Check Engine Codes

[bolokid]

I have a 91 Festie with a/c, p/s and an automatic. Just prior to driving to Westiva I rebuilt the B3 which now has about 2500 miles since the rebuild. Since the rebuild and on the trip to Westiva the car is only getting 30 mpg instead of the 40 mpg that it used to get with the old engine. The car runs fine, no missing, no stalling and no hesitation.

My problem is that the CEL randomly flashes on for about 5 minutes and then off. I replaced the O2 sensor and thermostat during installation of the engine. The temp gauge usually runs about 1/4 of the way.

I just grounded ST1 and read out the codes: 8, 15 and 17. I located on the forum where 8=air flow sensor, 15=O2 sensor and 17=O2 sensor.

Is it possible that there is one component that is bad that is causing a domino affect resulting in 3 codes?

How do I check out the parts (O2 sensor, VAF)? Why 2 codes on the O2 sensor?

Thanks.

[tooldude]

I believe that if you instantly get codes for two or more sensors at the same time, that will usually occur from one root problem. Yes, MECS and even the OBD I & II programs can set codes that are indirectly related to the problem rather than the actual cause. The common denominator here is the O2 reading in the exhaust. Maybe you should start by checking for an intake or vacuum leak. I was getting codes 15 & 17 and the intermittent CEL on my carb car because it needed a new O2 sensor. Since you changed that, make sure the connection is good (also on the VAF) -you are using the Denso sensor? The best way to check an O2 sensor is with an oscilloscope. Otherwise, you can check it with a DVM on the millivolt scale. With the engine warmed up, look for varying voltage between .1 and .9 volts. The spread should not be too narrow. You should see a minimum range of something like .3 to .7 Just as important is the switching speed from high & low, it should not be stuck at 1 or 2 cycles per second. When O2 sensor codes are set, the ECA operates in open loop mode which affects mileage. All I have time for.

[bolokid]

The O2 sensor is a Bosch, I will place the order for a Denso when I get home from work today. I do not have a scope at home but do have a DVM and will check out the O2 sensor tonight.

The switching speed you refer to is the readout from the O2 sensor? Where do I check for the voltage coming from the O2 sensor?

Thanks for the information.

[tooldude]

The switching ("crosscount") speed is the changing between the low & high voltages (from a 450mv center) the O2 sensor produces once it's hot (around 600*F). Set your meter for DC millivolts before connecting. Backprobe or (use a jumper) where the pigtail plugs into the harness, not on the sensor boot itself, with the positive DVM lead, the negative lead going to ground or negative battery terminal. The fastest switching speed occurs around 1500*F and around the stoichiometric air/fuel ratio (14.7/1). If you idle the engine too long and the sensor cools below the 600*F threshold, it will stop producing the required voltage and you will have to increase RPMs to heat it back up. You should see switching at idle occur at least once in 1-2 seconds and several times per second at cruising RPMs with a range of around 300-800 mv. I don't know if there is any exact spec on this, but you would need a scope or other than a basic DVM to use specs. A relative comparison from new, on an aging sensor, is sufficient because the ECA will set a code. There are other possibilities for these codes you are getting so it's not necessarily the O2 sensor, but I have had Bosch sensors to fail in relatively short mileage, especially the universal type to which you solder your own connector. The codes 15 & 17 seem to indicate the "lazy" slow-switching sensor. The 1992 Emissions Manual gives Code 15: EGO Sensor - voltage always below 0.55V (lean); Code 17: EGO Sensor - voltage has not changed 30 sec. after the engine exceeds 1,500 RPM. The 1990 Emissions Manual is slightly different but seems to indicate staying below 0.55V (lean) & above 0.55V (rich) for too long. I do not have the 1991 Emissions Manual to get the exact wording.

[bolokid]

I tried to check out the Bosch O2 sensor but was not successful. I changed out the O2 sensor with a Denso from Rockauto as you suggested, cleared the codes and have been driving for 2 days now without the CEL flashing on/off. I filled up with gasoline this morning and will check the mpg's. Thanks.

[bolokid]

Update on gas mileage

After changing the O2 sensor the gas mileage was still 30 mpg. I changed out the Mass Air Flow sensor with a working one from another Festiva and the gas mileage was still 30 mpg. I changed out the temperature sensor which is located by the injectors and the gas mileage was still 30 mpg. I rechecked the timing for about the 10th time and the white line was right on the pointer. I think Zoom Zoom suggested advancing the timing so I advanced the timing some (estimated 5-7 degrees) and checked for pinging and all was fine. Gas mileage for the next tank was 37.5 and the fillup today was 41.0 mpg.

Thanks to everyone for suggestions in finding the solution.