Garage > Mechanicals

6.9 K-jetronic rough and high idle cold start

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Jan S:
I'm starting to direct my attention to "the rough and high idle cold start" problem I mentioned before the summer. I will investigate step-by-step according to the manual "1976 1979 Mercedes Continuous Injection System Diagnosis & Adjustment Manual". I'm suspecting a few things at the front .... time will tell (the entire fuel assembly at the rear is new, replaced spring 2021). Will be exciting to follow the guide and learn.

Recently, a new annoying "problem" has surfaced:

- there seems to be fine pulsations/vibrations coming from the combustion, especially when the engine is cold and for the first hour driving the car.
- the pulsations seems to diminish after 1-2 hours (at least it feels that way).
- the pulsations can be felt through the seats when driving
- difficult to estimate the frequency .... I would guess 10-20 per seconds

I might stumble over the problem and the fix when following the k-jet testing manual, but I first wanted to check with you guys if this is a familiar problem.

If I were to guess .... maybe the fuel distributor (FD) is reaching its life ..... or the injection nozzles .... ?

Any tips to direct my assessments are much appreciated.

Well I would say 1 - 2 hours is a very long time to have warm - up related issues.  Usually your engine should come up to operating temp of 87C fairly quickly with sedate type driving within 5 minutes or so, depending of course on ambient temps.  It may still or course be a WUR or FD (or both) issue however you may need to describe more exactly what type of vibrations you mean.  Are you meaning like a misfire when idling, under load, or both?

Jan S:
I agree this is not a "normal" warm-up issue. The subjective feel is that the vibrations diminish after 1-2 hours, but maybe not ..... maybe I get used to it?

I did a new 30 min test drive this evening. The pulsations are apparently at their strongest around 50-60 miles/hour with a certain load on the engine (e.g. uphill).

The pulsations are fine, not dominant, and yes - it reminds me of slight misfire .... or constraints/variations in the fuel supply to the combustion chamber.

I didn't noticed this before the summer (again subjective feeling), and since then I haven't worked on engine related things, fuel supply etc. -- only suspension, steering, brakes, etc.

-- could it be the pressure damper sitting between FD and WUR ..... maybe the membrane is shot, which means the fuel pressure to WUR is not that stable, and the leak from the pressure damper leads fuel back to the air inlet, which makes the car run much richer I guess, which means revs should go down I guess?
-- the electronic ignition?

A lot of guessing at the moment :-)

 "1976 1979 Mercedes Continuous Injection System Diagnosis & Adjustment Manual" is in the technical library !!!  ;)


--- Quote from: TNNBENZ on 23 September 2021, 05:58 PM --- "1976 1979 Mercedes Continuous Injection System Diagnosis & Adjustment Manual" is in the technical library !!!  ;)

--- End quote ---
Yes, this. ^^^     I think this is, by far, the single most useful guide on the site as it applies to an enormous fraction of the issues discussed here.  Start at the beginning and work your way through in sequence.  Chasing specific problems such as this or jumping around will just make you nuts.


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