| 51f1 | -- 02-02-2014 @ 8:49 PM |
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I have an 8RT engine in a '51 F1. I rebuilt it in 2009 (with a new oil pump), and it has 4,900 miles on it. From the beginning, the dash gage indicated 50 PSI, which is what the pump should be producing, until recently when the NOS oil pressure sending unit quit sending. I replaced the NOS sending unit with a new unit. It was defective and indicated no pressure. I bought another new sending unit. This unit indicated slightly less than 30 PSI. I replaced that sending unit with another new unit. This one indicates slightly more than 30 PSI. To check the new sending unit, I applied 50 PSI air, and the dash gage indicated 30 PSI. I also temporally installed another NOS dash gage which also indicated 30 PSI. The only other thing I can think of to do is to replace the electrical gage with a mechanical unit to see what I get. Any ideas? (All of the sending units are marked 80.) Richard This message was edited by 51f1 on 2-11-14 @ 8:29 PM
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| supereal | -- 02-03-2014 @ 6:50 AM |
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Did you use Teflon tape or other insulator when you installed the sender? The sender requires a good ground to work properly. Is the ground strap between the engine and firewall in place, and with clean and tight connections? Almost all of the electrical problems in old cars are due to poor grounding. Is your oil pump a 50 or 80 lb unit to match the 80 lb sender? As you confirmed the pressure with a mechanical gauge, i'd assume the engine is OK. If the new senders didn't work, it is likely either the gauge is faulty, or the wire to the gauge is loose or broken, or the power to the gauge is not constant. This message was edited by supereal on 2-3-14 @ 6:58 AM
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| TomO | -- 02-03-2014 @ 8:19 AM |
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Richard, My guess is that the original sending unit was giving incorrect readings. A poor ground or corroded contacts inside the sending unit can cause high readings. The corroded contacts seems more reasonable as the unit quit working. If you are still puzzled, post the engine speeds for the readings and whether you have a 50# or an 80# pump and sending unit. The sending unit must match the pump output. Tom
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| JM | -- 02-03-2014 @ 12:43 PM |
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I believe you have already stated that all sending units were 80# and you have not yet confirmed your oil pressure readings with a known good mechanical gauge. I would suggest adding that mechanical gauge now (at least temporarily) and leave the electrical gauge and sending unit in place as well. This can be done by plumbing in a "T" at the threaded oil ports at the rear of the engine block. If this gives a 30# reading as well, I would go with the previous comment that your original sending unit was not functioning properly when it was giving a 50# reading. A 30# oil pressure reading on a flathead engine is good! John
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| JM | -- 02-03-2014 @ 12:45 PM |
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I sure wish that posting a duplicate message when I do a "return key" on my computer could somehow be prevented on this website. This message was edited by JM on 2-3-14 @ 12:49 PM
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| joe b | -- 02-03-2014 @ 12:46 PM |
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Also I believe there are 2 different PSI sending units. One is 50 PSI and One is 80 PSI. some vendors don't list the different pressures. This happened to me. I installed a new unit on my V-8 simply because my old unit was dented and did not look good. Had a noticeable drop in oil pressure. Put the old one back on and oil pressure was back to 50+ PSI when driving. This is when I found out about the 2 types. Just a thought
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| 51f1 | -- 02-08-2014 @ 12:59 PM |
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I bought a new Melling pump for a 1951 engine. I assume it was the right pump. That's been almost 20 years ago (I restore trucks very slowly). Both of the new sending units, have indicated 30 psi on the dash gage. I substituted another NOS oil dash gage and still got 30 psi with the new sending unit which indicates to me that the dash gage is good. I still need to substitute a mechanical gage and check the pressure, and I'll do that shortly. If the new sending unit is right, the pump is putting out a steady 30 psi. I'm not going to worry about it. A lot of old flatheads are operating fine with a lot less than 30 psi. I'll let you know what I get with the mechanical gage. If it turns out that I really have 50 psi, a lot of people are selling a lot of bad Chinese sending units. Richard This message was edited by 51f1 on 2-11-14 @ 8:38 PM
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| TomO | -- 02-09-2014 @ 9:10 AM |
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Richard, Your F1 came with an 80# pump and an 80# sending unit and dash unit. It should show 50# oil pressure at 2,000RPM using 30 weight oil and normal operating temperature (160-180). Your air pressure test proved that the gauge and new sending units are pretty accurate. There are many reasons for low oil pressure readings. The first thing to do is to use a mechanical gauge to determine the true oil pressure. If it is an incorrect reading problem, check the grounding of the sending unit, engine, dash and gauge. Teflon tape or other sealants should not be used on the threads of the sending unit or block. If you do have low oil pressure, you can check out the pump, oil pressure relief valve in the pump and the oil filter inlet orifice size. If these are all OK, you will have to look at internal engine clearances. You can cut a small V shaped groove on the bottom of the cover of the inoperable sending unit and then pry the cover off. This will let you examine the internal workings and you can see what caused it to fail. Tom
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| 51f1 | -- 02-09-2014 @ 2:50 PM |
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I put a good mechanical gage on the engine. When the electrical dash gage reads 30 psi, the mechanical gage reads about 55 psi, as it should. (If I recall correctly, the pump is rated at 57 psi.) I know the dash gage is good, and I positively grounded the sending unit. So where do I find a new sending unit that is correct? The two I got from Carpenter were bad, and everybody else probably sells the same unit made in China I'll call Carpenter again, but I'm sure that'll be useless. I did some more testing. The dash gage using the new sending unit reads 50 psi when I apply 80 psi air pressure. These new sending units are definitely not right. I called Carpenter again. They, of course, have no solution. They suggested I call Green Sales and see if they have a NOS sending unit. All Green Sales have are new Carpenter units. I once modified the reading with a temperature gage sending unit that read high by putting a resister in the line to the gage. Since this gage is reading low, that won't work. Anyone have anymore ideas Richard This message was edited by 51f1 on 2-10-14 @ 12:04 PM
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| TomO | -- 02-10-2014 @ 7:59 AM |
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Richard, You might be able to repair you old sending unit. If you do not want to tackle it, try Baxter Ford Parts. (785) 842-9256 Tom
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