Why is Engine running rich after O2 sensor replacement?

[Californian]

I replaced two pre cat O2 sensors on my car (1998 328i 4D Sedan with 120000 miles on it) in a BMW shop last week. I did it becasue these O2 sensors needed to be replaced as a part of maintenance per operator manual. The post cat O2 sensors were replaced last year, no problem.

My car was running smoothly before the pre cat O2 sensor replacement. After the replacement, however, following problems occured:

1) Now engine is running rich and the fuel consumption has gone up to about 22 miles/gallon from previous 28 miles/gallon on highway driving.
2) Gasoline smell inside and outside the car when engine started (never had this before).
3) Rough idle, especially when stop in the red light with shift in “D” and “R” position. As a result, I can feel that car was slightly vibrating /shaking. Sometime I had to put shift in neutral or park to get a smooth idle. Is this also a sign that engine is running rich?

Your help would be appreciated.

[stripedape]

the same exact thing has happened to me car ran great until i put 2 new before cat 02 sensors in now it runs so rough its a chore to drive it.did you figure out what is going on.does it take a few cycle for the computer to calibrate to new parts if anyone has any input it would be greatlyt appreciated thanks................

[JDStrickland]

I think this is a new record!

You replied, "me too," to a post that has not been active for more than 6.5 years.

[dangerwillrbnsn]

Hahahaha. At least he's using the search function. Poor guys here cant' catch a break... if they post a new thread with the same old thing, they get beat with "SEARCH". If they search and post to an old thread, the get lol'd at for posting to a 6 year old thread.

Anyway, I hope you figure it out man. And if you do - post back with results so someone else out there having the same issues gets some ideas. Even if it is 6 years later.

[JCooper]

I had this problem recently! Turned out, i didnt gap the plugs at all which led to me blowing the porcelain around the center of the electrode of the spark plug! This ended up causing a misfire on multiply cylinders which turned out to be the cause of the rough idle and lack of power!





If your plugs aren't blown and turn out to be just fouled Id suggest swapping a new fuel pressure regulator! Mine went bad at the same time! The diaphragm in the regulator failed and instead of in holding for example 30psi to the fuel rail @ xxx RPM it suddenly held 50psi which would lead to the injectors dumping more than enough fuel into all the cylinders! This would no doubt give off that fuel smell in the cabin, kill the fuel consomption, and foul the plugs up, which would lead to a misfire!

[kiwi325i]

i feel like it would take a few cycles for them to read right.
also like jcooper said, checking the plugs never hurts

[Javier H]

Another 6 year bump!

Seriously, no one ever answered the question.

Was the car stumbling because the 02 sensors were replaced and were just adjusting?

[MazerRackham]

Another 6 year bump!

Seriously, no one ever answered the question.

Was the car stumbling because the 02 sensors were replaced and were just adjusting?

He says in the second post his FPR died causing plug fouling. He also did not buy pre-gapped plugs which caused one of them to implode.

[Javier H]

Thanks MazerRackham but you appear to be mistaken.

"Californian" , the original poster that started the thread on his 328i sedan asked a question about an 02 sensor.

"Jcooper" another entirely different member chimes in and writes that he had the same issue after changing the spark plugs not an oxygen sensor.

OP never said what the issue was, let alone how it was resolved.

Are you saying that a faulty FPR caused both problems??? Exactly how would you know?

The FPR caused the plugs to go bad is my take on what Jcooper wrote.

BTW to clarify, I am asking because the same thing just happen to me.

Took my car is for preventive maintenance (pre cat 02 sensors) and now that car's idle wobbles/stumbles a bit.

I'll drive it a bit more....but still no one appeared to have addressed the 02 sensor. Perhaps the car just needs to adjust a bit?

My plugs were replaced 5 months ago, car has been driving good until the 02 sensors were replaced.


Javier

[Javier H]

Wanted to just follow-up and close this thread.

Shortly after both 2 pre cat 02 sensors were replaced, I experienced misfires etc.

Mechanic found the plug to the fuel rail was loose. Plugped that in....still misfired and stumbled.

Finally got a CEL. Code pointed to bank 1 as in coils.

Replaced the coil and the car has been running perfectly for over a month now. Power was restored, car drives smooth.

Interesting how these things surface. Hope this helps someone in the future.