Piston will not come up to top of Cylinder #6 2000 Sierra 1500 5.3 please help

[Bradamoki]

Hello, I have a 2000 GMC Sierra 1500 5.3Liter with 246,000 miles. I have a code P0306 for misfire Cylinder #6. I have changed the spark plugs and wires. Then did a compression test and cylinder #6 had low compression at 120psi. All other cylinders had between 180-190psi. Did a leak down test and heard air flowing through the oil filler spout when #6 was at TDC on the compression stroke. Removed the head, valves looked good. When I cranked the engine by hand, the piston for cylinder 6 would not go up all the way to be flush with the top of the cylinder like the other pistons were, it would come up to 1/8” before the top, the go back down. What causes this and how can I fix it? Do I have to replace the piston? Or??

Thank you for taking the time to read this, any input would be greatly appreciated.

 

[CamGTP]

The connecting rod is probably bent if the piston does not come all the way up to top dead center.

 

Or the piston has broken where the wrist pin goes through. Either way you need to remove that rod and piston to check it out. Pull the oil pan and windage tray out, unbolt that connecting rod and push the piston out through the top.

[dna9656]

Well without a broken crank (you obviously don't have one) there are 2 other possibilities; the journal where your connecting rod lives is worn (the diameter of the journal gets smaller with wear) thus increasing the stroke of the piston (the piston would travel up (relative to the crank) on  a down stroke and would travel less far on an up stroke (again relative to the crank) and that's why it's short of the deck on the up stroke. So my guess is you need new connecting rod bearings and generally if you need those you need to get the journal built back up (welding) and machined or a new crank. With the miles you got I think your due for a Good wrench engine. 3 year warranty and I think 100,000 mile.

[Bradamoki]

16 hours ago, CamGTP said:

The connecting rod is probably bent if the piston does not come all the way up to top dead center.

 

Or the piston has broken where the wrist pin goes through. Either way you need to remove that rod and piston to check it out. Pull the oil pan and windage tray out, unbolt that connecting rod and push the piston out through the top.

I wasn't sure if I could remove the piston without removing the engine. Thanks CamGTP for the info, I'm going to remove it today and check it out.   

[Bradamoki]

On 7/16/2021 at 7:17 PM, dna9656 said:

Well without a broken crank (you obviously don't have one) there are 2 other possibilities; the journal where your connecting rod lives is worn (the diameter of the journal gets smaller with wear) thus increasing the stroke of the piston (the piston would travel up (relative to the crank) on  a down stroke and would travel less far on an up stroke (again relative to the crank) and that's why it's short of the deck on the up stroke. So my guess is you need new connecting rod bearings and generally if you need those you need to get the journal built back up (welding) and machined or a new crank. With the miles you got I think your due for a Good wrench engine. 3 year warranty and I think 100,000 mile.

Thank you for your reply, great info. I agree and would love to put a good wrench engine in, slightly out of my budget. When I was removing the oil pan. I remembered my truck having a knock and it turned out to be a counter weight barely hitting the bottom of one of the pistons. I reached out for help and someone said to grind the counter weight so it no longer makes contact. It was barely making contact so I did that and thinking about it now what I took off is probably the same measurement of how short the piston is from the deck. So I'm assuming these are related? Pardon my ignorance.

  

[CamGTP]

Of course it's related. Things don't just move on their own like that. The piston got closer to the crank for a reason. My guess of a bent connecting rod is still in my mind.

[txab]

If you removed material from the crank, I'd strongly suggest you get the crank re-balanced

[Bradamoki]

17 hours ago, CamGTP said:

Of course it's related. Things don't just move on their own like that. The piston got closer to the crank for a reason. My guess of a bent connecting rod is still in my mind.

Your guess was correct, 

 

17 hours ago, CamGTP said:

Of course it's related. Things don't just move on their own like that. The piston got closer to the crank for a reason. My guess of a bent connecting rod is still in my mind.

That was a pretty awesome guess man, thanks. I got the piston and connecting rod through the top. I can totally see the bend in the connecting rod. Is it ok to just put the new one in or am I screwed for grinding away that little bit from the counter weight? Appreciate your input.

[CamGTP]

Just slap one new connecting rod in there.

 

The minimal grinding on the crank means nothing. I seen some weird stuff done to LS motors that are far "worse" than a little metal off the crank. I'm talking mismatched pistons, flat tops and dished, mismatched cylinder heads so the compression ratio's weren't even close to the same on each bank. All these engines ran just fine lol.

 

Take a picture of the old rod sitting next to the new rod whenever you put it back together and post it up.

Edited July 19, 2021 by CamGTP

[dna9656]

Well that something was hitting some thing else was the first indication something's wrong, things aren't supposed to hit things in engines. LOL!. I would get a new piston, con rod and bearings. Be sure to check the bearing clearance on the crank with some plasti-gage, if it checks good with the new bearings press on; if not I guess you'll be pulling the crank and if you're going to do that that you may as well get the crank completely serviced, all the rods and pistons checked and while there get them balanced.

[Bradamoki]

On 7/18/2021 at 6:18 PM, CamGTP said:

Just slap one new connecting rod in there.

 

The minimal grinding on the crank means nothing. I seen some weird stuff done to LS motors that are far "worse" than a little metal off the crank. I'm talking mismatched pistons, flat tops and dished, mismatched cylinder heads so the compression ratio's weren't even close to the same on each bank. All these engines ran just fine lol.

 

Take a picture of the old rod sitting next to the new rod whenever you put it back together and post it up.

[asilverblazer]

If I was in this deep with that many miles I'd be doing a full rebuild.

[CamGTP]

Sometimes the less you do the longer they last lol.