BMW e46 engine fault code 171, and lambda sensor

DiarmaidGNR

Hi all,

The engine management light came on in my (2000 E46 318i) dash yesterday (a few hours after getting the lambda sensor replaced).
I brought it back to the garage today to get the diagnostic scan done to see if it was the lambda sensor again, - but it wasn't, it was a new fault code "171" which is translated into something along the lines of - "air fuel mixture problem" or "throttle control unit".

Anyone have any ideas?

Could this code be because of the replaced lambda sensor? - The new sensor came out of a scrap yard.

Also, the mechanic said that a NEW pre-cat lambda sensor could cost over €200, is this true? - I've seen them on ebay new for about €60.

crosshair1

This is the 1.9 engine right?
Assume the fault code you have is P0171, system too lean.
Possible causes for this particular code on this car are intake leak , pcv valve , MAF, O2 sensor.
A s/h o2 sensor is not ideal as any hard knock renders them useless.
I would not put any non genuine sensors on this as an attempt to remove the code, from experience it will not clear.
The most common fault is the breather pipe from rear of rocker cover to PCV on manifold, this rots through allowing unmetered air be drawn into inlet,
next to this is MAF, again non genuine will drive ok but P0171 will return believe me.
BTW this should not be rocket science for your mechanic, It may involve a bit of time spent diagnosing the exact problem but should not turn into a "lets replace this and see what happens"

CaraFawn

+1 what crosshair1 said

Personally I would question this lambda sensor. Buying used lambda sensors is not the best thing to do. I got mine, brand new Bosch from local factor, for less than 100 euro.

In the mean time, check for cracked hoses around MAF etc. You can use some soapy water solution and put some around the hoses to see if they make bubles...

Either way fix it quick, if this is a faulty pre-cat lambda sensor, it will dramatically increase your fuel consumption (Like 50%) and will damage your cat too.

DiarmaidGNR

Thanks for the great advice!!!

DiarmaidGNR

crosshair1 wrote: »

This is the 1.9 engine right?
Assume the fault code you have is P0171, system too lean.
Possible causes for this particular code on this car are intake leak , pcv valve , MAF, O2 sensor.
A s/h o2 sensor is not ideal as any hard knock renders them useless.
I would not put any non genuine sensors on this as an attempt to remove the code, from experience it will not clear.
The most common fault is the breather pipe from rear of rocker cover to PCV on manifold, this rots through allowing unmetered air be drawn into inlet,
next to this is MAF, again non genuine will drive ok but P0171 will return believe me.
BTW this should not be rocket science for your mechanic, It may involve a bit of time spent diagnosing the exact problem but should not turn into a "lets replace this and see what happens"

Update on the code, turns out the code was on the diag report I got done before changing the sensor, the sensor was a different code (now gone) The original code:- "171 Check, Fuel supply system (EUIII code: p0170"

crosshair1

Would treat a P0170 just the same its triggered by the fuel trims (ecu's own fuelling adjustment to remedy fault) reaching their limits.
Basically the ecu is adding or reducing fuel to attempt to have an ideal air/fuel mix but has added/reduced as much as it can and still doesnt achieve control therefore triggering EML.

DiarmaidGNR

crosshair1 wrote: »

Would treat a P0170 just the same its triggered by the fuel trims (ecu's own fuelling adjustment to remedy fault) reaching their limits.
Basically the ecu is adding or reducing fuel to attempt to have an ideal air/fuel mix but has added/reduced as much as it can and still doesnt achieve control therefore triggering EML.

You clearly have an in depth knowledge!!

I posted this on e46fanatics.com -

http://forum.e46fanatics.com/showthread.php?p=11835629#post11835629

The link has a video clip of engine noise if that is of any use to you!

crosshair1

If you remove the cover over the pollen filter, 3 clips (at the rear of the engine under front window) take out filter, remove 4 torx screws and remove housing you will see a pipe going into the top of the engine check that there is no crack/deterioration at either end of this.

DiarmaidGNR

crosshair1 wrote: »

If you remove the cover over the pollen filter, 3 clips (at the rear of the engine under front window) take out filter, remove 4 torx screws and remove housing you will see a pipe going into the top of the engine check that there is no crack/deterioration at either end of this.

Thanks! - You are extremely helpful! I will do that first thing tomorrow - (depending on severity of hangover!)

shamwari

Regarding above comments re Lambda sensors, I agree fully with Crosshairs advice and diagnosis, especially regarding 2nd hand sensors not being a great way to go. As he says, some cars don't work right if you fit a replacement sensor, even if it is manufactured to OEM standard. Certain Honda's and Toyota's are a case in point. The original part from the main dealer is often the only way to go. I'm not sure if this applies to Beemers though.

Also beware of Ebay, there's a lot of sh!te up for sale there in the guise of genuine parts.

CaraFawn

shamwari wrote: »

Regarding above comments re Lambda sensors, I agree fully with Crosshairs advice and diagnosis, especially regarding 2nd hand sensors not being a great way to go. As he says, some cars don't work right if you fit a replacement sensor, even if it is manufactured to OEM standard. Certain Honda's and Toyota's are a case in point. The original part from the main dealer is often the only way to go. I'm not sure if this applies to Beemers though.

Also beware of Ebay, there's a lot of sh!te up for sale there in the guise of genuine parts.

It depends what brand you go with.

If you go with an aftermarket one that was designed for this particular car and equipped from factory, then you will be grant and even better off since BMW for example only provides a 2 year warranty on parts, while the original manufacturers often provide much better warranties (10 years, lifetime etc) for exactly the same part.

