1983 Mondial – diagnosing a miss/hesitation at low RPM | FerrariChat

1983 Mondial – diagnosing a miss/hesitation at low RPM

Discussion in 'Mondial' started by Spacecowboy, May 4, 2021.

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  1. Spacecowboy

    Spacecowboy Rookie

    Oct 21, 2011
    20
    #1 Spacecowboy, May 4, 2021
    I have a 1983 Mondial cabriolet that I’m attempting to diagnose a miss/hesitation coming off idle, from about 1,000-2,000 RPM. Once it’s past 2,000 RPM it runs fine. I live in Minnesota so the car gets parked for the winter months. It ran fine when it was parked last fall. This problem surfaced during my initial drive this spring. Gas tank was down to about ¼, after the initial drive I filled the tank with fresh fuel (always run non-ethanol fuel) and a bottle of Techron and have driven about 75 miles since and the problem continues. The car starts fine and runs strong with the exception of the hesitation in the 1,000-2,000 RPM range. The car had a major service 2 years ago. At that time spark plugs and wires were replaced, plug extenders and distributor caps were inspected and deemed to still be in good condition.

    I’m looking for suggestions on where to start diagnosing the problem. Is it most likely fuel, ignition or vacuum related? I’m trying to avoid just starting to throw parts at the problem. Thanks in advance for any assistance.
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
    25,133
    30°30'40" N 97°35'41" W (Texas)
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    Steve Magnusson
    #2 Steve Magnusson, May 4, 2021
    Does it have the same symptom both at cold start up and when fully warmed up?
     
  3. Spacecowboy

    Spacecowboy Rookie

    Oct 21, 2011
    20
    #3 Spacecowboy, May 4, 2021
    Immediately upon startup it seems to run fairly smooth, maybe a slight hesitation. Within a couple of minutes of running the issue is consistent.
     
  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,133
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #4 Steve Magnusson, May 4, 2021
    Last edited: May 4, 2021
    Your 1983 has only one "computer" -- the WUR (and it also serves as the "accelerator pump" to add enrichment during acceleration). I'd suggest that you have the cold and warm control pressures measured at the various vacuum levels as shown on pages D38-D41 of the Mondial8/QV WSM 281/83, and ensure that things are still "in bounds" as the first diagnostic step. Good Hunting!

    And some more questions:

    1. Is the warm idle smooth?

    2. If you hold it at a constant 1500 RPM in neutral (so no load) when warm is it OK or not OK?
     
  5. Spacecowboy

    Spacecowboy Rookie

    Oct 21, 2011
    20
    #5 Spacecowboy, May 4, 2021
    Thank you for the recommendation. I did install a rebuilt WUR 3 years ago which solved a rich running issue. I do have a fuel pressure testing gauge kit. Do I need the specialized KDEP Pressure Test device shown in the WSM to accurately complete the tests or can it be completed with my regular testing gauge?
     
  6. Spacecowboy

    Spacecowboy Rookie

    Oct 21, 2011
    20
    #6 Spacecowboy, May 4, 2021
    To answer your additional questions,

    1) the warm idle is not smooth, it seems the longer it sits at idle/low RPM the worse it gets and the longer it takes to clear once back at higher RPM

    2) at a constant 1,500 RPM with no load there is still an issue but holding it above about 2,000 RPM the issue seems to go away and run smooth
     
  7. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    25,133
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #7 Steve Magnusson, May 4, 2021
    Any typical CIS Pressure Tester should be able to be used to measure both the cold and warm control pressures, but don't know what "kit" you have. The CIS Pressure Testers have a 3-way valve that makes it easy to switch from measuring the regulated supply pressure to the control pressure, but even a simple gauge (with no 3-way valve) can be used if you plumb it in correctly to the right (different) places to measure both.

    Don't like those answers, as they don't exonerate the fuel distributor, but you've got Chapter D.
     
  8. Spacecowboy

    Spacecowboy Rookie

    Oct 21, 2011
    20
    #8 Spacecowboy, May 4, 2021
    Thanks for the information. I'll see if I can complete the pressure tests.
     
  9. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
    3,859
    #9 spicedriver, May 8, 2021
    Most likely you have fuel varnish on some component from the car sitting unused.

    You can try just running the car for some time with fresh fuel + Techron to see if that fixes it. Or start tearing things apart. If you tear things apart, I would focus on the fine mesh filter screens inside the various components. Injectors, WUR, Fuel distributor. If the fuel filter hasn't been changed in over a year, I would start with that. It would also give you an indication of varnish in the system. You don't need to be running an Ethanol fuel mix to get varnish. Then I would clean or replace the fuel injectors. Then clean the filter inside the WUR (requires disassembly).
     
  10. Spacecowboy

    Spacecowboy Rookie

    Oct 21, 2011
    20
    #10 Spacecowboy, May 9, 2021
    Thanks for your suggestions. At this point I haven't had a chance to look at it any further. Maybe a long drive is needed to see if that helps resolve the issue before I begin tearing things apart.
     
  11. afterburner

    afterburner F1 Rookie
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    Jun 20, 2008
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    #11 afterburner, May 10, 2021
    I've had this on my car in the past and it was an ignition issue:
    first time: one of the crank sensor dying
    second time: distributor rotor arm shorting to the cam shaft end
    third time: dying HT leads
     
  12. Spacecowboy

    Spacecowboy Rookie

    Oct 21, 2011
    20
    #12 Spacecowboy, May 10, 2021
    Thanks for you input. All crank sensors and the HT leads were replaced two years ago during an engine out major but I guess one could be failing again.
     
  13. afterburner

    afterburner F1 Rookie
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    #13 afterburner, May 10, 2021
    Oh then I wouldn't worry too much about the ignition side.
     
  14. 85MondialQV

    85MondialQV Karting

    Feb 20, 2021
    98
    Full Name:
    Matt Drake
    #14 85MondialQV, May 10, 2021
    I got the same issue on my 85 Mondial. Exact same symptoms except at 3,000rpm. I mean exactly the same. I’m trying to figure it out. Mine sat for 2 years before I bought it.


    Sent from my iPhone using FerrariChat
     
  15. afterburner

    afterburner F1 Rookie
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    #15 afterburner, May 10, 2021
    1) running rich and fouling plugs?
    2) running lean and not burning properly?
     
  16. Spacecowboy

    Spacecowboy Rookie

    Oct 21, 2011
    20
    #16 Spacecowboy, May 14, 2021
    I am happy to report that the fuel system cleaner appears to be working to help resolve my idle/hesitation issue at low RPM. I took a couple drives of about 75 miles total this week and the issue has improved significantly. I'm hoping that with some additional miles it gets completely back to normal. Thanks to everyone for your input to help me resolve this issue.
     
    85MondialQV and afterburner like this.
  17. LA_Rari_22

    LA_Rari_22 Rookie

    Nov 3, 2019
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    Len U
    #17 LA_Rari_22, Jun 1, 2021
    I had the same issues but mine was from 3-5k rpm. if the Techron doesn't clear it up completely and your oil smells like gas (smell the dipstick), your problem is the fuel distributor. probably seals going bad inside or the plunger is getting stuck. I just got mine back from rebuild recently and it fixed the problem.

    Keep us updated and look for my post, as I will update it and should be on top of this post
     

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