I have a ’92 GMC Sierra with a 4.3L engine. I bought it about 4 years ago with 93k miles on it. It runs like a champ, or rather it did till last week. It just hit 150k on the odometer. It was starting to run a little rough and was shaky at idle so I decided to give it a tune up by changing the belts, plugs, wires, distributor cap and rotor. I started with the belt. Changed it out and started it up. It ran like it was before, with a little shake in the idle. I then went on to change the sparkplugs. I replaced them one at a time, taking a wire off, removing the old spark plugs, putting in a new one, replacing the wire and moving on to the next wire. After replacing all the sparkplugs (but leaving the original wire installed) I started the truck. It fired right up and was running smooth as silk. I this point I should of stopped, but I has already bought new wires for it and decided that I would replace them. Someone had previously marked there placement on the cap before I bought it. I mapped them to the correct piston and then wrote there position down on a piece of paper. After double checking everything I went and removed the old wires from the engine and laid them out longest to shortest, then matched them up with the new wires. I then labeled them, and installed them on the engine. I went and started it up. It was a hard start, and it sputtered and spit fuel back up the carburetor as it was starting, but after several seconds it did eventually start. It ran really rough though and would give a little pop out the exhaust every now and then. I double checked all the wires, there were going where I had them wrote down to. I tried to swap some around, and things only got worse. I replaced the distributor cap and rotor, and still had the same problem. Then, thinking the problem was the new wires I put the original wires back on and that didn’t work. I even got another, top quality set of expensive wire, and they have the same problem.

When writing down the fire order, the piston firing order was 1,2,3,6,5,4. I thought that maybe I had crossed a wire while tracking it with my finger behind the looked up the fire order for my engine and checked with my local mechanic while is a friend of mine. He and I bought found that it comes stock with cam that fires 1,6,5,4,3,2. He stopped by and I fired it up for him. It started after several seconds and then it idled rough. For giggles we routed the wires to fire as the default manufacturer installed cam showed online and in the mechanic’s program. I started it up and fuel shot out the carb and started a fire in the carb. We put the fire out and then went and put the wires back. After placing the wires back to where I had them mapped to, it started and ran as mentioned above.

I then went and unhooked it one plug at a time, starting it up between plugs. When I unhooked cylinder #3 it started up great and ran smooth. It ran very smooth, but seemed to start running rougher the more it ran, although it was never as severe as before. I hooked #3 back up and started having problems the original problems again. I went through all the cylinders and #3 seemed to be the only one that it had any positive change of results.

I went on vacation and took the truck to my mechanic, and after working on it for a week it’s still not solved. My mechanic is great, but is a younger guy and hasn’t had much experience working on vehicles without the fancy computer monitoring going on in them. I’m hoping that someone with some knowledge and/or ideas might be able to give me some direction to help me solve this. My next step is going to be testing the compression of each cylinder. I’m thinking that maybe putting on some newer, better wires might of put more pressure on seal and blew it out (it has been leaking oil through a valve seal since I got it, but not enough to justify fixing since it was only ½ quart every 3500 miles). I'm really needing to get this truck back on the road soon. I don't have the money right now to get a new vehicle at the moment and it can't be something that bad. There has to be something that I'm missing. Thanks in advance.