Jeff, N2008T, 78 Sundowner O-360 engine with 400hrs:
After being on the ground for six weeks getting a new interior and windows done, I still have high oil pressure. My mechanic has worked on it two times this week. His gauge says 85 PSI; my gauge inside still goes way above 100 PSI. He says he has the oil pressure adjustment backed all the way out. He ran it on the ground at as high as 2,000 RPM with a warm engine. I flew it today and in cruise the oil pressure was showing above 100. My oil temp also only shows at the very bottom of the green in cruise. It has straight 50W oil in it, as I live in Orlando, FL. I know the gauge is off, but I think the 85 PSI on his gauge sounds high with the adjustment all the way out. Any ideas on what may be causing this?
Technical Editor:
True high oil pressure on a warmed-up engine is rare. 100 PSI is the max target setting on a lukewarm engine. On a cold engine and single-weight oil, engine RPM should be limited to prevent over-pressure until the oil warms up a bit. It would be normal to see 100 PSI on the takeoff roll (full power) with the oil warmed up to 90 degrees or so. Normal hot pressure in cruise is 65 PSI to 85 PSI (185 degrees oil temp), depending on RPM and internal clearances.
Do you have any reason to believe that your oil pressure gauge is reading correctly, and that his is the one that is wrong? The original oil pressure gauge is mechanical. The one he checked it with is probably also mechanical. If his is a true calibrated test instrument, you need your gauge overhauled. If his was not a calibrated test instrument, I would check the pressure again using several different mechanical gauges. If they all read close to one another, and yours is wildly different, yours needs overhaul. Most of the original gauges in these planes have long since become inaccurate. There is info on this elsewhere on the BAC website. One overhaul facility is Airparts Of Lockhaven.
Most owners, and many mechanics, misunderstand both the role and functionality of the oil pressure adjustment. With a given spring installed, it will only adjust the oil pressure within a certain range. There is a plain, a purple, and a white spring available from Lycoming. I don’t have the number ranges here with me for each spring. I seem to recall that the adjustment range is no more than 10-15 PSI for a given spring.
The oil pressure adjustment will have virtually no effect on a fully-warmed engine. The internal leak-down rate through the bearings, etc. will virtually always determine the oil pressure. That is why the oil pressure rises and falls with RPM, once the engine is hot. The exception is when there is a flaw in the relief valve or valve seat, letting it bypass oil all the time even when the valve is closed. This flaw results in low OP, not high OP. On a hot engine, OP may range from 25 PSI at low idle, to perhaps 85-90 PSI at 2,500+ RPM, on a fresh, tight engine. Only uncommon engines with special high-volume oil pumps, designed to provide oil to operate or lubricate other accessories, have the potential to produce red-line oil pressures at high RPM on a hot engine. I have never seen that happen on our small, direct-drive, non-turbocharged engines.
The primary purpose of the OP adjustment valve is to limit the maximum OP during the takeoff roll on a lukewarm engine (approximately 90 degrees F oil temp, or more). On cold single-grade oil, you can exceed OP red-line if you rev the engine before it warms up. The OP adjustment valve just can’t flow enough cold oil to control the OP, and you can split the oil filter housing. As a general rule, multi-grade oil won’t have this problem; one of the many good reasons to use multi-grade oil during the winter months in cold country. I prefer the straight-weight 50 in central and south FL year-round.
I strongly suspect that your oil temp gauge is also in need of overhaul. I will also hazard the guess that your fuel gauges are inaccurate, too. Most of these gauges are all untouched originals, and most are inaccurate. Your oil temp needle should be at the dot in the green in cruise (plus or minus a sixteenth of an inch). That is at about the 185-190 degree point. You can get a rough idea of how far off the temp gauge is using a simple test, if the weather is still warm enough where you are. I think that the gauge markings begin at about sixty degrees. Go look at your plane while it is sixty degrees, and see where the needle points with the Master on. This is most ideally done with a battery charger connected, so that normal full voltage is on the buss. If it is sixty degrees out, and the gauge needle is far below sixty degrees, then the temp gauge is ‘way off too. This is very common.