Missing O2 sensor?
 

Should my 84 GMC with a 305 4 bbl have an O2 sensor? I can't find where one was if it did. I've ran my hand all along the exhaust from the manifolds to the cat and there is no hole or O2 sensor. My truck has failed emissions test and I'm been trying to figure out why. I've only had it over a year and it past when I first bought it. I've not made any changes either. I've actually not driving it very much. The exhaust is stock and has no holes. Its a little lose aft the muffler but not enough to leak. The second time I had it tested, they said it wouldn't even read??? The first test the hydrocarbons where a little high so I started studying it and I changed the EGR value and replaced a few vacuum hoses before my second test. So after it failed again, I thought i would check the O2 senor next but to my surprise there isn't one. The cat is there but what the heck no O2? Anyway the exhaust appears to be all stock and doesn't look like anyone has ever messed with it expect from the cat and the muffle on back. Any ideas? Thanks.

Re: Missing O2 sensor?
 

My '86 does not have one, and never did. I'm pretty sure yours is not supposed to unless, maybe, it has California emissions.

Slonaker

Re: Missing O2 sensor?
 

I may be wrong, but I believe it would only have one if it were fuel injected, which yours shouldn't since it's not an 87 or later model.

Re: Missing O2 sensor?
 

I agree with slonaker... my 84 has a 02 sensor and a ecm... to set off the 'check engine light'... if it's burning incorrectly.. it is more of a California Emmisions thing... does your truck have a ecm?.. it might not.. and probably doesn't need a 02 sensor.. if it's a non-california emmisions truck

You know... a lot of it could be just adjustments on the carb too... a lot of times.. those testing centers won't even touch the carbs... and just test em how they are..
More often than none, those shop guys don't know how, or don't like to fiddle with these q-jets (or any other type carbs, don't know what you have) anymore. They're used to those fuel injected in and out jobbers in these new cars.

Knowledgable 'old school'.. shop guys who know what's up with the old carbs are a dying breed.. (over here they are anyway)

Re: Missing O2 sensor?
 

Everyone is right there is no O2 sensor on mine. I read in the Haynes that it is on the manifold for 8v's but never says Cali trucks only. So I looked again and there isn't one on mine. So now I've been testing the EFE "heat riser" value and it appears to not be opening own its own. It's either the temp switch or the heat riser value. When I try to move it by hand its very hard to open. If I use a screw driver, I can open the value. I've replaced the vacuum hose but i can't tell when i remove it if there is vacuum or not. Is this why I'm failing emissions test?

Re: Missing O2 sensor?
 

Actually EFE (Early Fuel Evaporation) can be considered one of the most lame ideas that GM came up with before fuel injection. The EFE bolts to the right exhaust collector. EFE is flapper valve - designed to restrict exhaust gas flow when coolant temperature is cold forcing instead exhaust to flow through intake manifold exhaust cross over passage. In grand scheme of things it should result in faster warm-up. Once warmed - the vacuum control switch would shut off vacuum and spring loaded EFE would move to a fully open position. I had one of these things on '76 305 Chevy Malibu - never worked right as spring and vacuum control modulator were flaky (and that was 20+ years ago).


//RF

Re: Missing O2 sensor?
 

RF-Did you remove the EFE?

I pasted the emission test today though, Yeah! All I did was add a bottle of rubbing alcohol and I also went to a different testing center. Figured the other one might be bad luck.

I'd still like to know if removing the emission equipment except the cat would be worth it.