Reflex

Thought I'd give a quick follow-up to the problem I was having. To briefly recap, my M20C was registering high fuel pressure at low throttle settings. I inspected multiple items without finding anything that would contribute to to the issue. These inspections were cursory as I don't believe I'm qualified to do an in depth check. In the end, I considered the aircraft not to be airworthy until I found a solution. After making multiple phone calls to various mechanics all who had an opinion on potential problems/solutions, I found that each believed the aircraft to be safe for flight as long as the fuel pressure was under 6 psi when at cruise settings. With this in mind, I made an appointment and took a short flight to a qualified mechanic. The aircraft was pulled into the shop and the cowling was removed. The mechanic I was working with spent a considerable amount of time checking everything from the electronic fuel pump to the carb. In addition the screen was removed from the carb and flushed. Hoses were pulled, fittings were checked, the hose to the fuel pressure gauge was checked and bled, crankcase pressure checked, etc. I was quite impressed with the amount of work that was done. No clogs, no anomalies, no traces of any issues were found. In the end, it was decided that the gauge is going bad. A new gauge was ordered. When I got back into the airplane and headed home, the gauge read "normal" while taxiing. Pressures previously ranged from 6 - 7.5 psi. Readings were 4.5 psi at idle and in cruise and just about 5 psi with the boost pump on. A few minutes later the gauge read 2.5 psi. I'd never seen a reading that low before. I flipped on the boost pump and got a reading of 6 psi. A few minutes later we were back to 4.5 psi. All of this took place with no change in prop setting, additional leaning, or a change in manifold pressure. The engine ran smoothly and never missed a beat. During landing, when the electronic pump was turned on, I saw no change in pressure. After landing with the pump off, no change in pressure. While taxiing to the hanger, the fuel pressure rose to 7 psi with no change in sound, rpms, engine temps, or egt. Although no "smoking gun was found", given the extensive search for a problem, the fact that the entire system between the electric pump and the carb were checked, I've decided that there can be no other answer but a bad/intermittent gauge. Reflex

A little update, but not much new data. We've had a short stretch of warmer weather here so I decided to take a run out to the airport and mess with the plane. The low the night before was 38 degrees and the OAT at the time of the following test was 63 degrees. The low temp for the previous 48 hours was 38 degrees. Removed the cowling to look for obvious issues and/or blue stains. After an exhaustive search, I found no issues or places where the fuel was leaking. However, I could not see the top of the fuel line or the spot where the pressure gauge T's off. I reinstalled the cowl, pulled the airplane out of the hanger and when through the starting procedure. Here's what I found: 1) Engine off, pump on, throttle closed, mixture lean = 5 psi (which is 1 psi under the maximum allowed pressure and is in the green arc.) 2) Started the engine without the electric pump running, 1100 rpm fuel pressure reading = 6psi exactly (right on the red mark) 3) Turned on fuel pump and reading increased roughly .2psi to 6.2 psi (.2 over maximum) 4) Fuel pump turned off fuel pressure returned to 6 psi 5) Allowed engine to warm up and come to operating temps 6) Increased throttle setting to 1700 rpm fuel pressure dropped to top of green arc or about 5.8 psi 7) While at 1700 rpm, turned pump on fuel pressure increased to 6 psi 8) While at 1700 rpm, turned pump off fuel pressure went back to 5.8 psi while at 1700 rpm While the problem didn't exactly repeat, the fuel pressure is still high for this particular aircraft. To recap the issue, fuel pressures on the last flight climbed as high as 8.5 psi. Normal fuel pressure for this plane is 4-5 psi. While it's possible the OAT has an effect on the problem, my "guess" is that the warmer temps could have allowed the fuel line to expand just enough to lower the pressure. It's my belief I still have a problem. Any chance anybody out there has a fuel system diagram for a M20C with a O-360? Thanks, Reflex

A little repetitiveness here, the link below gives an excellent explanation as to how these pumps work. While it's for laymen, it expands on N201MKTURBO's diagram. https://www.youtube.com/watch?v=PYKoLUsTJU4 One of the things that has me baffled is why I would have normal fuel pressure with everything off and the pump on; and high pressure with the engine running at idle with the pump off (7.5psi). When the pump is turned pressure runs up to 8.5 psi. '68 M20C

Very interested to hear more about this topic. My M20C just started doing this very thing. What is concerning is that until a week ago, fuel pressures remained in the green at all settings. The problem was noticed when I turned on the fuel pump prior to landing. When switched on, the pressure reading was right at 8.5psi. I decided to stay up and "tinker" with various throttle settings. Here's what I found: 1) At cruise settings fuel pressure was at the high end of the green arc (roughly 5.8 psi). Normally I see about 4.5 - 5 PSI. 2) When power is pulled back, fuel pressure increases over the red line (6.0 psi) 3) On the ground at low power settings, fuel pressure is roughly 7.5 psi. The engine runs smoothly and transitions well. EGT's are in the normal range and can be adjusted with the mixture and I see no change in how much I must move the mixture to achieve the desired EGT. One other observation. Yesterday, after the aircraft had been sitting for a week, I made sure the throttle and mixture were all the way back and then flipped on the Fuel pump without the engine running. My thought was that with everything shut down and the throttle and mixture all the way back, I should have been able to repeat the problem. No demand should equal high fuel pressure. With only the pump running, I got a reading of 5 psi. Exactly where it normally is before start-up. 321 SMOH I haven't flown the aircraft since discovering the problem I've contacted my mechanic and he's scratching his head. Again, everything was fine until a week ago. Had this been the norm for this aircraft, I wouldn't be quite as worried. I consider this aircraft not to be airworthy at this time. I look forward to additional thoughts about this problem. Reflex