Rear Engine wants to Die!

[side241]

I have a '68 T337C that has 950TT, 100 time on turbo's. The engines start easy, good compression, and run strong. I use it to commute to work about 8 hrs flying a week, usually at 7,500 ft. During takeoff and climb, no problems. However in cruise, about every second flight or so the rear engine will start to die. Let me tell you, the first time was a bit nerve-wracking, but now it has become just an annoying source of concern. What happens on the rear engine, now that I have had a chance to evaluate it many times, is this: I will notice the EGT start to climb for about 10 seconds, accompanied by the fuel flow drop from 12gph to about 8gph. Then the rear motor starts to die, which I quickly remedy by turning on the fuel boost pump to high. Then I operate the engine with the boost pump on low, and it doesn't happen again. Only happens every other flight or so. I don't want to always use the boost pump, since I don't want it to wear when I might REALLY need it. It happens regardless of if I am on the mains or aux. tanks.

I have a few theories, but I am no mechaninc. All help appreciated.

[Jerry De Santis]

From your description, the engine is going lean. TIT rising while fuel flow drops. Sound like it can be your engine driven fuel pump. Since it is not a constant problem perhaps on certain flight conditions the engine driven fuel pump gets hot and excess bypass occurs. Frankly, under the condition you have, I would have it check out asap and not fly it until problem is solved. Big problems start small.

Good luck

Jerry

[kevin]

That would be my opinion as well, get it checked before further flight.

Kevin

[Kim Geyer]

Things I would check would be 1 the fuel vent system, 2 The line that goes from the throttle body to the fuel pump, 3 The seals in the fuel strainer, they could be letting air in the system, 4 The pump.
Hope this helps
Kim

[side241]

Hi all,

This is an additional comment after this am.'s message. The plane flew 5 hours today. On preflight, I noticed that a drip every 10 seconds from the exterior terminus of the line that comes out of the bottom of the electric fuel pump. My mechaninc on the field said it was the diaphram weeping, and we are replacing it. Anybody out there think this could be the problem? Also, during the four 1.2 hr legs we cross-fed the engines with no problems, but on the last 1/2 hr I went to the aux. tanks, and after 15 mins, sure enough, it happened again. It has happened on the mains also, but no question it is much more prone to have this problem occur on the aux. tanks. Anyhow, read below. Thanks,
Ben
____________________________

thanks for the input. Actually, I am a VERY conservative flyer, and won't go if there is any problem and have it fixed ASAP. BUt this problem has occurred since I have had the plane, and it really is no big deal. Just turn the fuel pump to "low" and it has never happened.

Actually, in reading this and other forums on 337s, it confirms my instinctual diagnosis of what is going on. It very much acts like the rear engine fuel system is vapor locking. I read on the other site that a few years ago California planes were having this exact scenario occur at higher altitudes, the reason likely that CA fuel had a change to its refining parameters. Rear engine is higher, so less pressure to the engine driven pump, etc. The fuel line for the rear engine is routed where it can definitely get hot, and vaporizes before it gets to the pump. Will replace the fuel line with an insulated, fire-resistant line next week.

Any thought on this would be appreciated.

Belive me, I am going to take all of your suggestions and check all of those items also, its not only an annoying problem, but makes the passengers a bit nervous!
Thanks,
Ben

[rick bell]

several years ago, mine did the same, always at cruise (75%)
between 8500-12k. insulated all the fuel line and it has never
done it sence.

[Kim Geyer]

Ben
You may also want to check and make sure the fuel selector is going in the detents correctly.
Kim

[Richard]

Sounds like vapor locking to me. Very common with the Turbo models. Some careful thought to insulating fuel lines will more than likely solve the issue. Even now... now and then I get a little vapor locking if I climb for longer periods (15K+). I run the boost pump on low until I level out and gain some speed to cool things down. Then switch off the pump and enjoy.

[hharney]

WHAT MATERIAL IS USED TO INSULATE THE LINES?

[side241]

The engine sputtering has never happend on climb, I think because the fuel flow is at a high level for the climb (14.5 gph each). (BTW,it has never actually come close to dying, as I instantly turn on the boost pump). It only happens at cruise, and the more I evaluate it when it occurs, the more I am convinced fuel is vaporizing in the fuel line before it reaches the rear engine. The fuel lines are fairly new, so is there a way to insulate the lines without replacing them with the fire-proof lines, which cost a bundle?

[Richard]

http://www.aircraftspruce.com/catalo...ermoshield.php

Worked well for me.

[Paul462]

Ben,

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

[side241]

Hi,

I just yesterday got the plane out of annual, and installed new insulated fuel line on rear engine. Only one flight, but the rear engine did NOT ellicit the problem, so I am optomistic and will report at a later date if it solved the problem. BTW, during this past winter I noticed that the problem occurs much less frequently, but now that things have warmed up it is as frequent as last summer. Also, at higher altitudes (above 15,000 ft) the problem ocurred often. This again concurrs with my theory that the hot engine causes the fuel to vaporize in the higher (less gravity flow), slightly warmer (less air flow), and longer fuel line.

I have never had a problem with the front engine, and it doesn't make sense given my theory on the matter. I may be all wet here, I'm not an A&P, but have you tested the elec. fuel pump and especially filter to make sure there is no impediment to flow? Air is about 600 times less dense than avgas, so just because you can blow thru it, obviously the fuel is still impeded. Also, my mechanic told me a story of a fuel flow problem that could not be found, vents seemed fine. But after repair / replacement of the vents, just stabbing at a possible fix, the problem was indeed repaired. I am fairly confident that my theory on the fuel system is correct, especially in light of some of the responses I have received on this thread from people way more knowledgeable than myself. Thus I would recommend going thru your system completely.

These 'Gremlins' are tough to find. Best of luck and safe flying.
Ben

[big al 08]

if it not vapor locking, then most likely the system is sucking air at the low settings.
start looking for fuel stains as when it is not sucking the higher setting will make it leak fuel. fuel controller, fuel strainers use "o" rings to seal the shafts i have more problems with old hard "o"ring than anything else. they get hard and crusty with 30/40 years

[Walter Atkinson]

**This again concurrs with my theory that the hot engine causes the fuel to vaporize in the higher (less gravity flow), slightly warmer (less air flow), and longer fuel line. **


The problem can happen on engines that do not have fuel problems in the system. (those should be eliminated as per the other suggestions in this thread.) The REAL problem is the engine-driven fuel pump. It gets heat-soaked and will vapor lock. The problem is indequate cooling to the fuel pump. There should be a cooling shroud and blast line to it. The cooling shroud is frequently inadequate. Also, as a matter of routine, I turn on the low boost at abotu 5000 feet and go to high aux boost at 10,000 feet in my airplane. This stops the problem, but is only a bandaid as fixing the cooling shroud is the real answer.

I have yet to see this addressed and not have it fix the problem.