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M20E Engine won't die at Idle Cut Off


TheAv8r

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We seem to have some fuel system gremlins we're trying to work out on a M20E with a fuel-injected IO-360-A1A. It's always run rich, but now when you put the mixture to idle-cut off, the engine won't die, it still combusts, I've been shutting it down with the fuel selector and by going mixture idle, throttle full forward to create an incombustible mix. First thing we checked was the mixture cable rigging, and it's set correctly, at idle cut-off, it's all the way against the stop. The issue seems to be escalating, at first it would run rich but die just fine, now it's refusing to die, and the last couple flights it's started to backfire a bit and run rougher in cruise. The fuel injector nozzles and spider were recently overhauled. Next thing we want to check is the fuel valve setting to set the values by the book, and check all fuel lines to ensure they are secure. What else should we look at?

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When my E was behaving that way in 2017 it turned out to be the fuel servo. When Lynn disconnected the linkage that lever on the servo did not move freely. For the Lycoming the servo is not field adjustable so we did an exchange. More recently, when we installed the SureFly EI we had a similar issue. This time is was easily fixed - idle mixture was too rich and idle speed was a little high. According to Kromer this weekend the idle should be below 700 rpm and leaning the mixture at idle should not raise engine rpm more than about 50 rpm.

Understand, I am relating my experience, I am not a certified anything.

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Just like @Bob_Belville, mine was the fuel servo.  Then when we got that replaced the idle was too rich and too fast.  Mechanic adjusted it while I pulled all the levers and restarted it a few times.  No issues since.  Only my experience, not saying that is what's wrong with yours.

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Thanks all. I found this great article: https://www.cessnaflyer.org/maintenance-tech/item/1150-understanding-your-lycoming-fuel-injection-system.html , that also lines up with what everyone is saying, sounds like the fuel servo center seal is going and the servo needs OH/replacement. We will run that test and get it replaced/fixed!

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Poor Idle cut off can also be caused by a slicking plunger in the flow divider on the top of the engine.  Remove the safety wire and the four screws that hold the cover on, carefully lift the cover off, more carefully lift the diaphragm and see if the plunger is free in the body.

Clarence

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Thanks Doc. We had another A&P and his engine guy tear into it for a little bit today and they're pretty convinced it's the fuel servo, though they were also a little amazed at it. They couldn't get any rise out of the mixture cable at all, his guess is the fuel is bypassing the servo since the mixture cable is correctly traveling to the stop. They couldn't get the thing to die - which I'll take opposed to the other option! We checked all the fuel lines and vents and the spider, they all looked good. The fuel servo was recently overhauled about a month ago, we think it was done incorrectly and are contacting the guy now.

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  • 1 month later...
On 9/30/2019 at 2:53 PM, M20Doc said:

Poor Idle cut off can also be caused by a slicking plunger in the flow divider on the top of the engine.  Remove the safety wire and the four screws that hold the cover on, carefully lift the cover off, more carefully lift the diaphragm and see if the plunger is free in the body.

Clarence

That was my thought too. Its my understanding that the divider actually cuts off the flow below a certain flow.

-Robert

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On 9/30/2019 at 7:07 PM, jacenbourne said:

Thanks Doc. We had another A&P and his engine guy tear into it for a little bit today and they're pretty convinced it's the fuel servo, though they were also a little amazed at it. They couldn't get any rise out of the mixture cable at all, his guess is the fuel is bypassing the servo since the mixture cable is correctly traveling to the stop. They couldn't get the thing to die - which I'll take opposed to the other option! We checked all the fuel lines and vents and the spider, they all looked good. The fuel servo was recently overhauled about a month ago, we think it was done incorrectly and are contacting the guy now.

In the end it probably doesn't make a difference. While you have it apart its a good idea to send the servo, divider, and the injectors to the fuel shop for flow testing, etc. You'll be surprised how much better everything runs.

I had an issue whereby if I moved the mixture back to the 3/4 position the engine would die (not fun on departure). I sent everything in and the guy said he could save me money and rather than overhaul it he could just fix the issue. I agreed to that. A couple weeks later the engine died again (this time I leaned overhead the airport). Next time I sent it to a fuel shop Maxwell uses and just told him to do a full overhaul. Didn't have any issues for several years until the engine threw a rod for unrelated reason. This engine was 1050 hours SFNEW from Lycoming.

-Robert

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