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Mike V.
Mike V., Auto Service Technician
Category: Car
Satisfied Customers: 25183
Experience:  25 years of experience, Certified Subaru Technician and Nissan factory trained.
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97 mazda 626 2.5L v6, idles rough, misses under acceleration,

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97 mazda 626 2.5L v6, idles rough, misses under acceleration, trouble code P0300 with Fuel system 2 OL-Fault
HI there! Are you seeing codes P0171 and P0174 together?
Customer: replied 8 years ago.
The codes I had before I replace the crankshaft sensor were
P0173, P0300, P0302, P0304, and P0335
OK. Did you change the plugs as well or just the wires?
Customer: replied 8 years ago.
Changed the plugs also, put in NGK platinums
OK. Here is what I would do. First, check for any signs of a vaccum leak on the front bank of the engine. Spray some carb cleaner around the intake and see if the idle smooths out a bit or increases in speed.
If that doesn't show any leaks then, I would replace the front O2 sensor(the one in front of the converter) on that bank and clear the codes. This will reset the fuel trim so the computer isn't using the stored bad fuel trim to run. It should reset the fuel trim back to 0 which will make it learn new fuel trim.
Hope this helps! Thanks
Customer: replied 8 years ago.
found a vacuum leak, hose that connects to bottom of air cleaner assembly. I reset the codes, but experienced the same problems. rough idle, and cutting out on acceleration. When you say replace the front O2 sensor, please tell me where the other O2 sensor is so I get the right one. Also, can I wait to accept the answer until I have tried your suggestions?
OK. The O2 sensor I am speaking of it the first one on the front of the engine. It may be right in the exhaust manifold. It will be before the converter on that pipe.
Try disconnecting the battery for the code reset. I am saying this because the vaccum leak should be what caused the fuel trim code. You might want to check the plugs in the front of the engine and make sure they are still in good shape.Clean I mean, no carbon buildup yet or fowling. You can most certainly wait to accept. Like to have you fixed first! Make sure the firing order is correct as well. I think you are dealing with a couple of different problems here,so we have to take it one at a time. Make sure the cap and rotor are in good shape to.
Here is the firing order
Click image to see an enlarged view
Customer: replied 8 years ago.
Thanks, XXXXX XXXXX give these suggestions a try, and let you know.
Customer: replied 8 years ago.
OK, I checked the distributor cap and rotor, the firing order, and rechecked all vacuum lines for leaks. Didn't find any more leaks, however, I replaced almost all the vacuum hose with new hoses just to make sure. I disconnected the battery and reconnected. I erased all the DTCs from the onboard computer, and started it up.

The engine idled smoothly at about 1000 rpm. As the engine warmed up it idled down to about 750 to 800 rpm. Still reasonably smooth. I took it out for a spin, and found that while the engine was cold I could accelerate reasonably well. As the engine warmed up, it became apparent that the car hesitated on acceleration, especially when I really put my foot into it. I found if I only used about 1/4 the pedal, acceleration was smooth and no hesitation, but if I put my foot into it, the car would hesitate and buck, rpm would drop, so I backed off on the accelerator, and found that the engine started responding well again if I was back to below 1/4 pedal.

No new DTCs.

What do you think?
OK. I think you need to check fuel pressure. It sounds like the fuel pressure might be to low for the wide open throttle jump on the pedal to keep up with the demand. It also appears it may be temp related which could be the ect sensor. Here is some info on the temp sensor. I am not sure if mazda uses this for both the guage and the engine computer or just the computer. It sounds like you still have a lean fuel mixture happening. Hope this helps!

The coolant temperature sensor is a variable resistor that is influenced by temperature. Which means, as the temperature changes, so does the resistance inside the sensor. The computer reads this change in resistance to determine the operating temperature of the engine. It uses this reading to determine proper air/fuel mixture and ignition timing settings.


See Figures 1 and 2

This test can also be performed with the sensor still installed in the motor, provided the engine is at the same temperatures as given for the specifications.

Click image to see an enlarged viewFig. 1: To test the sensor while still installed in the engine, verify the temperature of the sensor then take the resistance reading
Click image to see an enlarged viewFig. 2: The end of the temperature sensor can be submerged in cold or hot water to check the resistance
  1. Remove the thermosensor.
  2. Place the sensor in water with a thermometer and heat the water gradually.
  3. Measure the resistance of the sensor and compare to the following values:
    • -4°F (-20°C)-14.6-17.8 kohms
    • 68°F (20°C)-2.2-2.7 kohms
    • 104°F (40°C)-1.0-1.3 kohms
    • 140°F (60°C)-0.50-0.65 kohms
    • 176°F (80°C)-0.29-0.2 kohms
Customer: replied 8 years ago.
I tested the heater element resistance on the front O2 sensor (3 wire on the front and 3 wire on the back) resistance between A (left) & B (center) 6.4 ohms, the temperature of the O2 sensor was probably about 20 deg F, the resistance between B (center) and C (right) was infinite. I have not performed the sensor output voltage test yet however.

