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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 01-07-2011, 04:06 PM
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adjmcloon adjmcloon is offline
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Mein Auto: '99 528iT sport
87 octane, cel (cat inefficiency) disapears

I have had cat inefficiency codes popping up consistently on bank one for a while. I filled up with Texaco 87 today and my cel went off after a quarter tank or so. This is the first time that's happened. Previously the cel would stay off after I cleared it with my peake tool, unless we drove it @ highway speed for more than about 15-20 miles. If we made short runs around town we would not get the cat codes. We have used super unleaded up until today.

I have done the ccv, new plugs (there was a bit of oil), oil filter housing gasket, valve cover gasket and vanos was done by the previous owner.

I am just wondering if there's someone who might have some suggestions as to why this happened. Could a small vacuum leak make the car run rich? If I have a vacuum leak its small because I have replaced most every hose I could find and I don't hear any obvious air leaks. I know that the i6 will give slightly better emissions on lower octane fuel. Since we are supposed to use super, something isn't right. I have some cats coming but I am worried about running rich and fouling the new cats. I don't have any oil leaks.

My only thought is maybe some residual oil in the plug boots could be causing a problem. I cleaned them but didn't replace them with the spark plugs.

Other symptoms are a small hiccup in idle and sometimes when decellerating. My trans has an issue with being slow to engage reverse but it shifts fine. Otherwise the car runs pretty well and I average around 24mpg in mostly highway driving
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2001 530i tiAg sport (sold)
2002 540iA sport (sold)
2006 330i sport (sold)
2008 535i-6 sport (sold)
1999 528iT sport (sold)
2002 525iT alpine sport
1990 325i sedan
1990 325is coupe
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  #2  
Old 01-07-2011, 04:09 PM
edjack edjack is offline
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How old are the O2 sensors. front and rear? Some advocate replacement at 100k.
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  #3  
Old 01-07-2011, 05:42 PM
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Mack Mack is offline
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Mein Auto: '02 525i
Why would you fill up on 87 reg??
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[---Quote (Originally by Tex330i)---
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  #4  
Old 01-07-2011, 06:13 PM
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Premium fuel has additives which retard detonation, allowing for greater spark advance at higher compression ratios without pinging (thereby giving better performance and throttle response). In an engine with a compromised emissions system (failing 02 sensors or cats), this works against you....a lower octane fuel will burn more easily and completely (albeit with somewhat reduced performance).
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  #5  
Old 01-07-2011, 07:06 PM
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Mein Auto: '02 525i
So you're advocating running low octane fuel for a car that was built for and recommends running premium fuel? Also ami I missing something here? Op you previously ran premium until now and when you filled up with 87 you threw the cel. So either your car doesn't like the low octane fuel (most likely) or you got a bad batch of gas. Just my $.02.
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[---Quote (Originally by Tex330i)---
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  #6  
Old 01-07-2011, 07:14 PM
mujjuman mujjuman is offline
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Quote:
Originally Posted by Fast Bob View Post
Premium fuel has additives which retard detonation, allowing for greater spark advance at higher compression ratios without pinging (thereby giving better performance and throttle response). In an engine with a compromised emissions system (failing 02 sensors or cats), this works against you....a lower octane fuel will burn more easily and completely (albeit with somewhat reduced performance).
Wow awesome peice of information!!!
You know your stuff
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  #7  
Old 01-07-2011, 07:59 PM
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bluebee bluebee is offline
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Quote:
Originally Posted by adjmcloon View Post
I know that the i6 will give slightly better emissions on lower octane fuel
I don't know the answer ...

But ...

Kudos to you for doing your homework!!!!!

- Octane emissions smog test results

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  #8  
Old 01-08-2011, 12:51 AM
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adjmcloon adjmcloon is offline
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Location: Texas
 
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Mein Auto: '99 528iT sport
I filled up on 87 as an experiment, because of what I read about lower octane fuel giving better emissions. I have put a lot of time and sweat into the car, and that cel annoys the **** out of me, even if it is something relatively "minor". I am trying to get the "wagz" up to 100%, but she's a stubborn old bitch. (But I do love her)

My pre-cat sensors are new and I'm going to replace my post cat o2 sensors with the cats I have coming. The code is consistently bank one cat inefficiency. I am sure replacing the cats should solve the issue, but the weird little hiccup at idle and on deceleration has me wondering if there's not something else going on. I will report back after I get the cats and new post cat sensors in place.
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2001 530i tiAg sport (sold)
2002 540iA sport (sold)
2006 330i sport (sold)
2008 535i-6 sport (sold)
1999 528iT sport (sold)
2002 525iT alpine sport
1990 325i sedan
1990 325is coupe

Last edited by adjmcloon; 01-08-2011 at 12:53 AM.
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