Low oil pressure when hot

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Ann Serow
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Low oil pressure when hot

#1 Post by Ann Serow »

My '58 Speedster's engine was rebuilt recently and all is well, except a green oil pressure light. Everything's fine when the engine is cold (40 psi), then 20-30 psi. But when the engine is fully warmed up and stopped or going very slowly, the green light comes on. The oil pressure has fallen below 7 psi. Upon accelerating, the green light goes out. But the oil pressure never is above 10 psi and the green light goes on frequently, with the warmed up engine. The idle seems OK. I did some research: may be typical of a small oil pump. It's distracting and I'm worried. I'd appreciate any expertise that you can offer. I do better talking with my mechanic when I have some specific information for him rather than just "green light comes on and then goes off." Thanks very much.

Ann Serow in West Hartford CT

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John Brooks
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Re: Low oil pressure when hot

#2 Post by John Brooks »

Look at the oil pump cover for the wrong gasket or too much wear.
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David Jones
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Re: Low oil pressure when hot

#3 Post by David Jones »

Ann, If memory serves me correct it actually states in the drivers manual for the "A" that the oil pressure warning light can come on when idling in hot weather. One thing that can help is putting a heavier weight oil in the engine. I would suggest a 15/50 if you have something lighter in there right now. Shell Rotella would be my recommendation. Do you know what oil is in it? Who built the engine, was it a reputable shop/mechanic? My "A" did that before I rebuilt the engine but it was quite worn.
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C J Murray
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Re: Low oil pressure when hot

#4 Post by C J Murray »

Hi Ann, I would talk to the mechanic soon. Driving the car with low oil pressure could damage your expensive new parts that were installed during the recent rebuild. If the oil pressure was fine when the rebuilt engine was first installed and it is now low I suspect that it will need to come apart to find the problem The longer you wait the more expensive the repair will be.
Sorry for your troubles.
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Neil Bardsley
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Re: Low oil pressure when hot

#5 Post by Neil Bardsley »

"My '58 Speedster's engine was rebuilt recently and all is well, except a green oil pressure light. Everything's fine when the engine is cold (40 psi), then 20-30 psi."

I would be concerned if my engine was at 20-30 psi when running.

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Re: Low oil pressure when hot

#6 Post by Dick Weiss »

Ann,

The small oil pump assembly is critical to produce proper oil pressure and flow to the engine; Any shaft-wear--especially the bushing/spacer (next to the spiral gear fitted to the bore in the pump's cover)( to keep the shaft parallel to the driven gear shaft, and uneven wear on the
gear's pitch line, matching gear lengths, and worn shaft clearances to their bores and excessive end-play/clearance when assembled.
If the regular .008"-to-.010" gasket was used, the endplay will be very excessive and oil slippage will occure I've made .004" paper gaskets
to suit the clearance specs and had success w/rebuilds producing 10 PSI @ idle and 40-48 PSI when hot w/20/50 oil.

Dick

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Harlan Halsey
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Re: Low oil pressure when hot

#7 Post by Harlan Halsey »

Ann,

I think Dick is correct, a freshly rebuilt engine should show 40-50 psi hot at 3000 rpm using a 20W/50 oil. after break in. The most important thing is to know that you always have at least 40 PSI hot above 3000 RPM. I use SWEPCO 20W/50 or Mobil 1 15W/50, and the green light never goes on once the engine starts. When these engines were built, Porsche recommended 30 W oil. (20W in the winter) The 30 W would thin out as it got hot and there was a possibility that the low pressure light would come on at idle. Modern multi grade oils are thinner when cold and thicker when hot. The cold thinner oil gets throughout the engine faster on startup. When hot it stays thicker than the 30 W and so results in higher pressure at hot idle. On a new rebuild I put a pressure gage in place of the pressure switch so I can determine the pressure vs RPM. This is easy to do as the pressure switch thread is close enough to a 1/8" pipe thread so a standard pressure gage easily screws in.

Dick is also correct that the condition of the oil pump is critical. The gears can be easily inspected and shafts the shafts measured. If the gear housing is worn, that needs to be corrected. Some gasket kits have been found to have gasket which is too thick too thick resulting in too much gear end play. I think the end float spec is .004". (I've actually never found anything unusual with the pressure gage, it is just one of the start up checks I always do with a fresh rebuild. But then checking and repairing the oil pump is part of a rebuild. )

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Re: Low oil pressure when hot

#8 Post by Al Zim »

When I first started working on these cars in 1962, oil's were terrible compared to what they are today. When a customer complained that the oil light was on, we removed the oil pressure switch which was adjustable and set the pressure lower so the light would be off. I only worked there for 3 more years, and do not remember any engines blowing up from low oil pressure. At an idle with a 7.5 to one compression ratio, not much oil pressure is needed! al zim
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Ann Serow
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Re: Low oil pressure when hot

#9 Post by Ann Serow »

Thanks a lot for all the interest and assistance. With your help, I hope to have the Speedster on the road in the near future, with proper oil pressure.

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Paul Lima
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Re: Low oil pressure when hot

#10 Post by Paul Lima »

I think Dick is right on. The clearance between the pump gears and the 3rd piece and pump cover are critical to getting good oil pressure. I just finished this step in building my engine and I think I settled on 0.05 mm (.002") clearance. Measured dry with no gasket (gears protrude from case), then calculated gasket to get 0.05 mm clearance. I also found that my gears were slightly different lengths and I had to work some trades to get 2 gears that were the same length.

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Re: Low oil pressure when hot

#11 Post by VinceFinaldi »

Hate to be the "bear-er" of bad news bit it also could very well be the bearings. Worn bearings will cause low oil pressure. Gets worse as engine heats up and viscosity drops. Sounds like your issue. Was the engine dyno'd? I have seen this happen when people are too "enthusiastic" on the dyno before proper break-in. That's why I am not a big proponent of dyno "max HP" runs on a new engine. Also can happen if someone does not follow proper break in, or even if wrong tolerance bearings are used.

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