Scott

I have been chasing down a strange oil pressure problem for over a year. It started when we through the engine together after a spring and lifter failure to make it to the end of the season. This is when I first saw the Redline gauge start acting up. It would show bouncing pressure while out running never below 30psi while rpm's were around 3000 to 4000 the light would blink on and off on the gauge even though is was set to go off at 10psi. I ran a mechanical and it confirmed that I had pressure. That winter I did a rebuild on both motors they started life as HP500 (carbed) and stayed the same except cam and head change. The bottom end was re-balanced and put back together with new bearings both had the same size bearings.
All last season I chased down this strange oil pressure problem thinking it was the Redline gauge I tried two new gauges and a new sending unit (not to mention a ton of swaps in between) I think part of it may have been electrical and I have been able to stabilize the gauge and now it matches the mechanical gauge I used to test with. (Sounds good right ).
I have confirmed that once that engine heats up (idle to temp) that the oil pressure falls to 30psi at idle and runs up to 60+psi when revved up to 2000rpm. The other engine holds a steady 60psi. After a run the pressure drops down to 7 or 10 psi at idle causing the Redline light to come on but does not sound the alarm (it must be borderline close). One more thing different about that engine is that metal had gone through it when the spring failed!
To sum it up I loose pressure (too much) when the oil is hot and no I don't know the exact temp but I did shoot the remote filter block and fittings with a IR temp sensor and got 230deg or less.
I plan on changing out my oil pump (never should of put it back in after metal went through it) and of course looking for bearing and clearance problems. I will also change the By-Pass valves to 30psi valves in case the wrong ones or the bottom end builder installed the wrong ones.
Would you do something different? Am I missing something? Should I look for something else? Or am I on the right track?
Sorry this was so long but I wanted to give up as much info as possible.
Thanks, Scott.

GregP

What did you find "electrically" to help the gauge. One of mine still goes bezerk most times when I fir the motor, then "gets well" and works fine after 10-15 minutes of running. Mine never "bounced", just read high.

Can't help you much with the other oil pressure issue, mine drop to about 50 psi at idle after a nice long run, then come back up to 60-70 psi in about 2-3 minutes as the oil cools.

Thanks - Greg

Scott

Greg,
I found the voltage on that side was low by about 1.5volts compared to the other side. I then pulled the main engine plug and started checking continuity back to the dash. I did find a problem with the switchable exhaust hookup pulling the power down on that side. I hooked everything back up and the voltage came back up (never got a clear answer) But at this point I was able to confirm that the pressure gauge was working right.
I hooked a cable from the dash to the sender to confirm.
I hooked a cable from that sender to the other gauge to confirm.
I hooked up a mechanical gauge to compare readings.
If you do these three things you will eliminate all possible problems. I am going to guess if you pull your sender out and make sure the threads are clean and pull your main motor plug off and make sure its tight and clean you may find your problem. BTW if the gauge input sees a direct ground it will peg so you may have a bad sending wire.
Scott

Bad Girl

from the description I understand the oil presssure is different after a run and a few other things but it may be possible you are overheating the oil somewhere in your motor breaking down the oil. depending on how your system is laid out, it may be possible you are overheating the oil, cooling it it in your oil cooler then taking the oil temp.
Also a thought that if your oil pump pickup is mounted a little too high in the pan you could be intermittently sucking air. this will do two things, give you erratic pressure reading ans raise the oil temp by physically beating up the oil.

give me a call. as usual yes I am off today! ha

dog

Scott

I tried to call yesterday as usual you were not answering

You know the history and that the plumbing is right and that the pump and pick-up are original. I think you should come up on Friday and help

GregP

Scott,

I've already done the bypass the harness, hook to the other gauge, new sender (nothing at all on threads), different gauge stuff all to no avail. Volt meters both read the same, but havn't checked with my DVM. The "new" gauge does different things/readings than the "old" gauge. but they both sometimes read wrong and have the low pressure light come on when the pressure isn't low.

Since I have an "extra" gauge (replaced it thinking it was bad, but new one does "different" flaky things) I played with sender to ground, sender good but no ground etc and could never get a "gauge high/light on" responce from it.

If it keeps acting up I'm just going to dump the red line (and probably Gaffrig too) gauges.

-Greg

Scott

Greg,
Did you replace the sender?
I did everything in your first paragraph including a sender swap from one motor to the other. It would still give bad readings (intermittent). I replaced the senders with the ones from Gaffrig and the gauge started to act normal (this is when I found out I had bad pressure after heating up).
I would also consider using a mechanical gauge to confirm your readings or just replace them with mechanical gauges like I plan on doing (someday).
I understand your frustration you have to think just how hard can this be it's a simple circuit and how is it possible for the gauge to show a lot higher than 10psi (the light setting) but that damn light keeps flashing

GregP

Yes, I replaced the "old" Gaffrig sender with a "new" Gaffrig sender and that wasn't it either (I knew it wasn't really the sender as I'd already wired the sender to the other side gauge and it read fine). I've gotten to where I really don't worry about it too much, just check the mechanical gauge back at the motor and go boating. It usually "regains it's senses" and reads ok by the time I idle out and get up on plane. Once it starts to work it keep reading ok (at least so far).

-Greg

shawn

Are the Gaffrig senders different from the Merc ones?

Scott

Yes they are supposed to be different. They do look a little bit different but my best guess is two different suppliers make them to the same specs. I do have to admit my gauges seem to have a bit more realistic readings with the new senders. (A tad bit closer to the mechanical).