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Discussion Starter #1
Hi all
Just acquired a 2001 in Anthracite. Being my 16th M5 i'm no stranger to these cars nor the common faults we experience with them. However this one has me stumped.
Symptoms:
-Poor MPG, around 15-18 maximum
-Very VERY strong 'rich' smelling exhaust and thick black sooty exhaust tips
-Definite lack of power
-Only codes are post-cat O2 sensors (both banks) (surely no effect on fuel trim?? just there to measure cat efficiency??)

New parts:
-Spark plugs
-Fuel filter
-BOSCH pre-cat O2 sensors

I have also done a MAF test and got 132 l/hr which I understand is healthy.

I have new BOSCH post-cat O2's on order and will be with me tomorrow but I don't hold out much hope for those being the cure.

Now, the car has a few mods. The main ones are Supersprint headers, and a custom exhaust from the downpipes back (still has cats). It is re-mapped by a reputable UK company and the map is only active in sport mode (i've only been using it out of sport mode so far).

I usually have a muffler delete on my M5's and have had the odd different system, but this one sounds extremely "burbly" and gives a distinct "dub-dub-dub-dub" thunder like an american muscle car. All well and good yes, but could this point to a bad exhaust CPS? Would a bad CPS cause mixture problems or just confuse the VANOS?

Anyway, M5 experts please chime in. I'm anxious to fix this ASAP as I've not been able to enjoy my new toy so far. Will post up pictures of it as soon as its behaving itself again.

Regards
Ben
 

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Hi all
Just acquired a 2001 in Anthracite. Being my 16th M5 i'm no stranger to these cars nor the common faults we experience with them. However this one has me stumped.
Symptoms:
-Poor MPG, around 15-18 maximum
-Very VERY strong 'rich' smelling exhaust and thick black sooty exhaust tips
-Definite lack of power
-Only codes are post-cat O2 sensors (both banks) (surely no effect on fuel trim?? just there to measure cat efficiency??)

New parts:
-Spark plugs
-Fuel filter
-BOSCH pre-cat O2 sensors

I have also done a MAF test and got 132 l/hr which I understand is healthy.

I have new BOSCH post-cat O2's on order and will be with me tomorrow but I don't hold out much hope for those being the cure.

Now, the car has a few mods. The main ones are Supersprint headers, and a custom exhaust from the downpipes back (still has cats). It is re-mapped by a reputable UK company and the map is only active in sport mode (i've only been using it out of sport mode so far).

I usually have a muffler delete on my M5's and have had the odd different system, but this one sounds extremely "burbly" and gives a distinct "dub-dub-dub-dub" thunder like an american muscle car. All well and good yes, but could this point to a bad exhaust CPS? Would a bad CPS cause mixture problems or just confuse the VANOS?

Anyway, M5 experts please chime in. I'm anxious to fix this ASAP as I've not been able to enjoy my new toy so far. Will post up pictures of it as soon as its behaving itself again.

Regards
Ben
If you dont know the history of CPS being replaced, then do all 4 and make sure your thermostat is in working condition, and not sticking open too early.

Also do a Vanos test to be safe. My M5 showed no codes, but my bank 1 failed the test. I had someone to replace all vanos rubber seals.
 

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Engine should reach 79* C (174.2*F), rather quickly (5-15min after cold start depending on conditions) and stay at least this temperature. If this is not the case, start with diagnosing the cooling system.
 

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Discussion Starter #4
If you dont know the history of CPS being replaced, then do all 4 and make sure your thermostat is in working condition, and not sticking open too early.

Also do a Vanos test to be safe. My M5 showed no codes, but my bank 1 failed the test. I had someone to replace all vanos rubber seals.
Will look into the VANOS. The thermostat is fine and it comes up to temperature fairly quickly.

Engine should reach 79* C (174.2*F), rather quickly (5-15min after cold start depending on conditions) and stay at least this temperature. If this is not the case, start with diagnosing the cooling system.
As above it reaches operating temperature as it should. The cooling system is in good condition throughout.
 

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MAF Sensor
 

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As I mentioned I've done a MAF test and saw 130+ l/h at the redline in 3rd gear

Cheers
Ben
The "MAF test" name is a misnomer. Its a measure of fuel flow. Because you are running rich, high fuel flow is expected. I suspect you have an issue with air measurement (MAF failure), fuel delivery or O2 sensors.
 

