camhar25

Took the car to the stealer last week for diagnosis.They said the airflow meter numbers were way out of wack.Changed the airflow meter and the car is still running rich and the idle is up and down or stalling out.Blowing black smoke when idling and still no check engine light.Any ideas?I also instlled a new idle air control valve,and a temp sensor thinking these could be bad.In all my experience the check engine light should be on.

cliffyk

Sorry I missed this--the O² sensor heaters are all connected to the same +12V source, the same source that powers the EVAP stuff, and switched to ground via MOSFET transistors in the PCM.

If the heater is not being powered as it should then there is a wiring problem with the +12 V source or the switched ground wire running back to the PCM--or, sit down for this, the MOSFET in the PCM is blown.

Here is a kludgy sort of test/solution. Get an older 4-wire O² sensor with a 6.0 Ω heater; on our stock sensors the heater is 3.3 Ω, more about this in a bit. The Bosch 13275 is a good pick as it was used on a bunch of mid-90s cars and is usually cheap--$25 to $30.

The 3.3 Ω heaters cannot be continuously powered without burning up, however the older 6.0 Ω units can be--so installed the older sensor with the sensor output wires connected normally, but with the heater wires connected to ground and a switched (HOT in run) +12 V source that can supply at least 3 A.

This will result in a functioning, continuously heated O² sensor that will make the PCM happy...

cliffyk

They didn't charge you for the new MAF did they? If so then at least get the old one back from them.

As I said in an earlier post failing front O² sensors would cause this, and could do it for a while without throwing a DTC; monitoring the short term fuel trims will also reveal this situation. The O² sensors cannot be properly tested in situ without an oscilloscope, however armed with a scope their output at idle should look like this:



The signal will oscillate ("switch") 3 to 4 times per second between 0.2 V and 0.85 V. However the voltage range can be as low as 0.0 V to 0.6 V and still not throw a DTC, thing is this will be telling the PCM the mix is lean and it will compensate by adding fuel--up to +25% before throwing a code.