Code 51, horrible mileage...

[Mike M]

This is on my 88 Plymouth Sundance RS. It has an '89 2.5L (BONE stock, lower miles, resealed including headgasket) with mitsu and all, non intercooled. Also has a '89 2.5L SMEC installed. Before I get into the problem, the vehicle had a new Mopar 3-wire 02 sensor installed, when it was installed I was getting the best mileage the car had ever gotten, even with the 2.2T1 or the 2.5T1.

The issue: This last tank of gas has been the worst I've ever got... rough estimation of about 10-12 MPG. This is normal driving (not being overly agressive), cold weather, car runs for extended periods of time before moving (have a command start). I know the cold and extra running can impact mileage, but this is just stupid.

I decided to bring it into the shop at lunch today (limited time here!) - found a new code I'd never had before: Code 51, too lean too long. Well this is obviously an issue, its reading lean thus dumping in extra fuel to compensate resulting in shite gas mileage.

Checking the "datastream" on the scan tool showed the 02 sensor holding LEAN, and the 02 voltage on the scanner read 0.00V. Checked the 02 sensor with a multimeter, the heater circuit appears to be working normally - one 12V and one ground. Checking the signal wire is what is really confusing me here - with the sensor unplugged I'm getting 1.02V on the 02 sensor connector itself. Cecking the connector on the harness side I still have 1.02V... when I plug the connector in and backprobe it... 1.02V.

I know I'm getting the difference between scan test 02 voltage and actual 02 voltage but... I was pretty much ignoring what the scan tool said and am judging by the multimeter readings.

So what is going on there? The 02 is supposed to generate its own signal of 0-1V depending on the oxygen content of the exhaust. So why do I have a constant 1.02V on the harness side of the connector without the sensor itself plugged in? Is there any kind of rerence signal from the ECU at all? (I ask this because I was told that GMs use a .5V refence voltage for the 02 sensor until the 02 sensor warms up and starts to work). Or is it possible I have a short or wiring issue? Or just a bad ECU? Or is it just a bad sensor?? (Which I just replaced 5-6 months ago or so).

Any input or help guys? I'm wondering what path I should take next. Should I test all the wiring between the sensor and ECU? Then I can at least try and ECU and sensor with a little more certainty.

Thanks for any and all input!

And sorry for making this so long. I may be an apprentice tech but crap like this still confuses me!

[turbovanman²]

It sounds like you have either a bad computer or the heater pos wire is bleeding into the signal wire. I would carefully go over the wiring and trace it back to the computer. If ok, check all the grounds, I like to use a highbeam headlight as my tester-hook one wire to power, then use the other wire to check your grounds. This will put 15 amps or so thru the grounds and checks them properly. If the headlight bulb lights bright, ground is good, if dim, ground is bad. I bet you'll find bad wiring, bad ground or if not, computer took a dump,

[GLHNSLHT2]

+1 on checking the wiring very good. You can check the codes with just the ignition key, don't need a scanner