407 plugs and performance
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Thread: 407 plugs and performance

  1. #1
    Fellow Frogger!
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    Default 407 plugs and performance

    We serviced the 407 (V6 petrol) on Saturday. A minor service but thought we might remove a plug or two and have a look. Bought the wagon with just over 100,000kms on it and have now done 40,000kms but there were no records of when or if the spark plugs had ever been changes or even been looked at.

    What we found was that 5 of the 6 plugs were not screwed home and the last 8 to 10 threads of the plug were "proud" of the head. The plugs looked like they had been scalded around the hex nut area and up half the porcelain. Cannot be sure if they were fitted like that or had only been done up finger tight and were slowly unwinding themselves.

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    Don't like using a tap in an alloy head to clean up the threads (presumed bottom 8 or 10 threads where the plug had not gone had crud in them too) so cleaned the thread on one of the plugs, top 10 threads were caked in I don't know what, then cut a groove down the cleaned tread and screwed that in and out of each a couple of times like a tap. Put 6 new plugs in and they went in ALL THE WAY without any dramas and were just given a slight nip up and not just left finger tight (don't like over-tightening either so just a little nip with a plug spanner).

    The difference in the wagon is remarkable. Gone is the almost sluggish take off (thought it was because it was a wagon as it was much slower than the V6 407 sedan we had before it), the gear changes seem happier and the wagon runs much quieter and goes like a bird.

    So my advice to anyone with a V6 petrol 407 or whatever is check the plugs in it you never know what you might find under that coil pack. It might even improve the performance of the car/wagon just like it has done with ours.

    FLASH
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  2. #2
    Fellow Frogger! Roland's Avatar
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    Great result!

    Loose plugs, to the extent you have stated would have had a very significant effect on the size of the combustion chamber, increasing it markedly, and having the effect of lowering the compression ratio.
    This would have given you the resultant low performance, especially when taking off.

    Also having the plugs "recessed", as much as that would have affected the combustion process and flame front, again affecting the power available.

    I have driven with a loose plug before - a Peugeot 505 estate, while in England.
    The plug gradually completely worked itself out of its thread.
    Power was down but the noise of the loose plug gave me cause to think the exhaust had fallen off.

    Luckily I was able to get of the carriageway and park up on a dirt road 100 metres past a house that had police cars allover it.

    As I was doing repairs and screwing the plug back in, I was approached by one of the policemen to find out what I was doing. Satisfied that I was not a threat, he turned to go back to the house.
    I inquired as to what was happening and he told me there had been a murder in the house overnight.

    Got the car going super quick and left the scene rapidly.

    Cheers
    Roland

    Land Rover Discovery 4
    406 Coupe D9 - Manual (2002)
    307 Hdi Estate - Auto - (2007)
    307 Hdi Sedan - Auto - (2007)
    505GR Estate - Personal Import from UK 1971cc Manual (March 1986)
    405 Mi16x4 (all the parts ready to install into the LeMans body)

  3. #3
    COL
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    Default

    On the spark plug stories, just over 20 years ago I bought a Renault R12 that someone had put Renault R16 plugs in it. As Renault people know from that era is that the R12 has short reach plugs and the R16 has long reach plugs. Anyway decided to remove the plugs and check the gap. Went to undo the first plug and in would hardly move, after about half an hour of screwing the spark plug backwards and forwards I managed to get it out. I also managed to remove the other three plugs all without damaging the threads in the alloy head. I well and truly used my get out of jail free card that day.
    Regards Col

    1973 Renault R12 Station Wagon
    1976 Renault R12 Station Wagon
    2002 Renault Laguna V6
    1973 Alpine A110

    http://alpine-a110.weebly.com/

  4. #4
    Fellow Frogger! Roland's Avatar
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    Quote Originally Posted by COL View Post
    On the spark plug stories, just over 20 years ago I bought a Renault R12 that someone had put Renault R16 plugs in it. As Renault people know from that era is that the R12 has short reach plugs and the R16 has long reach plugs. Anyway decided to remove the plugs and check the gap. Went to undo the first plug and in would hardly move, after about half an hour of screwing the spark plug backwards and forwards I managed to get it out. I also managed to remove the other three plugs all without damaging the threads in the alloy head. I well and truly used my get out of jail free card that day.
    Hi Col,

    Well done getting the wrong plugs out!

    How did the car perform with the "too long plugs"?
    They would have had the effect of filling in combustion space and raising the compression ratio.

    Did you notice any difference in running after the correct plugs were installed?

    Cheers
    Roland

    Land Rover Discovery 4
    406 Coupe D9 - Manual (2002)
    307 Hdi Estate - Auto - (2007)
    307 Hdi Sedan - Auto - (2007)
    505GR Estate - Personal Import from UK 1971cc Manual (March 1986)
    405 Mi16x4 (all the parts ready to install into the LeMans body)

  5. #5
    1000+ Posts driven's Avatar
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    If using a tap in alloy head ensure you grease it well so the metal swarf / dirt is captured in the grease

    Go slow and change grease often

    Bush trick, don't have a tap, cut grooves with grinder into bolt / old plug threads

  6. #6
    COL
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    Quote Originally Posted by Roland View Post
    Hi Col,

    Well done getting the wrong plugs out!

    How did the car perform with the "too long plugs"?
    They would have had the effect of filling in combustion space and raising the compression ratio.

    Did you notice any difference in running after the correct plugs were installed?

    Cheers
    Roland

    I can't remember, the car was used as a hack and was not the most powerful R12 around.

    The only thing I really remember is it was a prick of a job to get the old plugs out
    Regards Col

    1973 Renault R12 Station Wagon
    1976 Renault R12 Station Wagon
    2002 Renault Laguna V6
    1973 Alpine A110

    http://alpine-a110.weebly.com/

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