I have not been back to the Forum for over two years and wish to thank all those who had assisted me with my last issues awhile ago but now I need help to assist or troubleshoot the problems with my 1993 Volvo 940. It started when Idrove the car for a few blocks when the car started to hesitate and shake until it stalled. Stop the car and re-start. Again the car started to run rough and wants to die. Pull aside and now the car refuse to crank (turn-over). Wait for the next day and try to (cold) start-up but took many tries to eventually get it to run (at park idle). The car was shaking and stall some more. The car's exhaust was making 'backfire' noise and shaking while idling. Once it warm up a bit (without stalling), the shaking stopped but I notice exhaust noise (leak) coming below the engine/ manifold area. I track the leak at the front exhaust header pipe but was not due to crack or rust out but a 'nickel' size opening which seems to be 'missing' a plug of some sort on the header pipe. I do not know how or why it has gone missing or what is the function (if any) of this plug hole as all the exhaust leak pressure are coming out from this opening. I checked and compare the same position of the front header pipe on my other Volvo (1994 940 model) and realized the missing plug or nut. I am not sure if anything is attach to this opening (like sometype of sensor perhaps)or just a plug hole. Is this what cause the backfiring due to the exhaust leak? I try to first diagnose this by retrieving the trouble (fuel) codes from the diagnostic unit in socket #2. The red LED light indicated (flash) with a 2-2-1 which means something to do with the Heated Oxygen Sensor and Rich mixture/part throttle according to the code. The Ignition trouble code reads normal (1-1-1). Also as the car did warm up, I also see the "Check Engine" light remain lit and notice the temperature gauge has not move at all. My problem or question is it a faulty O2 sensor (never been replace with current mileage at 110K) as well as coolant temperature sensor (since the temp. gauge is not working or faulty perhaps)or possibly air temperature sensor or pressure sensor pertaining to the control unit need to be replace (from what I read but not fully understanding it all pertaining to load information of how the control unit is programmed)? Do I need to replace the complete front exhaust header pipe or something I can replace or plug up the opening since the pipe is still in very good condition?I don't think even a new O2 sensor will work properly as long as there is an opening for the exhaust to escape without properly heating the sensor. Which is the correct replacement for the O2 sensor base on models with Regina or Bosch and the front header pipe with or without EGR but with Bosch or Regina Fuel Injection? Does this vehicle has EGR or not and how do you find out whereit should belocated? What will be your prognosis regarding the shaking, stalling & backfiring, check engine light on, temp. gauge and cost effective step by step resolution. Please help.

 

Is your O2 sensor still in the exhaust?
If you have EGR there is a metal pipe that comes off the exhaust and goes around the back of the motor to under the intake manifold where the EGR valve is located.

 

Greetings Tech:

Thanks for your response. To answer your question, the (original) Heated O2 sensor (with the wiring still intacted or connected) is still attached to the front exhaust pipe close to the catalytic converter. The round opening hole or presumably a plug hole at the pipe is where the exhaust (pressure) is coming out. As stated I check the test (fuel) diagnostic code reading at 2-2-1 and no other code came up. The test (Ignition) code indicates normal at 1-1-1. The trouble code only tells me (according to the code guide) something to do with the O2 sensor and rich mixture (?) / part throttle (?)which I do not fully understand as to what needs to be adjusted or replace. I think the exhuast leak is what is causing the stalling and backfire (besides hard to start up the engine). Once the car was able to turn over completely and stalling residedand warm up, the 'backfire' stop while the 'Check Engine' light stay lit and the temperature gauge did not inch up at all. Do I need to find a way to 'plug' up the hole opening (to allow heated exhaust to go through the O2 sensor properly unless the O2 sensor is faulty already and need to be replace before going any further)provided the hole is not used foranything else of which I cannot visually detect? A new front exhaust pipe that I have checked on-line shows some sort of plug attached but not detailed or decriptive to tell me what it's function (if any) as is same for my newer 1994 Volvo 940 as well. Maybe to change other sensors associated with the control unit (according to the functionality of this system) as well? What is my next course of action or steps to take as process of elimination for diagnostic? Thanks.
Terry