I'm pulling 3 check engine codes.

-Secondary O2 malfunction
-Primary O2 Malfunction
-MAF malfuction.

I'm thinking it's the Secondary O2 sensor that has gone bad and is tripping the other 2. I haven't had the chance to test anything but here is my reasoning: When the engine is cold it hesitates/bucks really bad around 2500 rpm unless I put the gas pedal to the floor. after the engine has warmed up then the bucking/hesitation stops. In other words once the catalyitic converter is hot no more bucking.

Is my reasoning valid? Also, could this cause the other 2 sensors to pull codes? I don't want to have to buy all 3 sensors if I don't have to. The MAF is working because when I unplug it the car idles noticably worse. Right now the car idles fine and still gets 30 mpg on the highway.

Any pointers are greatly appreciated.

Originally posted by sr20detutah
I'm pulling 3 check engine codes.

-Secondary O2 malfunction
-Primary O2 Malfunction
-MAF malfuction.

I'm thinking it's the Secondary O2 sensor that has gone bad and is tripping the other 2. I haven't had the chance to test anything but here is my reasoning: When the engine is cold it hesitates/bucks really bad around 2500 rpm unless I put the gas pedal to the floor. after the engine has warmed up then the bucking/hesitation stops. In other words once the catalyitic converter is hot no more bucking.

Is my reasoning valid? Also, could this cause the other 2 sensors to pull codes? I don't want to have to buy all 3 sensors if I don't have to. The MAF is working because when I unplug it the car idles noticably worse. Right now the car idles fine and still gets 30 mpg on the highway.

Any pointers are greatly appreciated.

My FSM says that under normal operating conditions the Rear O2 sensor is not even used. It is used only for onboard diagnostic of the catalyst (page EC180 1995 FSM). It also says in the overview that if the Front O2 fails to keep the mixture correct the rear will be used to keep it in stoichiometric. This is called Closed Loop operation. It uses the feedback between the O2 sensors to determine the best amount of fuel to provide. It also states that during a malfunction, warm up, acceleration and a variety of other conditions feedback control stops. This is called open loop. So if your car is not working well during warm up (open loop time) then it seems there might be a problem other then the O2 sensors since these are not being used at this time to adjust the fuel control. Once your car is warm it goes to closed loop and tries to use the O2 sensors to control the amount of fuel provided. Under Rear O2 sensors section it says to check these items: Harness+conectors, Fuel Pressure, Injectors, Intake air leak, Rear O2 sensor. Under the front O2 sensor it says to check Harness+Conectors, Front O2 sensor. To check the Front O2 check the resistance between the outside terminals. It should be 3.3-6.3. To check the rear O2 check the outside terminals and the resistance should be 5.2-8.2. This is all out of a 1995 FSM so your numbers may be different.

I have been reading the two post and trying to figure it out. I would check the Front/Rear O2 sensor using a ohm gauge. You don't need to take them out. I also suspect a leak in the plumbing someplace. You have a turbo (I remember that red Valve Cover Right??). Could you have a leak. A small one. Maybe during warm up when the computer is not making adjustments it runs poorly because of the lack of compensation. During full throttle it is to small to make a noticable difference. Once warm it compensates and again doesn't make a big difference. Just and Idea. Both my suggestions are free of cost except for your time involved.

Thanks Steve. I will check the resistance of the O2 sensors. I should have mentioned above that I have a BB SR20DET with a JWT reprogrammed 97 ECU.(I believe)

Yea, I see you remember that picture of my engine bay from a while back. I have had leaks where the manifold meets the downpipe before, but those have been fixed. Unless there is another leak. On Monday I'm going to go nuts on this problem and try to figure it out. Thanks again Steve.