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    surging while holding speed, more

    Hey, hope someone can help here. I've had some drivability problems since saturday, and maybe you can shed some light on them. Kind of a long post but I'm putting out all the pertinent info I can right away.

    Symptoms: surging/unsteady engine output while holding speed (mild throttle)... not while decelerating or mildly accelerating. Also, it's cutting out under full throttle, but not all the time and not consistently.

    Background on the car: 220k+ M20b25, but runs very well (until now). m30 AFM + 19# injectors, with MarkD chip for that application (many miles of smooth running with that setup), new fuel pump and filter last summer, etc. 2.5" single magnaflow setup, no cats.

    Saturday I ran at a local autox, and everything seemed fine all day. On the way home i bought a couple of gallons of 93 at a busy gas station i do Not usually go to. Midway home from there I had to get around traffic and hit it - 3000 rpm or so it starts breaking up; alternating power and no power, as if i was misfiring badly. I get out of the throttle and relax the rest of the way. It seems to be surging a bit at steady speed, powering on and off somewhat as I hold speed.

    Got home, pulled the plugs to look - they looked decent, worn, but not fouled. Light grey / slightly tan deposits, maybe a bit of oil residue but not much at all. I replaced them with some copper Champions (the store wouldn't have bosches until today) and found I had the same issues.

    Considering the fill-up (which was only about 3 gallons) I added a can of Heet in case of water in the gas, and also added 3 gallons of (known good) 100 octane racegas to fight detonation. The car seems to run better now, a bit, and doesn't seem to be breaking up under load like before.. However, the surging at steady speed is still there.

    Last night I replaced some vacuum lines (fpr, brake booster) that looked a bit hard, and also checked for intake air leaks. Filter, AFM, elbow boot, IAC, all seem tight and leak-free. Same surging. Tonight I am swapping the champion plugs out for bosch supers, cleaning the AFM and IAC, and doing a compression test.


    Is there any place else to look? I've had bad fuel issues before, and the symptom was just like the breaking-up under load. I should run this tank out in the next day or two, and I'm betting that's the main issue.

    However, the surging is new to me - seems that an intake tract leak would cause that, but not sure where it's coming from. Is there a way to locate any other vacuum/intake leaks?

    Any other thoughts?

    PS: I trophied on saturday :) and now with the race gas, everywhere I go I think there's a badboy in a blown LS1 stalking me!
    Jay

    #2
    update - still cutting out under load on the way home from work today... but not every time. very strange.

    also, coil, wires, distro cap and rotor are all new with less than 5k on them. OE distro, IE wires, MSD coil with no issues prior to now.
    Jay

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      #3
      last night I changed All the vacuum lines - FPR, brake booster, charcan, pcv, everything. Cleaned the AFM, IAC, TB, checked all for leaks or weirdness. All connectors (purge valve, TPS, IAC, AFM, c191) are clean. I also teflon-tape wrapped the IAC fitting into the TB as it was just a bit loose.

      I ran out most of the gas last night and added some known-good 93, and still i'm having a small bit of surging. I'll change the fuel filter tonight, but after that I'm stumped.

      Is there any way to check a manifold/tb gasket without removing everything?
      Jay

      Comment


        #4
        it sounds like you aren't gettting enough fuel.

        change the fuel filter, and make sure your fuel pump is working. after 220k it's possible that it's gone bad.
        Build thread

        Bimmerlabs

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          #5
          I thought about that too, but fuel pump and filter were new last summer, and now have no more than 20k on them. Can a fuel pump really go toast in that short time? The filter's an easy cheap swap, but the pump is expensive- any way to test output?

          I also considered the FPR. I may pull it and see if it holds vacuum, and see if I can locate a pressure gauge.
          Jay

          Comment


            #6
            I doubt the pump has gone bad in that much time. they seem to last 200k pretty easily.

            but it definitely sounds like lean surging. you could get a used FPR to test with pretty cheap too.
            Build thread

            Bimmerlabs

            Comment


              #7
              My car is doing the same thing, but I have a stock engine. Changed the fuel filter and it still happened. Any more ideas? ps, I noiticed it right after I installed my intake.
              Byron
              Leichtbau

              Comment


                #8
                mine is having similar problems, it will majorly lean out on light throttle causing it to surge and such, my thoughts are on a tps at the moment, if its sending an idle signal with the throttle still open the fuel will cut out but more air than needed will be going in. well see as soon as i can get another TPS

                what does the ecu do if the TPS is unplugged? itll just idle like shit and WOT will be super lean?
                Your signature picture has been removed since it contained the Photobucket "upgrade your account" image.

                Originally posted by TimKninja
                Im more afraid of this thread turning into one of those classic R3v moments, where Pizza gets delivered.

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                  #9
                  with the TPS unplugged it will run in open loop, and yes your idle will suck/lean full throttle. but mid throttle should be the same.

                  oh, have you tested for smoothly increasing resistance on your AFM? the tracks may have worn out, causing it to send bad data to the ECU.
                  Build thread

                  Bimmerlabs

                  Comment


                    #10
                    that's a good idea, i'll check that tonight. do you know offhand which of the pins to check? i dont have my bentley with me.
                    Jay

                    Comment


                      #11
                      also does anyone know if there's a way to test the charcoal can itself?
                      Jay

                      Comment


                        #12
                        not important, atleast i dont think it is, i ditched mine but i was having these problems before hand anyway
                        Your signature picture has been removed since it contained the Photobucket "upgrade your account" image.

                        Originally posted by TimKninja
                        Im more afraid of this thread turning into one of those classic R3v moments, where Pizza gets delivered.

                        Comment


                          #13
                          Originally posted by fretburnr View Post
                          that's a good idea, i'll check that tonight. do you know offhand which of the pins to check? i dont have my bentley with me.
                          the first pin is ground, 2 and 3 are from the AFM and 4 is the temp sensor
                          Build thread

                          Bimmerlabs

                          Comment


                            #14
                            thanks man! I'll break out the multimeter when I get home.
                            Jay

                            Comment


                              #15
                              new problem is it related or not?
                              cold start my car runs really rich, causing it to run very rough, just started after work yesterday. it usually corrects itself when it starts to warm up a little and i start driving
                              Your signature picture has been removed since it contained the Photobucket "upgrade your account" image.

                              Originally posted by TimKninja
                              Im more afraid of this thread turning into one of those classic R3v moments, where Pizza gets delivered.

                              Comment

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