Replacing O2 sensor

['89Z71]

How do I know when its time to replace my O2 sensor? I have an '89 GMC Sierra Z71 w/ the 350 but my engine is out of a '68(or around that year). I have heard that a bad sensor can hurt gas mileage, is this true? Any other info on them would be greatly appreciated. TIA, DK

[Gee_Emm]

you have a 68 engine in a 89? The o2 sensor should be on the exhaust manifold. They make o2 sensor sockets or you might be able to get to it with a wrench. But if you have a 68 engine in the truck is the rest of the computer parts hooked up to it? I would think that if you have a o2 sensor still it won't be hooked up to anything.

['89Z71]

Yea the engine is about a '68 and I know the O2 sensor is there I just cant remember if its hooked up. The computer and everything is still hooked up because of the '89 TBI and the stock intake manifold. I'm just wondering if its nessecary to replace my O2 sensor?

['89Z71]

BTW: with that '68 engine matched up with the stock 5 spd, My truck really gets up and moves

[cableguy0]

ok so you reused the tbi intake and harness and everything just used the long block from the 68 am i correct? if you are using the fuel injection and its running crappy theres a good chance the o2 is bad but if its running ok dont mess with it

['89Z71]

cableguy0- Thats pretty much the extent of it. The old carb and intake was taken off and replaced by the stock '89 TBI and intake. Everything else on the engine is '68 except for the belt ass. on the front. My truck runs fine but gets bad gas mileage and I've heard that O2 sensors can affect gas mileage(is this wrong???) and if mine is bad and I replace it will I get better gas mileage?
I appologize for the stupid question, but the more Questions I ask the more I learn about how my truck works

[cableguy0]

lots of things can cause bad gas mileage but the 02 is the place to start in most cases. is the check engine light on or has it came on at all in the past

['89Z71]

Yes my check engine light comes on now and then, sometimes after only 4 miles of driving, sometimes after 20, sometimes it doesnt come on. If I turn off my truck and turn it back on the light will be off.....

[Longhorn Man]

Before spending any money on sensors, you may as well put a scanner on it. Auto Zone can do it for free, but I'm willing to bet that they will tell you stuff that isn't wrong with it just to sell parts. (I may well be wrong though)
Being that your service engine soon light does come on, there is at least one code in the memory.
You could throw a 30 - 50 dollar O2 sensor at it...but it could be something else all together, and then you'd just be throwing money away.

[cableguy0]

the biggest problem is that truck is pre obd2 and they usually dont save codes.you need to put it on the scanner while the check engine light is on. also any decent shop should have a snap on scanner basically the only one worth using. with that scanner you can watch the o2 sensor switch which it should do every couple seconds switch high then low to keep the fuel mixture right

[68LSS1]

Codes will be retained in the computers memory on any GM. These are known as "hard" codes. "Soft" codes are ones that reset. I have a Snap-On scanner and it was probably one of the best purchases I ever made. OBDII is primarily for emmisions and it's suppose to let you know that there is a potential problem prior to a sensor going bad. It sounds to me like '89Z71 may have a bad O2. I'd bet what's happening is the O2 isn't going into closed loop after it warms all the time like it should. A lot of times a bad O2 will not set the check engine light.

[Phillip Johnson]

Your 89 saves the codes. I know because I has a pos 89 crew cab duellay that GM bought back. All you need to do is use a jumper wire in the terminal under the dash and then count the blinks on the light and that will tell you which number to look in the book for the code. You will need a haynes or chilton book to know what terminal to short out and to read the correct number. As for the 0 2 censor, if you replace it, dont touch the end that goes in to the exhaust. it will fubar it. If you are driving an oil burner and throwing alot of oil smoke, chances are your 0-2 is toast. Why did you put a 68 motor in a newer truck? How did you get past your local smog man?

['89Z71]

I didnt put the engine in there, the PO did prolly cuz they blew up the orrigional and found this one to swap with. It does burn some oil but only smokes on start up. I live in Wisconsin so there isnt a smog check... unless you get pulled over... then if your missing your cat or mufflers you'll recieve a fix it ticket(luckly I havent been pulled over yet)

[ddsmith]

Here is the easy way to check engine codes. Find the ALDL connector(where the computer would hook up). It is normally under the dash by the steering column and has a plastic cover. There are a bunch of female ports in the connector. Put the ignition in the run position with the truck off. Take a metal paper clip. Not the coated ones just a plain old cheap metal paper clip. Bend it into a U shape so that the two ends are close together but not touching and are the same length. Insert the paper clip into the upper two right hand female ports which are normally referred to as A and B. Basically jumper the two together. The check engine light will flash 1 long flash and two short flashes three times. It will then flash any additional codes set into the computer. Even with the check engine light not on the code that caused the check engine light will be present. Count the number of short flashes and long flashes and that equates to the code. For instance three long flashes and then 4 short flashes is a code 34. They always have a series of three. Continue getting the codes until code 12 flashes again and that is all that is set in the computer. Just for information the electric cooling fan runs when doing this so don't be frightened by it coming on. I hope this has been helpful. I just get a kick out of those code readers they sell for $30-40 when a paper clip will do the same thing. I'll also come back with a site for telling you what the code are if you don't have a factory service manual. If the O2 sensor is not hooked up I would understand why mileage is crappy. The computer controls the TBI based on it's input along with a bunch of other things. If it is sensing a lean condition it will richen the mixture and kill mileage. Good luck.

[ddsmith]

Here is a link to a good site for trouble code info and it discusses the procedure I explained in my previous post. Trouble code information Happy troubleshooting.

['89Z71]

Thanx for all the great info so far!
ddsmith- I will try that tomorrow if i get time. Thanx for the detailed info and the link