You learn over time what was used in your car etc. and so you know what brand is good or not for your car. When you service your car and change parts, you see the brand log stamped on it along with the car manufacturer logo.

Now of course some specific parts will only be made by the car manufacturer, but most of them are made by 3rd party companies.

shamwari

As I said, some cars wont work right even if you buy an non-genuine one that is manufactured to OEM spec. Have seen cases with some Toyota's and Honda Civics where you can buy an OEM one that swears blind in the catalogue that it will work.....and then the funny fault codes appear which don't add up that tell you otherwise.

Some makes are ok and some aren't. On the plus side. I have used a Bosch OE replacement for a Marelli sensor used in my sisters Stilo. No probs with that in the last 6 months thankfully...

DiarmaidGNR

crosshair1 wrote: »

If you remove the cover over the pollen filter, 3 clips (at the rear of the engine under front window) take out filter, remove 4 torx screws and remove housing you will see a pipe going into the top of the engine check that there is no crack/deterioration at either end of this.

There is mild deterioration to this pipe (slightly soft to the touch) but there are no cracks to be found.

crosshair1

With the engine running remove the oil cap, is there a change to the engine note? Is the cap sucked back down?

DiarmaidGNR

crosshair1 wrote: »

With the engine running remove the oil cap, is there a change to the engine note? Is the cap sucked back down?

The is a change in engine note (like a hole in the exhaust sound), and the cap gets sucked back in.

crosshair1

If the cap is difficult to pull away from the engine that means there is full vacuum in the engine and most likely is the pcv (valve) at fault

DiarmaidGNR

crosshair1 wrote: »

If the cap is difficult to pull away from the engine that means there is full vacuum in the engine and most likely is the pcv (valve) at fault

The cap is not difficult to pull off so the might be a vacuum leak somewhere. Can I just use any hose to replace the lines, - or do I need to get the part numbers etc. and get the proper ones? - How much are they anyway?

I bought this yesterday - http://cgi.ebay.ie/002-BMW-Car-Diagnostic-Scanner-Tool-1-3-6-1-36-PA-SOFT-/220616008177?cmd=ViewItem&pt=UK_Diagnostic_Tools_Equipment&hash=item335dbd21f1

So hopefully it will help me out a bit.

crosshair1

What was the original fault with this car that determined that the o2 sensor was to be replaced. Has the o2 sensor voltage been checked by the mechanic in live data?

DiarmaidGNR

crosshair1 wrote: »

What was the original fault with this car that determined that the o2 sensor was to be replaced. Has the o2 sensor voltage been checked by the mechanic in live data?

There was a separate fault code for the O2 sensor - P0130. - now gone.

I don't think live data was checked.

Do you know of any links where I can get live data from a car working 100%? (so I have something to compare to)

Also, I am going to change the vacuum hose you mentioned before (it's on its way out anyway) - Is this a good price? -

http://cgi.ebay.ie/BMW-CRANK-CASE-BREATHER-HOSE-M43-E36-E46-Z3-GENUINE-/270576755852?cmd=ViewItem&pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3effa1a48c

And would it be a good idea to change the ccv valve at the same time? -

http://cgi.ebay.ie/BMW-CRANK-CASE-BREATHER-VALVE-HOSE-E36-E46-Z3-NEW-/270576757293?cmd=ViewItem&pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3effa1aa2d

crosshair1

From memory I dont think those parts are that much more expensive in the dealers here.
If you dont have full vacuum to the engine then replacing the pcv is not necessary. The pipe I certainly would change.

I'm not sure if its even viable to attempt to fix this without the necessary equipment, not just diagnostics.

If this were in my workshop I would carry out the following
Pressure test intake for unmetered leaks
Pressure test exhaust for leaks before o2 sensor
Vacuum gauge on inlet to confirm mechanical functions
Scope the o2 voltage vs MAF Voltage to confirm that the o2 is capable of reacting and that the MAF voltage is within range.

Replacing components based on the fault code alone I think at best would give 50% success, this is why a lot of mechanics would shy away from a p0170 as its definition doesnt refer to a specific part being faulty
I'm not trying to knock you back on this but once Ive connected diagnostics and see a p0170 then thats pretty much as far as I can go with diags. and have to rely on other tests to determine the fault.

DiarmaidGNR

crosshair1 wrote: »

From memory I dont think those parts are that much more expensive in the dealers here.
If you dont have full vacuum to the engine then replacing the pcv is not necessary. The pipe I certainly would change.

I'm not sure if its even viable to attempt to fix this without the necessary equipment, not just diagnostics.

If this were in my workshop I would carry out the following
Pressure test intake for unmetered leaks
Pressure test exhaust for leaks before o2 sensor
Vacuum gauge on inlet to confirm mechanical functions
Scope the o2 voltage vs MAF Voltage to confirm that the o2 is capable of reacting and that the MAF voltage is within range.

Replacing components based on the fault code alone I think at best would give 50% success, this is why a lot of mechanics would shy away from a p0170 as its definition doesnt refer to a specific part being faulty
I'm not trying to knock you back on this but once Ive connected diagnostics and see a p0170 then thats pretty much as far as I can go with diags. and have to rely on other tests to determine the fault.

I was actually looking into getting my hands on a smoke machine to try and find any leaks, but could I just use a normal compressor and see if I can hear the leaks? (maybe use some soapy water as well) - or is it a bad idea?