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Oil Pressure Gauage / Switch 80psi |
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jmesser 
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Joined: March-20-2004 Location: United States Status: Offline Points: 84 |
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 Topic: Oil Pressure Gauage / Switch 80psiPosted: May-28-2023 at 8:37am |
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Again, I come to the group baffled-
Set up- I have an early 80s 351 PCM on a stand with a PCM 40i. I have a gauge panel from an old Fish Nautique with the original factory warning harness connected. The engine fires right up and runs great (drives my neighbors crazy). Problem-The oil pressure reads 80, no fluctuation. Diagnosis thus far- 1. Removed the oil pressure sending unit and replaced with one exact matching one, then one of the newer small sending units. No change- pegged at 80. 2. Removed the gauge and replaced it with a new one, no change. (note, I have also taken the sending lead off and direct grounded it resulting in a 0 to 80 reading immediately) 3. Rigged up a new sending wire from the sending unit to the gauge "S" terminal completely bypassing the wiring harness. No change 0-80 immediately. Am I crazy, but after reading all the diagnosis from all the post, I should have found the problem by now, right? What am I missing? |
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Johnnie Messer
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KENO
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Joined: June-06-2004 Location: United States Status: Offline Points: 11116 |
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Posted: May-28-2023 at 4:56pm |
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Assuming the gauge terminals are labeled, are the 3 wires on the right terminals, positive, negative and signal? When you turn the key to the Run position without starting the engine, does the gauge stay at zero or does it instantly go to pegged high at 80 psi? From your description of things, I'm guessing that it pegs instantly If pressure stays at zero till the engine is actually running, then goes to 80, I'd check your pressure with a mechanical gauge in place of the sending unit Are you sure the sending unit is for what I'm guessing is a 0 to 80 psi gauge?
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Jonny Quest
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Joined: August-20-2013 Location: Utah--via Texas Status: Offline Points: 2979 |
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Posted: May-28-2023 at 9:41pm |
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Direct plumbed oil pressure gauges are available. Some call them, “mechanical” or “manual” oil pressure gauges. You can also get an oil pressure test kit that screws into the oil pressure sending unit port. That would provide some feedback without the variables of sending units, wire, harness, etc.
Do a search on Amazon or Summit Racing and you’ll find several options. JQ
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2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
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MrMcD
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Joined: January-28-2014 Location: Folsom, CA Status: Offline Points: 3750 |
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Posted: May-29-2023 at 12:20am |
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If you actually have 80 PSI at idle someone may have installed a High Volume High Pressure oil pump or your oil pressure relief valve is stuck in the bore giving you max pressure all the time. In the worst case scenario if you need to fix the oil pump at least this engine is on a stand where the job is pretty simple. If it is 80 PSI I am thinking the pressure relief is stuck closed.
We did not ask what weight of oil you are running? I know the factory said to run 40W but with your oil pressure you might drop down to a 10-30 and see how it does if you don’t want to pull the pan. If a mechanical gauge is available that is the best first step. Make sure what you have before wrenching.
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bwinn
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Joined: July-13-2009 Location: ct Status: Offline Points: 396 |
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Posted: May-29-2023 at 9:54am |
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Is it possible the wrong sender is installed? I had a leaky sender while on vacation and installed one that I could source at the time. It was an on/off type essentially not a variable pressure type as originally installed. It would read full on the gauge when running. You could remove the sender and test it with an air blow gun with a rubber tip and a multimeter.
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jmesser
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Joined: March-20-2004 Location: United States Status: Offline Points: 84 |
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Posted: June-03-2023 at 8:05pm |
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I'm back at it.
-Keno, yes sir the three prongs on the gauge are Pos, Neg and Signal. -I got a pressure tester (Maddox from Harbor Freight) and oil pressure shows 50psi with a little RPM, but close to 40psi at idle. -So I'm sure I don't have a High Pressure pump installed. -This gauge pegging to 80 has me baffled though. |
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Johnnie Messer
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KENO
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Posted: June-04-2023 at 2:52pm |
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You forgot to answer a question from earlier When you turn the key to the RUN position without starting the engine, does the gauge stay at zero or does it instantly go to pegged high at 80 psi? That answer will help a lot Multiple choice.....pick one  1) stays at zero and then goes up to 80 when the engine is started (or goes to some number that's less than 80) 2) goes right to 80 instantly when the key is turned from OFF to RUN without starting the engine |
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jmesser
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Posted: June-07-2023 at 10:05pm |
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My apologies, I thought I hit post on this earlier...
1) stays at zero and then goes up to 80 when the engine is started I don't get a twitch on the gauge going from off to run on the ignition switch, but when I hit start- 80 |
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Johnnie Messer
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KENO
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Posted: June-08-2023 at 6:06am |
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Staying at zero tells me that the wiring is OK. If you had a ground, it would peg high when you went to Run.
So, I figure that you have a gauge and sending unit that aren't meant for each other Is it the original temperature gauge in the panel? As far as the sending unit, I checked the resistance of the unit on a handful of PCM engines I have access to and with the wire disconnected from the sender and engine Off, they were all in the 230 to 275 ohm range and read OK on their gauges. I'd check the resistance with a multimeter to see what you have with the engine off and then with the wire still disconnected, start the engine and get another reading with the multimeter. Then fill in the blanks  Engine Off reading________ Engine running reading_________
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