Fuel flow problems

[rwenner]

I'm having problems with my 1967 T337B fuel flow at alt's above 9500
conditions: 30 min into flight after cruse pwr and leaning (26" 2300rpm and 10.3 gph) fuel pressure and flow drop and the engine tries to stall out. This does not seem to happen at lower alt's and can be fixed by turning on the low boost aux pump.
Items checked or OH to date:
Engine driven fuel pump OH and retested.
Upper deck pressurers checked and adjusted
Fuel distributor inspected
Vent lines and check valves tested clear
Fuel strainer cleaned and OH
Fuel filters replaced
Injectors cleaned and O-rings replaced
New plugs and ignition harness.

Where next guys?

[WebMaster]

My first guess would have been vent lines, especially given that it is 30 minutes, and higher altitudes.
I'd re-check all of them. I'd bet that you have a clog, also check the caps.

[big al 08]

had a simular problem a few months back. it was sucking air from the top of the fuel strainer as the top "o" ring was cracked, old and very hard. at full or high cruise there is an abundant amount of in the system despite a leak, however at low setting and low fpressure it starts sucking air which leans it out. diid they completely oh the fuel strainer? or maybe you have a small leak, start looking for fuel stains on strainers, fuel controlers ect

[Paul462]

Dear rwrenner:

Have you found a solution to the problem yet? I've been experiencing the same thing - description and attempted solutions follow:

I've been experiencing a front engine anomaly since buying the aircraft two years ago. N462DA is a 1968 C337C which started out life as a turbo-charged model, and which was subsequently converted to normally aspirated engines, about 10 years ago. So its fuel system is that of a turbo-charged model, although its engines are normally aspirated Continental IO360s, both factory re-mans. The front engine has about 1.5 years and 140 hours since factory re-man; the rear one has about 1150 hours since factory reman.

I usually climb at 25in/2500RPM to 6,000-8,000 feet, then set 65% power @ 50 degrees rich of peak. After 30 - 45 mins the fuel flow to the front engine starts oscillating, dropping slightly below 9.6 GPH and then right back up again, too slightly to notice. After a few minutes (maybe 5 mins) of gradually increasing amplitude, the anomaly becomes noticeable as a slight power drop, then recovery of power. This gets more and more noticeable, as the fuel flow drops to around 6 GPH. Application of the low boost pump for 30 -60 secs cures the problem, and the engine runs fine for another 30-45 mins, after which the anomaly recurs. During the anomaly EGTs rise and CHTs descend. An EDM 760 with fuel flow option is installed, and I've graphed the anomaly.

We checked the fuel tank vent system, and it's fine, as are all the tank cap vents. We removed the vapor return line from the engine driven fuel pump and blew through it, and heard bubbling in both the left main and left aux tanks, so it appears the vapor return line off the front engine driven fuel pump is un-blocked. I've switched fuel tanks from left main to left aux to X-feed during the anomaly, and none of this helped, so it appears it's not a problem with the fuel system upstream from the fuel selector valve. The anomaly has occurred while feeding both from the left main and from the left aux tanks. Opening the cowl flaps doesn't help. I insulated the fuel supply line from approx. 2 feet behind the firewall all the way up to the engine-driven fuel pump.

The front engine driven fuel pump was removed and sent to Continental, who bench checked it. The 600 RPM setting was fine, but the 1600 and 2600 RPM settings were approx. 10 - 20% low. Continental set these flow settings to midrange, and the fuel pump was re-installed on the aircraft front engine.

A few flights back I tried climbing at full power, then setting 65% power and leaning to 50 degrees rich of peak, and the anomaly occurred almost immediately, and also occurred on the rear engine for the first time since I've been flying the aircraft. Application of low boost pump to both engines cured the problem, albeit temporarily - it re-occurred in the front engine within 30 mins. The next flight I climbed at 25/2500, set 65% power and leaned to 50 degrees ROP, and the anomaly occurred in its usual pattern: in the front engine (after a 30 min. delay), and did not occur at all in the rear engine. Is it possible the problem is vapor lock somewhere, and climbing at full power heated up both engines more than the usual 25/2500 climb power did, and this exacerbated the problem in the froth engine, and caused it to occur in the rear engine also? Continental's position has always been that the problem couldn't be the fuel pump because this isn't the way that this fuel pump would fail - it has no altitude compensation - it just turns its little vanes, has the fuel/vapor separator on top, a jet (that wouldn't go bad with such regularity), and a fuel return line down its side to its lower housing.

The problem persists. I've installed GAMIjectors and now run approx. 20 degrees LOP (that's as lean as I can get without running rough), and the problem still occurs regularly.

Let me know how your efforts are going.

Thanks!

Paul462

[rwenner]

Paul

I have settled on the by-pass valve in the Aux pump as the problem. We recently checked the flow of each Aux pump (gravity flow into a bucket) and found the flow was considerably less thru the front aux pump, so we switched the aux pumps front to rear. The problem moved with the pump to the rear engine. I just finished installing a new aux pump in place of the one with low flow. ground check and low alt flt. test were ok, but that is not the conditions I was having the problems with. I will be going to Houston this week and will let you know the results. I did note a difference in the mixture controls this last flt....they are evened up again for the same gph flow where as before I had to leave the front set higher to get the same gph flow

[rwenner]

Paul,

The problem resurfaced again on the front engine, with the remaining old aux pump I have replaced it and we will see.
Reading the comments on the other thread, I'm willing to buy into the vapor lock thing. We have been thru my airplane with a fine tooth comb and a load of $$$$$$, I have nothing left to replace