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Old 07-21-2019, 01:55 PM   #1
Newc
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Default ?? Oil change and pressure dropped??

Background; Bought an [ unknown history] Flathead car with a nice 59ab... high oil pressure showing 45-50 hot, Ok oil was clean maybe the later pump... Then [250 miles later] went in for an oil change used 10-40 conventional 5 qts. usual pressure 45-50 hot. Got about 10 miles and pressure drooped to 30 with 20 idle? Level never changed, oil is super clean. Seems that some thing cleaned out or the valley relief started working? Any ideas barners?? more testing to follow. Newc
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Old 07-21-2019, 02:26 PM   #2
ford38v8
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Default Re: ?? Oil change and pressure dropped??

Your unknown history engine may have had diesel oil in it, which would be thicker. Your 20-30 lbs pressure now is as good as can be expected.
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Old 07-22-2019, 11:40 AM   #3
J Franklin
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Default Re: ?? Oil change and pressure dropped??

Maybe the new oil was detergent and the old was 40Wt non-detergent and had gummed up the passages that are now clearing up?
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Old 07-22-2019, 12:41 PM   #4
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Default Re: ?? Oil change and pressure dropped??

I use 20w50 in my flatheads, in the south with lots of heat oil pressure is around 40/45 running down the road and 20/25 idle hot
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Old 07-22-2019, 07:27 PM   #5
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Default Re: ?? Oil change and pressure dropped??

Quote:
Originally Posted by Newc View Post
Background; Bought an [ unknown history] Flathead car with a nice 59ab... high oil pressure showing 45-50 hot, Ok oil was clean maybe the later pump... Then [250 miles later] went in for an oil change used 10-40 conventional 5 qts. usual pressure 45-50 hot. Got about 10 miles and pressure drooped to 30 with 20 idle? Level never changed, oil is super clean. Seems that some thing cleaned out or the valley relief started working? Any ideas barners?? more testing to follow. Newc
This can be as simple as changing brands. When I had attended GM school, we were taught to keep the same type (brand) of oil for the life of the car. Not necessarily the same weight but positively the same type (brand). In later years I'd seen numerous folks that would come to my shop with complaints of oil pressure issues, oil consumption issues...often enough it was traced back to changing brands.

Detergent oil? Nope - that is not the cause in this case. And, most likely, the relief valve did not coincidently just start operating.

I'd suggest you change the oil again and install 20w/50. See what, if anything changes.
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Old 07-22-2019, 08:19 PM   #6
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Default Re: ?? Oil change and pressure dropped??

I agree with everyone saying that you could have been using straight 30w, now with 10-40w you see some difference. Is 20 at hot idle bad? My thought is that is pretty decent for an unknown engine.

Last edited by Tinker; 07-22-2019 at 09:35 PM.
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Old 07-23-2019, 12:04 AM   #7
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Default Re: ?? Oil change and pressure dropped??

Yea Fellows; The new oil pressure is right for a stock eng. I was spoiled by the high readings of the old oil and will try a different weight when changed again. Thanks Newc
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Old 07-23-2019, 07:22 AM   #8
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Default Re: ?? Oil change and pressure dropped??

Just a side note: my new 59A hot idle is 35 lbs as soon as I move first to second it goes
to 60lbs and stays there all day all night. It has the origional oil pump too. I use 20-50
Valvoline racing oil with zink from Napa I use this oil in everything. Even as cold as it
gets here it starts instantly in our January deep freeze (car sleeps out side too) sam
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