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Misfire and Weird Compression After Oringing

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    Misfire and Weird Compression After Oringing

    Hey everyone, I am a long time reader of this forum (lot of good info!) but posting for the first time as I am a little confused by my car currently.

    I have a 1989 325i that I turbocharged myself last year and last summer all was well, made about 280whp at 13 psi for most of the summer.

    This winter I oringed the block, changed the HG (Elring), and put in a 284/280 (Ireland Engineering) cam as I want to get closer to the 400whp mark. After putting it all back together, firing it up, and making some Megasquirt tuning changes for the new cam, I feel like the car has a misfire at idle around 8-900rpm. It does not have a completely clean lumpy idle like I would expect, seems like it misses here and there. I do not notice this past like 1200 rpm or so and there is no hesitation to rev or anything else weird. I decided to do a compression test after checking the spark plugs, wires, distributor etc. and finding nothing. I got about 9 bar of pressure on cylinders 1,2,4,5,6 and 11 bar on cylinder 3.....I know the factory spec is 10-11 bar and it had prefect compression on all cylinders at the beginning of winter before it all came apart. I am assuming the lower compression result now is due to my longer duration cam but why do I have such a different and high number in cylinder 3? I checked my cam timing and it was correct. Could the difference in compression be related to the misfire I think I am hearing? Any idea what the correct compression test pressure should be with a 284/280 cam and why there is such a difference with 1 cylinder?

    I have not taken the car for a drive yet this year, just idling in the driveway really because I am trying to understand what is going on.

    #2
    I've never had a 100% perfect idling M20, so I'm probably not the best person for this, but could it be a valve adjustment issue?
    Originally posted by priapism
    My girl don't know shit, but she bakes a mean cupcake.
    Originally posted by shameson
    Usually it's best not to know how much money you have into your e30

    Comment


      #3
      Sorry I actually posted this thread twice yesterday due to the site issues (i think?).

      I have checked the valves a couple of times and they are all right at 0.010-0.011". I just wanted to have that nice lumpy cammed engine idle but it certainly is more important that it pulls well up top! Maybe I won't worry about it too much if a perfect idling M20 is rare.

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        #4
        My 284/280 cammed M20 stroker seems a bit rough to me too around idle - but I have put a few 1000km on it so far without issue. I will take a video sometime tomorrow, maybe a good reference point (and maybe we both have something wrong! lol).

        Comment


          #5
          post a video? might be chasing ghosts
          89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

          new build thread http://www.r3vlimited.com/board/showthread.php?t=317505

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            #6


            This is a video I had from last week (had to get it off my phone). To me it sounds like there are parts of the video where it sounds rougher than others. And it is definitely more noticeable in reality.

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              #7


              Finally got to it today. Tried to stand in a few spots for a bit so hopefully you can get an idea of sound from that. Also ongoing oil leaks and I forgot to thread on my coolant cap when I moved it a few days ago...

              I let her warm up first. She is a bit rough right now, just pulled her out of the shop on saturday and she is going back in shortly, right in the middle of tons of shit but sound has pretty much always been like this, even regardless of valve adjustment etc.

              Comment


                #8
                A lot af things will cause your issue most of them are tuning related. Are sure it is tuned good in the idle area?

                M20s like idling on the rich side, Tune idle till your engine sound happy.

                Comment


                  #9
                  Last year, I had the engine idling around 12.5-13:1 AFR with about 15 degrees of timing advance. It seemed strange to me to have it idle so rich so I am trying to idle it around 14.5:1 now but I need about 28 degrees of advance to do that somewhat smoothly (like in the video) Sounds like it would be better to go back to the richer idle tuning. Any idea why they prefer a richer idle mixture? What AFR/timing do you find to work best? I also have 800cc injectors and my duty at idle is around 1.4%. I suppose it is possible that this will cause a rougher idle compared to a set of smaller injectors too.

                  Comment


                    #10
                    Originally posted by turbo6cyl View Post
                    Last year, I had the engine idling around 12.5-13:1 AFR with about 15 degrees of timing advance. It seemed strange to me to have it idle so rich so I am trying to idle it around 14.5:1 now but I need about 28 degrees of advance to do that somewhat smoothly (like in the video) Sounds like it would be better to go back to the richer idle tuning. Any idea why they prefer a richer idle mixture? What AFR/timing do you find to work best? I also have 800cc injectors and my duty at idle is around 1.4%. I suppose it is possible that this will cause a rougher idle compared to a set of smaller injectors too.
                    I burned valves trying to tune leaner, My exhuast was poping pulsing till it lost power.
                    lesson learned.

