M104 Duty Cycle thoughts ...

[ray_hennig]

Just a quickie:

My 300ce-24 has a catalyst.

Cold start excellent and smooth, as always.

It has some hesitation problems when warm so I checked again the duty cycle, fully warmed up.

Ignition on, engine off - 50% solid.

Ignition on, engine idling - 50% +/- 4 or 5%. - settles back to this after revving.

Does this sound right?

Thanks in advance.

RayH

 

[grober]

This of any help? PeachPartsWiki: Mercedes-Benz Driveability

 

[kth286]

coolant sensors go out of spec and will feed wrong signal back to ECU which in turn will cause under/over fuelling for the actual engine temp.

 

[ray_hennig]

coolant sensors go out of spec and will feed wrong signal back to ECU which in turn will cause under/over fuelling for the actual engine temp.

Things rings true for mine. Last year I adjusted Duty Cycle and noted the following:

Ignition on, engine off - around 75% solid.

Ignition on, engine idling - 47% +/- 3-ish%.

I wonder is somethings's failed to cause the engine off value to alter?

RayH

 

[Number_Cruncher]

Over longer time periods when the engine load and speed aren't altering much, a working lambda sensor and a fundamentally operational fuel system will converge back towards a reasonable duty cycle.

When you ask the engine to make a rapid change, however, the lambda sensor is almost useless, and the other sensors around the system provide some "open loop" information to bring the injection volumes close enough to allow good performance.

I say "open loop" in quotation marks, because the system doesn't necessarily work in open loop mode - more a case of using the open loop fuelling values with closed loop corrections from the lambda sensor.

Consider an engine managed solely by the lambda sensor. After you put your foot down, opening the throttle, the only message the lambda sensor can give is "weak!", it can't say how weak, or how help provide any meaningfull estimate of required fuel volumes - the engine would be near enough undrivable.

 

[alexander patie]

must echo number crunchers point about the narrow lambda range, i use the analogy of trying to balance on a knife edge - this is why in closed loop we see the lambda values fluctuate above and below stochiometric, the ecu cannot balance on the thin edge.

i think on the cat cars the duty cycle output should actually read lambda voltage when all sensors are ok and the ecu sees no faults. on earlier cars 50% denotes all is ok.

ray, the duty cycle read form the socket is usually stable, are you reading duty cycle, or rms voltage and doing a calc?

 

[ray_hennig]

ray, the duty cycle read form the socket is usually stable, are you reading duty cycle, or rms voltage and doing a calc?

I am reading duty cycle and it fluctuates around an estimated mean of 50%.

Lambda sensor was replaced 18 months ago.

Sould the 'engine off' value be 50% or 75%?

I recall mine was 75 and is now 50.

Thanks all for your interest.

RayH

 

[alexander patie]

sorry ray, i am not sure without googling it, but if you spend 15mins with our friend you will have all the answers.

 

[ant500sl]

Engine off value is a fixed value of approx 70%, this denotes that there is no speed sensor input.

50% with engine off would either be a fault in the self diagnostic system or a wiring problem.

To test turn ignition on but do not start the engine. Press the accelerator pedal to maximum which will open the wide open throttle switch. As the engine is not running the system should change to another fault number which denotes a problem with the throttle switch. If there is no change then there is a fault with the self diagnostic system or more likely a wiring issue.

Perhaps something was disturbed when the lambda sensor was changed?