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Stuck Valve

Discussion in 'Technical Q&A' started by spicedriver, Oct 27, 2015.

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  1. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
    3,859
    #1 spicedriver, Oct 27, 2015
    I've had a Mondial 3.2 for some 20 years now. It's not a daily driver, but I do start it up and run it regularly. Yesterday upon startup, it made a mechanical plinking noise that matched 1 cycle of engine revolution. I also got a lot more black smoke than normal. The sound went away after the engine ran for a few seconds. The engine revs smooth, but the idle is a little rough. I've always had a butter smooth idle after warm up.

    I'm thinking that there was a valve sticking open. Being an interference engine, the piston was hitting the valve. There's not doubt that a broken timing belt, with a cam lobe holding the valve open would cause the piston to bend the valve. But what if the valve is just stuck in the guide, would the piston still bend the valve at that point, or just whack it shut ?

    Also, has anyone else had their valves get stuck open like that ?
     
  2. Rifledriver

    Rifledriver Three Time F1 World Champ

    Apr 29, 2004
    37,288
    Cowboy Capitol of the World
    Full Name:
    Brian Crall
    #2 Rifledriver, Oct 27, 2015
    Second question no, first question yes.


    Valve angle and stem diameter pretty much guarantee it. Also the piston shape dictates the hit will be a small point at the extreme edge of the valve head.
     
  3. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
    3,859
    #3 spicedriver, Oct 27, 2015
    Last edited: Oct 27, 2015
    Thanks Rifledriver. I think you are correct. I'll have to do a compression test.
     
  4. PV Dirk

    PV Dirk F1 Veteran

    Jul 26, 2009
    5,401
    Ahwatukee, AZ
    #4 PV Dirk, Oct 28, 2015
    Check all your extenders real good when you go in to do compression test. It's all guessing but an arcing extender might cause similar issue. Odds of a stuck valve near nil I'm thinking.

    I believe Rifledriver was saying never seen a valve stuck on these engines but if it did stick open it would bend.
     
  5. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
    3,859
    #5 spicedriver, Oct 29, 2015
    Thanks PV Dirk. But this noise was much louder than a high voltage arc.

    Anyway, I think I've found the problem at the AC compressor belt. Very weird noise for a belt to be making.

    1 Min Pinging Sound from Cold Start in 3.2 - FerrariChat.com

    Also, the black smoke, and rough idle continue. The plugs were carbon fouled. I removed the cold start injector to see if it was leaking, but it was not. Allowing more air into the air box smooths out, and increases the idle speed. So there must be something that is causing an overly rich mixture. I do not want to adjust the air/fuel mixture screw, because that would likely mask some other problem.

    If anyone has had their 3.2 motor start running rich all of a sudden, please LMK what caused it. Thanks.
     
  6. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
    3,859
    #6 spicedriver, Oct 30, 2015
    Got her going again.

    Cold start rough idle, and low idle (1,000 RPM)

    Two things I did that helped.

    1. Depressed the Air Flow Sensor Plate all the way down, and let it snap back. I did this several times.

    2. Tapped the AAV with a rubber mallet a couple of times.

    Now cold start goes up to 1,500 RPM smooth idle. Gradually lowers to 950 RPM as the motor heats up, and the AAV closes. Butter smooth idle is back. No surges or dips. Great throttle response.

    These problems showed up after letting her sit for two weeks. Will promise more attention in the future.
     

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