Erratic Fuel Flow

[Scott Grand]

1965 PA24-260 with fuel injected IO-540. Fuel flow has been erratic, swinging as mich as 1 GPH up, then back down. It does this fairly often, even in level cruise flight. The fuel flow change is indicated both on the panel mount guage as well as the fuel flow on the JPI EDM-700, as is confirmed by the associated change in EGT's. Two mechanics said the fuel servo needs to be overhauled. So, I had one of them send it our for overhaul. After installation, there is no improvement. Turning on the electric boost pump has no effect on stabilizing the fuel flow. Any suggestions?

[N3322G]

Hi Scott,

I bet you've already cleaned the injectors.

With digital fuel flow, I always notice when they run up and down in the twin. I got some bad gas and while the engines never coughed, the fuel flow was erratic until I got that gas entirely out of the system. I drained a LOT before and after every flight. Eventually the fuel flow settled down.

Worn wire or bad grounding has also shown as erratic fuel flow - although if you also see this on the JPI - it may be one of the above.

These three things may or may not be your problem but this is what has fixed a similar problem in the twin and hope it helps you.

[9089P]

Hi Scott,

We have 1400 hrs on a factory new engine in our '66 260B. From day one when we reduce power and lean at the top of climb to a cruise setting the flow will fluctuate for several minutes and then settles out. There is no effect on actual power output. During this period of time any adjustment to the mixture will result in a momentary reverse reaction on the gauges (factory and jpi). We noted to the a/p who advised not to worry about it.

Good luck, Don

[Zach Grant L1011jock]

How old are your fuel hoses? Bad hoses can introduce air into the line and cause all sorts of wierdness. It would also be helpful to "T" in another fuel pressure gauge to see what the pressure was doing. You may have a shot boost pump as well, as no change to the fuel pressure happens with it on (and it could also be a source of air entrainment). Do you smell fuel in the cabin after the pane sits closed up? That is a sign on a fuel seep in the fuel selector or in the boost pump (assuming you do not have gushing leaks on the fuel bladders).

Zach

[Scott Grand]

The fuel swinging is not just after initial leaning after leveling off. It continues for the duration of the flight. The fuel hoses could be old (I have only owned the plane for 2 years), and I do smell fuel in the cabin after it sits up for a while. While the boost pump does not produce a change in fuel flow on the panel guage, the A&P did confirm that it is putting out sufficient flow. And the main symptom I have in cruise flight is that after leaning to the desired point, the fuel flow then starts to swing high by about 1-2 gph, and the EGT's drop off 50 degrees or so, then the flow flow drops back down to the initial setting. Over time, it will on occasion drop down below my initial setting, and I have to enrich the mixture a bit. Another thing I notice is that the actual fuel flow and corresponding EGT change is initially very sluggish when I screw the vernier mixture knob in or out, and then all of a sudden changes dramatically. We have checked the operation of the cable and linkage, and there is no slack.

If you think the condition of the hoses, boost pump, and selector are a possible contributor, I will pursue replacements / repairs.

Thanks for all of your input!

[Manche 68]

I have a 1968 260B with an EDM 830 and Gami's. I see at least a .5 GPH swing all the time. I had Denny Haskins look over EVERYTHING and he found nothing. We changed hoses, cleaned injectors, looked at wiring...you name it and nothing came about. I have been flying it for about a year with the EDM 830 and it has alway had some swing in GPH. I would love to know if you found a fix for this because it bugs the daylights out of me. Thanks

[MULEFLY]

One issue that was contributing to my erratic readings was that the fuel flow inducer had 90 degree fittings on both the input and output sides. Somewhere I read that they should be straight fittings to provide more even flow thru the unit. It helped!
Jim

[skipsouthernsky]

To all,

Although many (maybe all) single Comanches have fuel pressure guages, my understanding is that most of the twins only have a fuel flow guage with no fuel pressure guage. I personally think that is a shortcoming. On my other plane, (not a Comanche, but lycoming powered with Bendix fuel injection) I have fuel flow via JPI and fuel pressure via factory installed guages. I have found that operationally, especially in warm environments, upon level off at 7000 to 12,000 or higher, the fuel pressure will bobble quite a bit with a related fluctuation in fuel flow. Normally I believe this to be due to entrapped air in the lines (maybe there since previous hot shutdown) or possible fuel vaporization. This seems to be a normal occurance. 5 to 10 seconds of fuel boost pump (pumps working normally) will normally stabilize the situation and things will settle down and then pumps can be turned off. Sometimes this happens every 10 to 15 minutes for the first hour of flight and then dissappears. Hardly ever happens at lower altitudes. I think you could learn a lot about whats going on with your engines and fuel flow if you had fuel pressure guages. I have seen other scenarios of engine complaints, that IMHO could be much better diagnosed if a fuel pressure guage were present.

It might be problematic and troublesome to do, but I heartily recommend trying to get fuel pressure information into the cockpit

Sincerely,
Skip Dykema