Garage > Mechanicals

6.9 K-jetronic rough and high idle cold start

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Jan S:
Thanks to all of you for your assessments and tips - much appreciated.

To daantjie: the rough running is slight engine vibration at 1500-1600 rpm (noticeable more than when warm, warm is smooth). Yes, my assessment is that it's running a bit rich. Regarding the warm-up regulator: In April the engine showed the same running pattern as today. In April I measured the system pressure 5.2 bar (5.2-5.8 bar is the design spec), control pressure cold at around 10 deg C 1,0 bar (spec says 0.7-1.1 bar) and control pressure fairly warm 3.2 bar (spec says 2.8-3.2 bar NO vacum and 3.4-3.8 bar WITH vacum). That tells me the WUR was fairly ok in April. I haven't measured it recently. Yes, I would love to reduce it to 700 rpm, but I need to be making the right adjustments.

To Robert: good tip about the AAV. Is the cleaning and testing of the AAV a fairly straight forward DIY job, or can other issues emerge? I'm planning a longer road trip in 4-weeks time and I will not risk being stranded in my own garage waiting for parts etc. This might be a job for later this fall.

To s-class: yes, I probably has to get some professional help with the ignition, idle speed and warm running fuel mixture.

To Randys01: the rough running and high idle has been there from the start (two years ago when I bought the car). I started at the rear and replaced the entire fuel assembly. That fixed a lot, e.g. easy to start both cold and warm. Now I have to fix the correct idle speed and fuel mix when warm. I haven't measured the fuel pressures since April.

Next step: I believe I will start by checking the ignition, set the correct idle speed and fuel mix when warm. And see what happens with cold start.

You're certainly on the right track.  A couple thoughts:

1.  I'm no expert, but my experience has been that everything on these engines is interrelated.  Unless it's all right, nothing is right and it's very easy to go down the wrong path and make yourself nuts.  You already did the rear end of the fuele system.  So keep going:

2. Get this manual if you possibly can:  "1976 1979 Mercedes Continuous Injection System Diagnosis & Adjustment Manual."  I have it as a set of pdf files, can't remember where I got it.  Unfortunately files are too big to download.  It should be in the technical section but is not.  Anyway, it runs through the whole k-jetroninc testing sequence in an ordered way.  It changed my life, it's that good.  PM me with your email and I'll send it to you in bits.

3.  As suggested above, also check timing.  But I'd take it one step further.  Your distributor is very old and almost certainly filthy inside.  This can cause timing to be unstable, so clean it up.  Do a search, there's a detailed thread on the process.  It isn't very hard and afterwards you'll know it's working right.  The difference it made on my car was NOT small!

If I had done the above all at once instead of messing around bit-by-bit it would have saved me a lot of grief.  Stick with it and Good Luck!

Hi Jan,
To answer your question. Removing/testing/cleaning/lubricating the AAV implies no risk for your trip. It’s all quite simple and it doesn’t involve buying other parts. There’s a small gasket that goes with it that might be deteriorated, if that’s the case you can easily make one (we’re talking air at not very high pressure (vacuum actually) so no big deal) to ensure no uncontrolled air leaking in.
I insist, leave your WUR alone for now, your engine starts as it should and your fuel pressures (all 3) are where they suppose to be by spec. Timing, if not done already, is your next step after AAV to improve your low idle/cold issue. There’s a procedure for that (attached) which involves not only idle check but 1500 and 3000 rpm too. That’s important because it will tell you whether other components (i.e. vacuum advance, membranes, distributor springs/weights, etc) are doing it’s job. However, warning, I won’t open the inside of the distributor before your trip, it can be a pandora box if it’s your first time! It’s worth doing it but without pressure after your trip. The condition of the cap/rotor you can check, but I wouldn’t venture to get to the guts. Your low idle speed when cold issue is not coming from there anyway.
There are two styles of approaching these type of improvements: changing parts left and right and crossing your fingers is one. Some call that changing parts “by principle”.  If you feel like "praying" each time you change a part, there’s your confirmation you’re on style 1.
The second one is to understand the mechanics of a system and its components to be able to diagnose, test, root cause and change the right part(s) with certitude and without crossing the fingers. The advantage of the latter is the gaining in knowledge, the learning experience and the possibility to solve many issues in the future thanks to that know-how base. You’ll save hours of time actually. Both schools are valid, you need to understand which is yours.
Please keep us posted on the results.

.".but my experience has been that everything on these engines is interrelated.  Unless it's all right, nothing is right and it's very easy to go down the wrong path and make yourself nuts. "

Never a wiser passage has graced this Board................!! ::)

Jan S:
Great advice from all of you - thanks a lot!

Conclusion is I will start tackling this issue after my 5000 km road trip, i.e. it will be my fall job. Fingers crossed.

To Robert: I'm trying to be a style 2 guy, I like the learning experience  :) It worked well with the replacement of the fuel assembly - not enough fuel pressure (i.e. the pump was toast) and the system couldn't hold the pressure (i.e. the accumulator was toast). Fairly easy to diagnose after measuring the 3 pressures. Let's see how it goes this time  :-\

I will keep you posted, be patient.


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