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  1. #1
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    Member No: 103264 tdhardin is an unknown quantity at this point tdhardin's Avatar
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    compression test....

    I have recently been trying to detirmine the overall health of my motor, which was broought on by a faulse oil pressure warning...oil pressure and temp gauges were installed and the show the bottom to be in good health, and the comp readings were ranging from 155-165 which seems low but at least its fairly even the motor has around 200k on it ( the odom) broke at 111k so the timing components are going to happen , but all else looks good.


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    Member No: 45949 Philo is an unknown quantity at this point Philo's Avatar
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    Do a leakdown to tell for sure

    You gotta do a leakdown test to know for sure. A compression test gives you a reading for dynamic compression seen during running conditions.., but to determine the overall condition you need to find out how well the engine is sealing. This determines clearances in the bores, valve guides and valve seats.

    165 across all cylinders is a bit low, I have 190 -200 on all 6 with 260 k on the clock, but as long as they are even your ok..., but odds are you've got worn piston rings / bores.

    Also,if your M5 has the headers with the access ports, during the leakdown you can take a plug out and listen for air passing the exhaust valves. Remember, most high mileage S38s fall to burnt exhaust valves.

    Philo.

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    Re: Do a leakdown to tell for sure

    hey philo,
    I do understand that...wouldnt it be fair to assume that the valves are rings are at least wearing evenly? which (hopefully) indicates that the valves are probably ok? I am most concerned with the timing components if this is all true...also, is there a good write up on doing a leak down?
    thanks to all!
    Damion

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    Member No: 45949 Philo is an unknown quantity at this point Philo's Avatar
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    Re: Do a leakdown to tell for sure

    whichever leakdown tester you buy will give instructions on how to do the test. Or search the archive or google.., there's a ton of info out there.

    Engines don't always wear evenly. If you have an injector that runs lean you can do all kinds of damage to just one bore. Same with valves, if the lash isn't maintained and one exhaust valve gets tight... this is the beginning of a burnt valve seat.

    But in general you want to do the leakdown to see how much blow by you have at the rings and at the valves and guides. After I burned three exhaust valves I rebuilt the head and the car runs great. 260k on the clock.

    Philo.
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