4.3 vortec

My GMC half ton v6, stick, 2 wheel drive, has a misfire when running at a level speed. It doesn't matter what speed I am going. Though it idles smooth as silk. It has great take off and pick up. But when travelling at a level speed, it starts misfiring. The only code that comes up is "Random misfires all cylinders". I have replaced the following with no improvement; plugs, wires, cap, rotor, fuel filter, air filter,pcv. I have tried a different coil, a different distributor, cleaned the injectors, checked and double checked all connections, updated the software on the ecm. Still misfires. I took it to the best diagnostics guy in the area and I stumped him. He spent five hours on it and was at a loss except to say that there is a very slight possibility that there is wear in the timing chain and gears but he was puzzeled as to why it was idling so smooth if that was the problem. Anyone have any ideas?

I may have the same problem. I have a '02 Silverado 4.3, automatic 2wd with 47,000 miles. Last week, I noticed a slight surge while on the interstate. The next day, on the way home from work I noticed the service engine soon light was on. Hooking it up to a code reader said random misfire.

My truck idles fine and so far, I've only noticed the misfire when the engine is warmed up. The shop I took it to told me to run some injector cleaner throught it. Any help from anyone would be great. If you've had this problem and fixed it, let us know. Thanks.

oper8tor, sounds like a distributor gear problem, I think the gear itself is about $30, the vehicle will run great until you get to light load cruise- say 45 mph on a slight downhill angle when the timing will really advance. Have your diagnostics guy check the cam retard on the scanner, if it's out of spec on the negative side, I'd start by checking the distributor gear

Have not seen any distibutor gear problems on the V6 engines as of yet (doesn't mean it couldn't happen)
Quite possibly if this problem is happening when the engine is hot and at highway speeds or under a load you may be experiancing a condition where the valve stems are sticking in the guides causing your misfire.
I have seen a few of these as of late and the repair is to remove the heads and ream the guides.

GMMerlin:

I originally thought it was only happening when the engine is warm, but I did some more checking. I removed the MAF and it looked spotless. I let the engine idle and with it cold, I held a continuous RPM, first 1500 and then 2000. After 3 or 4 seconds at each RPM it would start misfiring. Give it more gas and it would rev up without hesitation.

I checked with the closest GM dealer and asked about cleaning the injectors (a reply of yours on a previous thread) and they said the only injector cleaning they do is a "carbon flush." Know what they're talking about?

My family and I are leaving on a 2 week trip (1000 miles total) and we'll be pulling a trailer. I'd really like to get it fixed before then. Any more suggestions from anyone would be helpful. Thanks.

As far as injector cleaning have them reference this TSB #00-06-04-003B

Thanks GMMerlin. I've fixed my problem by simply running some Techron through the tank (about 3 bottles total). I haven't felt the misfire in about a week and I had it hooked up to a diagnostic code reader and it didn't show any trouble codes. If it happens again, I'll be sure to give that TSB# to the service desk at my local dealer.

i had the same problem with my 4.3 liter, tookit to the dealer and they replaced my fuel system. said that i got into some bad gas and gunked it all up. has ran fine ever since