Sounds like it's missing when in open loop. Our 92's have a
heated o2 sensor, high idle at start up to help warm the
o2 as soon as possible along with the heater and to get
the CTS warmed up so it can go into closed loop after
the timer settles down. So with all this, how old is the
o2 sensor ? They only go for about 65,000-70,000
before they start to go biased and the older they are
the longer it takes for them to send a signal to the ECM
that it is hot enough to go into closed loop.

Are any codes stored ?
Do you have a way of reading a data stream while it
is heating up and then going into closed loop ?

 

Im not sure exactly how old the o2 sensor is , Ive only owned this truck for a year or so and dont know much about the repair history . It very well could be old. I have recently scanned it for codes and non came up but I currently dont have immediate access to a scan tool, would you suggest me replacing the o2 sensor? thanks

 

I would simply for the fact that o2's will slow down switching
from 0.1 volt up to 0.8 volts and can go biased lean or rich.
One wants an o2 switching at about 1/2 to one second high
and low. Old o2's used to use a galvanic battery for power
and that's why they were just single wire. Also an o2 uses
a minute hole to compare ambient atmosphere to
air fuel mix.

A new one will be fresh and will
do the proper switching it's suppose to. I have used
Bosch, Standard Motor Products and AC/Delco o2's. I feel
it's worth the money to go with an AC. They aren't
that expensive. AC Delco is at the bottom. Click
"more info" for a pic. In fact click the second link.

http://www.rockauto.com/catalog/x,carcode,1060557,parttype,5132
http://www.rockauto.com/catalog/moreinfo.php?pk=87304

Though this may not cure your misfire, it will definitely help with
performance and mileage.

 

So ive been really keeping my eye on this rough running thats happening in the morning on this truck and I decided to buy a new o2 sensor, when I pulled out the old one it was a bosch sensor, didnt look really old, maybe a few years, but I replaced it anyway with a new NGK sensor, and so far no change in the running condition of the truck.
I want to now say that the rough running doesnt completely clear up after its warmed up, after its hot it idles a little rougher than it used to but still very noticibly worse when its cold in the morning. I also checked manifold vacuum and its at 17.5 inches of vacuum. Im thinking about running a compression test and seeing how the cylinders check out. Any more suggestions guys? Thanks

 

How do the plugs look?

How does the CTS test out (compare to the link in my sig.)

Is there an exhaust leak?

 

Also, have you checked for a plugging up cat?

http://www.secondchancegarage.com/public/186.cfm

 

I just recently changed the plugs, wires, cap , rotor about a month ago,
There are no exhaust leaks up at the manifolds, the exhaust system is definitly older, original cat id guess, the muffler and tail pipe look good on the outside but I suspect its rusting from the inside out, Its become more loud and throaty over the past year than it used to be. Ill check the cts today and see if I can see any signs of a clogged exhaust with my vacuum gauge. thanks

 

Just becaue the ignition was done a month ago, doesnt mean it is good or wouldnt give indication to what is going on....

Let us know.....

 

update, so I checked for a plugged exhaust with a vacuum gauge and all looks well. I changed the ignition wires. And still no change ! Plus I need to say that the truck puffs blue smoke out the exhaust after its sat for 3 to 8 hrs. Im sure that its either valve seals or guides, or maybe both with 136000 miles on it. so now im thinking could this be the problem? Is oil leaking down into the cylinders and fouling the spark plugs up ?

Today I drove the truck with the misfire for 5 minutes and didnt let it clear up completly, then I came back 6 hrs later and drove it again and this time the truck really ran like garbage, misfiring constantly, stumbling and leaving black sooty marks from the exhaust on the pavement. So I brought it home and pulled all the spark plugs and found #4 and #6 were really black (not oily). all the other plugs were like new still, so I quickly ran a compression test on #4 and #6 cylinder hoping to find something. and both were equal at 180 psi. So I put it all back together and drove it for 30 mins. The misfire finally cleared up and ran not bad after that but I still could feel it wasnt running as smooth as it used to. P.S, Im not sure if ive said before but the truck pings under acceration, and timing is dead perfect @ 0 degrees

So this is where Im at, should I look into a pair of rebuilt heads and slap some on. or should I keep diagnosing? thanks

 

Why would you need heads?

If #4/6 Were black, those cylinders were either misfiring or running rich....

Have you run some SEAFOAM in the brake booster line to clean the intake?

Have you replaced the Cap/Rotor again? (use an ACDELCO CAP/ROTOR).

What brand of Cap/Rotor did you use?

 

I thought if I needed valve guides and seals it maybe easier or cheaper to just pick up a set of re-man heads instead of rebuilding mine..

Also I have recently done a seafoam treatment 2 months ago, I found the seafoam fouled up my old plugs with deposits after.

I replaced the cap and rotor three weeks ago, I used bosch brand, its got brass terminals. whats so great about the acdelco brand?

thanks

 

People have issues like you describe with NON OEM caps, thats the big deal about Delco stuff......

Yeah, seafoam can kill the plugs if there is alot of gunk.....

As far as the heads, sounds like alot of extra work and cost tho...... Have you taken some vacume readings?
http://www.secondchancegarage.com/public/186.cfm
#8 is for worn guides.......

 

when i did the vacuum reading, the gauge read steady at 17 at idle, didnt bounce around at all, didnt do any weird things going up to 2000 rpm or higher.

what about throttle body issues? my injector spray patters look good the same, i pulled off the tbi this summer and cleaned it really good and cleaned all vacuum passages, the tbi has no leaks from the regulator, however this spring i did install a "super sucker" throttle body spacer from HVH. im not sure if that would make it run like it is. just information for you guys to consider.

also checked the cts and all was within spec

MAP maybe ?

 

Is it using any coolant? You said the plugs were blacker than the rest on #4/#6?

 

If the plugs are black, are they a shiny black? Perhaps the missfire is due to the oil fouling after it drips down into the cylinder?

 

im 99% sure its not using coolant, i have noticed in the past the coolant reserviour isnt as full as it used to be but never empty.

the plugs were not shiny black, more dry sooty black

the blue smoke I see might be an exhaust valve seal or guide, the oil might not ever see the plug.

this problem is stumping me but im gearing myseld up to possibly pull the intake off and look at things, like the cam lobes and push rods then maybe pulling the heads off, im not shy to get in elbows deep haha

 

Have you pulled the egr off to take a look at it. Maybe a piece of carbon stuck in there holding it slightly open or maybe a weak spring in the diaphragm?

 

last year when i did the intake gaskets i had pulled off the egr and cleaned out carbon in the intake passage ways and cleaned the valve itself. but how can I test for a weak spring?. also if there were something holding it open wouldnt the engine run crappy always, mine runs better after its driven for a while.

I pulled off the belt the other day to get a good listen to the engine, the motor doesnt sound very quiet, some sort of valve train noise im hearing, no knocks or anything. just lighter sounding roughess. im wondering if ive got a worn our roller on a lifter, then maybe it would be wearing down a lobe. maybe im stretching too far?

 

If compression is fine on those cylinders..... Well.........

I would say that the engine runs with a miss untill the plugs get defouled enough to run good, I would try some thicker oil/ or some oil stop leak....... The oil could be leaking down into the 2 cylinders on that side after shutdown and upon startup it fouls up those 2 plugs and untill it runs a bit and the fuel helps clear the cylinders out...... get the idea.......