505 STi low compression problem
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  1. #1
    Fellow Frogger! TassieExec's Avatar
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    Default 505 STi low compression problem

    Hi, would appreciate some input into a problem with my sons 505STi A while ago the car started running rough and despite checking all the usual fuel injection and tuning issues with these cars we failed to fix it. Then in checking the spark plugs we found that the plug in number one cylinder,nearest the flywheel, was oiling up quite badly. we then did a compression test and found that it was down to 25psi, others ok, the oiled up plug and low compresion led us to think broken rings. As a result we have just pulled the head of and surprise, surprise all looks well in the ring department although I have not taken the rings of and measured the gap in the bore yet. Does anyone have any ideas on what else might give the above symptons as obviously I don't want to put it back together, even with new rings and find the problem is still there, any help would be greatly appreciated.

    Regards
    Neil

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    Quote Originally Posted by TassieExec View Post
    Hi, would appreciate some input into a problem with my sons 505STi A while ago the car started running rough and despite checking all the usual fuel injection and tuning issues with these cars we failed to fix it. Then in checking the spark plugs we found that the plug in number one cylinder,nearest the flywheel, was oiling up quite badly. we then did a compression test and found that it was down to 25psi, others ok, the oiled up plug and low compresion led us to think broken rings. As a result we have just pulled the head of and surprise, surprise all looks well in the ring department although I have not taken the rings of and measured the gap in the bore yet. Does anyone have any ideas on what else might give the above symptons as obviously I don't want to put it back together, even with new rings and find the problem is still there, any help would be greatly appreciated.

    Regards
    Neil
    Has it possibly had a valve stem seal or PCV failure causing oil to leak into the intake and result in burnt valve on No1 Cylinder?

    Usually pays to do a wet compression test on the low cylinder.

    If a quirt or two of oil in the plug hole prior to another comp test don't bring up the compression substantially, you can usually rule out rings.

  3. #3
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    Default Low compression ?

    Hi,
    If the compression is down to 25psi I would expect you will have no problem in finding a problem. How can the rings be checked until the piston is out. The piston could be "burnt" on the ring lands.

    As suggested the valve might be "burnt out" or the like. The head could be cracked, the gasket blown, the bore excessively worn or scored. A full diagnose of the engine is always the answer rather than just guessing or, I might suggest asking for remote guesses.

    Cheers jaahn

  4. #4
    1000+ Posts Beano's Avatar
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    There's some kind of story there....A causes B, which in turn causes C.

    The valves for that cylinder are OK ??
    No 1 cyl always gets hottest.

  5. #5
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    Default 505 STi low compression problem

    Thanks for the replies although I guess I did not make it clear enough for some people, I have pulled the head of and removed the the piston from that cylinder, although as I said I have only done a visual check and will check the rings in the bore to check the gap. Thanks for the suggestions re valves and head, will make that my next thing to check, I had considered these possibilities, but did not think that they would cause severe oiling up of the plug which occured to the extent of needing to replace the spark plug within 5 to 7 days.

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    Default Oiled Plug !

    Hi TassieExec ,
    If the compression is so low the combustion in that cyl is allmost a non event. That's why it runs bad etc. So the oil which normally gets up there does not get burnt off the plug. Made worse by a worn motor of course. The poor combustion also soots the plug with carbon too.

    You could blow down the ports to see if the valves are bad. Or just lever them open to have a look at the seat area. Hope it's not too bad. Perhaps a valve grind might just smarten it up for a few more years.

    jaahn

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    Default 505 STi low compression problem

    Hi Jaahn

    Thanks for that info, that could well explain the oiling up, will carryout further checks on the valves and rings, which we will definately replace anyway, now that we have the motor stripped down.

    Thanks
    Neil

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    Fellow Frogger! stew's Avatar
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    I bet the engine breather in the valve cover is blocked, too. Also, was the hose fitted to the cover down to the block? If it wasn't, the engine pumps the oil in the tappet cover into the inlet and fills the manifold. This also happens if the tappet cover is blocked with carbon. Hot tank is the only fix for that. It looks like Jo doesn't need the Hastings rings I have, so if you need a set I have some!
    As for strange running, air leaks at the injector tubes, heating for the control pressure regulator, poor injector spray pattern, faulty mini switch on the throttle can bring on the deceleration valve thus giving lean running. Ignition can do it. Measure the resistance of the dist. coil in the dist. That is called a reluctor winding. It should measure over 1000 ohm to 1200. If it is 800 that is the start of tune problems if it is less, losing spark. I can get those coils rewound.

