R129 Exhaust Cat gone walkabout

[Rogerkuala]

1995 500SL car has a new stainless exhaust front to back but the first catalytic converter has been removed before I bought the car and replaced with two silencer boxes - this is Malaysia with no MOT tests and little about emissions!

The car has an issue with low power when it warms up. Lower rev power and response seems really "soggy" comapred to "crisp" when cold. The knock sensors are failing but I have eliminated a misfire by swapping out the EZL. My question is, will the absence of the front box cause a difference in back pressure for it to cause the ECU to cut back on power or any other symptoms?

 

[Petrol Pete]

Your drawing shows an exhaust sensor that is not fitted to your new silencers..has it been mapped out ?

 

[grober]

Your drawing shows an exhaust sensor that is not fitted to your new silencers..has it been mapped out ?

If that is the lambda sensor its absence may have an effect on fuelling. While the car is warming up its in open loop mode which does not use input from the lambda sensor. Once its warmed up however it switches to closed loop mode in which the lambda sensor acts in a feedback loop to adjust the mixture strength. This might explain the different running characteristics between hot and cold states

 

[Doodle]

From experience, I can tell you that the 500 will run surprisingly without complaint even when the lambda sensor is totally non-functioning. However, your AFR will be out by miles, which isn't going to make for the best characteristics.

The original systems are furiously expensive and were in short supply at one point, but dropping in a sport cat and reinstating the lambda sensor would be a good start.

 

[poormansporsche]

You could try buying a cheap rear lambda sensor bypass chip thing. Had one on a old car years ago

 

[ray_hennig]

but dropping in a sport cat and reinstating the lambda sensor would be a good start

The original picture looks very similar to the arrangement in my 300CE24. The O2 sensor is upstream of the Cat. So, I think you could install an O2 sensor again and that should enable the correct fuelling. I would certainly try that before installing another Cat.

Also, I suggest you measure the fuel delivery duty cycle (should be circa 50%), assuming that the system ever reaches closed loop which it sounds as if it does.

Good luck.

RayH

 

[Bellow]

My question is, will the absence of the front box cause a difference in back pressure for it to cause the ECU to cut back on power or any other symptoms?
]

It isn't only back pressure that affects engine performance but the presence/absence of pulses rebounding throughout the system when released on valve opening.
In your case, it is probable that your silencers present lower back pressure than the cat would. That is an assumption though.
Pulses are reflected on meeting changes in cross sectional area and reflect different polarities depending whether the CSA increases or decreases. The speed of the pulses in the pipe are temperature dependent (faster when hot, slower when cool) and thus their transit times (ie when they arrive back at the cylinder) vary with temperature.

Probably not your problem but you did ask!

 

[Rogerkuala]

From experience, I can tell you that the 500 will run surprisingly without complaint even when the lambda sensor is totally non-functioning. However, your AFR will be out by miles, which isn't going to make for the best characteristics.

The original systems are furiously expensive and were in short supply at one point, but dropping in a sport cat and reinstating the lambda sensor would be a good start.

I'm handicapped by the fact I have never driven, or even been in, another 500 so I am not sure what the normal response is. I have a 300, 12valve version but it's a totally different car in the way it feels. 500's are rare as hens teeth here and I know of only one I can try to borrow. I'll get there in the end and thanks for the suggestions!

 

[Rogerkuala]

The original picture looks very similar to the arrangement in my 300CE24. The O2 sensor is upstream of the Cat. So, I think you could install an O2 sensor again and that should enable the correct fuelling. I would certainly try that before installing another Cat.

Also, I suggest you measure the fuel delivery duty cycle (should be circa 50%), assuming that the system ever reaches closed loop which it sounds as if it does.

Good luck.

RayH

Really basic question here sorry, surely if a sensor was missing, the ECU/PCM would read it as a fault and report as such? The garage is not getting any error messages from the STAR that show anything other than knock sensor failure now. Engine seems to shift to closed loop condition based on the very predictable timing, circumstances and symptoms of the power loss. Beyond that is beyond my limited abilty to understand

 

[grober]

The sophistication of error reporting/adaptation on these early cars is pretty basic.
http://www.diakom.ru/CARS2/pribor/cs1000/manual/cs1000_mb.pdf In the absence of a lambda sensor the ECU may revert to open loop running for example or as mentioned it may have been fitted with some sort of faux feedback device. Bosch will have spent years of development on these injection systems to permit their efficient operation under varying conditions- low/high altitude- hot/cold temperature- low/high humidity- good /poor fuel quality etc then some bright spark comes along and says- you don't need a feed back loop just use a resistor to fool the system. Well -- what might work in a temperate climate may not work too well in the tropics for example and fiddling with the factory setup may have unexpected consequences in some environments?