Brokenls

Have a 5.3 that is misfiring on cylinder 4. Plug looks good. I put on new wires and new coil packs and put in New injector. It runs and idles fine, but around 45 to 50 it hesitates and generally throws the code for cylinder 4 or sometimes random misfire. I have a new fuel pump and pressure regulator and have checked pressure. The only thing out of the norm is I am pretty sure I have a bad valve seal because it smokes on start up. Could this be causing the misfire? Plug is not fouled, so I didn't know if it could be the cause or not

cookseyb

Knock sensors pulling timing, try premium fuel and see if it persists. Valve stem oil seal should not be a factor unless it is so bad it is fouling the plug..

Brokenls

Ok I will try fuel and see. I hope it's not a knock senor. I replaced sensors less than a thousand miles ago. I used ac delco sensors and harness

ChopperDoc

Normally misfires are mechanical/electrical in nature, but it sounds like you've checked the blocks. However I will also add that before changing anything else, it's worth a shot to check the wiring on your coil packs. It's free, and a quick easy way to do this is to switch the banks and see if the problem follows. Take the coil brackets and switch them. Keep in mind when you do this you have to flip them. Plug 1 and 8 both use the purple reference wire iirc.

If your problem switches banks then obviously you have a harness issue. That's what I would do if all the parts are good.

Brokenls

Just to makes sure I am not misunderstanding, but I took the entire bracket with the coil packs off my other truck and put it on the passenger side and still got cylinder 4 misfire. I'm at a loss. It runs great at idle, gets up and goes when you accelerate, but 90 percent of the time right about 50 mph it gets choppy but quits after a bit or if you slow down some

99 Black Bird T/A

iTrader: (4)

Just a guess here. I would replace the plug in #4.

I had an almost identical issue with #2 cylinder and went through replacing coil pack, swapping plug wires, swapping coil pack harness and the issue remained. #2 plug looked good and was correctly gapped. Replaced #2 plug and the issue went away and didn't return. That was 4,000 miles ago.

There was some sort of issue with #2 plug. The plug only had 7,000 miles on it. Not sure exactly what it was wrong with that plug but it had an electrical issue. Maybe it had been dropped in shipping and cracked the insulator inside where it wasn't visible.

My next guess would be #4 fuel injector.

qweedqwag

Bad injector/from the parts store it'll **** you up and make you second guess.

Brokenls

I will try a new plug. I put a working injector in from another motor. I have 3 vehicles with 5.3 swaps so I have a lot of parts. Ha

ChopperDoc

Let us know if the new plug fixes it. Sounds like you've done just about everything else.

qweedqwag

Good you have extra parts. LUK

Brokenls

Well it looks as if spark plug was the cause. Put a new one in and drove it hard with no misfire. Thanks for the help

Brokenls

Gave it another run this morning in the cold weather and it runs like a top. Wish I had considered changing the plugs sooner, I just didn't think to since they were gapped correctly and clean. Live and learn

ChopperDoc

Guess you're going to have to change your name now LOL

Brokenls

No, 1 down, 1 more to go. Ha

99 Black Bird T/A

iTrader: (4)

Glad to hear the misfire is gone.

My TA smoked and burned 1 quart of oil ~ 500 miles oil at one point turned out it was the valve seals. The black GM factory ones are fair. The blue Viton rubber valve seals are much better quality.

Best wishes

Brokenls