406 coupe mystery inactive cylinder
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  1. #1
    Tadpole
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    Default 406 coupe mystery inactive cylinder

    Hi all,

    I hope someone can can help me. I have a 406 coupe (V6 manual D9 model) that recently came up with a Catalytic Converter fault and an anti pollution fault. I took it to my trusty mechanic, connected up a diagnostic computer to clear the faults and solve the issue. The fault pointed to cylinder 4. He initially thought that it was the ignition coil on cylinder 4, but the coil was in good working order. He then took out the plugs in the back row to check and found that the plug for cylinder 4 was clean with no carbon build up, he showed me the other plugs as reference. He checked the compression of cylinder 4 and found it to be normal. He checked the injectors and found them to be working properly. I had a chat with him today and he cannot find and solve the fault, saying that it could be electrical (gulp).

    Has anyone else had this problem? Any advice would help me out a lot.

    Thanks,
    Nick.

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  2. #2
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    Does the mechanic know the French cylinder numbering system? Does the engine misfire?

    (PS Number from the passenger's side, so 4 is at the back near the transmission)
    Last edited by seasink; 18th June 2018 at 08:37 PM.

  3. #3
    Tadpole
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    Yup, he did a bit of research, he was surprised by the layout. There wasn’t a misfire issue, but when the fault warnings appear, the car did feel less responsive.

  4. #4
    1000+ Posts Beano's Avatar
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    As the computer says it is a catalytic converter fault, it may be that there is a small head gasket leak and water is getting into that cylinder. That would account for the clean spark plug. Water then goes down into the exhaust pipe and causes a fault warning in the cat converter.

    I would have thought that your mechanic would suggest this.

    (If there was recently a decrease in spark, you would think that there would still be some carbon on the plug from previously.)

  5. #5
    Fellow Frogger! dmccurtayne's Avatar
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    Did he swap coil to another cylinder
    How did he “check” the injector
    What fault codes are there

  6. #6
    1000+ Posts Beano's Avatar
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    I'm going with the gasket leak.....if the diagnostic codes say Catalytic Converter and an anti pollution fault.....that means something.

    And they would probably show up with water getting into the exhaust system.

  7. #7
    Demannu-facturing! Demannu's Avatar
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    If your car has the earlier engine (up to about mid-2001, aluminium inlet manifold), dismantle and check the contacts in the large black connector on the timing-belt end of the inlet manifold. It has a red slide lever, pull it out to separate the plug. They are notorious for breaking internally and allowing the contacts to withdraw from the back of the connector - breaking the circuit to one of the injectors.

    If you have the later engine (after mid-2001, plastic inlet manifold), check the connector to the throttle body. They have a similar problem to the above, but have smaller, more delicate pins. It is a known problem with the harness; in some applications I believe there was a recall but the 406 missed out. The symptoms, depending on which pins aren't contacting, is a hunting idle, power loss and misfire, with an anti-pollution fault.
    Scotty

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  8. #8
    Tadpole
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    Thanks fellow Froggers.

    Just an update, I spoke to my mechanic today, asking about whether the gasket leak could be the issue, he does not believe the gasket is the case as he checked the compression and said it was normal and he was happy with the seal. He tested the no.4 coil on other cylinders and found it was functioning. He said that the injectors were functioning normally and even checked with the spark plug removed to see the fuel entering the chamber.
    I will be collecting the car tomorrow and I will chat to my mechanic about the connector to the throttle body. The coupe is a D9 MY02 so hopefully i be able to see if that is the problem.

  9. #9
    Fellow Frogger!
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    could the fault be a manifold vacuum leak at no.4 cylinder? could give similar symptoms as what you have.just a thought.........jim

  10. #10
    Tadpole
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    Hi all,
    I picked up the coupe today from my very excited mechanic. He had been a bit dejected about being unable to find the ‘smoking gun’ to the problem and was going to return the car to me. So, he re-sealed the manifold and plugged everything back into place, made sure all the connectors were clean and plugged in well. Took for a spin and found the cylinder 4 firing again! He wasn’t 100% certain what solved cylinder 4 problem, but he believes as he put everything back in its place clean and making good contact and seals, this provided the solution to the problem.

    I took it for a drive, 50+km, and the performance was back to normal. I will see how it goes after a bit of driving.
    Thanks for all the advice froggers,
    Nick.

  11. #11
    1000+ Posts Beano's Avatar
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    Quote Originally Posted by Demannu View Post
    If your car has the earlier engine (up to about mid-2001, aluminium inlet manifold), dismantle and check the contacts in the large black connector on the timing-belt end of the inlet manifold. It has a red slide lever, pull it out to separate the plug. They are notorious for breaking internally and allowing the contacts to withdraw from the back of the connector - breaking the circuit to one of the injectors.

    If you have the later engine (after mid-2001, plastic inlet manifold), check the connector to the throttle body. They have a similar problem to the above, but have smaller, more delicate pins. It is a known problem with the harness; in some applications I believe there was a recall but the 406 missed out. The symptoms, depending on which pins aren't contacting, is a hunting idle, power loss and misfire, with an anti-pollution fault.
    It may well be the problem Demannu described above. My own reply was simplistic, but known and notorious problems as described will trump this any day. If it ever happens again, look into the above.

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