1994 GMC Sierra C1500 5.7L Runs Rough

[KLHill]

The above mentioned truck came to me running rough. Replaced the distributor cap/rotor and plug wires. Checked the plugs (NGK iridium) and they appear to be fairly clean. Three had oil on the threads but the gaps looked clean. Swapped out fuel regulator and injector assembly with same make model truck that runs well. None of the above has improved the rough run. At idle, roughness is not as noticeable, once in gear it becomes very noticeable. It does start easily.

Forgot to add: shows a code 13 for oxygen sensor. It does have a new sensor installed by the previous owner. Swapped it out with a known working one to no avail.

Edited April 25 by KLHill
missing info

[richard wysong]

first thing to do is figure out which cylinders are misfiring. When you replaced the wires did you do them 1 at a time or take them all off and start anew? Possibly someone had already messed up the firing order before you got it so double check that. 1-8-4-3-6-5-7-2 clockwise rotation, #1 should be around the 5o'clock position on the cap. If that checks out check the compression, where you already had the manifold apart you probably already checked for vacuum leaks. If all that checks out pull the valve covers and check the valve train and camshaft lift. Note: aftermarket ignition control modules have caused this problem for me in the past

[asilverblazer]

I had put new plug wires on one time and one of them was bad out of the box.

 

Careful throwing parts at it, just more variables that could complicate matters.

 

As noted above, try to pinpoint a specific cylinder.

[KLHill]

On 4/25/2026 at 7:03 PM, richard wysong said:

first thing to do is figure out which cylinders are misfiring. When you replaced the wires did you do them 1 at a time or take them all off and start anew? Possibly someone had already messed up the firing order before you got it so double check that. 1-8-4-3-6-5-7-2 clockwise rotation, #1 should be around the 5o'clock position on the cap. If that checks out check the compression, where you already had the manifold apart you probably already checked for vacuum leaks. If all that checks out pull the valve covers and check the valve train and camshaft lift. Note: aftermarket ignition control modules have caused this problem for me in the past

Double checked the plug wires. 7 and 5 were swapped. The miss is gone only to reveal a shudder when accelerating with slow acceleration almost like no fuel. It has a new fuel filter. I'll need to pressure test the pump.

[asilverblazer]

2 hours ago, KLHill said:

Double checked the plug wires. 7 and 5 were swapped. The miss is gone only to reveal a shudder when accelerating with slow acceleration almost like no fuel. It has a new fuel filter. I'll need to pressure test the pump.

How is compression and timing?