Need input on codes

[vix_98_318ti]

Hey Forum!
I need some help here, I have a auot/98ti and I've been getting VERY, VERY bad MPG. I am going to guess about 15 mpg! (city-if that). I recently replaced the all spark plugs (4+ Platinum), oil change not too long ago, tires are in good shape, air filter is good shape, and car appears to run no major noticeable problems. In the last week or so I have cleared the following codes:
1. 0A - PreCat O2
2. 28 - Cat efficiency (CEL on)
3. 1b - Fuel Trim QL additive

Anybody have any ideas what could be my problem? It seems everytime I clear the codes, I get a new one. I know something is killing my gas consumption and mostlikely related to one of the culprids above!

Any good ideas to try or look for would be greatly appreciated! I need to improve those MPG back to 25-30 mpg!

So much for having a 4-cyl efficient car!

thanks ppl

[BonesOfSteel]

I believe you need a new catalyst converter

[vix_98_318ti]

that sounds like fair assesment. How about if I told you that I replaced the CAT about 2 yrs ago with an aftermarket one. After learning that the OE would cost me about well over $1k!! California has some very strict rules about who works on cars when it comes to smog relaged parts, like the CAT. Luckely I have a contact who will hook me up with replacing the CAT with another 3rd party. If anyone has a spare laying around that doesn't have too many miles, I might consider taking it off there hands if the price is right. Let me know anybody?

Any other ideas?

[lilblkbmw]

hi! im new to this forum. if you can help me, id very much appreciate it! my CEL is showing the same codes as yours;
1. 0A - PreCat O2
2.1b - Fuel Trim QL additive (what's a fuel trim?)
3.50 - secondary air control ( secondary air pump?)
i was wondering what you had to replace to fix these problems?

thanks

[1996 328ti]

02 sensors go.
There were problems with the cat in 98s.
There is a letter from BMW somewhere around here.
http://www.318ti.org/forum/showthread.php?t=11810

[vix_98_318ti]

I did a tune up, which didn't really help much....I got rid of two of the codes except for the bad pre cat sensor code. I then bought an O2 sensor at Autozone (OE Bosch sensor) but the problem came back. I kept clearing the code and it would come back after about 1 day of driving. I think it only comes back when I am driving in stop and go traffic. Some how, it is not getting hot enough. I drove about 400 miles about a month ago and I had cleared the code before we left, well it never came ON! Until I drove around locally. Anyways, I have bought another O2 sensor just to rule out the problem is in the sensor, this time I purchased it from an bmw auto dealer online for $70. I'll post any replys if I end of determining what fixed my problem.
Good luck!

[bullmand]

Someone will correct me if I'm wrong, but I believe that if the cat is not working properly the post-cat o2 sensor can cause the computer to run the mixture too rich. This can then kill your new pre-cat sensor. That also seems to fit with your highway vs. city experience. I think this may be the fuel trim code you're getting. The computer can't reconcile the readings between the two o2 sensors so it richens the mixture to protect the engine. Or not. Someone will have more to add to this.