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No vacuum at TB port

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  • No vacuum at TB port

    I just finished putting the TBs (VAC 48 mm) on the engine and noticed that there was a significant sucking sound at cylinder #1. The engine also behaved like it was running on 3 cylinders.

    I took the CF airbox off to check the TB gaskets and found that one of the nuts (the one under the FPR) was not torqued. I tightened that up and then fired up again. There was a big improvement but still wasn't running quite right.

    I then sat back to think about all the items I had addressed since starting this project back in April and realized that maybe my valve job had gone way wrong. The valve cover came off and I checked the clearance for cyl. #1: intake between .28 and .30 mm, exhaust about .32 (this is for Schrick 284/276 cams, measured dead cold). Also checked the spark plug and that seemed to come out too easy when I removed it. With a flashlight I could see a lot of carbon on top of the piston, more on piston #1 than the others.

    During my initial TB setup and all along this process I've gotten zero vacuum on TB#1 with a Carbtune gauge hooked up. The other 3 TBs all were drawing a vacuum with TB #2 showing a little less than 3 and 4. I should also note that there was some vacuum at TB #1 back in August, but it was about half or less of what the other TBs were showing.

    I have not done compression or leakdown tests. I'll probably have the car hauled out this weekend to have that done on Monday.

    What do you guys think I'm looking at here?
    carbon on a valve not allowing full seating
    burnt exhaust valve

    I have a feeling I'll need to be sitting down when I read the responses to this.

    Bill Molina

    Keeper of the Car

  • #2
    I think you should press ahead with the compression test!


    • #3

      I agree, compression and leakdown tests. They should tell all.

      Ron ///Man

      • '91 Gr-A Former CiBiEmme / Ravaglia - Sold
      • '90 M3 Faux EVOII Alpineweiss 36K Orig Owner - The Queen
      • '91 M3 Faux EVO III Brilliantrot Euro Driveline - The Rocket
      • '91 M3 Faux Gr-A Club Racer DM - The Alter EGO
      • '89 M3 M3T / ITR Club Racer
      • '94 Spec E36 - Eh....
      • '09 M3 - Tarmac Terrorist
      • '04 330Xi Sport 6 Speed - Snowmobile
      • '07 530 Xi - Highway Star


      • #4
        I realized I never posted the answer to this mystery, so here it is:

        OK, after 6 weeks of tinkering, disassembly, reassembly, and much cursing, I finally found the cause to the big air leak at cylinder/TB #1. Look at the picture here:

        Inside the mounting hole of the TB gasket you'll see some metal that shouldn't be there. The hole is big enough to fit over the stud on the head but the metal caused the gasket to bind on top of the dowel (BMW lingo for a bushing around the stud) so that no amount of tightening would allow the rubber to create a seal. This is on a brand new, dealer part. The solution was easy enough: round file used to open up the hole.

        In addition, I used the Turner TB gasket kit for added security.

        Big Thank You to Ron ///Man who talked me off the ledge and made me go over every little detail.

        Bill ///M
        on the track 2 days from now
        Last edited by drwillb; 12-06-2012, 06:32 AM.

        Keeper of the Car


        • #5
          Good job troubleshooting! Glad you found the problem and glad it wasn't more serious. Did you drive the car much before you found the vacuum leak?



          • #6
            Didn't drive the car at all during the troubleshooting. It was that obvious that something was wrong and that there was a massive vaccum leak (LEAN) condition.

            It took a while to figure out because it wound up being a brand new factory part. I really didn't consider it initially. Who'dve thunk it?

            Keeper of the Car


            • #7
              I think I may have something like this. an intermittent misfire on #1 has grown worse over the years, to the point where I ran a compression test this weekend. Thankfully that was all fine. Just shy of 180 on all 4 cylinders.

              But when staring at the engine I noticed a sheared stud and missing chrome nut at the far (cylinder 1) end of the throttle body assembly.

              So my plan is to dismantle it all, deal with the sheared stud (likely needs a retap), replace gaskets / o rings and re-assemble. I am happy to plod on with that but would welcome tips or observations.

              Many thanks.