Intermittent Temporary Rich Engine Start-Up Condition?
 

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?

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.

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.

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?

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.

throttle body
 

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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HOW TO MAKE A VAPORIZER

Update on fuel draining
 

I have a new exchange fuel pump and throttle body on the front engine. In earlier post I had the fuel divider overhauled and still had the draining issue but worse than before it was overhauled. Sent it back to the repair station and they bench tested it and said all was fine. So that's when I sent everything, pump, throttle body and divider valve (injection lines and injectors) to ACI in Texas. They said that the throttle body was old and obsolete, had the wrong adjuster set screw in the servo, fuel pump was also obsolete (all these were original) and they charged me a bunch of money for new exchange units off their shelf. Well we got it all back together and low and behold the drains still leak. Post flight or engine operation, fuel will leak out of the cylinder drains. ACI did not mess with the divider valve because it was under warranty and they said their bench test was good.

The divider valve will hold 4 PSI under operation and 2 PSI while static. We put a gauge on the line from the servo and it reads just barley 1 PSI. We also left the line off the servo overnight with the fuel selector on to see if the fuel could be coming from somewhere else but there was not fuel leaking in that procedure. So after spending well into $2500 it still leaks. Yes I have a new pump, throttle body, divider valve, etc. but this whole issue started with the fuel leak that is still there. Next step is to send the divider back to the repair station and bench test again. It has to be passing through the divider.

On a second note, we did some fuel pressure adjustments too. It was suggested by the Texas shop to just verify their factory settings. The fuel pressures were low so we did some ground testing and adjustment. The one thing that is concerning is that you can't get full RPM on the ground. There is an correction table that provides multipliers for compensating but it only indicates to -120 RPMS max and I could only get 2550 RPMS. The procedure calls out 2800. Well that's -250 RPMS. We did some calculating and came up with the multiplier and set it. When we flew the aircraft it achieves the 2800 on take off but the fuel flow was pegged on the engine we adjusted. We turned it back down a little and I need to fly it again to see where it's at. Next month we are installing the JPI 760 with fuel flow that I purchased at Oshkosh this year. I really don't trust the fuel flow gauge in the panel for accuracy.

Anyone else ever have fun with fuel pressure adjustments?

And to end it all, I can't get my G5 power mac to start up, really weird thing going on there too.

Stay tuned.

Herb:

After a flight last weekend, I have noticed a very very slight drip of fuel coming from the cylinder drain tube from my rear engine. I have never noticed this before. I sat and watched the tube for about 10 minutes and witnessed maybe one drip per 90 to 120 seconds on average. I was reading this thread and was wondering if you ever resolved all this with a high confidence level as to the cause?

Thanks, Ed

After having the fuel divider rebuilt 3x, replaced the throttle body and fuel mixture control, beat my head against the wall, replaced the check valves, checked fuel head pressure on divider valve, adjusted fuel pressure and beat my head against another wall, IT IS STILL DRIPPING!!!!!!

Granted not as bad and only out the port side, front engine. At one time after the first divider rebuild it started dripping out both sides.

I'm into this thing at least $3K. Granted the throttle body and mixture control really had nothing to do with the drip but it was all tied in together.

Frustrating. Last time I talked to Paul he said his just quit for no reason.

Thanks for the reply Herb. I will simply watch mine for awhile and see what transpires. It is such a slow rate drip and my shop did not seem concerned at this point. They said "go fly" and see what happens so that is what I am going to do. I will keep you posted. Meanwhile, if you ever "get to the bottom" of your drip, please share. I do feel for you my friend. It is things like your situation (which I have experienced with my plane many times) that drive me nuts. I enjoy my plane but it is a roller coaster of emotions. The highs are high but the lows are low. There seemingly is no in between. Thankfully, it is the only part of my life that is like this.

Good Luck,

Ed

It does not drip in flight only when shut down. I just made a habit of turning off the fuel valves for now.

Drip in flight
 

Herb, How do you know if it is not dripping while in flight? Is it because engine is running?

Jerry

A couple of weeks ago I was talking wth the female pilot for the Right Whale project in St. Augustine, and she showed me the fuel throttle body they had just replaced on their Turbo 337. The fitting on the end of the fuel line going into the throttle body had broken off at the threads. But the most interesting part was that the throttle body also had a crack in it starting at the fitting, which their mechanic surmised was due to over torque at the factory.

The one thing that jumped out at me was that there was the classic blue dye streak on the underside of the housing, which showed that this had probably been cracked for some time, and had a slow leak.

I didn't ask her if they had experienced any fuel or pressure flow issues prior to the break in the fitting, but if I see her agian I will ask and let you know. In the meantime, use the old method of looking everywhere for any blue dye, to try and locate a random leak.

Jerry

I have not noticed any fuel/oil deposits on the belly after flight.

Update: I flew yesterday and today, about 3.5 hrs total. It appears that the dripping on the rear engine has ceased. I looked closely at the front engine and after both flights, I could see a drip hanging from one of the drain tubes but it didn't fall after watching it for about five minutes. I got bored and quit watching after that. Unless I see some dramatic increase in dripping 100LL from either engine, my gut tells me this is a non-issue. The lack of concern from my shop helps my gut feeling as well. Who knows.

Ed