Very rich burn......? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 05-11-2007, 03:23 PM   #1
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Default Very rich burn......?

On the '98 SSEi.... I'm getting all of the signs of a rich burn. Two month old tune-up parts like the O2 sensor and front plugs have carbon on them. I think the exhaust smells rich and there is carbon on the exhaust tips.. Combined mpgs are around 20... 50%/50% highway/local. that seems a bit low.

I have an Actron scantool which isn't best for OBD II. But, maybe readings will supply some direction.

Front O2 sensor is at .1v to .9v., never in the negative. The Actron manual says .4v is even burn. (I need to confirm that). So it looks like the burn is in range, but the short term trim is continually pulling back the fuel supply. Neither of the other cars works so much to keep the fuel down. My boost gauge jiggles at idle, because of the up and down.

The long term fuel trim is up at +10,+15% all of the time. Ignition advance is +20, +35. I have no standard numbers to evaluate those.

I need some problem solving steps and will answer any questions if more info is needed.

Thanks.
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Old 05-11-2007, 04:10 PM   #2
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Ok. 20 does not sound very low at all for 50% city. Heck, I usually get about 18-20 for about 70% highway. But of course I don't know how you drive.

Anyways. Those numbers, were those recorded after the car was fully warmed, in closed loop and after being driven? I imagine that would make quite a difference in o2 reading.
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Old 05-11-2007, 05:08 PM   #3
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Quote:
Originally Posted by wjcollier07
Ok. 20 does not sound very low at all for 50% city. Heck, I usually get about 18-20 for about 70% highway. But of course I don't know how you drive.

Anyways. Those numbers, were those recorded after the car was fully warmed, in closed loop and after being driven? I imagine that would make quite a difference in o2 reading.
Yes.. the O2 sensor has to be well heated, and it was closed loop. After a half hour of local driving, I hooked it up and tried to get a good look at the values, without driving into a tree.

Putting mpgs aside, there is way too much carbon on the plugs and O2 sensor for such a short time in. Just a few weeks ago I was trying to help a friend with his LeSabre L36. He was getting terrible mpgs. Did the BC tune-up for him. Finally figured out his cat was gunked. He had this much of a burn present.
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Old 05-11-2007, 11:53 PM   #4
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Have you cleaned the mass air flow sensor lately?
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Old 05-12-2007, 08:26 AM   #5
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Quote:
Have you cleaned the mass air flow sensor lately?
Better give him instructions on how to do that without damaging it.
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Old 05-12-2007, 08:47 AM   #6
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Quote:
Originally Posted by charliemax
Finally figured out his cat was gunked. He had this much of a burn present.
Hmm...and what about YOUR cat? You are OBD2, right? Is there a way to look at the rear 02 data to see if it is mirroring the front 02? If it is, the cat is either gone or going.
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Old 05-12-2007, 12:13 PM   #7
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Quote:
Originally Posted by 2000SilverBullet
Quote:
Have you cleaned the mass air flow sensor lately?
Better give him instructions on how to do that without damaging it.
Good idea. If you haven't done that lately - check out this techinfo article. May as well clean up the throttle body while you're at it.

http://www.bonnevilleclub.com/forum/...e=article&k=64
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Old 05-12-2007, 04:27 PM   #8
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A couple of months ago when I got the car, I steadily went about doing what I consider the BC Best Practices Tune-up.

1-New Beldens, NGK-TR55s, PCV(delco), front bank O2 (delco), fuel filter(delco), oil/filter
2-Cleaned battery cables, TB, MAF, IAC, (BTW, it is difficult to find around here, but Gunk'* Intake Cleaner for FI Engines cleans out the TB twice as fast as any other I have used).
3) Gutted the airbox, and dropped in a K&N. This is when I got the only unexpected result. My boost gauge started to read -9 instead of -10 at idle. Went over all the vacuums and replaced many. Enough people reported the same issue, that it is on hold for now. If anything this would mean some unmetered air, and a lean condition.
4) TBD... the 180* stat and swap out the Dexcool.

Only other i thing i did was put in a new steering pump.

The rear bank O2 sensor was giving me a relatively steady .7v reading. I have no idea what that should be. That has no impact on fuel/air mix, though. That'* why I'm putting mpgs aside. i'm more concerned on the general mix coming in. My gut tells me the long term fuel trim is positive high for some reason, and the short term trim has to continually pull it back down. I may be way off base, that'* why I wanted some informed opinions and directions to chase this down.

BTW, I have been trying to keep in grandpa mode while I'm getting a baseline for gas mileage.

Now I know why you guys have always said it'* hard to not lean on the throttle.
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Old 05-12-2007, 07:16 PM   #9
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Did you disconnect the battery to reset the pcm after you installed the new O2?
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Old 05-12-2007, 07:41 PM   #10
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The "base" voltage for the O2 sensor is 450mv or .45 volts. From that voltage, it can swing down to 0, or up to 1 volt, and should do so fairly quickly. A higher voltage indicates a rich mixture, and a low one, a lean mixture. If the voltage seems to hang longer in the above .45 voltage area, that would confirm your feeling that you are running a bit rich. Was this the same car that had the intake problem? If so, was the O2 sensor replaced prior to that repair?

The O2 sensor works by comparing the oxygen content of the exhaust with a "clean air reference." While investigating this, I found that it uses the wiring for the O2 sensor to sense the clean air around it. So, chafed, burned, pinched or damaged wiring can cause it to malfunction. That would also explain on why it'* important to keep the spark plug wires (#6 being the usual culprit) away from the wiring.

Other things that can cause a problem with the sensor is a leaky or restrictive exhaust system, or an evaporative system problem.
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