About 4 months ago I replaced all 6 injectors, coolant temp sensor, and O2 sensor. Although replacing 2 bad injectors helped, it still lacked power, especially when warm and it hesitated when starting from a stop. Thinking one of the two sensors were lemons, I disconnected the O2 sensor and performance improved. Not to 100% levels, but maybe 80% and no hesitation. Since the O2 sensor was still under warranty, I exchanged the O2 sensor for a new one but it fell back to the same poor performance when installed and improved performance when disconnected.

Any ideas why disconnecting a good O2 sensor would increase performance like this? It seems to point to a different problem, but I can't put my finger on it.

 

Have to checked to see if your cat was clogged?

 

I checked for tel-tale signs of a clogged cat, but it seems ok. If it was clogged, I'm not sure how disconnecting the o2 sensor would make a significant improvement. I didn't remove the sensor (other than to put the new one in) I only disconnected it.

 

Might be filthy or misconnected wiring.

 

+1

If it ran better discconnected then that's sounding like the source of your headache.

 

Thanks for all the suggestions. I cleaned and check the contacts on the coolant tempature sensor since this was the other thing I replaced when my car started going down hill and I checked the distributor since I had to pull the distributor to replace rocker cover gaskets when I was also working on it a few months ago. At that time I was carefull to put the distributor back in the same place with the rotor in the same spot, but when I inspected the rotor today I noticed I good easily move it about 60 degrees. It wasn't entirely loose, but it turned very easily. I pulled the rotor and noticed the screw down the shaft was loose. Onced tightened and the rotor reattached, there was no more play in the rotor. I'm not sure if this had anything to do with my loss of power or whether it was a possible bad coolant sensor connection, but when I took it for a drive all my power was back—a hill that I struggled to pass at 55mph, I could now easily do so at 80mph. I then reconnectd the o2 sensor and everything still worked fine. Crossing fingers and hope it stays this way. I plan to check my timing tomorrow given the rotor issue.

 

Loose rotor.