Hi,

My Mazda 626 1995 GE 2.0 l auto is idling roughly. It isn't missing - just rough, particularly when idling in drive with the airconditioning on.The engine does not run as sweet as it didat all speed ranges but, at idle, engine shake is now very evident. Sometimes the engine hunts at idle and stumbleson driving off.

Ihave just about run out of ideas as I have replaced the spark plugs and leads, checked fuel line pressure, done a hold test on the fuel pressure, checked the manifold vacuum. I have also changed the EGI temperature sensor, done a compression test and the checks I have made seem to indicate that the ISC valve is working. The throttle position detector has the correct resistance when there is no throttle and on full throttle. The off-throttle switch is functioning correctly. I can disconnect the vacuum hose to the EGR and it makes no difference. The PCV is fine.

If the engine had a carburretor I would think the mixture was incorrect but of course it is not adjustable with fuel injection.

Therefore I am getting very frustrated and would greatly appreciate any assistance.

Thanks
Glenn

 

Could be a exhaust leak or oxygen sensor is bad. Thats about the only thing I could add.

 

Have you tried adjusting the base idle speed by removing the two wire 'spout' connector behind the left headlight area. Will just be two wires Green and the other one Yellow looking as if they are just plugged into a plastic plug. Pull that shorting bar plug out and make sure base idle speed is around 700-750 and base timing is 10' BTDC. Base idle is adjusted by a air bypass screw on the throttle body right near where the throttle cable connects up.

 

Thanks Babyhuey,

Yes, I have adjusted the idle speed as per the genuine Mazda Workshop Manual. Yesterday I cleaned the airflow sensor using CRC electronic circuit board cleaner. I must admit, until I found this forum a couple of days ago, I did not know it was something one had/should do. The A/F sensor came up pristine clean and I think it has marginally inproved the running of the vehicle but there is still a significant fault.

It is this.It is the situation whenthe car is operated until the engine reaches normal operating temperature and then the engine is turned off for a few minutes. When the car is restarted, it starts immediately but after a few seconds the idle speed drops right down well below 700 rpm until it almost stalls. Alternatively the engine will hunt badly. If the accelerator is touched, it idles satisfactorily again and it will be OK when one drives the car but with some engine shake at idle when in drive/reverse. Any clues would be greatly appreciated as I am fast running out of both ideas and patience.

Regards
Glenn - Brisbane Downunder

 

The Epilogue

I cleaned out the throttle body which I found was caked in carbon and muck. I expected it would fix up the problem but it made no difference.

By connecting a voltmeter to the oxygen sensor at the ECU, I observed the output voltage at different driving conditions and as it swung from close to zero volts to about 1.0V several times in a 10 second interval it seemed OK. However, when the car was started after the enginehad beenhot buthad beenleft to cool for 5 to 10 minutes, the voltage from the O2 sensor started out at greater than 3.0V and progressively fell over a few minutes presumably as it heated up. I suspect that the primitive software in the ECU saw the 3.0V signal as an indication of a super rich mixture and then attempted to cut down on injection time. In reality this resulted in a very lean mixture causing the engine to idle poorly and to start hunting. Anyhow replacing the O2 sensor fixed up the idling problem.

I understand that the maximum output of the O2 sensor is 1.1V but as I could not measure any leakage from the heater to the sensor part I am at a loss to explain how the output voltage could exceed 3.0V. Can anyone please explain this phenomenon?

Regards
Glenn