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Vacuum fluctuating at idle

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  • Vacuum fluctuating at idle

    I have had an idle problem ever since I bought my car a couple years ago. The basic description is that it idles low and fairly rough (around 600 - 700 RPM) when cold and then idles much better, around 900 RPM when it is warmed up. It also does not compensate for load - when I turn on the A/C the idle drops down significantly, but I have confirmed that the 12V signal to the DME for A/C on is getting to the DME connector.

    I have checked the cooltant temperature sensor, confirmed that the ICV is buzzing, applied power to the ICV out of the car to confirm that the valve moves freely, and even connected an oscilloscope to look at the ICV control signals from the DME. I have had the base CO set as well.

    I finally decided that it was time to try to synchronize the throttles so I went out and picked up a cheap vacuum gauge. When I connected it to the vacuum ports at the throttle I got a wildly fluctuating signal. When I added a bit of throttle I could get the vacuum gauge to settle down a bit. This was true of all 4 ports and was the case both when the engine was hot and cold. I shot a quick video to better explain what I saw:

    What does this mean? Do I have a vacuum leak somewhere? How can I find it? I have tried spraying carb cleaner around common leaks but never seem to have luck with that method.


  • #2
    I think thats normal if you put it on just one port, you may have to Tee all the four throttles into one vacuum, but it still will not be as smooth as a single throttle at idle, you may get around 7 in. hg vacuum.


    • #3
      it's normal, after all each cilinder sucks in air once every two revolutions. Normally you would have some sort of way to squeeze the hose in order to dampen the pulses and have a much more steady ready, comparable to what you see at slight throttle openings.


      • #4
        fwiw, my car acts just like yours. even swapping ICV's and DME's hasn't helped (i have spares for both). visibly my DME's are fine and the electrical wiring all checks out so i'm stumped. i have BDX53 transistors ready to go into one of the DME's, but have never gotten around to that task.


        • #5
          I purchased a glycerin filled vacuum gauge to do this part. The fluid damps out much of the vibration and it helps a ton. Found it on ebay for a few bucks.
          1987 Euro M3, Jet Black
          "BMW M cars: technically advanced, beautifully engineered, and the choice for total gits" -- Top Gear, Summer 2008


          • #6
            Thank you so much for the input guys. This evening I tried 2 different things to get a better reading and both seemed to work fairly well. First I put a check valve in line with the gauge so I could capture just the maximum vacuum rather than seeing the fluctuation. Next I took the check valve back out and just squeezed down the hose to dampen the vibrations. By slowly squeezing it I could make the fluctuations decrease until they completely stopped at the center point, presumably the average vacuum. The numbers I got were:

            Numbers are: cm of mercury (millibar)

            1--43.5 (580)--16 (213)
            2--42.0 (560)--16 (213)
            3--44.0 (587)--17 (227)
            4--43.0 (573)--16 (213)

            The throttle adjustment procedure posted here ( indicates that the throttles should be adjusted within +/- 5 mbar so it looks like I can use some adjustment, but they don't seem to be too badly out of synch. My tamperproof caps are yellow instead of blue so it seems that perhaps the adjustment has never been done on my car.

            However, something I read somewhere else, as well as the example numbers from the procedure indicates that the vacuum numbers should be around 300 mbar. Does that refer to the maximum or the average? Granted the gauge I'm using was purchased from Harbor Freight for $12.99 so I don't necessarily trust the accuracy of the measurement, but it still seems that I might be pretty far off from 300 mbar. The instructions do say to remove the adjustment screws and clean off the carbon. I haven't done anything with the adjustment screws yet so maybe when I clean them and screw them all the way in per the directions the vacuum reading will go up.

            James, I find it very interesting that your car has the same symptoms. Does that include the failure to accomodate a load (like turning on the A/C)? I also have a set of transistors ready to solder in, but when I visually inspected the DME the transistors looked ok. Also, when I probed the signal to the ICV with an oscilloscope they were definitely getting switched from 12V down to ground, so it seems that the transistors must be switching properly. What I found most strange is that the duty cycle on the ICV control signals didn't seem to change much. In particular I would think the duty cycle should increase (try to open the ICV more to increase idle speed) when the A/C button is pushed. However, the duty cycle remained the same.

            At this point it seems like there are 3 things that could be possible causes:
            1) There is some sort of leak that causes the idle to be poor at cold temperatures, but then it seals up due to expansion of components with heat so things run better when warm.
            2) There is some chip on the DME that controls the output duty cycle to the ICV and it is not correctly controlling the signal to set idle speed properly.
            3) The signal that goes to the DME to enable the idle speed control loop (presumably an engine speed input signal) is not getting all the way to the control chip properly.

            It's interesting and disturbing that changing the DME did not change your symptoms. I haven't yet had a chance to try swapping the DME. Did you by chance have any other damage to your DME? I know that mine had some components related to the purge valve relay burned up so the valve was open all the time. I have since repaired that part of the circuit so the valve operates properly but the strange idle remains.

