Doing without the 'Evaporative Loss" system

[S80]

Hi All,

Now that those long summer nights are here (!), I've had a chance to look at some 'problem areas' on my M104-engined W124 280E.

The first concerns the evaporative loss purge valve (on the LH inner wing). Despite repeated cleaning, it still has a habit of sticking in the 'open' state - giving rise to an unstable idle due to the admission of unmetered air.

My question is: notwithstanding any legal or 'green' issues, can the evap system be dispensed with by plugging the hose from valve to manifold and venting the charcoal canister to atmosphere?

I'm not sure whether the engine management system actively 'looks' for a change in fuelling when the purge valve is operated, or whether the closed-loop system just does its stuff whatever happens.

I have tried this out, and no fault codes are logged (I kept the valve connected electrically, so the ECU doesn't squeal on that front) - but will something nasty be happening elsewhere?:cry:

I'll keep an eye out for a suitable second-hand replacement valve - they seem a bit pricey new for what they are.....

 

[television]

You ask a hard question, but if the car runs fine with your mod, then thats OK and if its running sweet then nothing can happen, it it cannot damage anything

 

[roofless]

it is a good way to check if the valve is at fault in the first instance but as malc say's it's not clear if it's going to have any long term affect on the fuel system but most cars have been venting to atmosphear for years but i would guess it's a US emissions thing

 

[roofless]

some info http://www.youtube.com/watch?v=IzfR84T6pBY&feature=related

 

[S80]

Thanks Roofless - it's amazing what you can find on YouTube:-D

I don't think the EVAP system on the W124 is as 'clever' as the OBD-II variety - no tank vacuum (vac-you-um in the YouTube vid!) sensor for example.

The whole EVAP system seems like a lot of fuss about nothing, TBH

 

[roofless]

Thanks Roofless - it's amazing what you can find on YouTube:-D

I don't think the EVAP system on the W124 is as 'clever' as the OBD-II variety - no tank vacuum (vac-you-um in the YouTube vid!) sensor for example.

The whole EVAP system seems like a lot of fuss about nothing, TBH

i agree with you on that but the basic principle is the same

 

[Number_Cruncher]

It's a bad idea, because the fuelling will be affected. When the valve is open, the ECU is expecting some unmetered air and fuel vapour, and provides compensation for it.

As you've found when the valve sticks open, the ECU isn't expecting the unmetered air, and so, the engine does not run properly.

Despite the system having a lambda sensor, the lambda sensor doesn't (can't) have full authority over fuelling, and it is only there to allow small changes on either side of the set point based upon the ECU's open loop mappings, plus any ECU mapped corrections for throttle opening, EVAP valve state, engine temperature, etc, etc.

You might, so far, have not triggered a fault warning light, but, you will be pushing the lambda trim values off centre, and so, you might find lambda related fault codes eventually come up.

Keep looking for a good secondhand valve, and allow the system to work properly would be my advice.

 

[S80]

Some good points, N_C, though perhaps you could consider the following points:

1) If I disconnect the wiring connector to the valve, the ECU does detect this (throws a fault code). If the ECU 'knows' that the valve is disconnected, will it still apply any evap-specific modifications to the fuelling?

2) The volume of air/fumes drawn from the evap canister is small relative to that consumed by the engine in normal use.

3) Would this generation of ECU be able to determine the relative amount of fuel vapour present in the gases drawn from the canister?

I'll still keep an eye out for a good second-hand valve, mind

 

[Number_Cruncher]

Hi,

1) If I disconnect the wiring connector to the valve, the ECU does detect this (throws a fault code). If the ECU 'knows' that the valve is disconnected, will it still apply any evap-specific modifications to the fuelling?

No, but I'm not sure exactly what mapping the ECU will switch to - would the fault be considered serious engough for the ECU to switch to a limp home map? Off the top of my head, I don't know.

2) The volume of air/fumes drawn from the evap canister is small relative to that consumed by the engine in normal use.

Yes, but, the closed loop system can still be sensitive to small air leaks, as the lambda sensor is for making only small corrections from the map, a small error can be important.

3) Would this generation of ECU be able to determine the relative amount of fuel vapour present in the gases drawn from the canister?

Yes, indirectly via the lambda sensor. One problem with the valve being always open is that the richness of the vapour will decrease with time, where with the pulsed operation, as the valve opens, a fairly rich slug of vapour is available to be burnt - again, it will just mess up the fuelling, and will tend to make the car less drivable, more jerky on acceleration.

In summary, you'll probably get away with it!, but, the car will be better if repaired correctly.