This month’s questions for the tech come from the pilot of a vintage twin Cessna with Continental IO-470 engines. A gear-up landing required engine teardown and ever since, one engine developed attention-getting oil pressure fluctuations. IA Mike Berry helps sort it out. And, a Mooney owner battles GPS navigator reception problems, and Larry Anglisano points the troubleshooting in the right direction. First, the oil pressure trouble.
Upon cold startup, the engines’ oil pressure goes to the upper middle of the green, nice and quick.
When warm and in cruise below 5,000 feet, both engines stabilize at the lowest end of the green. The CHTs (320) and oil temps (middle) are good. I run at 50 degrees lean of peak. My first two oil analyses looked just fine (I’ve put 50 hours on it—oil burn with Aeroshell 100W appears to be one quart every seven hours on the left engine and every 12 hours on the right). Compressions are good, no metal contamination, and so on.
The weird part comes when I climb to higher altitudes—the oil pressures on both engines drop below the green by almost one “tick.” At 12,500 I get a little nervous and every change in engine sound startles me. When I descend, the pressures go back into the green.
My A&P tells me that these particular engines don’t have the adjustable “spring” for oil pressure. We can take the units out and try to stretch the spring, but if that doesn’t help, my only alternative is to rebuild the engines. So, two questions: What is causing the drop in oil pressure as I climb? And, can these engines be adjusted?
The drop in oil pressure at altitude is the result of a slight increase in oil temperature. Thin air at altitude changes the cooling characteristics of the oil.
Baffling and cooler surface area don’t respond the same to thin air as they do to “thick air” found at lower altitudes. It only takes a slight change in oil temperature to cause a significant change in pressure—especially on a marginal engine. The relief valve in your engine oil pump contains a spring that is no longer produced. You can add washers under the spring to get some added pressure or you may want to try replacing the spring with one of the styles used on some Lycomings. A caveat and potential deal breaker, depending on whether your mechanic would sign it off for testing: It’s not an approved change and you would want to experiment with it, but I have heard of people doing just that.
The other option is to replace your oil pump with a used pump of a newer style—one that contains the adjustable relief valve. Serviceable pumps can be purchased from salvage suppliers (Wentworth Industries first comes to mind) for about $1000 and the new relief-valve kit will run about $500.
I recently bought an older Mooney that has a Garmin GNS 430W GPS navigator. This GPS started life as a non-WAAS GNS 430 until the previous owner had it converted to the WAAS-equipped GNS 430W. It was done by an avionics shop and the logbook entry said that the unit was shipped to Garmin for the WAAS upgrade and the shop replaced the GPS antenna.
The problem I’m having is that when I transmit on the secondary navcomm (a King KX155), the GPS signal on the GNS 430W drops off. It takes over one minute for the unit to reestablish a lock-on. Is the problem in the KX155 or the GNS 430W?
It might not be in either unit. A WAAS GPS upgrade to the legacy GNS navigator not only requires changing the antenna, but it often requires changing the coaxial antenna cable that runs from the antenna to the GPS.
Many early non-precision approach-capable navigators were installed with non-shielded antenna cable. That spec was tightened up with WAAS certifications and you’ll want to be sure the installation in your Mooney has low-loss twin-shielded coax.
While you’re at it, check the condition of the comm antennas and their cabling. They can be a source of stray RF and will shut down a GPS receiver.
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