johnw413

Have you cleaned the servo strainer? It's very, very fine. A&Ps often use Hoppe's to dissolve any lead accumulations. Your readings are much better than I saw with the bad FP sender. It might be the bellows on the servo. If it's been years since OH or has a lot of hours you might consider that route, though you might want to talk with a Bendix tech about your symptoms. There are some o-rings that can cause internal leakage that may not be visible at first glance. But, at least all your readings are in the green.

So, any blue stains around the servo? Sometimes the servo has internal leaks and a worn bellows which can cause a fluctuation. Does the engine sound good? EGT OK? What's the lowest FP reading? Does it drop on full throttle or at least on runup?

The fluctuation sure sounds like my problem, which was a bad sending unit. It's cheaper to replace the sending unit than doing anything else. I promise you that once you start troubleshooting servos and mech pumps you're in for many wasted AMUs. You can buy the sending unit from Aircraft Spruce, $50 - $150, depending on paperwork; it's Mitchell either way. Replacing is quick and easy, though you probably should replace the wiring while you're at it. The unit's on the right side next to the fuel servo. John W

I sent my servo to Aircraft Accessories of Oklahoma in Tulsa, (918) 835-9924. They said it had internal leakage. It was not cheap to overhaul (surprise!). Unfortunately, it didn't fix the problem. The gauge still read 5-8 psi. I spent about $5k running this problem down, including replacing the mech pump, which is very expensive because the mags and vacuum pump have to be removed. So, what was the actual problem? The FP sending unit, which costs $50 at Aircraft Spruce, was bad and would give low or fluctuating readings. Now it reads 24 psi except when at idle. What did I learn from this? Start by working through things that are easiest and cheapest to do, like replacing the sending unit. Also, some A&Ps are not all that knowledgeable when it comes to the IO-360's injection system and will do normal troubleshooting, e.g. checking for clogged lines, bad fuel pump, etc. That will cost a you over a grand right there. If the servo is leaking you will see blue stains in the inlet, which should not be there since the venturi is for measuring air volume not dispensing fuel. If you have questions call Aircraft Accessories, they know way more than your average A&P about servos.

What is your cruising altitude? I live in the mountains and usually have to fly at 10,000+. My fuel pressure is less than 20psi at cruise. Replacing the mechanical pump and servicing the gascolator didn't help. The boost pump does not add much. I only see 25psi at runup.

I spoke w/AP Central and they're pretty sure it is the tach generator, with maybe a solenoid problem related to its sticking. I'll get it yanked and sent off to maybe put my AP worries to rest. Thanks!

So, what did it cost, if you don't mind my asking? I've never had AP problems before and they can be pretty scary in IMC.

I'll ask about that. He did remove it and tried to improve its alignment to the capstan. I'm pretty sure at this point the servo will get pulled again and sent to (probably) Duncan for OH. I'm not sure about AP Central in Tulsa. Some people think they're great but my avionics shop seems to prefer Duncan, perhaps in part because Duncan bought the servo biz from BK.

Thanks for the reply! They've had the KI256 on a roll table while running the AP/FD but that won't easily simulate a feedback problem like what I've seen. The tach in the roll servo does sound like a definite possibility. I've already had an issue with the roll servo, which has been a tendency to stick on, which naturally happened while trying to intercept an ILS LOC in IMC. I had to struggle turning onto course until it let go. Apparently the pinion gear which connects to a capstan would not retract when the AP was disconnected. Not sure if the problem is the servo or the capstan. The tech showed me some metal filings from the pinion gear in the servo. I'll ask about the KI256. It was not overhauled just a used inst from a Navaho undergoing glass panel upgrade. I had hoped to replace the KI256 w/a KI300 but I'd like to hear that people are using it and not having the problems I've read about with the Sandia 340A, not to mention that the AP adapter has yet to get FAA approval.

