Low Oil Pressure O-300D (2025)

As you may now I am putting back together a Cessna 172E that has been sitting for awhile doing a visual of the inside of the crankcase and putting new cylinders Superior. I was very excite to ground run and everything was fine. Since I had all the flight controls removed and no tail was installed when I took possession of the aircraft a check flight was conducted. During the flight around 3500 ft and about 80% power the oil pressure went lower to green and pilot reported that it appeared to go lower oil temp was fine. He pulled the aircraft to idle and of course the oil pressure went to the edge of the red he decided and do the right thing and landed the aircraft. I went to a few well known O-300 guys they said to remove the pressure regulator clean inspect plunger and seat and spring. After doing so I ran the engine and what the pilot saw I watched the oil do the same as described. Every one say to change the spring. I have ordered it so my question to the community how many has experienced this same thing and the spring was the actual cause. I just got a sneaky suspicion that it is the bearings. The reason I think so is the 5 washer under the spring and it was OH around 1996 and about 479 SMOH it just seems real soon to have so many washers under the spring with so low time.

I've never had a Continental with high oil pressure. In my opinion, they all seem to run a little on the low side, but consistent, and according to Continental, that's fine. As long as the pressure isn't gradually dropping, or going below the minimum, it should be good. Close monitoring would tell you if the pressure is gradually dropping between now and the next oil change

Verify the psi with a mechanical gauge first, then condemn other things next!

Put a mechanical gauge on the engine psi out port and verify the psi, so you know exactly what you have.
T it into the system so you can see instrument gauge and other gauge.
Then you will know just what you have.

Ok Ladies and Gentleman here is the results. I change the spring and plunger on on the pressure regulator ran the engine dipped below or at 25 psi at 2300 to 2275 static run Out side Temp around 65 to 70 F. And engine temp maybe 200 F to 190. Bought a proper oil pressure gauge for testing and hooked it up again around 25 on the first run Same rpm and outside 65 and engine better around 190 to 180. Second run of the day took some advise put 3 thick AN-960 washers under the new spring. Did quick run since it was already warm. Gauge read great at idle 700 RPM around 11 psi but failed on the power run as temp increased again making 2275 to 2300 I let it go to 28 to 26 psi still not making 30 psi ..Looks like daddy is at OH of the power section. I would be betting that the wrong bearings were installed at last OH since the engine only have about 500 hr SMOH

Keep us posted what you find! At least you'll know exactly what you have when it's all done. Peace of mind! Take a really hard look at the oil pump. Not to pee in any ones Cheerio's, pump gears from Continental were around $1300 for the two last I checked. Gears usually don't go bad tho. The magnesium housing is more likely to wear "if" there is anything wrong with the pump.

I'm guessing you've checked the suction screen, correct? So before you go farther, possibly your oil pump could be worn. Continental made an upgraded oil pump which may solve your problem. If your engine has the older square drive pump, the upgrade won't work, but you can still have your pump reworked. I can't say I have ever seen bearings on a Continental worn so bad it loses oil pressure.....but anything is possible. From your post, you mention you bought a proper (manual) gauge...so it is hooked up to the engine directly??