My friends 89 RS failed emissions due to running way to rich and I was wondering if a pinched PCV hose to the air cleaner on his 305 TBI was the cause of this. Also, the O2 sensor wasn't giving any errors, we think it's defective but won't that throw a code too? We're replacing the cat converter soon to see if that helps any, but we already did new plugs, wires, cap and rotor and that didn't really do much.

 

Its also a good idea to replace the O2 sensor before a tailpipe test.

 

A failing O² will not necessarily set a DTC. The common failure mode of a zirconia oxygen sensor is a progressively weaker signal, until the signal falls below the acceptable threshold for the ECM. Before the signal falls to that warning (error code) point, the lower signal will be an indication to the ECM that the system is running lean, so more fuel will be added to compensate. This creates a rich condition.

If the O² hasn't been changed in the last 30,000 miles or so, it's time.

 

ah, alright, thanks for the explaination. That would make a lot of sense. The emissions guy said it was basically throwing fuel out the tail pipe lol. Now I wouldn't mind that, so long as the car is making like 1000 HP but with 170 on a stock TBI it's a nuisance, thanks bro.