Changed my “fully” working O2 sensors, made a huge difference.

Interesting analyis..but makes sense that in order to trip the "check engine" light...the O2 sensor has to be way out of wack...but could be slightly out of parameter...enough to not drive ideally, but not enough to trip the "CE" light. What is the premise of only changing the "pre cat" sensors only, and not the "post cat" sensors as well? I'm assuming there are four, yes? Two on each side? Pre cat and post cat? Do the pre cat ones get fouled more easily than the post cat?

 

My 308 also running a little uneven at steady highway speed other than that everything is perfect, should I replace my O2 sensor too?

 

The post-cat O2 sensors are, indeed, only for monitoring the condition of the cats. By "monitoring" the old sensors, I assume this meant checking that they're toggling, but if they're slow in responding, it could cause a driveabilty issue.

I'm not confident that the 308 has O2 sensors...

 

QV's do....

 

My QV runs excelent during acceleration and at full throtle but not at a steady speed, feels like something is holding you back then let go repeatedly, so does this still sounds like O2 sensor?

 

I'd just go ahead and change it...you only need one, and it is a good preventive maintanence item anyway....you can buy a generic Bosch one, and splice on your connector for about $100. Bosch #15735 is the generic one. The specific one for a 84 308QV is Bosch #13957 (according to www.rockauto.com website)

Froogle says this is a great deal:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=33557&item=8072411785

Cheaper than the generic one, that would require you to splice on your old connector....

 

how did you "clean" you MAF...that would make a huge improvement in the car...I learned teh hard way that the MAF really controls your whole "Ferrari expereince" Ok..the engine part anyway....

 

I was assuming he meant cleaning the big connector to the MAF. I had a idle fluctuation from 750rpm to 1100 rpm that stabilized to 1000rpm after cleaning the connector with aerosol electrical contact cleaner and treating with Stabiliant 22A.

 

I used Stabilant 22 on the MAF connectors on the t Coupe and it smoothed out the idle, improved throttle response, got rid of the black soot coming out the tailpipes every time I nailed it and improved my mileage 25%! Amazing such a simple thing could make that much of a difference.

-Peter

 

as a recent experiment, I installed one of these in my '89 328:
http://cgi.ebay.com/ebaymotors/O2-OXYGEN-SENSOR-BECKARNLEY-SG45-FERRARI-328-GTB-V8-3-2_W0QQcmdZViewItemQQcategoryZ33557QQihZ019QQitemZ8051482057QQrdZ1QQsspagenameZWD1V

it works as expected. I also monitored the output for a while, and it's fine.

bench testing the original sensor showed that it seemed to want to stick around .6-.7V periodically, possibly due to crud accumulation from misfiring cylinder #7 (bad plug wire). Heating the tip red hot seemed to get rid of that habit, but the sensor wasn't capable of outputting more than about .9V. The el-cheapo replacement sensor easily swung from almost 0V to 1V.

I soldered the new sensor wires instead of using the crimp connectors for added reliability.

disconnecting the o2 sensor and watching the output open loop was also interesting. The original sensor was outputting around .65V, and the new sensor was more like .53V. With original sensor, hesitation on acceleration from stop occured periodically. That's gone.

Both sensors showed that open loop, when revving the car to around 2500 (no load), the output dropped to .2-.3V ... lean. Closed loop the lamba compensates. Haven't looked at the output under load, or tried getting readings at the sample tubes. Is this normal behavior, or is there an underlying problem that the lambda is masking?

if anyone in the sf bay area with a 328 wants to do some comparison measurements, pm me. I'm also interested in the vacuum readings taken at the plenum - the vapor canister connection is convenient - idle and around 2000 rpm.

 

Interesting that Rockauto is spec-ing the 4 wire O2 sensor...I would have thought the 308 uses the 3 wire like the 328....hmmmm.