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Thread: Compression Test

  1. #1
    Jedi Hopeful
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    Compression Test

    Recently performed a compression test as part of maintenance. All cylinders read between 180 and 182 psi. The manual states compression should be 120 psi. The tester instruction claims that if the reading is considerably higher, it may indicate carbon buildup. Dieseling does occur when shutting the car down. I've read that dieseling also indicates carbon buildup. Is this reading problematic and what is your experiences when performing a compression test?

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    Yoda CJD's Avatar
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    Re: Compression Test

    Sounds about right...unless the head’s been decked for compression, which is highly likely with engines this old. The manual even gives instruction on removing carbon.
    John

    1955 TR2

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    Jedi Hopeful
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    Re: Compression Test

    Of course I'm looking for an easier fix than pulling the head. Isn't there anything out there within the wonderful world of additives that will make the carbon go away?

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    Yoda CJD's Avatar
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    Re: Compression Test

    Sea Foam. I’ve got friends who swear by it. I’ve never gotten much help from it.
    John

    1955 TR2

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    Re: Compression Test

    Sea Foam spray is injected into a vacuum port in the manifold but there is no way to do this on a TR. Sea Foam liquid can be added to the gas tank but I doubt it would be as effective. I wonder if the spray can be sprayed in the carb? Has anyone done this?

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    Yoda poolboy's Avatar
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    Re: Compression Test

    Is this the first compression test you've done since you've owned the car ?
    The point I'm getting at is do you have a previous one to compare it to ?
    DRIVE 'EM IF YOU GOT 'EM

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    Re: Compression Test

    This is the first test I've made since I bought the car. I realize that the high reading doesn't necessarily mean that there is a carbon buildup inside the combustion chamber. But the dieseling issue does point to a carbon buildup. The head may have been milled but I have no way of knowing. I wondered what kind of readings other owners got when doing a compression test, especially with heads that have not been touched.

  8. #8
    Yoda CJD's Avatar
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    Re: Compression Test

    What speed is your idle? Deiseling can also be from a high idle speed.

    Oh...you have probably read this post...

    http://www.britishcarforum.com/bcf/s...-Slumbering-TR

    Check out the compression on this barely driven restoration.
    John

    1955 TR2

  9. #9
    Yoda poolboy's Avatar
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    Re: Compression Test

    Maybe some one can tell you the original thickness (height) of the cylinder head and with that you could tell if yours has been shaved.
    Another possibility is the octane rating of your gas, or the ignition timing.
    DRIVE 'EM IF YOU GOT 'EM

  10. #10
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    Re: Compression Test

    i would expect that if my head was suffering from carbon build up my compression test would vary more than 2 psi. across the 4 cylinders. Maybe not. My idle rpm varies between 800 and 900 rpm. I've got a problem with the rear cylinders being rich and the front being lean. I notice that if I let the car idle with the temp going up the dieseling is worse. And with the car idling there is black smoke from the exhaust, not bad but there. Do these TRs tend to run rich at idle? I've had a previous thread about doing plug checks and am still trying to get four plugs to look the same.

  11. #11
    Yoda martx-5's Avatar
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    Re: Compression Test

    The original head thickness is 3.330", measured from the head gasket surface to the valve cover surface. If you take the valve cover off, it will be very easy to measure that with a caliper.
    Art
    '58 TR3A TS236xxL
    '92 Mazda Miata -- Supercharged
    '07 Mazda RX-8
    '11 Mazda CX-7

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    Jedi Warrior RJS's Avatar
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    Re: Compression Test

    I have a '66 TR4A and get same readings of 180-182 psi across all cylinders. No dieseling however. Great power and throttle response. I am certain PO shaved the head down. However, if it was carbon build-up, where does it occur - on top of piston? On valves?

    I head the head off this summer and head/valves were perfectly clean. If the pistons had any carbon build-up it was probably less than 1mm on the surface of the piston.

    Bob
    1966 TR4A IRS

  13. #13
    Obi Wan
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    Re: Compression Test

    You probably know this: but anyways, diesels have glow plugs that glow red hot and when a gas engine diesels something is glowing hot enough to keep firing the plugs. However, like John suggested a fast idle can keep the engine running, so can timing and so can a distributor that slops around when the engine shuts down. What do the plugs look like? The carbon deposits glow and keep things firing. I have seen oil deposits glow and keep the engine running also.

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    Yoda poolboy's Avatar
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    Re: Compression Test

    Octane should be added to the contributing factors...
    DRIVE 'EM IF YOU GOT 'EM

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