Intermittent miss at light load.

Yes, I did. 3 times

Once for buying new plugs, once for wasting the labor, and once for not doing what I thought I should've done before. DOH!!

I did put the plugs in and when I found nyumber 6 I thiought, WOW is that plug messy. I got thinkiong "how long ago did I put them in" Well, it was about 4000 miles ago. I figured that I maybe overtightened or used a heat range too cold and with high miles maybe they fouled quick. 1-5 is nice and 6 was ok I guess.

Anyways I test drove and right at the point it goes to closed loop it fell apart again and missed although not quiite as bad. I think that a new coil, plugs, wires, cap and rotor makes it slightly better, but the problem is still there.

I got mad at it and "fixed" it. Tommorow I will verify and post if it is what I think.

I think I have worn out timing chain, dist gear, and dist housing.

Actually, my bad, they are all set to 0.045.

Mine were about .04 with the gaps not burned away a bit. My old , old ones I replaced were burned to .125. I tend to gap plugs on the tight side and mostly want to use the factory (plug manufacturer) gap. Only one electrode did I have to adjust sideways on the AC delcos.

Test Drive in a few hrs.

Right after my test drive with new plugs, I got kinda pissed and it was a bit hot and I don't have a distributor wrench anymore .

I was able to "fix" it by first using a large screwdriver and hammer and sharply cracking the housing tang that is clockwise of the roll pin holding the body from turning and then loosened the mounting bolt clamp. I rotated the body ccw until it hit the intake and retightened the clamp-down bolt.

It now runs perfect all the time. I expect I have sloppy stuff before the distributor shaft like gears, timing chain bushings and/or housing and expect sooner or later it will be back exposing the root problem.

One other indicator is the timing does not look right, I can't seem to locate a timing map, but this one is all over the place according to computer.

Take it to a shop, you are just blindly adjust things without knowing what the problem is.

Most of the shops would have r&r me to death replacing parts. Only one shop had any idea what I was talking about and that was the shop that mentioned distributor bushings. As it turned out I really needed zero parts and am sorry I replaced good used plugs, coil, coil wire. The plugs I suppose told a story, but I really didn't need to know it.

Blindly would have been pulling plenum and looking for fuel leaks, replacing parts that were suggested by some that do not have an understanding of the problem. It is possible that I didn't make myself clear when describing the problem too.

Anyways I will assuredely post if/when the problem comes back and if I ever find out the root cause which is probably as I stated (some slop in the mechanical system) That won't happen until stuff gets taken apart.

Thanks for your help, thanks to all who helped me.

But, did you check the timing after you made the adjustment/

Didn';t you say it was all over the place?

I have never checked the "actual" timing, but did run it with the lap-top and my scanner and freeware. All the timing readings are identical to what they were before. They are still all over the place and range from -38 at the top and lightest load to maybe -8 at wot and low rpm.

I think it is accurate and probably uses that crank sensor thingy to deliver these readings. I really don't care what the timing is and really don't care why it all over the place as long as it runs ok. I also won't waste my time investingating why the rotor/post relationship is so far off. I'll just wait until it acts up later. It may be a week, it may be a few years. Hopefully it will not be my vehicle when it does. lol.

Any body want to buy a "nice" LT ? lol.

I would get it to the dealer and have them connect a Tech 2 and diagnose it with everything set like it was before you fiddled.

Screwing randomly with a whole bunch of things will make this truck almost unfixable seeing it will be a combination of problems.

I actually only did one thing. That was to advance the dist body x degrees. It was very precise and purposeful (hardly random) Random was the plugs, coil, ect.

I'd bet money if I put it back the way it was I'd be out hundreds of dollars of new parts exchanged for my "good used" parts and still have an intermittent miss.

Hi Folks. My first post. I also have the vortec 4.3 in my 97' gmc sierra that also had, and has again the same problem. Did you figure out your problem yet? Are you losing a small amount of antifreeze? My truck was losing some antifreeze and missing when you are letting off the gas like when you have reached your desired speed and just leveling off to a constant speed, never at WOT or foot off gas pedal. Also missed at constant speed above 2k rpm's. Took to small shop close to my house and they determined that the intake gasket was leaking coolant onto the plugs, they were right. Now, after about 2000 miles, it's doing it again, I feel like the intake is warped too, I read that they are prone to do this since they are plastic, I didn't know this, I'm going to replace intake and gasket this time and see what happens. this might not be your problem but it's worth checking into. Before finding this out, I replaced air filter, fuel filter, cap and button, plugs twice, plug wires, throttle positioning sensor, MAF sensor, EGR valve. So before you replace these things it might be your intake gasket. Hope this helps!

Take it back to the place that changed the intake gasket. It is under warranty and they have to fix what they screwed up.

