so my 242 gt idled horribly, failed emissions, high HC only at idle. had to take it to a garage so that i could get a 'conditional pass' and they said i was running too lean. ok $180 later car failed emissions again, high CO, too rich. anyway got conditional pass so that's good. but now the car wont start without some serious peddle work. long story short, i want to install an oxygen sensor (my '79 doesnt have one) and hook it up to the ipd air-fuel mixture gauge so i can monitor it and adjust it myself. anyone know if you can just install an O2 sensor in the exhaust manifold, run a lead and a ground and hook it up to a gauge? does the O2 sensor operate independently of the computer and other electronics? i see from a wiring diagram that O2 is attached to an electronic module attached to the thermostat and frequency valve. i'm presuming that those connections are more so that the sensor can make adjustments to the mixture. all i need it to do is send a signal that the gauge can read. i read online that there is chemical reaction in the O2 sensor that results in varying voltage depending on levels of exhaust gasses, so it seems like it should be okay without the computer. any guesses? suggestion? opinions? thanks for any help
sp

Hi sp,

I also own a 79 B21F 49-state with no lambda and EGR. I wanted to do just as you want, measure the a/f ratio using a common narrowband sensor, noting the manifold has the EGR pipe screwed in where the sensor is on my later cars.

That's when I pulled the pipe and compared the threads. Looks close until you get them side by side. Then I figured to wire in a heated sensor to the downpipe, but there was no bung.

You could go beyond where I gave up by welding a sensor bung into your headpipe, or tapping one into the manifold (maybe) and then read your sensor with a voltmeter or one of those aftermarket a/f meters to zero in on the mixture adjustment on the airflow sensor. There's no need for the computer; the sensor will just output near 1V when rich and near 0V when lean. It will flip from one to the other very sharply when the mixture is near ideal for CO.

If you go the sane way and put a bung in the downpipe (a muffler shop should be able to handle that without a painful bill) get a universal heated sensor and power the heater leads.

Your presumptions about the function of the controller and frequency valve are correct. You will be manually taking the place of the controller with your corrections.

Here's a photo of what I made up to test, when I thought the downpipe's test port was an oxygen sensor's bung. The connector you see on the sensor's harness mates with an old vacuum tube voltmeter. I wasn't going to drive around with this, just use it to get by emissions, like you. Because I never used it, I diddled the a/f mixture screw listening carefully to the motor, learning to hear the change in its rhythm as it crosses from lean to rich during idle and back again. I took the re-test and passed.

nice, thanks for the info. i was thinking of pulling a sensor out of a wrecked newer vo at the wrecker, but i guess a new one would be smart. it was $180 (that's CDN coin mind you, so about $4 US j/k about $125USD) because they charged me $80 just to hook it up to their machine, and then the rest labour to mess around with it, then another $23 to re-test. air care in vancouver is such a scam. anyway i did just replace cylinder head gasket, intake manifold gasket is new, i did some mild port work on intake and exhaust, i think thats what leaned me out in the first place. #4 spark plug did look a little more gummed up than the others, but there was oil leaking from head gasket before, i have not had a chance to check it since the new head gasket. im assuming that the air/fuel mixture control is the black screww on the front of the throttle body that you can adjust by just twisting it with your fingers? all i could get out of the mechanic when i asked him was something about a female allen key??? i guess there is an allen key fitting in the screw, i should have bugged him more about it. anyway, looks like im going to go ahead with the sensor and gauge set-up, ill let you know how it goes!
sp

Hi sp,

I just made use of an old sensor, or rather intended to.

The mixture setting is the 3mm allen reached through the top of the airflow sensor, not the black knob used for base idle air bypass. If you don't have the books, there are some good websites devoted to k-jet afficionados explaining things like base idle and mixture adjustment along with k-jet theory. Either someone will add the links to this thread, or you might post another question more specifically asking about them.
--
Art Benstein near Baltimore

Be careful about reading health books. You may die of a misprint. - Mark Twain