Running lean, testing for air leaks

[Adam230K]

Hi,

Basically i've had the 'running lean' codes (2 of them) for a while now and i want to get rid of them before they develop into a misfire, i am inclined to think it is vacuum leak but i'm not quite sure how to test for this type of thing.

The car is running perfect and the only things i can spot are the following;

1. at very low rpm when i'm driving very gently, i can feel it 'stumble', or a bit of a judder if i try to build up revs slowly.

2. If i floor it from low down in 2nd gear its more noticeable, it will feel like its pausing for a split second and then it surges on through the rev range without any problems, its almost like a turbo lag (maybe i'm over-exagerating a bit, but i want to drive the poiint home).

I'm aware that these codes could be related to the 02 sensor (which has been checked by olly and seems to be on the slow side, but fine), or related to injectors or (hopefully) just an air leak.

What do u guys think? and how should i test an air leak and where are the hot spots?

 

[w124coupe]

Also could be dirty or on the way out MAF (if its fouled, it will under-read and lead to lean mixture).

I assume its a 230K - what mileage?

 

[Adam230K]

90k miles. The MAF seems to read fine when it was plugged into star though. The code says something like 'Self adaptation running too lean'

 

[lynall]

Here W124 MoT Emissions Failure - Mercedes-Benz Owners' Forums meybe some help?


Lynall

 

[Adam230K]

I think i'm going to check for lamda and o2 sensors... if they aren't expensive i'll replace them just to see if the codes go away... if not i guess i'll just leave it

I'm a bit an4l, i like my car to be nothing short of 100% perfect, and thats my downfall sometimes!

I'll have to wait til it develops into a physical problem if i cant solve it now

 

[vijilants]

Hi Adam,

I found Euros about the cheapest for the Bosch units. Also free nextday delivery on items over £50 and the wesite is cheaper than over the counter prices.

 

[lynall]

Dead sensor from a 2001 Bora might have something to do with the cat breaking up and the bits destroying it as they went past!

Fault was no output or similiar



Lynall

 

[nickcc101]

Good website for O2 sensor operation is www.autotap.com explains the operation of the sensor. American website that is aimed at the DIYer and mechanics. Seems that sensors should be changed between 60 and 100k miles depending on model of car.

 

[Dieselman]

Check MAS operation with a meter on pin5, check vacuum leaks either with a vacuum gauge or using carb cleaner sprayed round all joints and pipes, check the O2 sensor with a meter onto the black lead.

MAS should read 2v at idle rising to 4.5-4.8v at full revs, O2 sensor should alternate between 02v and 0.8v.
Perform both tests when driving to observe what happens under load.

 

[vijilants]

Check MAS operation with a meter on pin5, check vacuum leaks either with a vacuum gauge or using carb cleaner sprayed round all joints and pipes, check the O2 sensor with a meter onto the black lead.

MAS should read 2v at idle rising to 4.5-4.8v at full revs, O2 sensor should alternate between 02v and 0.8v.
Perform both tests when driving to observe what happens under load.

Now where have I seen this recently

 

[BlackC55]

You have had it re-mapped havent you?

These faults logged maybe because of that.

Is the inlet manifold leaking?

 

[Adam230K]

I'm going to need to check the inlet manifold somehow...

These lean fault codes existed long before the remap, thats why i'm not blaming the map...

Can one of you more clued up people explain to me why an air leak would throw a 'lean' code? If i think about it logically, lean means the engine is getting more air than fuel, but if there's an air leak wouldn't it be running too rich? or am i thinking about it in the wrong way?

 

[markmifsud]

yes, you have it back to front, your engine is running lean as its able to suck in additional air via the leak, rather than just the controlled flow via the manifold

 

[Adam230K]

So i'm looking for a leak before the MAF.... thats helpful since my MAF is right infront of my throttle body ...

 

[w124coupe]

So i'm looking for a leak before the MAF.... thats helpful since my MAF is right infront of my throttle body ...

No, you are looking for a leak AFTER the MAF - i.e. air that the engine is receiving that the MAF doesn't see and therefore doesn't read.

