Skipping 350

wmf1964 · September 9, 2008

hello everyone, i have a problem with my truck running rough and has a miss at idle and while driving. i have replaced everything i can think of that may cause this problem and its still there. the items ive replaced are cap,rotor,pickup,plugs,coil,wires,module,ecm,replaced intake and gasket, tbi gasket,replaced egr valve 3 times with known good uesd ones, replaced injectors twice with known good used ones, replaced fuel pump and fuel filter, used 7 cans of carb cleaner and 2 bottles of soapy water to check for vacuum leaks. i just noticed that when i press the brakes it changes the rpms of the engine,so i think the booster may be leaking vacuum. i blocked off the vacuum from the booster and it still idles rough with a miss. the engine is new with less than 300 miles and has 140-150 lbs of compression per cylinder, and does not give any trouble codes. ive readjusted the valves 3 times and all are moving like they should. i have made my living for 20 years as a mechanic but i need some help on this one. for some reason i cant seem to fix my own now. I REALLY NEED HELP WITH THIS ONE. thanks

95Sierra2500 · September 9, 2008

With everything you've replaced, I think it's time to start troubleshooting. Since you're TBI, you should be able to tell if you're getting good fuel or not based on how the injectors are spraying. Next I'd say pull each plug wire one at a time (replacing it afterwards) to see if engine behavior changes. It should be noticable with each plug wire coming off, but if it doesn't change on one particular cylinder then you've found the problem.

A few things to consider: Did you check the timing and set it properly? Are you sure you don't have a cracked head? What kind of plugs/wires did you use? There's alot of variables and without seeing the thing I have a hard time telling you where to start.

wmf1964 · September 9, 2008

With everything you've replaced, I think it's time to start troubleshooting. Since you're TBI, you should be able to tell if you're getting good fuel or not based on how the injectors are spraying. Next I'd say pull each plug wire one at a time (replacing it afterwards) to see if engine behavior changes. It should be noticable with each plug wire coming off, but if it doesn't change on one particular cylinder then you've found the problem.

A few things to consider: Did you check the timing and set it properly? Are you sure you don't have a cracked head? What kind of plugs/wires did you use? There's alot of variables and without seeing the thing I have a hard time telling you where to start.

ive done so much i just forgot to list that the timing was set at tdc with the wire unpluged. each plug wire has been removed one at a time and a difference was noticed. the heads were rebuilt at our local napa store and checked for cracks. there were given a clean bill oh health by a machinest with 40 years of experience, but that dont mean he didnt overlook something. a cracked head has been in my mind also but id like to see if i overlooked something else myself before removeing the heads. the passenger side exhaust has a miss for about the first minute or so when the engine is cold, the drivers side has a much worse miss for about 4 minutes and as the engine gets hotter it seems to get better but never goes away completely. i have noticed that both exhaust pipes has a little water that comes from them on cold start ups(condinsation i thought) but the drivers side does give off about twice as much. it does not run hot, get water in oil or anything like that. i placed some uv dye in radiator to see if water was leaking through engine somehow, it showed nothing. plugs were ac cr43ts then changed to autolite25 then changed again with another new set of ac cr43ts. wires were bought at orielly auto and then replaced with a nice set of summitt receing wires($80.00) all had no effect on the way it runs..... p/s...thanks for trying to help me