Emissions fail

[Carnut68]

My new-to-me 92 failed CT emissions today which is a bummer as I have still not even been able to register the car. It failed the "Cruise Mode" CO test (1.64% vs 1.20% limit). It passed the Idle Mode CO test as well as the Cruise and Idle HC tests. The car passed a NY state inspection last year which included an emission test of some sort. Mileage is 204k. The cats and plugs (Bosch Platinum +4's) were replaced @ 120k. No record of an O2 sensor change which is odd given the extensive maint history. I only have a few days left on a temp registration to resolve and retest before this becomes a major PITA. Thoughts from the resident experts?

 

[gsxr]

If the O2 sensor is old (or of unknown age)... I'd start by replacing that. Check out this thread:
viewtopic.php?f=19&t=901&p=5347

Although I doubt the +4 plugs are causing the emissions fail, there have been quite a few issues with these cars not being happy with resistor plugs. They are supposed to have non-resistor Bosch F8DC4 plugs. However if they are old... yank them out and pop in a set of F8DC4's (not anything else, unless it's a different brand of non-resistor plugs). If they are 80kmi old, they're due for replacement. ~$1.60/ea from Autohaus if you have time to wait for 'em.

 

[Carnut68]

Thanks Dave. Very helpful. I should have caught that in a forum search.

 

[Jimbo]

Platinum plugs ?!! Oh no you didn't just say the PO had those in there...definitely ditch them and go with what the car calls for.

Doubtful that is what caused your car to fail the sniff test tho. His car passed at idle but failed cruise right?...def sounds like the cat wasn't warmed up enough at the test....hard to think the O2 sensor would fail since the last recent test, but its possible. Any water coming out of the auspuff when its running?

 

[Christian_K]

In Germany every Mercedes specialist knows since ages that the Mercedes V8s starting with the M117 (or even earlier?) only wants to have Bosch Plugs for lunch.
Every single other brand will cause problems sooner or later in the Mercedes V8s. (Questionable of course, why the pre-owner got "Beru" Sparkplugs when he ordered 8 pieces directly at Mercedes?)
Why this is? I don't know... in forums people usually suspect that it might have something to do with the high temperatures the V8s are generating.

 

[Carnut68]

It failed at cruise only correct. Proper plugs and new O2 are en route. Once I have them in, I will retest and try to romp on it a bit more on the way over to the test center to make sure the cat is super hot. I guess I will put some fresh gas in it as well although I don't know if that will have any impact.

 

[gerryvz]

In Germany every Mercedes specialist knows since ages that the Mercedes V8s starting with the M117 (or even earlier?) only wants to have Bosch Plugs for lunch.

Actually this is 100% true back to the mother of all MB V-8s, the M-100 (6.3, 6.9).

The preferred Bosch plug on the 300SEL 6.3 and Grand 600 is the Bosch W7DC or W8DC or Champion N12 (if you can find them).

Cheers,
Gerry