I recently converted from FP to CS and ran in to a bit of a problem?. I am sharing this in case someone else runs in to similar symptoms and in also to get community feedback.
Engine: O-360-A1A (one regular mag, one pmag, overhauled, currently w/ ~460 hours)
Governor: Jihostroj (New)
Stainless steel prop governor oil line (New)
Prop: Whirlwind 74 RV (New)
First and second ground runs had expected engine behavior and oil pressures. Logbooks updated, FSDO notified and first flight was ready to go.
On first flight, the oil pressure was initially normal on initial take off roll but after 160 seconds post take off start the oil pressure alarm went off while engine still maintained power. (The oil pressure alarm was set to go off at 25 psi.) I immediately reduced power and returned to land but before I could land the oil pressure got as low as 6 psi but then recovered to about 30. I?m thinking I would have an aircraft covered in oil but there was not a drip to be found. Therefore there could be an issue with the oil pump, prop governor, oil pressure sender, bearing or something entirely else.
In order of diagnosis over several weeks:
1. Inspect oil pressure relief valve bearing and spring ? looked normal. Adjust oil pressure relief spring by adding washers and going to heavier spring. The idle pressure was now 50 psi but adding power resulted in oil pressure drop, not rise.
2. Swap out oil pressure sender. No change in readings.
3. Disconnect prop governor oil line from prop governor adaptor and perform pressure leak down test. Results 39 over 40 and probably closer to 40 over 40. This is definitely outside the 3 to 36 acceptable range.
4. Drain oil and inspect oil filter, oil sump screen. No particles found. (I change the oil and inspect filter with every oil change and the filter has always been very clean.)
5.Remove prop governor and installed cover plate. Reconnected prop governor oil line. Ran up engine (with new oil and filter of course). The PSI at idle was stable (~50 psi with heavier spring) but adding power resulted in lower oil pressure dipping down to the 20s. Also, at high manifold pressure settings, the prop maximum rpm was ~1300.
6. Disconnected and capped off the prop governor oil line and capped off the prop governor adaptor. Now the oil pressure to the prop can only come from the #1 main bearing oil passage. Ran up the engine with the same results as before. This time I noticed that after the engine was shut down that the prop remained in full course pitch. The crank was not releasing the pressure.
7. Remove prop and inspect crank plug. Still solidly there.
After takeoff the main bearing shifted causing the oil from the #1 main bearing oil passage to have a free pass in to the sump, when the PSI reached the minimum, the bearing assumed another position resulting in a higher but still unacceptable oil pressure. Tests 4 and 5 isolated the issue to the front of the crankcase. With the current position of the bearing, the oil coming through the #1 oil port is pressurizing the prop but the clearance is so tight that it is not allowing pressure to be released. The bearing is also in such a position that it is also sapping PSI. Although the bearing has been seated fine for the prior ~460 hours, the additional oil pressure coming from the prop governor imparted a new force caused the bearing shift.
My problem seems to be very similar to this one seen by Bob Martin:
I?ve already talked to an engine shop but they haven?t yet reached the same conclusion.
Step back and take a break for a while?..Then likely remove engine and determine root cause and damage. I think the likely cause is an undersized bearing. The damage may be crank case halves may need to be reworked, crankshaft may be damaged, and the main bearing is most certainly junk.