2014 Silverado 5.3 #3 cyl dead miss at idle only.

[LeBaron]

Plug, wire, and coil is good. 190psi compression.  Replaced all fuel injectors. Rockers moving normal visually during cranking with valve cover off.  #3 cylinder is not one of the cylinders that drop out during DOD.  192k miles.  Runs great off idle. OBD II shows no missfire above idle driving down the road but as soon as you come to a stop, #3 cylinder has a dead miss.  Any ideas?

[mjm-1957]

What about a vacuum leak? Has the intake manifold ever been off with close to 200,000 miles? Spray some brake cleaner around the intake manifold at number 3 and see if it straightens out.You could also monitor live O2 data while spraying brake clean and watch for bank one sensor one changes.

[LeBaron]

Yes, the intake has been off.  Everything pointed to injectors so I changed all 8 injectors.  Had to take off the intake to replace the injectors.  I installed new rubber gaskets.  Still has the dead miss at idle.

Edited October 9, 2024 by LeBaron

[rlcole321]

who did the injector replacement?, if you, did you look at the back of the intake valve (or all of them) for cyl# 3 to see if it had excessive carbon build up on it?, how many eng revolutions to get up to 190 psi on compression test?, 

try pulling at least 3 plugs (I would do all of them) and doing a compression test on all 3 cylinders and see how many piston cycles it takes to get to full compression, if it takes a lot more to get cyl. #3 to come up then you could have a minor valve issue. I'm guessing you tried swapping coil and plug wires from another cyl. to see if that made a difference? 

[asilverblazer]

Codes?

 

[LeBaron]

P0303. #3 Cylinder missfire. 

[LeBaron]

I replaced the injectors. I did not look inside the intake for carbon. I got 190 psi in 4 revolutions. Yes I swapped plugs, wires, and coils one at a time. I also just changed the plugs but didn’t help. I also made sure the butterfly was clean in the throttle body. It idles really low. About 550 and less. The miss goes away above around 750rpm. Maybe it is a valve not seating at idle. I just don’t see why it doesn’t miss out above idle. 

[asilverblazer]

Any change warm vs cold?

 

I would be thinking about inspecting everything from the cam to the valve. 

 

Maybe an injector balance test. 

 

Have you confirmed via a scanner the miss does go away off idle (or does it just become undetectable)?

[LeBaron]

Cold, no miss.  Yes, I have verified the miss goes away above idle with a scanner.  I have also cranked the engine over with the valve cover off and there is no excessive play in the rockers while cranking, therefore, a cam of lifter problem is not likely.

[newdude]

Sounds mechanical.  Bent pushrod, burnt valve, lifter eating the cam.  

[asilverblazer]

On 10/11/2024 at 8:02 PM, LeBaron said:

Cold, no miss.

Does the miss begin when the engine switches to closed loop operation or simply when things get warm?

 

For the sake of avoiding prematurely condemning major mechanical parts - re-evaluate injectors, coil, plug wire and plugs.

 

Were new parts genuine GM parts (not counterfeit)? Try swapping individually to a non miss firing cylinder. Understand you've done this once, but I'm reluctant to say a mechanical condition would clear up off idle and not be present when cold. 

 

Carefully inspect injector and coil wiring, where the fuel pipe connects to the injector, manifold gasket... anything isolated to cylinder #3. 

[LeBaron]

newdude,

 

I pulled off the valve cover and there is no excessive play in the rockers.  People keep saying it's mechanical.  If there were a bent push rod, worn cam lobe, or bad lifter, there would be noise or the rocker would be off the pushrod.  Also it would miss all the time.  I only have a miss at low idle.  I have confirmed this with a scanner. Also #3 is not one of the cylinders that drop out, so it should have regular lifters. I know there is a problem with the lifters that drop out (DOD) but #3 cylinder isn't one of those lifters. And if it were, I would hear obvious tapping and the miss would be all the time.  

 

Can anyone tell me if GM uses 8 regular lifters and 8 dod lifters, or do they use 16 dod lifters?

[newdude]

12 minutes ago, LeBaron said:

newdude,

 

I pulled off the valve cover and there is no excessive play in the rockers.  People keep saying it's mechanical.  If there were a bent push rod, worn cam lobe, or bad lifter, there would be noise or the rocker would be off the pushrod.  Also it would miss all the time.  I only have a miss at low idle.  I have confirmed this with a scanner. Also #3 is not one of the cylinders that drop out, so it should have regular lifters. I know there is a problem with the lifters that drop out (DOD) but #3 cylinder isn't one of those lifters. And if it were, I would hear obvious tapping and the miss would be all the time.  

 

Can anyone tell me if GM uses 8 regular lifters and 8 dod lifters, or do they use 16 dod lifters?

 

 

8 and 8.  Doesn't mean a lifter can't still spin in the bore or the roller fail and start hammering the cam.  

 

If the plug, wire and coil are good, its either carbon buildup, a burnt valve or a lifter spun and its eating the cam. 

 

Or, have you replaced the fuel injector on that cylinder or all 8 injectors?  

Edited October 29, 2024 by newdude

[LeBaron]

I replaced all 8. I guess I need to pull the intake. I don’t know what else to do. Maybe it is carbon build up. I’m puzzled as to why it only misses at idle. Thank you for taking the time to help. 

[KARNUT]

I’m a little different. When I had my 92 Chevy that was modified much to my disappointment it used oil. 1qt every 500 miles. 5lbs of boost or 12 same. 12 years later with me beating on it. I sold it to a family friend. Two years later same engine. I would run it. At that mileage I would do a total replacement when it failed. Depending on the shape of the truck that would dictate to that direction.