Remember the MAF tells the ECU the mass of air coming in so the ECU can balance it with the right injector timing for a correct mixture. The O2 sensor then allows the ECU to fine-tune the mixture by sampling the resulting exhaust gases.

I would still pop the MAF and check its clean - for the sake of 5 mins - if its dirty with oil from the crankcase breather or crap that's got past the airfilter (if its ripped or badly seated for instance) then it will under-read and cause lean running (but will still output a voltage that the ECU thinks is "feasible" and so no error code is shown). A leak before the MAF of un-filtered air will destory a MAF quickly from the impact of dust and grit on the sensitive sensing surface.

If its oil fouled (I think your earlier post said its fairly new?) then you may need to service or replace the crankcase breather system.

(as per other posts, when the engine runs, the column of air in the inlet pipe resonates because of the action of the valves as the engine runs. This means that excessive oil vapour from the breather - if the oil separator is clogged or faulty for instance - WILL be able to go backwards towards the MAF and foul it over time).

Using a can of carb cleaner as DM suggested is a good way to find leaks at the various inlet gasket points - the engine will stumble if it pulls in the cleaner along with the air.

You should also remove and then plug the mainfold ports for any vent and/or vacuum pipes to confirm they are not leaking "downstream" and allowing air in instead of maintaining a vacuum.

(by the way - you were very brave to take a re-map with "running lean" codes set - and so was the guy who provided the map.....)

 

[Adam230K]

Thanks for that, thats quite insightful.

Yes when i did remove the piping to put the MAF infront of the throttle body, it was very very dirty, with what looked like very old oil residues and some oil bits.

I'm a bit confused now because there isn't much AFTER my MAF, i'll show u guys a picture when i get home, but its literally a silicone hose from the MAF into a plastic pipe, into the throttle body. Sorry to sound stupid, but am i supposed to spray carb cleaner to clean it up only? i dont immediately understand exactly how this will help me check for leaks? If i spray it around the places that might be leaking, what will happen to indicate a leak? Do i have to rev it after i spray it to see anything?

Help is much appreciated fellas.

 

[markmifsud]

by spraying the carb cleaner you will end up with the engine sucking on the cleaner via the leak. So instead of sucking extra air, it will take and burn the cleaner so what you do it slowly spray all parts after the maf, including throttle body, inlet manifolds etc. move from section to section, when you get the spray on the leaking area, you will notice the engine behave differently as its now choking on the cleaner fluid for a few seconds

 

[w124coupe]

Just to be clear, it seems you have 2 modifications from standard (?)

1. Moved MAF from stock position to nearer the throttle body
2. Re-map

It would be useful to understand if the lean codes were there before both mods or not?

Can you post first 6 numerics from your VIN so we are sure of the engine etc you have?

The pipe from the air filter to the MAF needs to be spotless and so does the MAF. If they are not, then sort that out first. The piping can just be cleaned with detergent and water and then dried carefully . The MAF element ideally wants to be cleaned with a solvent for oil that doesn't dry to leave a residue - isopropyl alcohol meets the requirements and is a few quid from the local chemists (not Boots though, they don't seem to stock it).

Is the MAF the type that has a tube with mesh at the end and then an insert held by 2 screws?

Do you still have the crankcase breather entering the inlet AFTER the MAF?

 

[Adam230K]

The lean codes have been there since sept last year if i'm to be honest, but the car has never acted strange in anyway just didnt feel as eager as normal, only recently that 1 code has now turned into 2 seperate lean codes;

050 - Idle self adaptation too lean

Lower partial self adaptation too lean (or something like that?)

These were there before i modified the position of the MAF or the ECU Software.

I'm going to go home and give it a go with the carb cleaner to see if there's any leaks and i'll also just give the pipes a clean if needed, at least that way i can check if they've built up any filth since the last time i took it apart and give me an indication as to how the breather is doing.

EDIT: this is for you w124;

its the M111.975 engine, the earliest kompressor engine there was.

The MAF is the old type, circular connection, and its a tube with mesh at the end and held in by 2 screws, as u mentioned.

 

[andy27168]

Hi,
What fuel are you running it on?