Mercedes 450 Lunges/misses at constant Idle - No Load

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cjhols
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Mercedes 450 Lunges/misses at constant Idle - No Load

Post by cjhols » Sun 22 Apr, 2018 7:18 pm

Hi all,
I read a post somewhere (but I can't remember where) that is a fix for the 450 that lunges/lurches or misses when it is on a constant rpm (whilst not under load - does it at idle and around 100 kmh). I think it has something to do with the Hall Effects connector coming from the distributor and having to clean the connectors with an eraser???
Does anyone know about this issue???
Thanks
1973 SLC 450
1971 Clubman Mini

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Re: Mercedes 450 Lunges/misses at constant Idle - No Load

Post by AMG » Mon 23 Apr, 2018 1:03 pm

The D-jet trigger points should not be cleaned with an eraser, but removed and sprayed with CO cleaner.

the trigger points are the injector timing triggers for the ECU.

Without a more detailed description of the issue, it's not possible to say if it would influence or remedy the issue you have described thus far.

Numerous threads on the trigger points have been posted, and unless you are certain that the issue you have is caused solely by the trigger points, then you should follow the factory service manual procedures for diagnosing fuel and ignition related issues on M116/7 engines.

The reference library contains the information you need to study the subject and make a more detailed diagnosis, perhaps even rectify the issue you are experiencing.

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Chai
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Re: Mercedes 450 Lunges/misses at constant Idle - No Load

Post by Chai » Mon 23 Apr, 2018 6:36 pm

Try replacing the rubber vacuum tube from the intake manifold to the Manifold Pressure sensor in case there is air leaks affecting the EFI computer from this sensor. Is 8mm internal diameter as I recall.
Another hiccup while running at a constant speed issue may be the throttle control 'switch' (underneath the air filter). The copper slide contacts may have a corrosion at a specific area.
Recall mine occurred at around the 100 km/h. Had to either drive a little faster or slower to miss that area in the slide contact. A replacement throttle, NOS from a garage clearance sale, fixed this.
Chai
1974 450SLC

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Re: Mercedes 450 Lunges/misses at constant Idle - No Load

Post by cjhols » Mon 23 Apr, 2018 7:26 pm

Hi Chai,
This is exactly what it is doing ( the second part of your post).
Thanks
Peter
1973 SLC 450
1971 Clubman Mini

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Re: Mercedes 450 Lunges/misses at constant Idle - No Load

Post by Chai » Wed 25 Apr, 2018 9:27 pm

The D-Jet throttle switch is located beneath the air filter and on the other side of the rod where the throttle butterfly flap is rotated by the accelerator linkages.

For switch advises the D-Jet computer whether the accelerator is:
  • at idle
  • at full throttle
  • at a steady position in-between
  • moving - acclerating or decelerating
I think the engine lurches/hiccups because the slide contact point sits on a small pit on the copper strip. The computer senses the intermittent loss of electricity and reacts accordingly.
I tried rubbing an eraser on the copper strips inside the switch without any beneficial effect. I did not try using very-very fine abrasive (1000 grit perhaps), as I managed to buy a new throttle body with switch, but this slightly more agressive repair method may be tried.

The cover on the switch can be popped out for this 'repair' without the need to remove the switch from the throttle body. There's a tab that's visible on the top of the cover and a corresponding tab below the cover.

However, if you decide to remove the switch, take care to mark out the exact position before attempting its removal, otherwise it'll affect the at-idle signal, etc.

The arrow indicates the switch, viewed from the right-hand side of the engine.
DJet Throttle switch-1.jpg
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Chai
1974 450SLC

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Re: Mercedes 450 Lunges/misses at constant Idle - No Load

Post by cjhols » Fri 27 Apr, 2018 8:28 am

Thanks Chai
Excellent summary of the issue.
Thanks again
1973 SLC 450
1971 Clubman Mini

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