I recently bought a 89 300e that is not passing the smog test, the results were high on NO. As for the other gases they are fine. The person doing the smog check also said my car jumped in speed a bit meaning when he was at a steady 25, it would out of no where jump to 26 not allowing him to finish the test. I've already changed the oil, new spark plugs and catalytic converter hoping it would help but it hasn't. I also took it into a mechanic and the first thing he recommended was changing the 02 sensor, but i wasn't going to leave him the car just yet. any advice or help? thanks

 

I would have to agree with the Mech. Change the O2 sensor

 

on those older cars its very reccommended to get the exhaustvery, very hot..drive in 3rd or 2nd gear for a bit at high RPM and i bet you will pass then. tell them to make sure its hot, to drive it high rpm for several minutesand then test it.. you should pass then. Ive been through that many times. make sure you have a fresh oil change too. helps a little bit.

 

well i did go on the freeway before taking it to smog, ran it for about 10 min. but like i said the guy doing the smog said the car was jumping in speed. also how can i tell if an 02 sensor is bad?

 

I would replace the coolant temp sensor...it's by far the most important sensor for the injection system...plus it's only about $35 and easy to do....a bad coolant sensor will throw everything off...seriously