I have been having an intermitent miss at idle. It would also seem that when I unplug the 02 sensor that it goes away. Could this be caused from a faulty 02 sensor or is this more of a computer issue. I have check many of the other things that it could be and everything seems to be in spec. (IAC,TPS,Plugs and Wires,Cap Rotor, Fuel pressure) I am not exactly sure what the 02 sensor does? Any help would be great. Thanks

 

Unplugging the O² will force the ECM into backup fuel and spark mode. In this mode, timing advance is limited, and the air/fuel ratio drops to around 12 or 13:1 (more rich). If this condition causes a more reasonable idle, there could be any of several problems, or any combination of them, including:

1. Vacuum leaks;
2. Low fuel pressure;
3. Clogging fuel injectors;
4. Incorrect base timing;
5. Knock sensor / EST problems;
6. Poor fuel atomization;
7. Incorrect MAF or MAP indication:
8. Weak or failing O² sensor (creating a more rich condition than Backup Mode)

I'm sure there are more that I've overlooked.

 

by unplugging the o2, you are basically putting your car into an open loop mode. What happens then is the car does not/cannot use the o2 sensor to govern the amount of fuel injected into the engine. Rather, it relies on preset 'tables' that the ECM uses, generally under start up/cold start conditions, i believe (but not 100% sure).

With this in mind, and with what you have checked, have you checked the MAF sensor for proper functionality? While the car is running and the o2 sensor is plugged in, un plug the MAF sensor and see how it runs. If it runs better, the MAF or affiliated connection may be bad. If not, check all the air ducting down stream of the MAF sensor. It is important that there are no leaks. This means no cracks, no loose connections at the clamps, etc.

If the entire MAF setup is in good order, i would check the fuel system and see that there are no clogs, as Vader suggested.

However, bearing in mind as to what you are doing by unplugging the o2 sensor (sending the ECM into default fueling) and the fact that it runs better, leads me to believe your issue is in the MAF.

This is my opinion and i am not the most proficient with ECM's. So you can take my advice with a grain of salt, if you desire.

 

Still trying to figure this out. I guess it may help if I tell you what engine 1989 305 TPI. I think that i have tested the vacum I hooked up a gauge to the bigger nipple at the back of the plenum and it was about 17 and steady. As I did this I unhooked other vacum lines and plunged them. Is this good way to check vacum?

Fuel presure is at 34 psi at idle and steady.

timming is et to factory specs.. But found something esle that was interesting at this point. when I disconected the computer to check the base timming. THe Idle was steady as a rock? does this make heads or tail to anybody.

I am not getting any codes from the computer.

I did the knock test on the maf. Nothing and when disconected the car dies imediatley. (sorry about spelling)

replaced the 02 sensor with a new one. Now what.

As always very greatful for any help thanks
Kris

 

Kris85Rock,

The new O² nearly eliminates that as a possibility. The intersting part is your timing results. Check again, and watch for widely varying advance in timing whiles checking both with and without the EST bypass connected. I'm thinking you may have an EST or detonation sensor issue, or possibly a worn distrubutor/spark scatter/bridging problem.

 

Hello Vader
Thanks for the info. With the EST line connected, the timing mark moves off the the tab by about 1 1/2 inches advanced. ( If that Made any sense). With it disconected, it stays right where it is supposed to be. I am not sure how much the EST is supposed to advance the timing at idle? Detonation sensor being the knock sensor? Is there any way to test this? The car does have 127.000 miles on it. So I suppoes that the distributor shaft could have some play. Test this just by giving it a little wiggle?

vader's the man thanks for your help.