Car is running rough, if you unplug the O2 sensor it smoothes out.

Travis Tedrow

Asked by Jun 02, 2013 at 10:45 PM about the 1997 Chevrolet Cavalier Z24 Coupe

Question type: Maintenance & Repair

I am still diagnosing a rough running issue on my 2.4 motor in my cavalier. I have been
fighting a rough running problem. It is loading up on fuel and running rough and smoking
bad. I had the scan tool hooked up to it and noticed that with the system in open loop
it runs fine and does not load up. I tried running the car with the O2 sensor unplugged
which puts the system in open loop and it runs fine and smooth. I replaced the sensor
with another O2 sensor I had laying around but it had no affect on the car. I was told
that the issue would be caused by a sensor in the air and fuel system. This car uses a
fuel cell with an aftermarket pump on it. I have replaced the fuel pressure regulator, the
map sensor, the throttle position sensor and the air inlet temp sensor. I have also
swapped engine wiring harnesses and engine computers with no result. Do they make
an open loop O2 sensor for this vehicle?

10 Answers

sounds fishy to me...the 02 produces a minute amount of voltage with platinum emitting voltages under varying's like you're pulling the eyes out of the computer brain, yet it still walks---put it back the way it is supposed to be, and look for your troubles elsewhere in performance, like the TPS, the ambient air temperature sensor, the fuel pressure regulator...vacuum to this devices and all devices...replace the butyl rubber with silicone...may help these devices behave themselves~

5 out of 5 people think this is helpful. far as the oil consumption one could guess 250,000 miles could produce this...perhaps the days are numbered for your motor~

1 out of 1 people think this is helpful.
Travis Tedrow

I have replaced the tps, the temp, and the map with new sensors with no results the car still runs the same. I have replaced the regulator as well and it had no affect on the car. the vaccum is at 15 inches

2 out of 2 people think this is helpful.

....rough as in spark is dodgy and sometimes not there or rough like a stuck EGR valve forcing raw exhaust into the intake...because it's buggered up with carbon?

1 out of 1 people think this is helpful.
Travis Tedrow

EGR valve is no longer on the car. It acts like it misfires but it does not log a misfire code into the system. After 4000 rpm the engine begins to smooth out.

1 out of 1 people think this is helpful.

still sounds like a vacuum leak, ted---is there a vacuum manifold? why don't you wave an unlit propane torch over the idling motor to pipoint where the vacuum leak is?....expensive as they are silicone vacuum hoses will never let you down.

3 out of 3 people think this is helpful.
Travis Tedrow

we did the test of taking brake clean and spraying it all over the running engine with no change in the motor. There is no vaccum manifold on the car.

1 out of 1 people think this is helpful.

brake kleen has been made safer, to where it is no longer explosive, like it used to be...unlit torch with MAP gas should show a change in RPM to a leaker~...just haven't found the leaker, yet.~

1 out of 1 people think this is helpful.

As I recall this engine does have an intake (carburetor) with functional vacuum devices attached to it like the fuel pressure regulator and vacuum advance mechanism~ still the 02 sensor and the rest of the smog devices should not be ignored~ You need your EGR valve which engages when operating temperatures get too high...try to put it back the way it was.....everything~

2 out of 2 people think this is helpful.

No one has brought up a clogged /bad catalytic converter? If O2 sensor is before the catalytic it would read very high and change the air / fuel mixture to cause the poor performance you describe?

2 out of 2 people think this is helpful.

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