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Discussion Starter #8
The "MAF test" name is a misnomer. Its a measure of fuel flow. Because you are running rich, high fuel flow is expected. I suspect you have an issue with air measurement (MAF failure), fuel delivery or O2 sensors.
Always learning! Ok so under that assumption, if I unplug the MAFs, although it won't run perfectly, if some performance is restored then thats probably the culprit?

-Ben
 

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Always learning! Ok so under that assumption, if I unplug the MAFs, although it won't run perfectly, if some performance is restored then thats probably the culprit?

-Ben
Get some MAF cleaner (NOT carb cleaner) and clean the MAFs. If you see an improvement (which will only be temporary), then you have found at least 1 of the issues.

Regards,
Jerry
 

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I wouldn’t say 15-18 mpg was abnormal but it depends on driving style. I get ~17 mpg (imperial) town-driving quite aggressively. You are obviously used to getting better.

Can you read fuel trims? They’re a good clue to what’s going on. It’s quite rare for an M5 to run rich since there aren’t many ways it can run so rich that the fuel trims can’t bring the AFR back to the correct range, especially as your new pre-cat O2’s should be sending good signals to the DME.

And if the AFR was getting too rich for the trims to correct, you would be getting AE/AF or 90/91 codes, which you aren’t. (Unless you have run the car so little, it hasn’t had time to set the codes.)

It could be contaminated, partially plugged cats (stock cats, right?), which will reduce power big time. Normally you would get the B2/B3 codes for this but since your post-cats are bad, you wouldn’t. (I assume the codes you mentioned are specifically for bad sensors, and not the B2/B3 codes?). Is the smell a raw gas smell, a rotten egg smell, or eye-watering fumes?

MAF’s under-reading badly enough to max out the trims and cause real rich running would have brought in the AE/AF, 90/91 codes, so I don’t believe it’s the MAF’s, although running with MAF's disconnected as a test is so easy to do, why not.

Headers can affect the tune quite significantly and you can still drive aggressively in non-sport mode so I don’t know why the aftermarket tune would just work in sports mode. It’s quite common for a bad tune to cause weird fuel trim problems. You should talk to the tuner.

A bad VANOS/CPS (they go hand in hand) can definitely affect AFR’s, power and exhaust noise so a VANOS test is needed.

And never underestimate the sneaky way an M5 t/s can go bad and be quite hard to detect in warm weather. The key is the warm-up time. In 25C weather, it should only take about 5 minutes to come up to 79C. If it takes 15 minutes, it opening too much and overcooling.
 

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Discussion Starter #12
I wouldn’t say 15-18 mpg was abnormal but it depends on driving style. I get ~17 mpg (imperial) town-driving quite aggressively. You are obviously used to getting better.

Can you read fuel trims? They’re a good clue to what’s going on. It’s quite rare for an M5 to run rich since there aren’t many ways it can run so rich that the fuel trims can’t bring the AFR back to the correct range, especially as your new pre-cat O2’s should be sending good signals to the DME.

And if the AFR was getting too rich for the trims to correct, you would be getting AE/AF or 90/91 codes, which you aren’t. (Unless you have run the car so little, it hasn’t had time to set the codes.)

It could be contaminated, partially plugged cats (stock cats, right?), which will reduce power big time. Normally you would get the B2/B3 codes for this but since your post-cats are bad, you wouldn’t. (I assume the codes you mentioned are specifically for bad sensors, and not the B2/B3 codes?). Is the smell a raw gas smell, a rotten egg smell, or eye-watering fumes?

MAF’s under-reading badly enough to max out the trims and cause real rich running would have brought in the AE/AF, 90/91 codes, so I don’t believe it’s the MAF’s, although running with MAF's disconnected as a test is so easy to do, why not.

Headers can affect the tune quite significantly and you can still drive aggressively in non-sport mode so I don’t know why the aftermarket tune would just work in sports mode. It’s quite common for a bad tune to cause weird fuel trim problems. You should talk to the tuner.

A bad VANOS/CPS (they go hand in hand) can definitely affect AFR’s, power and exhaust noise so a VANOS test is needed.

