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Thread: Hesitation between 3-4K RPM on my M30

  1. #1
    Join Date
    Feb 2016
    Location
    Chicago
    Posts
    3
    My Cars
    1979 E23

    Hesitation between 3-4K RPM on my M30

    I've been having an issue where my '79 733i has been hesitating/losing power between 3-4k RPM. Below and above that the engine runs fine. It seems to be getting worse - it used to be somewhat intermittent but the last two times I took it out for a drive the problem persisted until the engine finally quit on me. I was able to get it restarted with a TON of throttle and keep it running enough to get home. During today's occurrence, I realized just before I got home that the engine was running fine again as long as I kept it out of that 3-4k RPM range.

    I've been replacing cracked fuel hoses on the car and originally was wondering if it was a fuel delivery problem. My initial thought was that not enough fuel was being delivered at 3-4k RPM causing the hesitation. Today it occurred to me that the issue might actually be spark. If insufficient ignition was occurring in that range and the engine was kept there long enough, too much fuel would build up in the cylinders causing it to die. Restarting the engine at high RPM blows out the cylinders and allows it to return to normal running conditions after a few minutes.

    Has anyone else run into this type of thing? Do you think my assessment is correct? If so, what is the most likely culprit? I'm thinking plugs, wires, or distributor. Going to try new plugs and wires tomorrow.

    Appreciate any advice. Thanks!

  2. #2
    Join Date
    Mar 2013
    Location
    USA
    Posts
    465
    My Cars
    E21/early e23/e34
    I've got the same car, could be a number of things, be helpful to know if auto or manual shift.
    Sounds/looks like it's a fairly new car to you, I'd seriously consider baselining the car so you know exactly what is or isn't in good condition
    For now I'd check;
    Fuel pressure
    Cap/ rotor
    Are both vacume lines hooked up correctly on both ends of distributor?
    Engine timing
    I'd also pull spark plugs and see how they look, should tell you if it's dumping too much fuel.

    Kew note on ljet cars, the ECU only controls fuel, not fuel and spark and this system is very sensitive to inputs from sensors, critical that all sensors are working correctly, CTS (coolant temp sensor) can cause some wierd shit to happen- on my car that sensor was faulty, and car would dump massive amounts of fuel into cylinders at just under 3k rpms. Car ran just poorly enough normally to not notice, new CTS changed the car instantly.

  3. #3
    Join Date
    Feb 2016
    Location
    Chicago
    Posts
    3
    My Cars
    1979 E23
    Thanks Kojo! The car is a manual.

    It is relatively new to my ownership, however my dad bought it new back in '79 so it's been around. =)

    I checked the distributor cap today and there was a bit of corrosion but not much. I cleaned all the points and checked that all the plug wires were seated. Then I took it out for a spin to see if that helped.

    The hesitation was still there, but I had a realization while I was driving - it only seems to happen at WOT! So I got back home and put a multimeter on the TPS and found that at WOT the resistance didn't drop to zero as it should, and more disturbingly, it seemed to jump around a bit from about mid-70's to ~190 ohms each time I left the WOT position and returned. I've ordered a new TPS that will arrive this week and we'll see if that remedies the issue. I'll let you know!

  4. #4
    Join Date
    Jun 2010
    Location
    TX
    Posts
    4,487
    My Cars
    1983/84/85 745i

    Thumbs up

    Quote Originally Posted by antiteam View Post
    It is relatively new to my ownership, however my dad bought it new back in '79 so it's been around. =)


    Badass! Have any pictures?
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



  5. #5
    Join Date
    Mar 2013
    Location
    USA
    Posts
    465
    My Cars
    E21/early e23/e34
    Quote Originally Posted by antiteam View Post
    Thanks Kojo! The car is a manual.

    It is relatively new to my ownership, however my dad bought it new back in '79 so it's been around. =)

    I checked the distributor cap today and there was a bit of corrosion but not much. I cleaned all the points and checked that all the plug wires were seated. Then I took it out for a spin to see if that helped.

    The hesitation was still there, but I had a realization while I was driving - it only seems to happen at WOT! So I got back home and put a multimeter on the TPS and found that at WOT the resistance didn't drop to zero as it should, and more disturbingly, it seemed to jump around a bit from about mid-70's to ~190 ohms each time I left the WOT position and returned. I've ordered a new TPS that will arrive this week and we'll see if that remedies the issue. I'll let you know!
    Should do it... Post up images.

  6. #6
    Join Date
    Feb 2016
    Location
    Chicago
    Posts
    3
    My Cars
    1979 E23
    It's fixed! Here are some of the things that were NOT the cause of the problem:
    • Distributor cap & rotor - Checked/cleaned the points and verified the vacuum lines were connected properly.
    • Spark plugs - Inspected all of the plugs. None were fouled but all were old so they got replaced regardless.
    • Fuel pressure - Hooked up a pressure gauge and tested both stationary (on and off vacuum) and while driving. Pressure looks good.
    • TPS - I was totally wrong about the problem only being at WOT. It happened even off of that point. The old TPS was giving an improperly high resistance at WOT, so didn't hurt to get a new unit in there regardless.


    And so the issue was...
    • AFM - I pulled the unit and tested the temperature circuit and did both a resistance test a voltage test on the door opening. All of the circuits checked out. While I was testing that I realized that the flap was sticking about halfway open and would be slow to close. After using some MAF cleaner on it and working some penetrating oil into the hinge I reinstalled everything and took it out for a test ride.


    The car hasn't run this well in YEARS. It's super smooth all the way to redline.

    Here are a couple of pictures. Needs some attention to the paint this summer but thinking just a buff and polish should make worlds of difference.

    Next thing to tackle - the moonroof motor. This one has the motor in the trunk (I think later years head motors mounted in the headliner, maybe?) and was working weakly until it finally just gave up.


    IMG_0828.jpgIMG_0829.jpg

  7. #7
    Join Date
    Jun 2010
    Location
    TX
    Posts
    4,487
    My Cars
    1983/84/85 745i
    Need a video of it ripping to redline now!
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



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