was recently rebuilt. How thoroughly, I don't know. But I recall threads during a rebuild.

At idle, with only 10 psi, if bearings are the cause, I would think some mechanical noises would be heard. Not necessarily knocking noises, but maybe sounding like marbles are rolling around in the crankcase.

Is it possible for some reason the pressure relief valve in the oil pump is stuck open? From my experience on stock and high volume oil pumps (not high pressure) the pressure relief valve starts to open in a range of 55 to 60 psi range. So at cruise his oil pressure is normally high enough to open the pressure relief valve. But, maybe it is stuck open? If so, that could be an inexpensive one day repair.
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#5
Your idle pressure is 'within limits'. If the relief valve was open you would get just about nil at idle.
Bring the idle up to 'book' and fit a new light sender. Your engine seems fine. Just to be sure, even though it might be new, change the oil filter and buy a good brand. I have been caught with a clogged filter - long story - changing the filter was all that was needed - oil and filter were 'fresh'.
                                   
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