Hi Folks. My first post. I also have the vortec 4.3 in my 97' gmc sierra that also had, and has again the same problem. Did you figure out your problem yet? Are you losing a small amount of antifreeze? My truck was losing some antifreeze and missing when you are letting off the gas like when you have reached your desired speed and just leveling off to a constant speed, never at WOT or foot off gas pedal. Also missed at constant speed above 2k rpm's. Took to small shop close to my house and they determined that the intake gasket was leaking coolant onto the plugs, they were right. Now, after about 2000 miles, it's doing it again, I feel like the intake is warped too, I read that they are prone to do this since they are plastic, I didn't know this, I'm going to replace intake and gasket this time and see what happens. this might not be your problem but it's worth checking into. Before finding this out, I replaced air filter, fuel filter, cap and button, plugs twice, plug wires, throttle positioning sensor, MAF sensor, EGR valve. So before you replace these things it might be your intake gasket. Hope this helps!

Hmm, thanks for the tips, sounds like your symtoms are same as mine were. Mine has run perfect for a month now. :grinyes: I never had any coolant leaking (that I know of)

534BC,
You may very well have something about the timing. I posted earlier in this thread and have also replaced many, many parts. After all was said and done, it ran much better and have only had the truck stumble at light loads twice in the last month. BUT, back to the timing thing, I was driving home from Washington D.C. Wednesday in the fast lane at about 70 to 75 mph when the truck just DIED. No warning, just died. Tried to restart after pulling off and the engine felt as if it would hit a compression lock after a revolution or two and kick the starter spline off the flywheel. Did some troubleshooting to verify my suspicions and found that the timing chain has jumped time. I called my brother who is a long time mechanic to reverify my findings and he agreed. He also said, and I'll find out for sure tomorrow when I tear it down, that Chevy made these engines with plastic timing gears. I don't know that for sure, but like I said, I'll find out tomorrow. The timing gears getting worn and/or the chain getting worn would explain a lot. I'll repost when I get finished to let you know what I've found.

Yep, I can sure go along with that.

Older chevies always wore out the timing chains because of plastic gears.

50 k - 60 k would see them very well worn and stretched. Mine has 180 k and now has run perfect, but wouldn't surprise me if it did the same thing as yours. I'll look forward to your post, Thanks.

180k, huh? When I was in D.C., I looked at the odometer and saw 188,600 miles even and thought to myself that I was actually going to get 200k out of it. Guess I'll know tomorrow if I will. It made it to about 188.7k. You may want to stick close to home. I was very lucky. I live about 100 miles away from D.C. and it waited until I was within ten miles of home when it quit. Good luck.

Update: Timing hadn't jumped. Since I had it open, I replaced it anyway. Chain was sloppy but the sprockets looked pretty good. NOT plastic as my brother thought. After more troubleshooting, determined that the distributor cap was shorting the coil input to the #4 plug, causing it to fire constantly and weakening or robbing the spark from the other plugs. Once I thought it must be what was happening, I looked under the cap and noticed that the #4 pathway runs directly next to the center post. Put a new cap on and with the new chain and cap, it purrs like it hasn't in quite some time. Noticed also that on the new cap, the #4 pathway has been designed with a seperation from the center post, must have been an issue. I don't know what it's like on models of different years, but the '96 was a BI**H. The oil pan was designed as a structural piece of the transmission / motor assembly. This made it virtually impossible to loosen the bolts and drop it a little. I got it together and it's running great, but it leaks oil at the oil pan to timing cover gasket. I'm done, I'm going to have a shop replace the oil pan cover tomorrow. I don't feel like unbolting the drive shafts, dropping the front pumpkin and axle assembly, loosening the motor mounts and raising the engine just to remove the pan. Anyway, I believe the timing chain and sprockets took care of the intermittant miss and slightly rough idle so it wasn't a total waist of time.

Regarding ignition timing, the computer controls and advances the timing until the knock sensor(s) complain then it backs it off then starts advancing it again, trying to maintain the maximum timing for economy & power. That's why disengaging the EGR system hurts fuel milage...without the exhaust gasses to "cool" the combustion process, the knock sensors pick up more detonation and the ignition timing spends more time retarded.
As long as the initial timing is somewhere in the neighborhood of OK, the computer takes it from there and constantly adjusts.
Fuel trims/O2 sensors are even worse...lol

ECM will only adjust so much. I had the distributor out about a year ago. When I reinstalled it, the truck ran but seemed a little rough. Just to be sure I had it back where it was, I moved it one tooth counter clockwise and it wouldn't fire. I then moved it two teeth clockwise (one tooth more than the first time) and it backfired. They weren't kidding when they wrote these engines are sensitive to the timing. I put it back where I had it the first time. Replacing the chain and gears, after 188,696 miles made enough of a difference that I could feel it the first time I drove it. Even if it was to the shop to fix the oil leak I created.

I bet you this is a bad TPS.

What's a bad TPS?

throttle position sensor.

Problem finally fixed by retarding the distributor shaft one tooth. Vehicle runs as perfect as it ever did. No more intermittant miss at light load. Man am I slow. Pic shows one broken tab for the first "fix" the second tab was broken today to retard the housing as well as the shaft. There's really not a lot of swing on this housing because of the intake.
http://img15.imageshack.us/img15/7668/picture206b.th.jpg (http://img15.imageshack.us/i/picture206b.jpg/)