                    Comment


                      #11
                      Originally posted by JehTehsus View Post
                      https://youtu.be/YLqU_3cMK4c

                      Finally got to it today. Tried to stand in a few spots for a bit so hopefully you can get an idea of sound from that. Also ongoing oil leaks and I forgot to thread on my coolant cap when I moved it a few days ago...

                      I let her warm up first. She is a bit rough right now, just pulled her out of the shop on saturday and she is going back in shortly, right in the middle of tons of shit but sound has pretty much always been like this, even regardless of valve adjustment etc.
                      For me this sounds as a slightly cammed 6cyl with one throttle body. Normal.

                      Comment


                        #12
                        I have a smaller cam and injectors (272 and 60lb/630cc) and I could never get the car to idle nicely around stoich, that should tell you something about trying to idle at 14.5:1 with a bigger cam and larger injectors. I could idle there and get an unstable and lumpy idle which sounds cool but I idle in the low 13s to stay smooth enough that it doesn't hunt with the AC on. Your 800cc injectors are partly to blame here, your pulse width at idle and 14.5:1 with batch fire injection is probably under 1ms. Unless they are high end units they probably don't like being below 1.5ms pulse width and your idle will go to hell as soon as you drop below whatever the minimum linear response pulse width is no matter what the injector. FWIW, 800cc/min is not necessary for 400whp, that's over 500whp worth of injector on gasoline and E85 isn't necessary to make 400whp with an M20.

                        As for the compression, re-test? Do a leakdown test? It is odd but there are confounding factors.

                        IG @turbovarg
                        '91 318is, M20 turbo
                        [CoTM: 4-18]
                        '94 525iT slicktop, M50B30 + S362SX-E, 600WHP DD or bust
                        - updated 1-26

                        Comment


                          #13
                          Originally posted by varg View Post
                          I have a smaller cam and injectors (272 and 60lb/630cc) and I could never get the car to idle nicely around stoich, that should tell you something about trying to idle at 14.5:1 with a bigger cam and larger injectors. I could idle there and get an unstable and lumpy idle which sounds cool but I idle in the low 13s to stay smooth enough that it doesn't hunt with the AC on. Your 800cc injectors are partly to blame here, your pulse width at idle and 14.5:1 with batch fire injection is probably under 1ms. Unless they are high end units they probably don't like being below 1.5ms pulse width and your idle will go to hell as soon as you drop below whatever the minimum linear response pulse width is no matter what the injector. FWIW, 800cc/min is not necessary for 400whp, that's over 500whp worth of injector on gasoline and E85 isn't necessary to make 400whp with an M20.

                          As for the compression, re-test? Do a leakdown test? It is odd but there are confounding factors.
                          You have turbo car.

                          Germans OEM's like idle "rich", around 12.8-13.5 in stock form. In the domestic market, EGR was added to gasoline engines early 80's, Euro cars generally left a large overlap instead. Most USM cars appreciate stoich idle, most German cars do not (even MKII single and dual cam VW's).
                          john@m20guru.com
                          Links:
                          Transaction feedback: Here, here and here. Thanks :D

                          Comment


                            #14
                            Thanks for all the help guys, I appreciate it!

                            I am definitely going to richen the idle to what I had it at last year and see where it seems the happiest, I had no idea that even in stock form they idled so rich. I thought it would potentially cause problems in the long run so I was trying to get close to stoich. My PW at idle was around 1.3ms so maybe some engine cycles were even to lean to fire properly and that causes the "misfire" that I think I hear. I am not trying to chase that lumpy idle just for the sound, I just thought that's what a happy engine would sound like.

                            I know the 800cc injectors are more than I would ever need for my goals but I got a great deal on them so I figured why not. A poorer idle quality did not occur to me when I pulled the trigger on them.

                            I am going to take the car for a decent drive tomorrow and then try the compression test again (I do not have a leak down tester unfortunately) and see what my results are and I will try a little oil in the cylinders with lower compression if I am still seeing a difference between cylinders. I want to take the car for a drive first and maybe clear it out a little bit since it has not done anything much besides idle at this point. I'd like to do a little street tuning for the new cam as well.

                            I will report back with results.

                            Comment


                              #15
                              Mine also idles rich, repeating what everyone above said basically. It is converted to full sequential too. I have quite a bit of engine related data logging (just no EGT yet) and I can put a log of it idling up with the current tune if you want another reference.

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