I pulled the coolant temperature sensor (I couldn't perform the -4 deg test), and the resistance was as follows:
68 deg F - .870 kohms
104 deg F - .440 kohms
140 deg F - .211 kohms
176 deg F - .140 kohms

I am leaving to pick up a new sending unit, and will try the new unit. Don't know if I will get it back together tonight however.
Customer: replied 8 years ago.
Tried the new water temperature sender. Didn't solve the problem. Still has hesitation when I push down the accelerator. When I back off to about 1/4 pedal, acceleration seems fine, but really cuts out badly if I try to push it.

No DTCs are showing up when this happens
OK. Did you clear the check engine light after replacing the ECT sensor. Sometimes you have to reset the fuel trim to start back at zero,otherwise it may be using old fuel trim which was calculated wrong.
Were you able to check the fuel pressure? I would say if the fuel pressure tests good,put a new mass air flow sensor into it. Bad hesitation is normally fuel pressure or mass air flow sensor. I think we hafve narrowed it down pretty good. I like to try and eliminate everything else before going with the mass air flow, it is a pretty expensive sensor.
Customer: replied 8 years ago.
I disconnected the battery after I replace the ECT sensor. I have not checked the fuel pressure, how do I do that?
You will have to rent or borrow a fuel pressure guage. Hook it in line with the fuel line going to the injectors. Then, tape that puppy to the winshield if you have to and drive the car. The pressure at idle should be the same as when you floor it. If you see the pressure drop off at the same time the engine stumbles,then you are looking at a fuel pump issue. If not then I really have to believe it's the mass air flow.
Customer: replied 8 years ago.
Just for the heck of it, I tested the voltage on the throttle position sensor. According to my meter, there was no voltage going through the throttle position sensor. Would this show up as a DTC if there was not voltage?

I tried it with the ignition on and off.
I am not sure on the tps. you should get voltage there. Here is some info I found for testing it. Might help. No voltage should throw a code when you turn the engine on.
626 and MX-6 with 2.2L Engine
  1. Remove the air hose from the throttle body.
  2. Detach the 3-pin throttle sensor connector.
  3. Connect the 49-G018-901 testing harness or equivalent, between the throttle sensor and the wiring harness.
  4. Turn the ignition switch ON.
  5. Verify the throttle valve is fully closed.
  6. Measure the voltage at the black and the red wires of the testing harness using a precision voltmeter with a scale of 0.01 volts, the voltage at the black wire should be approximately 0 volts and the voltage at the red wire should be 4.5-5.5 volts.
  7. If the voltage reading is not as specified, check the battery voltage and wiring harness, if these are okay, replace the engine control unit.
  8. Record the red wire voltage.
  9. Measure the voltage of the blue wire, verify that the blue wire voltage is within specification according to the red wire voltage. For example; if the red wire voltage reading is 4.50-4.59 volts, then the blue wire voltage reading would have to be within 0.37-0.54 volts.
  10. Hold the throttle valve fully open.
  11. Measure the blue wire voltage, verify that the blue wire voltage is within specification according to that of the red wire voltage. For example; if the red wire voltage reading is 4.50-4.59 volts, then the blue wire voltage reading would have to be within 3.58-4.23 volts.
  12. Check that blue wire voltage increases smoothly when opening the throttle valve from closed to fully open.
  13. If the throttle sensor does not perform as specified, adjust or replace the sensor.
  14. Turn the ignition OFF.
  15. Detach the testing harness and reattach the throttle sensor connector.
  16. Disconnect the negative battery terminal and depress the brake pedal for at least 5 seconds to eliminate the control unit malfunction memory.
Customer: replied 8 years ago.
This vehicle has a 2.5 liter V6, and I followed the procedure for this in my Chilton manual, it said to:
1. Detach the connector from the throttle position sensor.
2. Connect an ohmmeter between terminals IDL and Gnd
3. Rotate the throttle valve fully open, the ohmmeter should read 0.1 - 1.1 volts
4. With the throttle valve fully open, the ohmmeter should read 3.1 - 4.4 folts
5. If not as specified, adjust or replace the throttle position sensor

These are directly from the manual
It cannot be. If you unplug the throttle position sensor,where would the voltage come from? You can check resistance,but you have to look at the harness to supply voltage. Make sense?
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Customer: replied 8 years ago.
Yes, so how does the 49-G018-901 testing harness work? How is it connected?
Customer: replied 8 years ago.
I think this repair is beyond my capabilities and equipment. Thanks for all your help.