And never underestimate the sneaky way an M5 t/s can go bad and be quite hard to detect in warm weather. The key is the warm-up time. In 25C weather, it should only take about 5 minutes to come up to 79C. If it takes 15 minutes, it opening too much and overcooling.
I think I read the fuel trims in some way or another on my generic diagnostic tool. One bank was a + figure and one was a - (+1.2 and -0.53 if I remember correctly).

The smell is just eye-watering fumes. No rotten egg or raw gas/petrol smell.

I ran it with the MAFs disconnected. Ran smooth but not as well. Reconnected and the car was definitely better, so can cross those off the list. Got a few CPS sensors out up to now (not had time to do them all just yet as I'm working on the M5 in-between jobs). They are dated 2001 so obviously never changed.

I've done countless thermostats and know for sure this one is a good one. Even took a live data reading and it was at 79 degrees exactly within 5-8mins.

-Ben
 

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I suggest you read the fuel trims again, short term and long term. As I said, they provide some good insight on this type of problem.
 

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I ran it with the MAFs disconnected. Ran smooth but not as well. Reconnected and the car was definitely better, so can cross those off the list. Got a few CPS sensors out up to now (not had time to do them all just yet as I'm working on the M5 in-between jobs). They are dated 2001 so obviously never changed.



-Ben
Changing the CPS was smart, yours were original so that is a no brainer.
Your comment on the Mafs disconnected maybe right but could be way wrong. Unless you had them disconnected for 500 miles then you were still using the adaptions from when they were plugged in so the car will not run as well if rich. Considering you said the car ran better when you plugged them back in, there may have been some change in the fuel trim to less rich which would be better. I would suggest you do the test again and pay very close attention to any changes through the miles.It is also worth reading the fuel trim before you unplug them and then ever 150 miles until you get to 600. Then see what happens when you plug them back in. Fuel trim numbers really help.

When you are doing the Mafs unplugged test you are also looking for air leaks, the differences in the way the car preforms can tell you whether you have leaks and where they maybe.
 

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It could be contaminated, partially plugged cats (stock cats, right?), which will reduce power big time. Normally you would get the B2/B3 codes for this but since your post-cats are bad, you wouldn’t. (I assume the codes you mentioned are specifically for bad sensors, and not the B2/B3 codes?). Is the smell a raw gas smell, a rotten egg smell, or eye-watering fumes?
so what does this tell us?
 

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so what does this tell us?
Just looking for clues. To me, eye-watering fumes means combustion is occurring on all cylinders but the AFR is way too rich so noxious fumes are being created during combustion. Pure fuel smell is a bad misfire with raw fuel being dumped out the exhaust, and the rotten egg smell is cats on the way out.

The OP's response supports his original diagnosis of running rich.
 

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Discussion Starter #17
Changing the CPS was smart, yours were original so that is a no brainer.
Your comment on the Mafs disconnected maybe right but could be way wrong. Unless you had them disconnected for 500 miles then you were still using the adaptions from when they were plugged in so the car will not run as well if rich. Considering you said the car ran better when you plugged them back in, there may have been some change in the fuel trim to less rich which would be better. I would suggest you do the test again and pay very close attention to any changes through the miles.It is also worth reading the fuel trim before you unplug them and then ever 150 miles until you get to 600. Then see what happens when you plug them back in. Fuel trim numbers really help.

When you are doing the Mafs unplugged test you are also looking for air leaks, the differences in the way the car preforms can tell you whether you have leaks and where they maybe.


Just looking for clues. To me, eye-watering fumes means combustion is occurring on all cylinders but the AFR is way too rich so noxious fumes are being created during combustion. Pure fuel smell is a bad misfire with raw fuel being dumped out the exhaust, and the rotten egg smell is cats on the way out.

The OP's response supports his original diagnosis of running rich.

Thanks for taking the time to read through my problem and help guys. Much appreciated.

Now what kind of figures should I be looking at for ideal fuel trims? I'll openly admit that the whole AFR diagnosis and analysis is not my strongest subject on car mechanics.

-Ben
 

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Mr Bomber, I have a used set of Mafs with less than 10k on them, purchased from Bimmerzone. Let me know if you want them, I'm down in London
 
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