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M103, CIS and Injector Health

4K views 5 replies 3 participants last post by  LWB250 
#1 ·
I have a question regarding CIS related to the condition of fuel injectors on my M103:

I checked duty cycle and other related items today on the car. Duty cycle was pretty much on the numbers, maybe just a little low at 46.95 % at idle and 48.5 % at 2500 rpm, which according to the information I have is pretty good. Close enough that I see no reason to fiddle with any settings unless there’s an expert in the house that can tell me why I would need to change it.

However, I’m wondering how I could determine how well my injectors are doing by these values. That is, if I had a leaky or dribbly injector, or injectors that had a bad pattern, would that be reflected in the duty cycle, that is, would the system attempt to correct for overfueling, and if so, how that might manifest itself.

Drivability is fine with no performance issues whatsoever. Idle is not 100% smooth, as I can feel little “bumps” in the idle at a stop or just sitting parked out of gear. This is what has me wondering is I might have a injector or two that aren’t performing well. I’m assuming the injectors in the engine are at least 100k miles old, if not more. I know the head was redone at around 180k, but I have no idea if the injectors were replaced at that time.

TIA,

Dan
 
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#2 · (Edited)
Just like the O2 sensor, the injectors are up for replacement after each 100K mile mark, along with the seals and cups.

No amount of injector cleaner can fix them since they are fully mechanical.

The only real test you can do is attach each injector to a pop tester and check the spray pattern and/or leak pattern

I'd replace the O2 sensor first with a new correct Bosch unit (not the universal splice type) and recheck duty cycle before doing injectors and seals
 
#3 ·
New Bosch (OEM, not universal) O2 sensor was installed a week ago.

I realize that injector cleaner is more or less snake oil as far as a fix. That's not even a consideration. I'm just wondering if the system compensates for overfueling that might occur as a result of a leaky or poor injector. Since it's a closed feedback loop of sorts I would assume it does, using the O2 sensor voltage as one of the references.

Dan
 
#4 ·
... I’m wondering how I could determine how well my injectors are doing by these values. That is, if I had a leaky or dribbly injector, or injectors that had a bad pattern, would that be reflected in the duty cycle, that is, would the system attempt to correct for overfueling, and if so, how that might manifest itself ...
Questions as smart as these certainly deserve appropriate answers. :)

But unfortunately I’m too short of time right now to explane in detail how such an overfueling would manifest itself (in the duty cycle reading). I do, however, want to take the time for this answer:

The system would try to compensate for overfueling that might occur as a result of (a) bad injector(s) … which it might do successfully as long as the ECU’s ‘Lambda leaning limit’ (on your car -10 mA) via EHA is not exceeded.

In your case you can conclude that if
  • the setting of the Lambda adjustment screw is the same as it was when the car was new
  • and the fuel pressure (system pressure) is within spec
  • and you’re using fuel that contains maximum 5% ethanol
  • and the duty cycle at idle fluctuates with a constant frequency of ~ 0.5 Hz and an amplitude of ~ +/- 2% around ~ 47%
  • and the duty cycle at 2500 rpm fluctuates with a constant frequency of ~ 1 Hz and an amplitude of ~ +/- 2% around ~ 48.5%
  • and the aspirator line of the fuel evaporation system was blocket during the duty cycle check
your injectors are probably okay ... (unless there’s a false air leak that exactly compensates the overfueling caused by (a) bad injector(s) … what are the odds?)

If the setting of the Lambda adjustment screw has been changed, it depends on how far in which direction it has been changed in relation to it’s original setting, in order to make a statement about the injector’s condition (based on the duty cycle values). … If you don’t know whether / how the setting has been changed, it’s difficult to do that.

This, btw, illustrates that, in case of duty cycle deviation, it’s much better to start a logical search for its reason, instead of a thoughtless search for the 3 mm Allen wrench. :wink_2:

In this context I’d like to call attention to what I said about checking / correcting the duty cycle under “Please note” in post 2 of http://www.benzworld.org/forums/w12...97-ke-jetronic-lambda-control-duty-cycle.html

H.D.
 
#5 ·
Thank you for your response. I can completely understand, and as someone trained as a diagnostician and not a "parts changer", I fully grasp the direction of your post.

My next step is to fully inspect the injectors and induction system for leaks. I'll go around the injectors to see if any of them have leaky seals first, then look at areas such as the boot between the fuel distributor and intake.

I'm pretty well convinced at this point that the source of my mildly rough idle is a vacuum leak. I have no desire whatsoever to alter the duty cycle of the fuel system, as I suspect that would merely serve to mask the actual issue.

I'll continue to pursue other possibilities as I gather more information. For example, I am just now running through my first full tank of fuel under normal driving conditions, which will hopefully provide some insight as to fuel mileage/consumption. This will further define where to look for issues once I have the results.

Again, thank you!

Dan
 
#6 ·
Upon further investigation today I found a vacuum leak - my EGR does not hold vacuum. I have capped that line off for now.

Using brake cleaner I went around to all injectors, cold start valve, boot between the fuel distributor and intake manifold, etc., etc., and checked for leaks, none of which I found.

While the car does seem to idle a little smoother with the EGR out of the circuit, I realize it's not in play 100% of the time, so I'm not calling this issue solved just yet.

Dan
 
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