  9. #9
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    Default 505 STi low compression problem

    Hi Stew & others

    Thanks for all the helpful information, we have just cleaned up the head and the problem became obvious, the exhaust valve on that cylinder is split!!!! However I will of course still check the roker cover etc. Stew there is a breather hose freom the top of the rocker cover down to the sump. I have to get hold of a valve lifter so we can remove all of the valves for inspection to see what the rest of the valves are like, will eep you posted, Stew will send pm re rings.
    Regards
    Neil

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    Default 505 STi low compression problem

    Hi

    Further to finding that one of the exhaust valve had split, I wondered if anyone had some insights into what might cause it to split and how frequent a problem it is on these motors. in my experience of owning and maintaing 505 for nearly 20 years and with one of my 505's having done almost 500,000K/Lm, it's the first time I've coming across this.

    Thanks
    Neil

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    Default Exhaust Valve ?

    Hi,
    I would think the valve is not really split even if it looks like that now. From experience the seat seal failed and the valve "burnt out". The continued use "burnt" a groove through the valve with the high temperature gasses.

    The cause is allmost always a lean mixture on that cylinder, if the other valves are OK. Re-read some suggestions already made about possible causes. I would add suggestions, the brake booster hose if it is near this cyl inlet, or any other vacumn hose if near it either.
    Good luck
    Jaahn

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    Default 505 STi low compression problem

    Sorry Jaahn, but the valve is in fact split!! you can see daylight through the split, so the question remains, what might cause that?

    Thanks
    Neil

  13. #13
    1000+ Posts Beano's Avatar
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    The valve got old and worn. Then that cylinder got hot, as they always do for #1, being at the back. The valve starts leaking gas past. Then more (very) hot gas rushes through where it was not sealing perfectly. This creates and enlarges what looks like a crack.

  14. #14
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    Default 505 STi low compression problem

    While I really appreciate all the help and advice from people on this forum, I'm at loss to understand why people are insisting that the vale is not split!!!!! it is and has a definite crack right though the head of the valve where if you look through it you can see daylight. That said it may well be that the vbalve just suffered metal fatigue with age that was probably asssisted by that cylinder being a hotter one. I guess I would like to have a more definitive answer and some idea of how often people have seen this problem.

    Regards
    Neil

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    Quote Originally Posted by Beano View Post
    The valve got old and worn. Then that cylinder got hot, as they always do for #1, being at the back. The valve starts leaking gas past. Then more (very) hot gas rushes through where it was not sealing perfectly. This creates and enlarges what looks like a crack.
    I think No 1 is at the front for the Douvrin engines.

    Neil

    I was talking to another Pug owner in your area the other day, and he reckons the octance on the premium fuel must be wrong so perhaps it's related to the poor quality fuel. This car a 504 wagon had a burnt valve (though the head hadn't come off yet). We haven't experienced the fuel quality problem down south, as far as I know.
    Last edited by pugnut1; 7th July 2012 at 07:50 PM. Reason: More info
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    Quote Originally Posted by TassieExec View Post
    While I really appreciate all the help and advice from people on this forum, I'm at loss to understand why people are insisting that the vale is not split!!!!! it is and has a definite crack right though the head of the valve where if you look through it you can see daylight. That said it may well be that the vbalve just suffered metal fatigue with age that was probably asssisted by that cylinder being a hotter one. I guess I would like to have a more definitive answer and some idea of how often people have seen this problem.

    Regards
    Neil
    My second Douvrin engine suffered from a screw in the combustion chamber. I think it may have been in the rags that were placed in the ports by the PO to stop such a thing happening!
    I fitted the engine and after starting it there was a hell of a rattling sound.
    On taking the head off, the exhaust valve on one cylinder had shattered. Most knowledgable people are surprised when I tell them this so maybe these valves are made from inferior material?
    I have have had 403 exhaust valves that have split, these are inferior, Peugeot upgraded their valves in 1965.
    Graham

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    Default Split ?

    Ok ,
    I have not seen the valve or compared it to the other 3 in the engine nor looked at the general condition of the engine.
    So I will modify my remote suggestions thus;

    Quote Originally Posted by jaahn View Post
    Hi,
    From experience the seat seal failed and the valve "burnt out". The continued use "burnt" a groove through the valve with the high temperature gasses, OR caused the valve to split due to unequal heating of the head after the seat failed in one spot.

    This statement stands;
    The cause is allmost always a lean mixture on that cylinder, if the other valves are OK. Re-read some suggestions already made about possible causes. I would add suggestions, the brake booster hose if it is near this cyl inlet, or any other vacumn hose if near it either.

    There is a remote possibility the valve was also faulty in some way.
    Good luck
    Jaahn

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    Quote Originally Posted by pugnut1 View Post
    I think No 1 is at the front for the Douvrin engines.

    Neil
    try working out the front of an east west motor.
    No 1 is at the flywheel.
    That simple rule has seen many people trip up.
    Jo

  19. #19
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    Default 505 STi low compression problem

    Hi All

    Thanks for the responses, will continue to investigate, but have to wait until valve lifer arrives, I will be offfline all week so will not be able to respond but please feel free to continue to provide input, especially about cause of valves splitting and frequency of this occurring. By the way Gerry, No 1 Cylinder on these engines is definately at the flywheel end and therefore the rear of the engine.

    Thanks
    Neil

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