Intermittent Temporary Rich Engine Start-Up Condition?

[Paul462]

T337C N2576S Intermittent Idle Anomaly Description

Upon startup, the rear engine (Continental TSIO-360) sometimes runs rich, as evidenced by black smoke emitting from the exhaust. The throttle becomes very sensitive – the amount of throttle travel which would ordinarily yield an RPM increase from 1200 RPM to 1300 RPM actually elevates the engine speed to 1600 RPM; pulling the throttle back a travel amount which would normally reduce RPM 100 or 200 RPM then reduces engine speed to around 800 RPM. Bringing back the mixture control around two inches produces normal throttle operation.

The problem goes away before or during run-up. The engine runs normally during the rest of the flight. Normal and lean-of-peak magneto checks are normal. That would appear to leave either excess fuel or an air restriction causing this intermittent rich condition upon startup and for a few minutes afterwards.

We could run the fuel calibration process with the fuel flow gage, but that would only be effective when the intermittency is not occurring, which wouldn’t solve the problem when the intermittency occurs.

Any thoughts as to what could cause intermittent, temporary, abnormally rich operation for a few minutes following rear engine start-up?

[hharney]

Paul
Two items to consider, the fuel flow divider and the check valves. I am going through a similar scenario also but instead of the rich, flooded symptoms on start up I have fuel draining out of the cylinders through the small drain tubes from the engine cylinders. I have explored both items above. Changed the check valves, from the return fuel lines back to the tank, in the wing root and this did not solve the problem. You can check these pretty easy and pull them apart. The fuel divider on the top of the engine that distributes the fuel to each injector has a spring that holds a diaphragm closed when there is not pressure in the system. This spring or diaphragm can get weak / tear and will allow fuel to trickle into the cylinders while the engine is off. Are you seeing any oil or fuel dripping from the drain tubes on the rear of the engine cowl?

Here is a test you can try, use the aircraft and when you shut down the engine turn the fuel selector to OFF. The next time you go out see if that changed anything.

BTW, I had the flow divider rebuilt and it still did not quit. This is after I changed the spring, diaphragm and the stem. I had to pull the fuel divider back off and send it back to the rebuild shop and they tested it on the bench fixed it, not sure yet what, and it's hopefully solved the issue. I don't know yet because I am still out of the country but from what my mechanic says.......and Larry is suppose to fly it, it is not leaking.

Anyone else ever have this issue of fuel leaking from the cylinders? Looks like oil but it's fuel.

[Paul462]

Thanks for the input, Herb! Yes, the rear engine drips fuel when the airplane sits. We overhauled the fuel divider three times to no avail, and replaced the primer O-rings (also no avail). Nowadays I turn the rear engine fuel selector off, which minimizes the fuel drippage.

[hharney]

Sounds just like my issue with the front engine. We have to come up with something to solve this issue. The company in Texas (don't remember their name) says that the fuel pump could be the issue. The fuel is leaking past the diaphragm or valve or something but it's like $800 to rebuild the fuel pump. Have you looked at this?

[Paul462]

The rear engine drip we're getting can only come through the fuel divider or the primer. The fuel divider is upstream from the fuel pump(s), so if the fuel divider were doing its job, it would cut off the fuel flow, and stop the drip.

[rmorris]

Remember that there is another portion of the fuel system that you might consider....I overlooked it orginally when getting my system overhauled on the front engine. I was having issues with the high/low fuel flow settings, and they tried a bunch. I then overhauled the spider, fuel pump, cleaned injectors, adjusted fuel flow to book specs....then finally someone said - hey, what about the throttle body. Tried one from another Skymaster, and problem went away. I sent mine out for overhaul, and all has been good on that issue since (over a year now).

On a related note -- I mentioned all the issues to another a&p about a month later, without telling him the fix. he said, just replace the throttle body with an overhauled one. Says he keeps an extra one lying around for troubleshooting. Go figure, never know which mechanic will have which little jewel of information. Guess that's why we all try to share info on this website, eh?
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