Emissions: what causes high CO?
Thread Starter
Safety Car
Joined: Sep 2002
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From: Maryland
Emissions: what causes high CO?
I've been searching the archives, just out of curiousity on emissions.
E does drive my cam selection. I passed last time w/the heads/cam retuned set up but my CO was high. HC very good compared with what I've seen for the hotcam.
Anyway, I understand that high HC can be caused by the overlap from a cam, among other things.
What about high CO? The old discussions said A/F ratio off , but which way, rich or lean? Given that the emissions guys don't hit WOT, the chip (I have a 1993) shouldn't really come into play, would that be a bad O2 sensor?
E does drive my cam selection. I passed last time w/the heads/cam retuned set up but my CO was high. HC very good compared with what I've seen for the hotcam.
Anyway, I understand that high HC can be caused by the overlap from a cam, among other things.
What about high CO? The old discussions said A/F ratio off , but which way, rich or lean? Given that the emissions guys don't hit WOT, the chip (I have a 1993) shouldn't really come into play, would that be a bad O2 sensor?
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
As above - it's probably rich and a scan would help diagnose it. Usually, the ECM perceives a lean condition so the BLM's will be lean and it ends up pumping a bunch of fuel into it. I would scan it first and make sure it's not isolated to one side which if it was, might indicate an injector or valve train problem or header leak or faulty air pump check valve or perhaps an O2 (though unless the signal line has opened - and in spite of what Bosch claims, they seem to last forever - there's never been a recall for any car for high emissions due to a faulty O2). Assuming both sides show a lean mix, then suspect a vacuum or plenum leak or something with the air pump circuit. If the BLM's are normal, then more often than not, the CAT is shot (but a lot of CATs fail because of a rich mix). If the BLM's happened to be rich, then you'd probably have a code because the ECM isn't able to subtract enough fuel to get it back to 14.7 to 1 - a scan should give all the data you need to troubleshoot it so I'd hook one up and see what's going on.
Le Mans Master
Joined: Jul 1999
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From: Phoenix AZ
Originally Posted by sothpaw2
I've been searching the archives, just out of curiousity on emissions.
E does drive my cam selection. I passed last time w/the heads/cam retuned set up but my CO was high. HC very good compared with what I've seen for the hotcam.
Anyway, I understand that high HC can be caused by the overlap from a cam, among other things.
What about high CO? The old discussions said A/F ratio off , but which way, rich or lean? Given that the emissions guys don't hit WOT, the chip (I have a 1993) shouldn't really come into play, would that be a bad O2 sensor?
E does drive my cam selection. I passed last time w/the heads/cam retuned set up but my CO was high. HC very good compared with what I've seen for the hotcam.
Anyway, I understand that high HC can be caused by the overlap from a cam, among other things.
What about high CO? The old discussions said A/F ratio off , but which way, rich or lean? Given that the emissions guys don't hit WOT, the chip (I have a 1993) shouldn't really come into play, would that be a bad O2 sensor?
Thread Starter
Safety Car
Joined: Sep 2002
Posts: 4,030
Likes: 6
From: Maryland
Originally Posted by SunCr
As above - it's probably rich and a scan would help diagnose it. Usually, the ECM perceives a lean condition so the BLM's will be lean and it ends up pumping a bunch of fuel into it. I would scan it first and make sure it's not isolated to one side which if it was, might indicate an injector or valve train problem or header leak or faulty air pump check valve or perhaps an O2 (though unless the signal line has opened - and in spite of what Bosch claims, they seem to last forever - there's never been a recall for any car for high emissions due to a faulty O2). Assuming both sides show a lean mix, then suspect a vacuum or plenum leak or something with the air pump circuit. If the BLM's are normal, then more often than not, the CAT is shot (but a lot of CATs fail because of a rich mix). If the BLM's happened to be rich, then you'd probably have a code because the ECM isn't able to subtract enough fuel to get it back to 14.7 to 1 - a scan should give all the data you need to troubleshoot it so I'd hook one up and see what's going on.
Newbie question--what does BLM stand for?
Le Mans Master
Joined: Dec 2000
Posts: 7,839
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From: San Diego, Ca
BLM is the Block Learn Multiplier or Long Term Fuel Trim, measured in Cells from 1 to 16 which represents different engine speeds. Short Term is referred to as the Integrator. Expressed as number, 128 is considered ideal and represents a fuel ratio of 14.7 to 1. Above 128 is Lean, Below 128 is rich. The ECM will adjust injector On Time or Pulse Width to maintain a Short Term or Integrator of 128, based on the Long Term Fuel Trim or BLM and the O2(s) signal(s). Ideally, both should be right at 128, but a BLM 122 to 134 with an Integrator at 128 is considered within specs (though in my experience, a BLM of 134 can cause driveability issues). If you suspect your chip, scan the engine and capture some data. Then do the same with the stock chip and compare numbers - though I didn't think the '93 had a Chip - aren't changes made by reflashing the PCM? Also, I don't know of any reason to fatten up the fuel tables for idle or cruise and that's all that gets measured during emissions testing.
