Cessna 206 Fuel System Problems

[Flightist]

Hi People,

I'm flying a '71 206 that has had some interesting problems, apparently fuel system related.

On several occassions the engine suffered from very low power on the takeoff roll. You could taxi back and run up the engine again and power would be fine. Another pilot flying the plane and having the same power loss was able to restore power in flight by pulling the throttle to idle and then back full. In other words, an intermittent power loss. Our A & P, after talking to some other people, replaced the fuel pump.

After the pump replacement, fuel pressure was quite high. The pressure was reduced downward to the recommended 6-7 psi. Despite the pressure adjustment, the mixture control has to be leaned way more than normal to get fuel flow correct (about 2" out at sea level). I've had no more of the major power losses but the fuel flow is still way too high.

The plane has a digital fuel flow gauge to back up the steam gauge. And it has an EGT. I've run the plane just fine by setting the mixture with the fuel flow gauge and EGT but obviously this thing needs some further work. The idle mixture has been set so we're getting the recommended 50 rpm rise with the mixture idle cut-off.

Can anyone tell me what's happening here? Where can further adjustments be made? Could it be the mechanics pressure gauges are wrong and there is still too much fuel pressure. Something else in the system that's screwing things up? Any insights into this would be much appreciated.

 

[avbug]

Without having a chance to physically examine your airplane, let me first say it would be impossible to tell you what's wrong. However, from a historical perspective, three things leap to mind in the 200 series that could be your problem. Two are fuel related. One is indirectly fuel related.

In the Cessna 206, your air induction and fuel system are closely related by the fuel controller. A common problem is old induction tubing...the little blue, green, or grey rubber hoses that connect your induction sections together that you see when you open your cowling. These tend to age and clamps tend to loosen, and you get air leaks that can cause problems in your induction. If you're running a turbo, this can be especially noticable.

Atop your engine is a fuel flow divider. It looks like a little spider with six lines coming out of it that go to each cylinder (one to a cylinder). An additional line appears to come out of it, which sometimes runs out the cowl, or sometimes just stops...this is a diaphram drain line. The device is a rubber diaphram with a spring which serves to separate the fuel flow to each cylinder, and that diaphram leaks as it ages. The leakage can be seen in the overflow. If you're getting any fuel there at all, often evidenced as blue or green stains beneath the divider if you don't have an overflow line on it, then you've got a bad divider, and no ammount of adjusting of fuel pressure will fix it.

Another problem with the 200 series fuel system are the header tanks. Many pilots don't know they're even there, and don't know to drain them. Just beneath the point where the wing strut attaches to the bottom of the fuselage, on the bottom of the fuselage and just forward of that point, is an additional fuel drain. One on each side of the airplane. This drain comes from a small kidney shaped tank that receives fuel from the wing, via a fuel line that descends down the post at the aft end of each side of your forward windscreen. This little tank then feeds your fuel selector, which feeds your forward sump, fuel boost pump, engine driven fuel pump, and engine. The engine driven pump has a bypass which returns fuel not to the inlet side of the pump, not to the fuel tank...but to the header tank.

As the engine warms up and hot fuel is bypassed and delivered back to the header tank, it tends to develop vapor. This vapor has to go somewhere, and that somewhere is up. As the "up" is the same fuel line that's delivering fuel to the header tank from your main wing fuel tank, what you have is hot fuel and vapor trying to go up, and cold fuel trying to come down, and you can enter a vapor lock or reduced fuel flow situation, and end up with big fuel flow fluctuations or fuel flow stoppage. Cessna has a three step proceedure in the flight manual for FFF, which involves turning on your boost pump, adjusting your mixture, and switching fuel tanks. The Cessna proceedure is wrong, and can mak it worse.

What is required is reducing throttle, as this generally happens at a high throttle setting, and switching fuel tanks. If you turn on the boost pump, you only make it worse, by bypassing even more hot fuel, and contributing to the problem. What you need is an uncontaminated, fresh supply of cool fuel, and that you gain by switching tanks. Do that first, (after retarding throttle so you don't get a power surge and damage your engine), the use your boost and adjust your mixture as necessary.

Have you been draining those two kidney (header tank) sumps? Common place for contamination...a lot of 200 series pilots don't even know they're there.

In any event, these things appear to be an ongoing problem for you, and a known problem. Don't fly the airplane any more until you know what the problem is. If it's anything other than FFF, then you're facing a situation in which the airplane is talking to you, and you're not listening. Airplanes do talk to you, warn you, smooth talk you, try to wake you up. Then one day they kill you. You've been warned by the airplane. Listen to it. Problems do not fix themselves. The induction question, which doesn't sound as likely from your description, can be cheked with a vacum cleaner and soapy water. The fuel flow divider can be checked visually. It's unlikely unless you're also getting fuel flow fluctuations and transient power.

FFF is a possibility, I've experienced it as a complete power loss until the fuel tanks were switched, but often as not it's surging first. You may have an air leak in our induction with respect to your controller. You could have other problems too, which all need to be investigated per the maintenance manual. In any case, your have a known problem with no known soloution yet...don't fly until you do.

 

[Flightist]

Thanks very much Avbug,

I feel like I understand the system a bit better now. And yes, I do drain the header tanks. In any case, it looks we have some more investigating to do.

One more question though. Is there a way to adjust this excessively high fuel flow? It runs over 30 g/h (suppose to be about 24?) at full rich and definitley bogs down the engine if not leaned. The mixture control is definetely pulled much further back and more sensitive to adjustment. Or is the high fuel flow symptomatic of something else? I know it's dang hard to diagnose these things without see it. I'm just trying to learn all I can about the possibilities.

Oh, and for what it's worth I've never seen the fuel flow fluctuate in flight or on the ground. And the plane isn't a turbo.

Again, thanks alot for your time.

 

[avbug]

Fuel flow fluctuation isn't a function of a turbocharger.

Fuel flow can be adjusted; your mechanic should take care of that. It should be set per the Continental manual.

You indicated that a friend had experienced a power loss in flight and had restored it by pulling the throttle to idle and then pushing it back up again. What was his fuel flow doing then? What's it doing when you push the power up to take off and experience a power loss?

Has anyone examined the induction for a loss throttle plate, and the linkages for play? Has anyone verified the accuracy of your gauges?

High fuel flow in the injected engine often is a manifestation of a plugged injector. Intuitively, one expects it to be the other way around, as a plugged injector means less fuel is flowing. Truth is, however, that if you have a plugged injector, you're going to be indicating a higher fuel flow. Has anybody pulled the injectors and inspected them or cleaned them?

 

[Flightist]

We did get the fuel flow readjusted and seems to run great now. I'll ask the mechanic about the other items you suggested looking at. Thanks alot for your advice Avbug.

 

[avbug]

Don't adjust that fuel flow until you ensure a fuel nozzle isn't obstructed. You may be adjusting it too low.