I hooked up my laptop to my car today with the win aldl program to see if I could find out why it's running bad. The two codes I got were MAF voltage low, and that the 02 sensor was rich. Does anybody of any idea what this could mean.

 

check the grounds on the back of the cylinder heads (both sides) then do a search, this has been covered i believe.

 

Where are the grounds you're talking about exactely?
Any pics?

 

Yes. What this could mean is that the MAF sensor is not sending an adequate intake air signal to the ECM, as defined in the MAF tables in your PROM for a given engine RPM and throttle position. As a result, the ECM is ignoring the MAF sensor, and has reverted to using Backup Fuel and Spark Mode data tables to keep the engine running instead of leaving you stranded. These tables are necessarily rich so that the engine does not inadvertently go lean and cause mechanical damage. The oxygen sensor is detecting this exrta rich fuel mixture by the absence of oxygen in the exhaust gasses. Chances are that if you repair the MAF problem, the O˛ sensor problem will go away as well.

But that's just a guess.

Additionally, and as long as you have your laptop connected, what is the ECM reporting as a either a MAF output voltage of grams/second of intake air signal, and at what engine RPM and throttle position?

 

There is a short 3/8-16 bolt on the rear face of the right head that should have several ground wires attached to it. It is difficult to access, but you can often "clean up" the grounds there but slightly loosening, repositioning the wires, and tightening the bolt.

 

Thanks, I will look for that, maybe it wil clear my intermittend maf burn off problems!

 

If you are having intermittent problems with MAF burnoff codes, and have checked "everything", have you looked into teh auxiliary oil pressure switch? An unexpected input there during the burn off cycle can interrupt it and set the code.

 

No, I haven't, can you please explain more?
Where can I find this switch, what does it do, and how can it affect the burn of mode?

 

The auxiliary oil pressure switch is located (buried, actually) next to the distributor, adjacent to the oil pressure sending unit.



On some later engines, the switch and sending unit are combined into one unit, and it may be found either above the oil filter adapter or near the distributor.

In any case, the switch is intended to be a redundant path of power for the fuel pump, in case of failure of the fuel pump relay. It is [NOT a fuel pump safety shutoff to disable the pump on loss of oil pressure, as some might believe.



My understanding of the logic of the ECM is that the ECM burn off sequence is dependent upon several inputs:

1. The ECM must have had an ignition input, distributor reference pulses, and a MAF signal.
2. Once the ignition power signal is removed, the fuel pump circuit signal is removed, and distributor reference pulses stop (engine is shut off), the internal 20 second timer will begin.
3. Within this time, the MAF power relay is shut off, the MAF burn off relay is energized, and the resultant connections in the circuit will provide power to the MAF via the N.C. contacts of the MAF power relay.
4. The MAF burn off terminal AND the MAF power terminal receive 12VDC from the burn off relay, which causes the MAF hot wire to glow to an incandescent state to burn off accumulated deposits.
5. The current required to heat the hot wire to this state is considerably higher than normal operating current, which is what is actually measured by the ECM to determine intake air flow.
6. The ECM must read this current via the SIGNAL circuit to prove that the MAF hot wire has been heated successfully.
7. Any operation of the ignition switch during this cycle will terminate the burn off function and cause a repeat of the burn off cycle as soon as the ignition is shut off.
8. Any distributor pulses received during this cycle will terminate the burn off function and cause a repeat of the burn off cycle as soon as the engine is shut off.
9. However, any operation of the fuel pump circuit (terminal B2 on the ECM) during this cycle will interrupt the burn off function and may cause an error code to be set. Moreover, since the fuel pump is not a criteria for initiating the cycle, the burn off cycle will not repeat after the oil pressure switch opens again. This may cause a failed burn off cycle.

The ECM will tolerate several incomplete burn off cycles before setting an error code, which allows a short interval between restarts without error codes. However, the ECM logic apparently will not tolerate a failed burn off cycle. In this way, an oil pressure switch that is intermittent may interrupt the cycle within the 20 second span without initiating another burn off attempt.

I am not certain, but I believe this 20 second timer is the same internal timer used to prevent short-cycling the fuel pump on restarts.

 

Thanks for your explanation!
You say the ecm will tolerate several incomplete burn off cycles, but what happens with my car is as follows:
After shutting of the engine normaly I can hear the relay clicking twice with 1-2 seconds between the clicks, this is setting no code.
But sometimes I hear two clicks within one second, or no clicks at all, setting the code for the malfuntioning burn of system.
Again, this is intermittend and very irritating and has me puzzeled for a long time now!
Can the oilpressure unit you described above be the cause?

 

You should only hear one click. But instead of testing audibly, connect a voltmeter or test light. The fact that the relay is closing, then opening, then closing is indicative of an interrupted cycle. You can disconnect the large tan/white wire from the oil pressure switch to isolate it from the ECM. The engine should still run normally and doing that will set no codes.

 

Thanks, I will try that.
Suppose the oil presure switch is the problem, how should I cure that?
Just change it for another (new) one?
About the "clicks" I think I hear the relay closing (click one) and after about 1-2 seconds I hear the relay opening again (click two) burn off completed.
So it's in my opinion not starting the burn off cycle twice, correct me if I'm wrong.
But sometimes it's clicking very fast, opening and closing without starting the proper burn off cycle, and setting the code as a result.

 

That sounds like a plausible explanation for what you are experiencing. I simply wanted you to be aware of the possibility of the oil pressure switch contributing to the problem.