'85 QV Sudden Rolling Idle and Weak Low End

Discussion in '308/328' started by daynyc, Jul 20, 2020.

  1. daynyc

    daynyc Rookie
    Silver Subscribed

    Aug 5, 2016
    My ‘85 US QV seemed to be running fine. The only thing I did was, very gently, open the Digiplex compartment to peek at the boxes (which looked fine). Thereafter, at a stop the idle would drop down to around 600 RPM before going back to 1000 RPM and the car had reduced low end power. So I went back and connected some good new grounds to the Digi boxes. As a result, my idle was immediately much faster, but rolling, and, although the car runs, low end power is even worse. It seems to run normally over around 3000 rpm. There is no backfiring, obvious missing, smell of fuel, exhaust smoke etc.

    When I alternately pull the coil wires, each four-cylinder bank starts, idles and revs the same. I have done my research and realize, in theory, there could be scores of reasons for this problem. Coincidences do occur, but before I tear a bunch of things apart in phase two of my restoration this fall (new fuel pump, accumulator and filter, replace sensors, plug extenders, check distributors, check WUR), I'm trying to come up with something that may relate to the Digi box examination or the new grounds.

    If both banks individually run and rev similarly and fairly smoothly, I'm guessing the crank sensors or distributors are not the culprits. The original mixture screw plug in the fuel distributor at least looks undisturbed. Hoses, including the vacuum lines to the Digiplex boxes, seem fine. My next easy things to check are the cold start injector and auxiliary air valve, then a timing light. Any thoughts would be greatly appreciated!
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  3. conan

    conan Formula Junior

    Nov 13, 2011
    Maybe an old issue was not properly fixed, but now you have a good digiplex gnd and an adjustment somewhere you need to undo.

    When I changed intake gaskets, vacuum hoses and other bits here and there, the car starts easier, warms up as it should with a bit higher rpm etc. Subtle changes, but a clear consistency improvement.

    I think it is worth it to have a look at all vacuum hoses before doing some more advanced repairs such as wur refurbish etc. Maybe the throttle or airflow plate mechanism is slow at low airflow.
  4. daynyc

    daynyc Rookie
    Silver Subscribed

    Aug 5, 2016
    Thank you. That's along the lines of what I was thinking. That's why I was curious about a compensating mixture mis-adjustment. Even though I have done a good deal of work on older cars, the 308 is actually my newest and complex electronics are not my strong suit. I renewed all the hoses in the engine compartment last winter, but I will check them again. I also rebuilt the throttle body which I'm quite sure is working well.
  5. Brian A

    Brian A Formula 3

    Dec 21, 2012
    SanFrancisco BayArea
    Full Name:
    1983 US 308 GTS QV
    If it really truly was all that you did before the problem started then the smoking gun must be underneath that trunk carpet.

    I had a misfire issue once and swapped the two digiplexes. It didn't "move" my problem to the other bank but doing so showed me just how easy it could be to bump the connectors and, particularly, that vacuum hose thereby causing a problem.
    mike996 likes this.
  6. mike996

    mike996 F1 Veteran

    Jun 14, 2008
    Full Name:
    Mike 996
    Yep, I agree with Brian A. If it was fine before you did whatever you did, odds are that whatever you did (perhaps unknowingly) caused the problem. It's not impossible, of course, that something coincidentally failed at the same time but the 'usual suspect' would be related to what was done between the time that the engine was running right and the time that it was not.
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  8. 308 milano

    308 milano F1 Rookie
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    Jan 15, 2007
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    Yeah, I would give the vacuum lines into the boxes a second look.
  9. daynyc

    daynyc Rookie
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    Aug 5, 2016
    I definitely agree. I have triple checked the vacuum lines to the boxes, removed the box connectors (cleaned them, although they looked good) and will hopefully check the result later today. I'm wondering if someone far more expert in electronics than I might suggest how a better Digiplex ground, and possibly a resultant stronger "signal," could adversely affect something else. I will also check ignition timing with a light when I get a chance. Faster idle, poor low end power: my initial guess would be fixed advanced ignition. But it would have to be quite advanced and I am not getting any preignition pushback when starting.

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