Question about 1985 Nissan Pickup

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Pu trucks 4x4 24s motor

Terrible fuel econ. fixed the obvious, now what? will the 02 cause this?

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  • Nissan Master
  • 5,252 Answers

When the sensor gets contaminated with unburnt fuel deposits-covered in soot-I suppose it could contribute to poor economy, but I doubt it is the underlying cause.
Do you have the feedback carburetor, with an ecu for air/fuel mixture control through the air/fuel mixture ratio solenoid? This solenoid would be mounted on top of the carb. It can be checked for operation with a voltmeter, look for a pulsing signal-voltage swings back and forth from about 5 to 9 volts. That is, if you can find the diagnostic check connector-on my '87 Sentra, looked all over, finally found it by the fuse panel under the dash-a 7 wire connector, and the solenoid wire to check for the pulsing is a dark green wire.

You will find that Nissan had some real crappy systems in these years (and I'm a big fan of Nissan)-the ecu has no diagnostic code capability, but does have limited diagnostic functions. Do you see the little red and green led lights on the ecu? These are for the "limited diagnostic functions". Were you to go through the diagnostic systems check, the led lights would tell you first if the oxygen sensor is sending a signal to the ecu, then after another step, will tell you if 1) all ecu controllers, connections, and relays are good to go, or2) if some part of the ecu controls on the engine has a problem-little things like the egr solenoid vacuum controller, all the little solenoids, relays, and sensors that the ecu controls-won't tell you which one, just that a problem is up with some part. So much for "limited diagnostic functions"...Then you have to check each one.

Maybe you just need a rebuild kit for the carburetor. Or if you have the dual spark plug system (2 spark plugs per cylinder) you may need to diagnose that, and see if plugs are firing at right sequence. Only reason they put 2 plugs in each cylinder, firing one after the other? Some design flaw, where one spark plug did not get a complete burn of the air-fuel mixture. Nissan's answer was to put two plugs in each cylinder...
I can tell you one thing for sure: don't expect any help from a Chilton manual. Mine was useless. What I did was go to a website called eautorepair.com. This site is run by Mitchell-long known for auto repair manuals-and is very complete and helpful. It is a subscription site, the same one used by professionals with info on all makes. As a DIY-er, you subscribe and get complete info on your specific vehicle. Rates are cheap, too. $12.99/week, $16.99/month, or $25.99/year. Has complete and printable wiring diagrams-I had a 1 month subscr., and saved a dozen pages of wiring diagrams, component locations, and other repair info and help.
You and I kind of in the same boat: had you bought an '86 pickup, you would have throttle body injection-much easier to fix and diagnose. Had I found an '88 Sentra, rather than my'87, I too would have fuel injection, and we could both trash the "feedback carburetors".

Posted on Nov 23, 2013

  • 2 more comments 
  • keith jarvis
    keith jarvis Nov 23, 2013

    Thank you! I will check that out in the morning if the weather holds off. Sounds promising to say the least. I have saved the website to my favs, might be useful. Will surely let you know what I find out!

  • montehammons Nov 23, 2013

    Okay. Another heads up: A lot of wiring between your pickup and my Sentra will be some different. Sometimes those check connectors are under the hood, and sometimes the procedure for the mixture ratio solenoid test or check may be different. I recall some systems, where you connect a "check connector" together, and then check for a flashing light on the dash. I spent hours trying to make sense of this per my Chilton manual, then come to find out, the diag/. connector was under the dash, and I had no such light on my dash.

  • montehammons Nov 23, 2013

    It wouldn't hurt to start off with a known good oxygen sensor, as that is the most critical part of how the ecu maintains the correct air-fuel mixture coming out of the carburetor. The cost for one, if I recall, is about $50. I bought one first thing. If you been running too rich for some time, the sensor is probably coated with soot, Nissan would say contaminated, and should be replaced. They do stress the importance of the oxygen sensor, ecu, and carburetor controls for optimum fuel efficiency. You can tap into the sensor signal to the ecu, I used a sewing style T-pin right into the insulation on the white wire just before it entered the ecu connector, and with a digital voltmeter watch the action of the oxygen sensor readings. Good engine in good tune, understand, good gas mileage-the mixture is right-the sensor will just constantly shift back and forth between 100 and 900 millivolts, real close in that range. 500 millivolts is dead on the ideal ratio of 14.7 parts air to 1 part gas-theoretically the best burn ratio. The ecu varies the pulsing of the carb solenoid, to just run slightly too rich, then shift to slightly too lean- as in 100 millivolts from the oxygen sensor, and 900 millivolts is slightly too rich. So the ecu monitors the oxygen sensor and just keeps mixture right in that narrow range. How it's supposed to work. If you were to monitor your sensor, mine did this too for a time, and it says like 1.283 volts (1283 millivolts), means in my case a misfire, in your case "extreme rich- out of range" and if it doesn't cycle below 500 millivolts ever (o.500 v), running too rich, for sure.

  • Thomas Perkins
    Thomas Perkins Mar 04, 2014

    Go to you tube and see my weber conversion results,do it this way and it will run perfect,it's 85 nissan 720 and find picture of my carb.,it's listed under TommyPerkins,had it over 20 years.

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