R129 SL500 (1996) Failed Emissions

[DanMorgan]

Hey,

My R129 SL500 failed its MOT recently due to its emissions.

Is there anything obvious to check for this - I gave it a good old Italian Tune Up but this didn't help.

Looking at replacing the Lambda, does this SL have 1, 2 or 4 sensors, I have read many conflicting statements!

Cheers,

 

[alabbasi]

Looking under the car will tell you whether it has a pre and post cat 02 sensor. What did it fail on?

 

[DanMorgan]

The car has two sensors from looking underneath.

I have took it there again and the results were the same after taking it on a two hour drive as well as using some high quality injector cleaner.

I have also checked on the STAR machine that the lambda sensors are all working and they are passing too.

Has anyone got any ideas at all or experienced similar? The tester said that its unlikely to be the cats because the hydrocarbons (HC readings) were low.

 

[Doodle]

Post your emissions results, that will give a better idea of where things are going wrong.

 

[DanMorgan]

Idle Test:

CO = 0.24% (pass, limit 0.5%) (PASS)

Fast Idle Test:

CO = 0.37% (fail, limit 0.3%) (FAIL)
HC = 112ppm (pass, limit 200ppm) (PASS)
Lambda = 1.06 (fail, range 0.970-1.030) (FAIL)

Second Fast Idle Test:

CO = 0.32% (fail, limit 0.3%) (FAIL)
HC = 60ppm (pass, limit 200ppm) (PASS)
Lambda = 1.06 (fail, range 0.970-1.030) (FAIL)

 

[DanMorgan]

It only slightly failed (I think?) but I have tried all of the "usual" recommendations to lower the emissions prior to the MOT.

I have been told by multiple people that if the HC is low, it's unlikely to be the cats - would this statement be true?

 

[Doodle]

What does the temperature gauge sit at when the car is up to temp? I had results like this when the temp sensor on the top of the block died.

 

[DanMorgan]

Usually sits solidly at 80 (middle) once its up to temperature

 

[Stratman]

The lambda value measured during an MOT is different to a car's lambda sensors. The car's sensors directly measure the amount of oxygen in the exhaust and the ECU tweaks the fuelling appropriately. During an MOT the test kit calculates a value based on the relative proportions of different gases in the exhaust. A value greater than 1 would indicate a weak mixture. Air leak in the intake?

 

[E55BOF]

But if there was an intake air leak, wouldn't correctly functioning O2 sensor(s) detect the weak mixture and compensate for it?

 

[Stratman]

^^^You'd think so, but evidently they're not. I don't know the fuel trim range the ECU can cope with, perhaps 1.06 is its limit? Whatever's going on, there's too much air or not enough fuel getting into the engine which isn't being corrected by the ECU, so I'd be checking the basics first. Air leak, clogged fuel filter, perhaps incomplete combustion (ignition and spark plugs), stuff like that.

 

[zipdip]

Well make certain that you have no air getting into the enging that should not,if that was my car I would let the fuel get down to a quarter of a tank and put a whole bottle of cleaner in it somewhere quiet like a industrial unit or the like,and then rev and drive it in low gears around,I would find a test centre you can drive too and stay with the car running it all the time,you might want to put some good fuel in it as well,and then get the MOT there is nothing like having a hot engine for passing emmissions and you do not have to improve that much to get a pass.