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  • Compression Test - Engine Rebuild necessary?

    Dear all,

    Happy New Year to all of you. I am currently still checking several things on my engine since a Dyno Pull this summer with my 320is was not that good (171 HP instead of the 192 HP). Today I checked the compression on the engine (bone cold, around 35 degree F ambient and engine temp..). Two years ago the throttle bodies and valves were adjusted by a workshop. The results of the compression test are not that good (throttle fully open):

    Cyl 1: 168 Psi
    Cyl 2: 145 Psi
    Cyl 3: 151,5 Psi
    Cyl 4: 145 Psi

    The absolute numbers are not that well in my opinion but especially the deviation of cyl 1 (or of the other three) makes me nervous. Since the engine has now 146.000 km and a not completly known past I now more and more come to the conclusion to have a complete rebuild of the engine this winter. What do you think? Measure the compression again on a warm engine? What do you think the higher compression on cyl 1 might come from? Do you also recommend a rebuild? I already do some man math to justify the costs towards my wife ;-)

    Regards

    Nils

  • #2
    measure it again warm.
    do a leak down test to see where any compression loss is.

    is that dyno pull number at wheels or crank? assuming it is wheels, that is not a bad number for a stock engine. crank minus driveline loss (~15%) equals whp.

    and, 35deg F is pretty cold. oil thick and not flowing. not going to get good numbers.
    75 M2 Zinno
    88 M3 Henna

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    • #3
      I agree, the deviation between them does not paint a pretty picture.
      Did you have all 4 spark plugs removed when measuring each cilinder?
      What I tend to do when mesuring compression is take a video of the guage during the cranking. Reviewing it enables you to see and compare between cilinders
      A. what pressure the very first compression stroke generated
      B. after how many comp. stroked the max. pressure has been achieved.

      Consider buying an endoscope (peak tech 5600 got a good review in Oldtimer Markt) and have a look inside.

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      • #4
        How does it idle? A smoke test can reveal burnt exhaust valves.
        There'll be Spandex jackets one for everyone ...

        Comment


        • #5
          Hi,

          thanks for the replies. I tested with all the plugs out. Plugs look fine, not greasy or anything. Idle is OK, but a little high with around 1.000rpm when warm. Can not be adjusted below that value. The car has not a significant oil consumption and does not produce blue smoke, even on the dyno the car did not smoke. So I guess the problem more likely something in the area of the head and not with the piston rings I guess. @njboy: How is such a smoke test performed in order to reveal burnt exhaust valves? I only know these smoke tests for revealing leaks on the intake. @mlytle: the 171 HP on the dyno is the "calculated" engine power. They measured at the wheels and also measured the driveline loss when the wheels spun down after the pull. So the 171 HP is the wheel HP plus the driveline loss.

          Regards

          Nils

          Comment


          • #6
            Smoke test kits are pretty cheap. Set it up as if you are doing a intake manifold leak test. Turn the engine by hand so that you have the intake valves open and exhaust valves closed for each cylinder. If you have one or two bad exhaust valves smoke will exit the exhaust. Disconnect the header pipe where it connects to the exhaust system near the bellhousing for a quick result. Keep spark plugs in to reduce too much smoke in the garage. Nice side effect is that you reveal other leaks in the system. Your idle is too high. Large discrepancy in compression values will usually manifest in a bad uneven idle.
            There'll be Spandex jackets one for everyone ...

            Comment


            • #7
              An idle of 1000 rpm that cannot be adjusted down means there is an intake leak somewhere between AFM and intake valves.
              The smoke test will also enable you to find those.

              Comment


              • #8
                Did you do the compression test at wide open throttle? If the throttle is closed, it will lead to low numbers.
                '69 Datsun 2000 Roadster vintage race car (Street driven on a regular basis :taz
                '59 Alfa Romeo 101 Sprint (HUGE project :uhoh
                '88 M3

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                • #9
                  Originally posted by RAD2LTR View Post
                  Did you do the compression test at wide open throttle? If the throttle is closed, it will lead to low numbers.
                  Read first post....
                  75 M2 Zinno
                  88 M3 Henna

                  Comment


                  • #10
                    Hi there,

                    so today I measured the car warm (oil around 160 degree Fahrenheit) and got the following results:

                    Cyl 1: 168 Psi
                    Cyl 2: 155 Psi
                    Cyl 3: 165 Psi
                    Cyl 4: 161 Psi

                    I measured two times and got the same results. So much more even now but still too low in my opinion for a S14B20. Deviation of cyl 2 also too high I guess. I also took a look inside the plenum which is quiet oily or is this normal? My theory now is that the engine and piston rings are worn out and that blow-by gases push oil up in the plenum. What do you guys think?

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                    • #11
                      That oily film in the plenum is typical. The fitting you're looking at is where the oil separator routes gasses from the cam cover back into the intake tract.

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                      • #12
                        Dit you remove the AFM. vor compression test ?

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                      • #13
                        Again, do a leak down test.
                        75 M2 Zinno
                        88 M3 Henna

                        Comment


                        • #14
                          OK, did one last test with the now luke-warm Motor, this time not with the rubber-hose of the tester but with the solid tube and the rubber cone pressed in. Results as follows:

                          Cyl 1: 190 Psi
                          Cyl 2: 174 PSi
                          Cyl 3: 190 PSi
                          Cyl 4: 177 Psi

                          So next step is the leak-down test. Regardless of Motor warm or cold or if I used the hose or the tube Cyl 2 and 4 are the ones that are worst. Have to buy such a tester first....

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                          • #15
                            Is the camshaft and sprockets timing stock. OEM ?

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