O2 sensor causing misfire

[urdrwho]

I was reading on another Subbie site about a guy that had a misfire. He tried all kinds of things and the final fix was a new O2 sensor and reset the ECM.

 

I thought OBDII was supposed to monitor all this stuff such as a failing O2 sensor. I know sensors can be lazy but in a Subbie with OBDII could it be lazy enough to cause a misfire.

 

I believe that they were discussing the front O2 sensor.

 

BTW I'm not happy with the wires I chose. Because of the coil boots - I should have chosen the set with the 90 degree angled coil boots on the one side and straight boots on the other. I wonder if I could take them back? Probably not because they've been installed.

 

From the story of the guy and his misfire..............

 

" I had a similar rough running and misfires from idle through highway speeds. However, it would come and go. The rough running was significant - in some cases the car would kick and buck slowing down to 5-10 mph and then suddenly accelerate fine until I backed off the gas. My mechanic was stumped and he has been exceptionally good a troubleshooting over the years. He checked fuel pressure - ok, Manifold Pressure (replaced but would not charge me since it did not fix the problem), changed wires/plugs (needed this regardless).

 

Long story shortened - The cause was the engine control module was running the mixture extremely lean. I monitored with Harrison R&D OBD-II linked to the computer and both Short Term and Long Term trim would run to the high -20+ range with corresponding "misfires". No trouble codes.

 

The fix (been running about 4 weeks now with no problems versus daily before) was a combination of two changes. I don't know if both were required or which one fixed it.

 

The first change was the Front O2 sensor connection was cocked because the locking clip was worn. I removed the clip and tie-wraped the connection. Although cocked to one side, it seemed to me that the connections still should have been ok.

 

The second change was to reset the ECM. I disconnected the battery for a couple hours and shorted the disconnected ground wire to positive post. Cleaned the post and reconnected the battery.

 

The change was immediate and lasting. Short and Long Term Fuel trims stablized in the +3 to -7 range and the "misfires" stopped completely.

 

Maybe someone else here can narrow this down to the O2 connection or the ECM reset."

[snederhiser]

Hello;

Not knowing how dirty the battery terminal was, could have been this also. Extra grounding and keeping the electrical connections super clean will solve alot of problems. Steven.

[urdrwho]

I've always wondered about the engineering policy of having multiple grounding points on a car because it seems like it is a problem in waiting.

 

I've read and looked about cars that people made a one point grounding system. Makes sense to me.

 

Hello;

Not knowing how dirty the battery terminal was, could have been this also. Extra grounding and keeping the electrical connections super clean will solve alot of problems. Steven.

[vinnycogs]

I've always wondered about the engineering policy of having multiple grounding points on a car because it seems like it is a problem in waiting.

 

I've read and looked about cars that people made a one point grounding system. Makes sense to me.

 

 

Not disagreeing with you, but I'm just wondering why you think that?

[urdrwho]

Much easier to deal with all the grounds going to one body grounded busbar than to many different locations. It is easier to make that one shiny busbar to body than 15 different grounding points.

 

On my 1990 Jaguar it is insane how many different grounding points exist throughout the car.

 

I have tackled a few on the new-to-me Subbie but I am sure there are more of them.

 

Not disagreeing with you, but I'm just wondering why you think that?