Hey Ya'll, My KFC200 has developed a new feature. After recently changing the KI256 it has begun to exhibit a tendency for the wings to start rocking. I can engage the AP while things are nice and level but any roll change - e.g. turbulence, a turn, etc - will cause the wings to start rocking about once a second, like a metronome, which won't stop until I disengage the AP, stop the rocking and re-engage. It's very annoying. The roll is small, about 2-4 degrees. Has anyone seen this movie? The tech in the avionics shop has made adjustments but I've begun to wonder if it's an oscillation problem between the KI256 and KFC200. I'm thinking of trying another KI256 to see if that solves it but wonder if it's a gain setting in the roll controller. The flight director bars do not show it as a command. Even when a turn is commanded it rocks during the bank. Any ideas or solutions? TIA, John W 1981 M20J

OK, just came back from latest test of the fuel pressure. Here is what I found: With new pump it reads about 25 psi at idle, maybe a little higher. On takeoff it drops to about 20-22psi. The airport elev is about 2,300'. As it climbs above 6000' the pressure drops until it's about 17-18 psi @8500'. Not as high as some have noted but above the red line of 14psi. Switching on the booster adds maybe 1-2psi. I'll call these results acceptable. I'm wondering if having the cowl flaps open helped ( somehow the linkage was messed up and I couldn't close them ). Another A&P wondered if the pump vent was clogged but he wasn't sure if that would cause the symptom, viz dropping FP with altitude. I've thrown over 1 amu at this problem. Replacing the fuel pump may have helped along with blowing out the fuel lines, etc. I'll need some more time to figure this out but I'm moving on to other ways to shower my baby with amus. Thanks ya'll!

It's an '81 M20J, IO-360-A3B6D, I'll have to look into doing the avatar stuff. The gauge was check against a mech gauge and found OK. I'm going to da plane this morning and going over stuff w/theA&P. Here's hoping!

It doesn't bounce around so much as stay low. It was checked against a mech gauge and seemed OK. I'm going to da plane this morning and going over stuff w/theA&P. Here's hoping!

What I'm hoping for as a package deal would be 2 G5s (to replace KI256 & KI525A) and the AP adapter needed to interface with the KFC200. I'd think that would clean up with just the Mooneys and Bonanzas.

That's what I will discuss with the A&P tomorrow. It seems to account for the change in psi with altitude.

The main adjustment would be the mixture, which is a manual process. I'm concerned by the low fuel pressure, which I currently think might be due to an air leak. Another possibility might be that the fuel system is getting hot and causing bubbles in the fuel system, but I'm not sure that it would get that hot on first takeoff. If other people reported 15 psi as normal I'd feel better, but most report 20-25 psi.

That's what I hoped but replacing the pump didn't help. I'm now wondering about an air leak.

They checked that on the ground using a reference gauge. They checked the fuel flow by running the output into a bucket. When I run the boost pump at startup the pressure is good, about 25psi. It's only after takeoff roll that it drops and stays down. I'm starting to wonder if it's an air leak that only appears after the throttle is firewalled and it really starts sucking the juice. Maybe the gascolator? It was serviced at last annual. I'll call the A&P on Monday, but I have to wonder if it's air or vapor in the fuel lines. I'm not sure where the fuel pump is on the accessory case and whether cooling is an issue. I'll make another test flight this next week.

I have a similar problem, which is the FP looks OK at startup, drops to 20psi on takeoff and really starts to drop when I climb above 6000'. At 9000' it's about 15psi, just above redline. The engine runs smooth and all the JPI readings look good. Turning on the booster added maybe 2 psi. Some people suggest making sure the pump stays cool. There is a shroud you can buy at Aircraft Spruce to direct air around the pump. Other people thinks it's trash in the fuel strainers. Many swear that replacing the pump works wonders. My A&P doesn't see any problems. I had him replace the pump, which didn't cost too much (new Tempest $310) but I fear the labor is going to be multiples of that because the pump was hard to reach. Anyway, it didn't help. I honestly never looked that often at the fuel pressure as it was in the green, i.e. I don't think it was ever 25psi except at startup, otherwise it was just in the green somewhere. I'm not sure what to try next. To recap: I tried many different things, such as changing the throttle, prop speed, etc but the main factor seems to altitude. Could it be vapor in the lines at 10,000'? Maybe I shouldn't worry but an engine loss would ruin my whole day, especially in the mountains.

