Sympton of Oxygen Sensor Failure???

[gpdcop]

Again thanks for all of the support, the Jimmy is alive and running pretty good after the motor swap. Here is my question. Upon start up, the motor will run rough, hesitate and stall until the check engine light comes on and then runs fine. I had a new exhaust installed from the cat on back and there is evidence of running rich (black soot in tailpipe). I have been told that this is the symptom of a oxygen sensor bad. I do not have access to a code reader and was wondering if this was true before popping the $25 for a new sensor.

[geoffinbc]

No it cant be the oxygen sensor if it only happens at cold startup. It is more likely the MAP sensor or Coolant temp sensor.

The oxygen sensor only works when it reaches operating temperature. It takes several minutes after startup for that to happen.

You need to check the following.

Fuel pressure. Needs to be 9-13 PSI for TBI injection. Could be too high if your running rich.

Injectors. If you bought new ones you need to verify they are correct they may be flowing too much fuel. Check the part numbers on the injector with the parts store or with the company who makes them

ECT sensor. This sensor measures the coolant temp of the engine and relays it to the ECM. The ECM uses this to determine the amount of fuel to deliver. It sits in the front center of the intake manifold directly behind the thermostat housing. Yellow and Black wires. Try un plugging it first and see if the condition improves. If there is a significant change replace it. If not remove it bring it inside and let it get to room temperature. It should read around 3400 ohms with a Ohm meter attached to each pin. If it checks out then it is in working condition.

MAP sensor. This sits on the side of the air cleaner housing and is black with a green plug. There is no way of testing it without a scan tool. You need to replace it with a known good one. If you have other GM Vehicles of a similar era (1986 to 1996) then they should use the same MAP sensor and you can try one of them.

[gpdcop]

OK Thanks for the information. In regards to the coolent temp, should I unplug prior to start up, or after it is running rough or after it smooths out and the engine light is on?

[geoffinbc]

After it is running then you can listen for a change.