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Thread: Uneven idle, hesitating when accelerating, power is down

  1. #1
    Join Date
    Aug 2016
    Norway, Europe

    Uneven idle, hesitating when accelerating, power is down

    The title says it all. I've had this problem for a while - idle is uneven - engine is shaking a bit, it almost feels like it's missing, at certain throttle positions the car hesitates when accelerating, and - at least I feel - the car should be quicker. A friend owning a w124 E36 says the same.

    The car in question is a 1993 500E, 171k miles.

    What I have done:
    Replaced S29/3 Idle switch at accelerator pedal (Mercedes)
    Replaced S16/1 Neutral Safety Switch (Mercedes)
    Replaced Brake Lamp Switch (Mercedes)
    Replaced vacuum seals between lower and upper intake (Mercedes)
    Replaced rubber between ETA and MAF (Mercedes)
    Replaced a few vacuum hoses here and there, checked the entire vacuum system in the engine compartment
    Replaced gasket between intake and ETA (Mercedes)
    Replaced intake gaskets (Mercedes)
    Replaced distributor rotors (Bosch)
    Replaced distributor caps (Bosch)
    Replaced spark plugs (Mercedes) - they are the correct for the car
    Replaced spark plug wires and coil wires (Bosch)
    Replaced the oxygen sensor (Mercedes)
    Replaced air filters (Mercedes)

    I checked the MAF with STAR C3: Values are 1,50V and 21,8 kg/h at idle. Voltage rose to 2,0-2,1 at 3000 rpm.
    I have checked the fuel pressure; it's 3,65 bar with vacuum @ idle, rising for a second if I press the accelerator and then going back to about 3,65 bar (with vacuum).
    After shutting the engine off it stays at 3,65 bar for at least one hour (that's when I disconnected the metering device).

    What I have not done/ought to be investigated:
    Replaced the dust shields in the distributors - I have ordered them, but they have yet to arrive - could they cause this?
    I have some funny noises coming from the catalytic converters - could they be blocked?
    The car had been run with oily air filters, so the MAF was quite dirty. Could it be faulty - or how do the values look? I couldn't find anything in WIS.

    As far as I know I don't have any fault codes now.

    What else haven't I checked? Where do I go from here?
    Last edited by little_e; 07-04-2018 at 02:38 AM.

  2. #2
    E500E Guru emerydc8's Avatar
    Join Date
    May 2011
    Tucson, AZ

    Re: Uneven idle, hesitating when accelerating, power is down

    I experienced a similar acceleration issue with my E420 last month. This happened right after I changed the spark plugs with new MB F8DC4, gapped to 1mm as per Klink's SOP. I noticed a rough idle which seemed to go away after driving, but there was still a hesitation while accelerating, especially when the engine was hot. It was almost as if the ignition module was retarding the timing as it tried to accelerate. I could tell it wasn't accelerating linearly. At first, I actually wondered if the transmission was slipping.

    I installed another new set of factory F8DC4 plugs and the problem completely went away. So I don't know if it was a bad plug, right from the factory, or whether one of the suppressors just wasn't making good enough contact. I checked all the wires for resistance and they sat right at the 2K Ohm range, so I don't think the suppressors are bad.

    Has anyone experienced a bad MB plug right out of the box? Are German-made plugs better? Neither of the plug sets I installed was from Germany. Both were from France. In any case, plugs are cheap. Maybe try installing a new set in addition to checking your insulators behind the rotor caps.

    If you think the cats may be plugged, I did a post somewhere on this forum on checking the back pressure using a vacuum gauge tapped into where the HVAC pods sit.
    Jon D.
    1994 E420
    1995 E420

  3. #3
    .036 Hoonigan™
    E500E Boffin
    gsxr's Avatar
    Join Date
    Dec 2008
    Boise, ID, USA

    Re: Uneven idle, hesitating when accelerating, power is down

    MAF values, as a VERY ROUGH guide, will be: 650rpm idle = 20kg, 1000rpm = 30kg, 2000rpm = 60kg, 3000rpm = 90kg. Within ±10% of those numbers would be fine. If you see anything radically different, the MAF is suspect... the oiling doesn't help either.

    The catalysts should be silent when you whack them hard with your fist. If there is any rattle, the cats are also suspect. A proper test requires an exhaust backpressure gauge that you can view while driving at full throttle (this is important - can't check at idle, can't check in your driveway). That may explain the reduced power, but not the uneven idle.

    The uneven idle could be a vacuum leak somewhere, a smoke test is the easiest method to find leaks. In the meantime, keep driving the car, give it some full-throttle time through the gears, and check codes frequently on all modules (with SDS / HHT-Win, use the Quick Test).

    Oh, it would help if you mentioned the year / model being discussed here...

    Dave M.
    1997 E420 (Bugeyes)
    1994 E420 (Blondie)
    1994 E500 (Q-ship)
    1992 500E (Mach 5)
    1987 300D (Sportline Stage 2)
    Click here for my website
    Click here for my YouTube channel

  4. #4
    Join Date
    Aug 2016
    Norway, Europe

    Re: Uneven idle, hesitating when accelerating, power is down

    The car is a 1993 500E

  5. The Following User Says Thank You to little_e For This Useful Post:

    gsxr (07-04-2018)

  6. #5
    Senior Member Eno's Avatar
    Join Date
    Oct 2015
    Doha, Qatar

    Re: Uneven idle, hesitating when accelerating, power is down

    Have you pressure tested all the injectors to check if they are working properly?

    Sent from my iPhone using Tapatalk Pro

  7. #6
    Join Date
    Aug 2016
    Norway, Europe

    Re: Uneven idle, hesitating when accelerating, power is down

    No.. how do I do that

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