I just replaced the fuel pump in my '81 M20J because it stayed near the low red-line (14psi) and I thought it fluctuated a little but never really dived into real red zone. When I start the engine I get about 25psi, both with the engine pump and the boost pump. On takeoff at 2300' it goes down to about 20, which I attributed to the throttle being wide open. As I climb to 9-10,000' it goes down to maybe 15psi. I tried doing different things: change throttle,mixture,prop speed, etc but the only thing that changed the reading was altitude. At what altitude do you normally cruise? Does turning on the boost pump help? It changes very little for me, maybe 2psi. People keep saying the normal pressure is about 25 but I only see that at low altitudes. The A&P thought I was wasting my money changing the fuel pump. I'm not sure what else could account for the symptoms I saw other than the mech fuel pump. I would've thought the boost pump would help boost, but no. I'd never really looked that hard at the fuel pressure gauge before, just usually while priming or take-off. I thought the pressure was high but now I'm not sure. I've been told many times that if the reading is in the green it's OK. I'd be curious at what altitude you were reading it. I'd also wonder if your problem was the gauge, sending unit or connections. I'm guessing this is the kind of gauge that uses a bimetallic spring that's heated by current from the sending unit. In my case, it seems pretty stable. I really wish I understood this problem better. I believe some M20Js have a purely mechanical gauge but I'm pretty sure mine's electric.

From Garmin: " When paired with an affordable GMU 11 magnetometer ... G5 can serve as your primary reference source for magnetic heading,..." It sounds to me like you need the GMU11 to drive the G5 as its slave. I still haven't looked up the cost as I want to see a real product that does what I need. Hopefully they'll offer a package deal... I agree that a digital -> analog box would be easy, but most companies, especially Garmin, price according to what we're willing to pay, which depends on alternatives and the all-important installation cost, which is often more than the HW itself. I recently paid about $2500 to remove an old AM/FM cassette player and replace it with a black aluminum panel plate. I'm also guessing there will be a jam at the end of the year for people who have put off the ADS-B upgrade. Nothing boosts prices like a gov't mandate.

Sound advice, but the circumstances were that I entered IMC and after a few minutes started ascending rapidly. I was flying over Houston and ATC wasn't allowing much deviation off course. There was no turbulence, severe or otherwise, just an ascent that the altitude hold couldn't handle. It was over 20 years ago. My point had to do unexpected loss of AP control, regardless of why. In the more recent case there was no severe turbulence but the KI256 started to do weird things in mild turbulence. It would have been problematical whether I was hand flying or not.

I've wondered about a similar, dual-G5 setup but am concerned about replacing the KI525A. I haven't priced the cost of the Garmin add-on to replace the flux-gate setup of the KI525. Aerovonics might be competitive if it's not vaporware. Their timetable looks ambitious and I wish them well. All things being equal I'd prefer Garmin just because I have a 530W with 210 Bluetooth and a GTX335. As always, installation cost becomes a major factor. Hopefully the AP adapter will not be a deal-killer but never underestimate Garmin's ability to charge what the market will bear.

I share the same hope but feel pretty confident that Garmin will price it like a new iPhone. I've been wondering about the problems with the Sandia 340A and its failing during turbulence. My KI256 failed during turbulence with the KFC200 doing sudden steep banks in IMC, not a pretty picture. What I am wondering is how the supposedly "solid state" instruments fail. I'm guessing maybe the accelerometers used in these instruments have microscopic defects. Since they are rather small devices and probably pretty cheap, do the fancier (read:more expensive) new AIs have several of these devices and take a vote? I'm wondering what Sandia might have gotten wrong to make the AI fail. Just bad parts? One of my least favorite times is when the KFC200 resigns due to abrupt changes in altitude, etc, usually in a convective cell. The idea that a flaky AI could both knock out the AP and misinform the pilot sounds like something out of an NTSB report. I'd really like the the KI300 to be a viable primary AI and attitude source for the KFC200. I never want to appear in anything that says NTSB.

It does help. It'll be interesting once the KA310 is available. Maybe there will be news at Oshkosh. I can dream, can't I?