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.