Cessna 421c manifold pressure fluctuations

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    I am loving my 1982 421c but there is a gremlin in my rt engine. Its a factory reman with 500 hours. Up at altitudes over 15000 in cruise I get a slow, inreases in manifold pressure of about an inch, but it then returns to normal. It appears to be random in frequency but usually occurs around 31 to 32 inches and 1800 rpms. At most it happens every 30 seconds and then may settle down for a few minutes. At higher or Lower power settings it does not appear to occur. Many experienced minds have not found a solution. Here are some steps that we have completed in an effort to fix the problem.

    Turbo test per maintenance manual at altitiude
    Induction and exhaust leak checks with rep from pacific continental present
    Changed waste gate
    Changed controller
    Changed all upper deck seals
    Swapped governors (although props never go out of sync)
    cleaned injectors, plugs etc
    Changed rt header stack after believing slip joint was minorly leaking when cold
    Sent mp gauge to bench check although its clear by the TIT and EGTs that thiere is a real condition of fluctuation

    The right engine is also touchier in power changes. Especially from 30 inches to 29 or 28.

    Next steps are to swap sniffle valves from left to right and to throw spaghetti against the wall
    πŸ™ πŸ™
    I was told to maybe also try
    Checking the fuel aneroid?
    Aux fuel pump although it seems to work
    Eliminate the 6th nozzle start feature by capping all lines.

    Any input would be greatly appreciated.




    WOW, that is a comprehensive list. When I read the symptoms, I thought I would suggest “mouse milk”, that cured it when one of my motors did the MP dance. I would end up be fixated on the MP gauge. But mine was cured easily. I dont know if the 421 system is similar to the one on a 340.

    I wish I had something intelligent to suggest. I have been there where it seems like throw enough stuff at it until something works.

    I had an engine that surged at take off at about 20′ and I aborted and got back on the ground. It would do it on the ground too, so I did not want to fly it until we got it fixed. Your list looks like mine before I got back in the air. We never figured out what caused it, other then it stopped and did not re-occur after a basket of parts a bunch of hours. The engine was a 300 hour re-man at the time.

    Hope you sort it out.



    I am still trying to sort it out πŸ˜€


    Try swapping fuel pumps. This would be the easiest way to isolate a bad aneroid.


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