Very rough idle

[Jscott]

Hi again. 90 old C/S 3.1. Really bad idle. I replaced the o2 sensor and iacv. After installing the o2 it ran pretty good and then it didn’t. Back to rough idle. Any thoughts on what the issue could be? I’ve heard of bad pcv/ map sensor/ coil pack. The erg is not gummed up. However I just had my battery tested and came back bad. The car starts fine but they said it was bad. I’m not sure if that has something to do with it. Any ideas. Thanks. 

[55trucker]

My suggestion to you is to not throw parts at the issue, get a proper scanner on the engine, do a live readout & note in particular what the a/f display is showing you.

A failed o2 sensor will only prevent the fuel system from going into closed loop. If the sensor HAD failed then something upstream caused it to.

A check engine light will also light up. A bad map will completely throw off the ECM;s ability to configure a proper air/fuel ratio, that will be seen on a scanner as well.

Another small item to look at is the wiring harness condition to the engine temp sensor, if it's faulty then that sensor cannot send the proper signal to the ECM to configure A/F ratio due to inaccurate information on the engine actual temp. That will be seen on a scanner as well.

The 3 vital sensors needed for the ECM to calculate proper A/F ratio when the engine starts are the MAP, IAT, ENG TEMP.