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Slow idle when warm...

Discussion in '308/328' started by boatr2001, Jun 21, 2018.

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  1. boatr2001

    boatr2001 Karting

    Oct 3, 2006
    88
    Rockford,IL
    #1 boatr2001, Jun 21, 2018
    Good evening. A new problem has just suddenly developed. 1983 308qv Euro. I’ve never had any previous problems (nor made any adjustments) with regards to how the car starts, idles or runs.

    Strong cold start, good solid 1k idle. No misfires or lumpy running, motor accelerates smoothly through all rpm ranges. Cruises very, very smooth.

    The problem is now that when warm, if the throttle is released, the rpm decelerates to about 500rpm and SLOWLY recovers to only about 8-900ish (and not as smooth once there). Give it gas- and the smooth running motor once again appears.

    The fuel filter and accumulator were replaced about 2 years (1500mi) ago.....

    Where to start? Ignition? Fuel?

    Thanks so much!
     
  2. Iain

    Iain F1 Rookie

    Jan 21, 2005
    3,320
    UK
    #2 Iain, Jun 22, 2018
    It could be a bunch of things. I'd start by ruling out the easy stuff i.e. broken plug leads, bad spark plugs or burned through extenders etc. Then work your way back to the distributor caps. what's the state of those & the rotor arms? If alls well on the ignition front then vacuum lines, make sure they are all in tact & also breather hoses (air leaks in the CIS system can cause choas at low revs) .

    After that then the fuel system - injectors & maybe the CIS unit itself. Hopefully you will find it way before you get there though.
     
  3. boatr2001

    boatr2001 Karting

    Oct 3, 2006
    88
    Rockford,IL
    #3 boatr2001, Jun 22, 2018
    Thx Ian,- that’s the route I was leaning....I was just hoping that somebody had this exact situation before and could say- “I know exactly what that is” :).....ah, the joys of scraped knuckles.....thx again for the help!
     
  4. miketuason

    miketuason F1 World Champ
    Owner Silver Subscribed

    Feb 24, 2006
    15,810
    Cerritos, CA.
    Full Name:
    Mike
    #4 miketuason, Jun 22, 2018
    What would happen if you adjust the idle while it’s hot, will it not stay at the correct hot idle rpm?
     
  5. boatr2001

    boatr2001 Karting

    Oct 3, 2006
    88
    Rockford,IL
    #5 boatr2001, Jun 22, 2018
    Great question. It’s the only thing I’ve tried so far- with the car warm and idling, an adjustment to the idle speed results in a temporary (while I’m turning the air bypass screw) improvement, only to settle back into the 800rpm idle.... the new position of the bypass screw also does not prevent the rpm fall through 1000 (or even 800) when the gas pedal is released....

    I appreciate the idea though!
     
  6. Iain

    Iain F1 Rookie

    Jan 21, 2005
    3,320
    UK
    #6 Iain, Jun 22, 2018
    I would not adjust anything. If it was working fine before and is now not working fine then something has changed/failed. You may well be able to correct it by adjustment but then only mask over the real fault.

    By way of example, my 328 had a hole in the vacuum hose that leads from the plenum to the ignition box. That line is for the vacuum advance. The hole allowed additional air to enter the system so that at idle it appeared to be that the CIS was set lean. So some smart individual adjusted the mixture on the car till the CO level was right at idle. Problem was it was then , in reality , set way to rich - and because the vac hose had a hole in it there was no vac advance either.

    At idle the amount of air coming in via the hole was significant, higher up the rev range it was not....

    Result: lousy fuel consumption, poor running & very black sparkplugs.
     
  7. boatr2001

    boatr2001 Karting

    Oct 3, 2006
    88
    Rockford,IL
    #7 boatr2001, Jun 22, 2018
    I agree Iain, not going to adjust anything. I calculated that the bypass screw is a very simple “return to” adjustment.
    Thx for the thought..
     
  8. Ferraridoc

    Ferraridoc F1 World Champ
    Owner Silver Subscribed

    Jun 20, 2012
    17,220
    Gold Coast, Aust.
    Full Name:
    Patrick
    #8 Ferraridoc, Jun 22, 2018
    I'm chasing a similar problem, and I agree with Iain's point about air leaks - there are a gazillion hoses to check. I just had my AAV off to check it (BTW, apparently they aren't meant to be completely airtight), and am now moving on to the vacuum limiting valve. I should have it sorted by xmas :eek:
     
  9. miketuason

    miketuason F1 World Champ
    Owner Silver Subscribed

    Feb 24, 2006
    15,810
    Cerritos, CA.
    Full Name:
    Mike
    #9 miketuason, Jun 22, 2018
    Normally if you have vacuum leaks, the idle more likely to raise up. And if the vacuum hose of the Digiplex box is disconnected, the idle will raise too as it advances the ignition timing by a few deg.
     
  10. Archer911

    Archer911 Formula Junior

    Sep 26, 2016
    907
    New England
    Full Name:
    Tim
    #10 Archer911, Jun 23, 2018
    When my car was running I had the exact same issue (many do here as evidenced by similar posts) but the problem went away when I replaced the cracked distributor caps and semi-mutilated plug wires.
    I remember reading a post where brake application caused a dive in RPM. This was due to a leak in the vacuum assist circuit for the brakes
    My Digiplex units are currently out for testing. I have a feeling the test results will show a fault or two.
     
  11. boatr2001

    boatr2001 Karting

    Oct 3, 2006
    88
    Rockford,IL
    #11 boatr2001, Jul 1, 2018
    Hello again and thanks for everybody’s suggestions,
    Just to close the loop on this- problem solved.
    I went through both sets of leads, extenders and distributors. I had 2 leads on the 1-4 bank that ohm’d a little funky- not consistent. Snipped and re-pierced, steady resistance. 2 extenders were suspect (so hard to identify a pin hole in the black Kingsborne extenders), replaced those. Finally, because I had a new spare set, and some additional funky ohm readings on one, I replaced both rotors (the old ones had been in service about 7 years).
    Started right up, ran smooth in all rpm ranges, and idle drops right to 1000 and stays there- perfect.

    Thanks again for the help!
     
    Iain and 308 milano like this.

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