            Hopefully by putting our brains together we can solve the weird idle. Thanks again for the input.


            • #7
              Check the simple things first. I would make sure that you have no leaks before looking to ECU issues. One way is to get a can of WD40 and spray it around all intake areas that have gaskets and fittings. If the idle slightly improves your leak could be in the are you just sprayed. Also while the car is idling put some downward force on the airbox and see if the idle gets worse.
              There'll be Spandex jackets one for everyone ...


              • #8
                i know i don't have any vacuum leaks as i did that job and my symptoms didn't change. numerous tests of the ICV system, numerous DME's, and endless hair pulling haven't turned up anything. i didn't think much more of it until just recently when i got my a/c converted and charged. now when the compressor is on, the idle is way 600rpms. i have my hot idle at 1000 to compensate. ugh

                i'm hoping that by replacing the transistors i'll be straight, but visibly there is nothing wrong. i'll keep you posted.


                • #9
                  I bought my car new in 8/87 was built in 3/87. There were several issues with thw Fuel system when they first were sold here. At least several of us where I bought mine we all had some of the same problems. The idle condition that you are discussing was one of them as well as in hot weather they would not shut off. Most were all corrected but mine had several things changed. The Idle I still have as you described.


                  • #10
                    Originally posted by jht3 View Post
                    when the compressor is on, the idle is way 600rpms. i have my hot idle at 1000 to compensate.
                    Same here. I'll keep you posted also if I make any progress with this. Regarding the post below, what is your build date?

                    Originally posted by Oleman View Post
                    I bought my car new in 8/87 was built in 3/87. There were several issues with thw Fuel system when they first were sold here. At least several of us where I bought mine we all had some of the same problems. The idle condition that you are discussing was one of them as well as in hot weather they would not shut off. Most were all corrected but mine had several things changed. The Idle I still have as you described.
                    My car is a 10/87 build and I'm familiar with the problem regarding dieseling (engine not shutting off when hot). My car actually did that a few times the summer I first bought it until I repaired the DME wiring for the purge valve relay and replaced the relay with the updated version described in the technical service bulletin (I have some other very detailed posts here describing that). I had not heard that there were early build issue affecting the idle though. Do you have any more information on that?

                    Thanks again!


                    • #11
                      The idling there was not much ever let out. I was good friends with several of the techs as well as service manager at the dealership. All I could ever find out was that they had changed basically everything on the Fuel System and really did not make a lot of difference. It was one of those things that you learned to live with.


                      • #12
                        Hi Dominic

                        I am not an expert, but may I suggest that you pay a visit the the idle adjustment screw. My (very theorical) reasoning is that it might be almost bottoming in, requiring all the air to come from the idle valve, which in warm-up mode and A/C compensation mode, simly cannot pass enough air. I suggest (if I am right in my assumptions) to open up the brass screw to allow more air, then the DME will slightly close the idle valve in normal operating temps, allowing more margin in warm-up and AC compensation modes.

                        Also, this weekend at a track day, my idle was surging between 800 and 1200 RPM, which is very unusual as I usually have a steady 850RPM idle speed in any condition. It was caused by an unproper idle signal from the TPS.

                        Does anything make sense?
                        Last edited by LeeVuong; 08-27-2008, 01:20 AM.

                        [email protected]

                        1969 2002 racecar
                        1989 M3 racecar
                        e39 Touring


                        • #13
                          Lee, interesting theory. Unfortunately not applicable in my case. I checked my idle adjustment screw tonight and it was about 8.5 turns out from fully closed. While the A/C button is not pressed, I can adjust the idle up and down using the idle screw. However, when the A/C is active I was able to screw the idle adjustment all the way open with no increase in idle.

                          When I unplug the coolant temperature sensor (blue connector) the idle shoots up quite high, so the DME does seem to compensate for input from that sensor. Even when I had the A/C on the idle would still shoot up when I unplugged the CTS. So, it seems that it is possible for the DME to override the load that the A/C causes, but it chooses not to for some reason.

                          Does anyone know for sure if the DME compensates for cold temps (simulated with CTS unplugged) by increasing the duty cycle to the ICV? If so, that would seem to be a very simple test to confirm that the transistors are functioning and the DME can affect the ICV.


                          • #14
                            Set the base idle. Don’t just count the turns using someone else’s car as an example. The idle should be 880 +/- 50rpm. You do this by the following:

                            -Car at operating temp.

                            -Baro sensor unplugged.

                            -TPS sensor unplugged with all 3 wires at the TPS engine harness bridged together.

                            Start the car and adjust the idle screw to get the 880 +/- 50rpm (no additional loads i.e. a/c or headlights on)

                            Works best if you have a multi meter with rpm “smoothing” readout such as a Fluke 88. Once all that’s good then you can sync the throttle using a Carbtune or similar product.

                            Originally posted by drinaldis
                            I dated a girl who used to do the reverse grind. I kinda liked it.


                            • #15
                              Hmm, I've never tried unplugging the baro sensor while setting the idle.