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6.9 K-jetronic rough and high idle cold start

Started by Jan S, 10 June 2021, 05:36 PM

Jan S

I have a 1978 6.9 engine with K-jetronic (and a manually controlled cold-start valve, i.e. I activate the cold start valve by pushing the button for 2-3 sec before I start cold)

- The car starts immediately, cold and warm.
- However, cold start is a bit rough, idle is 1500-1600 rpm
- After 10-15 min engine runs smooth, idle is 900-950
- The ambient temp is 68-77 deg F/ 20-25 deg C.
- In April I checked the fuel pressures (system pressure, control pressure), they were ok (my entire fuel assembly at the rear is new)

Sounds familiar?

Could it be aux air valve, spray pattern injectors, vacuum leaks, ......?

A few tips would be great.
1975-mod W116 450 SE with 6.9 engine


You might have to define what you mean by "rough running".  Sounds like overly rich?  In my experience they tend to run a bit rich at warm up but 1500-1600 does sound a bit high at those ambient temperatures.
Sounds like you might have to take a look at the warm up regulator action.
With engine fully warmed up your idle should be lower @ 600 - 700 rpm.
1977 450 SEL 6.9 - Astralsilber


Very likely it's your AVV that's a bit stuck and not fully open when cold. Yes, the system is designed to run rich when cold for about 3-5 minutes. But if the AVV is working the engine will run at around 2500 rpm "hiding" the roughness of the engine running rich. There are some accounts that the AVV is reparable. I have simply removed them, cleaned  them and lubricated the visible part of the internal piston. It should slide freely when you submerge the bulb is a bucket with water and ice and immediately put it in a bucket with hot (90C) water. You'll see the piston moving to both extreme positions then you can lubricate the assembly. If your piston doesn't move with the practical test above or its displacement is limited to 2-4 mm then the hypothesis of a misbehaving AVV is confirmed. I have seen that a few times.
Your engine starts as it should cold and hot, so I would not mess with the WUR as it's doing its job just fine. However, you can play with your 3mm allen screw on top of the FD to fine tune the mixture cold/hot. That should help, but if the AVV is stuck that won't change matters significantly.
I hope all that makes sense.
Good luck

s class

The fact that you have a differential in idle speed from 1500 cold to 900 warm tells me the AAV is most likely OK.  I would regard a differential of 600rpm to be close to ideal. 

I would start with the basics - check and set ignition timing, make sure the vac advance is working.  Then reduce the warm idle speed to 650 and set the warm running fuel mixture properly using a decent exhaust gas analyser.  This will affect both cold and hot running.  It sounds to me like you have it set to rich.

[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL


I get the impression this erratic running has come on quickly rather than an observation over many weeks or months?
If indeed it's come on quick I would target air leaks in the inlet tract. Costs nothing..split hose..something's dropped off.??
The engine seems to be running 200 rpm more than it should at both cold n hot. Reduce hot idle to 700 and see next day what effect it has on the cold start idle. if it is still hi at 1500 double check your cold control pressure at the WUR. You said they were fine a few mths that still the case?

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.
1975-mod W116 450 SE with 6.9 engine


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!
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0


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.
1975-mod W116 450 SE with 6.9 engine

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.
1975-mod W116 450 SE with 6.9 engine


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?
1977 450 SEL 6.9 - Astralsilber

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 :-)
1975-mod W116 450 SE with 6.9 engine


 "1976 1979 Mercedes Continuous Injection System Diagnosis & Adjustment Manual" is in the technical library !!!  ;)
Neglected ~ Rescued ~ 1977 450SEL 4.5L  U.S.A. ver.
Project of Compassion ~ Respect
Silver Green Metallic / Toast
Left Hand Drive
Sliding Roof


Quote from: TNNBENZ on 23 September 2021, 07:58 PM
"1976 1979 Mercedes Continuous Injection System Diagnosis & Adjustment Manual" is in the technical library !!!  ;)
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.
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0