I'm diagnosing this issue utilizing the AJ6 4.0L Engine management manual guide. I'm at the point where I've tested the signal voltage with the signal spade disconnected from the EMS harness, ignition switch on, engine not running. I'm getting a reading of about .7 volts, which is above the .4 volts of where it should be. Could this be my issue? All my grounds checked OK,, as doe all my connections.

Any thoughts and advice would be helpful

Thanks

 

The way I read it, it seems to say that if the voltage is 'approximately 0.4 volts', then the sensor needs to be replaced ..the question, I suppose, is whether your 0.7 volts is 'approximate enough' and within the range of failure?

If it were me, I guess I'd just swap out the sensor. I know people rant on about aftermarket ones, but if you get the long tail three-wire one from Rock Auto, the cost is substantially less than the OEM one and probably made in the same factory.

tolerance should be clearer though, just what is 'approximately 0.4 volts' anyway?

good luck with it

Larry

 

Thanks for the reply Lawrence and that was my concern also!!! I'm thinking it's making it fail. I've seen in other repair diagnostcs where there would be a " + or - " voltage range, which is not stated here.
And thanks for the rock auto tip... Those prices are considerably less!

 

Hi Marco,

Your 0.7 volts is probably close enough, but the only question I can think of is: when you measured the voltage, had the engine been running long enough (just before you shut it off) for the O2S to be at operating temperature and the ECM to be in closed-loop fuel metering? Based on the sequence in the flow chart, that may be assumed, and may slightly effect the ECM signal connection resistance. Might be worth double checking just to see, but if your O2S is original to the car, it really is time to replace it regardless. Performance and fuel economy will likely improve even if the old sensor has not completely failed.

Cheers,

Don

 

Hi Don,
Yes i ran the car until it reached operating temp. This O2 sensor is only about a year old,,, it ohms out OK and gets battery voltage. While observing the switching voltage, the various readings are all under 1 volt, so I'm now researching the DTC 23. Im at the point where i need to attempt to adjust the feedback voltage via the MAF potentiometer, but my MAF doesn't have the adjusting screw and the area where it should be has a plug. So I'm at a bit of a standstill. Any thoughts where to go next?


Thanks
Marco

 

Hi Marco,

Did you tell us before that you also have FF23? Are there any other DTCs you haven't told us about?

Off the top of my head, I think the return voltages from the O2S should switch between just above 0 volts (say, 0.2 volts) and just below 1 volt (maybe 0.8 volts). Does that sound like the behavior of your O2S? I believe there is a chart in the EMS manual showing an expected O2S signal.

Only the early cars had that adjustment screw on the MAF sensor, which I seem to recall was for setting the base idle speed.

How is your battery voltage?

How long since you cleaned the ground points referenced by the engine managment system (the two on the intake manifold and the one on the firewall behind the cylinder head) and both ends of your engine ground strap?

Is there any indication that your engine coolant thermostat is stuck open or partially open or that your Coolant Temperature Sensor may be stuck on at a "cold" resistance? You can test the resistance when the engine is cold and then again when it's hot and compare the readings to the chart in the EMS manual.

Pull the vacuum hose off of the Fuel Pressure Regulator and look for signs of wet fuel that could indicate the diaphragm has failed, allowing unmetered fuel into the intake manifold. If you don't see any fuel, turn the key to Position II and allow the fuel pump to run through its priming burst, then check the FPR again.

Any indication one or more fuel injectors could be leaking or your carbon canister purge valve could be leaking?

How long since you did a basic tuneup - plugs, air filter, fuel filter, distributor cap and rotor, cleaned all the important grounds, cleaned the gunk out of the air intake plumbing, flushed the TPS, etc.? Sometimes DTCs are the result of multiple systems operating a little suboptimally.

 

Hi Don,
Yes,, all the tune up stuff was done. Last fall the car wouldn't start on me and it turned out to be a bad cap and rotor. Those were replaced. 3 weeks ago it wouldn't pass smog do to high hydrocarbons so it was given a full tune-up.

The only thing that I'm thinking the culprit might be are the cats... or the MAF sensor. I presently can't find CA legal cats,, so the MAF might be worth replacing,, even if its a used one to see if it changes anything.

I really appreciate your input as a second set of eyes are always helpful!!

 

So you decided the 0.7 volts reading isn't in the 'approximately 0.4' replace range, therefore it's OK?

Larry

 

Larry
I dont know if you read Dons reply he seems to think thats Ok. The sensor isnt that old and as i mentioned earlier it checked out ok.

 

Well if the sensor is ok, then I advise a good clean up / tighten up of all the connectors, relay sockets, intake cleanout, swap a few relays around on the firewall especially the oxygen sensor heater relay, check for vacuum leaks, make sure big concertina hose isn't buckled and letting in extra unmetered air, change air filter, clear the codes and drive her around Italian tuneup style to blow out cobwebs plus tighten up the maf connector and clean the pins.

Then see if the code returns ....

Larry

 

Well today i had a pre-smog test. It failed but Confirmed its running rich. We ruled out the cats since that portion of the test checked ok. So at this point i wil diagnose the DTC 23..MAF sensor...ECT sensor...and maybe replace the O2 sensor.

It just had a major tune up so those parts were replaced. Guess time will tell!!

Thanks for the resppnse!!

 

For rich running primary suspects are the coolant temperature sensor and a stuck-open thermostat.

Just because your CTS is new doesn't mean it is operating properly or there isn't a problem with the electrical harness. An open circuit in the harness equals high resistance equals the ECM seeing a "cold" signal from the sensor and applying cold-start fuel enrichment even after the engine is warm.

 

Thanks for that info Don. I will definitely run through the ECT flow chart and see how that plays out.

 

You don't necessarily have to go through the flow chart to do a basic check. Just measure the sensor resistance when the engine is cold, then again when the engine is warm and compare the readings to the chart in the manual. That will tell you if the sensor is working. Inspect the wiring harness near the sensor for signs of damage. Common problems include the electrical connector deteriorating and/or the wiring insulation hardening and breaking.