Exhaust Emissions Problems On 6.0l

V8MSY · February 18, 2011

Hi Guys,

Over here our vehicles get tested on emissions for their anuuanl inspection (MOT) and I'm struggling with my truck.

It's a 2003 2500HD Sierra with the 6.0L in it and it's giving a high CO reading on the test. The limit over here is 0.200% and the lowest they can get it down to is 0.217%.

Anyone got any suggestions as to what to look for. O2 and lamda readings are both fine suggestings those sensors are both fine. Air filter is also new. I've run injector cleaner and catalytic convertor cleaner through it and put high octane fuel in to try and get the cats as hot as possible to try to make them work as efficiently as possible.

I appreciate this won't make a lot of sense to a lot of people but thanks for looking, and possibly helping.

GMstrong · February 18, 2011

Seafoam might help.

HawkScot · February 18, 2011

A couple of suggestions.

1) whenever I have had rich conditions, it was because of the engine temperature. If the computer thinks the engine is not up to temp, it will run rich. Over 10 years ago, I thought I would put a performance thermostat in my 92 Suburban (160). It ran great I thought, figured the mpg was down due to more aggressive driving. Long story short, I ended up replacing with proper 195 thermostat, and replacing the O2 sensor and catalytic converter before it would pass emissions. Make sure the ECM temperature sensor is reading correct (I just replace with new) and the thermostat is the correct one.

2) some have run fuel tanks down to 1/4 and then added a gallon of alcohol. This reportedly lowers CO level in exhaust. I have never done this and probably never will, with all the sensors and expensive things that can go wrong, but perhaps someone there knows about it and has done it...?

jim brock · February 18, 2011

If your truck has an EGR Valve. Try cleaning it or replacing it. Also hotter plugs could improve this possibly. Good Luck...

V8MSY · February 19, 2011

Many thanks for the replies so far guys.

Seafoam looks similar to the injector cleaner that's already been run through it.

As for the alcohol idea that's obviously a similar idea to the catalyst cleaner that's been run through. You have to run the truck down to two gallons of fuel, add the cleaner and then drive for 10 miles to run it through.

The truck fast idles on start up and then drops down to standard tickover when warm so not sure about the thermostat idea but will check that out. They've had the truck, and cats, up to 101 degrees when testing so it's certainly getting hot.

EGR valve is a good suggestion, will have to check that out.

The truck had hardly been used for the last couple of years and was bought by me with a blowing R/H exhaust manifold gasket so I'd assumed it had just got itself clogged up and would clear quite quickly.

If it makes any difference I had also assumed that the ECU would self learn and was compensating for the exhaust blow. The truck was left with the battery disonnected for a couple of days to see if it would clear its memory a bit before any of the above work was carried out.