What causes a car to run rich? - The Unofficial BMW M5 Messageboard (m5board.com)

aaronmiles · 26th October 2007, 02:07

Hi all,

Im trying understand what my car is doing and Im in need of some help if possible.

I checked my car the other day with my obd2 reader and found that it was running at about -10 rich correction. I replaced the MFS and the air filters and using INPA reset the fuel adaptions back to 0.0. Drove the car to work today ( about 40-50kms) and its back to -5.3 so I doubt it was the MAFS.

The O2's and sparkplugs are about 40,000 - 50,000 KMS old so I doubt they would be on the way out already? FYI, the plugs were OEM NGK BKR6EQUP

any thoughts and suggestions as I am at a loss....

Cheers,
Aaron

gsfent · 26th October 2007, 02:58

Quote:

Originally Posted by aaronmiles

Hi all,

Im trying understand what my car is doing and Im in need of some help if possible.

I checked my car the other day with my obd2 reader and found that it was running at about -10 rich correction. I replaced the MFS and the air filters and using INPA reset the fuel adaptions back to 0.0. Drove the car to work today ( about 40-50kms) and its back to -5.3 so I doubt it was the MAFS.

The O2's and sparkplugs are about 40,000 - 50,000 KMS old so I doubt they would be on the way out already? FYI, the plugs were OEM NGK BKR6EQUP

any thoughts and suggestions as I am at a loss....

Cheers,
Aaron

I won't be much help.

Assuming the DME is OK, and plugs and O2 are OK, only thing that comes to mind is that air that is being "read" by the MAF's is not getting into the intake.

I guess there could be a leak where the two hoses go from the MAF housing to the plenum. Each one has a little rubber gasket. Could one be worn/missing?
Regards,
Jerry

jclyman · 26th October 2007, 03:08

I was a mechanic BEFORE the advent of O2 sensors ... but, it sounds like an O2 sensor to me ... just guessing, though ... I believe when an O2 sensor fails the car goes rich (by design) so it doesn't burn a piston.

aaronmiles · 26th October 2007, 03:10

Im going to pull one of them out tonight and see what it what (not that there is a lot to see... but ill take photos none the less)

no codes are being thrown interestingly enough

gsfent · 26th October 2007, 03:14

jclyman's post triggered another thought.

There has been some discussion that the post cat O2's affect fuel trim. You might do a search on "fuel trim".

Were all 4 O2's replaced?
Regards,
Jerry

THRILOS5 · 26th October 2007, 05:11

check your coils also, they are weak and they tend to fail often on BMW's
I had a similar problem last summer and it drove me crazy, but after a lot of trial and error it came down to bad coils, when I took them out you could see where the spark was arching (jumping) on top of the coil, if you look closely you'll see the blueish burn marks on them

chowdah · 26th October 2007, 05:18

bad thermostat could create a rich condition.

aaronmiles · 26th October 2007, 05:22

Quote:

Originally Posted by chowdah

bad thermostat could create a rich condition.

Water temp is normal (luckily)

aaronmiles · 26th October 2007, 12:12

Coolant temp is 79-80c which is perfect... o2 sensors are definatly black (which confirms richness I think) but they should still function... they dont look like they are 'blocked' as such.. im happy to replace them of course but i dont know if it will help.... no air leaks it would appear but i will pull the intake apart tonight to make sure.

both sensors were the same so it does not appear to be a 'bank' issue, but more over an entire system issue?.... im going to check plugs tonight

Could the DME be screwed? (doubtful I would think)

jclyman · 26th October 2007, 18:09

Quote:

Originally Posted by aaronmiles

Coolant temp is 79-80c which is perfect... o2 sensors are definatly black (which confirms richness I think) but they should still function... they dont look like they are 'blocked' as such.. im happy to replace them of course but i dont know if it will help.... no air leaks it would appear but i will pull the intake apart tonight to make sure.

both sensors were the same so it does not appear to be a 'bank' issue, but more over an entire system issue?.... im going to check plugs tonight

Could the DME be screwed? (doubtful I would think)

The blackness could be a sign of richness, or slow driving or extended idling prior to removal, or normal (I'm not sure that tells us anything). You haven't mentioned if your ACTUAL gas mileage has changed recently. If it has dropped, that would certainly indicate richness (along with some other potential problems).

The only O2 sensors I have replaced (and examined) were on my 1980 320i. And, I did it a couple of times due to high mileage (320,000). They appeared rusty with a little black soot on them. (The car was driven like it was stolen.) Even though they had already failed, they never looked blocked.

Unfortunately, I don't believe you can tell much by looking at them ... Most likely something has worn out or broken (from heat stress) inside the "probe," which is inside the shield with the holes in it. If it were me, and I was "curious," as you are, I would try (heck, I don't know as I indicated above, but am willing to try almost anything to learn) to check them with a multimeter ... see if they all have the same resistance ... might be a clue.

Certainly, an O2 sensor can fail prematurely ... so mileage since installation doesn't mean much to me.

As to the DME going south, I would reserve that for the last item on your list to check.

Jasoncrx · 26th October 2007, 19:41

Just as a quick list

Vacuum leak
O2 sensors
Clogged air filters
MAFs

Most likely I would say what you see on your O2's is normal. Also, the rear O2's will not affect fuel trim, so don't worry about those. They are emission devices only. If they weren't, installing O2 sims could change the running of your engine along with turning the cel light off.