leftlanetruckin

Got a LQ4 with a LS6 intake injectors, etc, and a 4L60E in my Range Rover.
When warm, and in gear with the brakes on, there is a noticeable miss in the engine. Can feel a "thump" kind of deal on your feet and see the tach dance a little.
No misfire codes, live data looks good, no CEL.
The intake etc was used, and sat when it was here for a couple of months. So no telling how long it has sat overall, along with the injectors.
Did a quick test with a timing light, and there was no variation in the flashes on any cylinder.
Checked injector plugs with noide lights, all pulsed fine.
Unplugged injectors one at a time, and when I unplugged No 7 it took the thumping right out of it. Actually idled smoother with no 7 unplugged! Double checked the pulse again and it was fine still.
So, bad injector?
I will get a full set of 8 and get a flow matched set.
Agree with the bad injector?

Martin

RockinWs6

I wouldn't think bad injector....unplugging the injector unloads the cylinder. You probably know what that means...........

leftlanetruckin

Doesn't sound like a rod bearing though, the knock isn't constant like the bad rod bearings I have dealt with......

Martin

RockinWs6

Well hole number 7 has a bad habit of melting-damaged pistons. If the piston-rod assembly is quiet unloaded<injector unplugged> safe bet it has damage. I'd investigate it more, maybe scope the cylinder before bad BIG things happen. I had part of number 7 piston go out the exhaust once......didn't do big damage but I knew it was coming. Number 7 was noisy, knocking thumping etc. Also sometimes if the piston breaks wrong it can crack the cylinder liner etc.

Number 7 and 8 run hotter than the rest because of the intake design. Piston-rings overheat, rings over expand POOF ring land breaks off the piston. I'm not trying to scare you but or worry you but bad injector? noway.

Another thing that comes to mind is maybe a spark plug or lifter problem. Did you pull the plug and look at it? Do any compression or leak testing? The THUMPING noise concerns me for sure. Cracked -broken pistons can thump but good.

leftlanetruckin

Hmmm, good point.
Looks like a motor being pulled apart......again.
Why do I do this again?
Engine number one munched a rod for some mysterious reason that no one can figure out. Now this one (not rebuilt, JY motor) has a rod/piston knock.....

Martin

RockinWs6

Crazy thing on mine even with part of the piston in the exhaust it had no blowby, but number 7 spark plug was oily.

leftlanetruckin

Compression test worth doing then?

Martin

RockinWs6

I'd look for oil on the spark plug and compare compression and leak down with the other cylinders.

leftlanetruckin

Compression 155-170 psi cold.
Porcelain is broken on number 7 spark plug though! No damage or discoloration on the tips.
My eldest kid did the test as I had to leave out this morning. New set of plugs going in.
Will advise the results

Martin

RockinWs6

Post a picture of the broken plug......it will tell a lot. I would scope the cylinder, look at the edges of the piston. If it melted the porcelain sure bet it melted the piston.

When I lost number 7 the porcelain was melted and the cylinder was oiled down. Compression was 160psi

The car had 130k with a "tune" in it, ran great but it smoked number 7. The GM stock tunes keep number 7 and 8 alive for a long time.

leftlanetruckin

It's running great again!
No plugs were fouled at all.
The porcelain looks like it broke, not melted etc.


Martin

RockinWs6

You meant the outer porcelain, I thought you meant inside the plug. Great can't beat that.

leftlanetruckin

Sorry, should have been clearer.
Maybe a little excited at dodging a bullet though!
Thank you for your help

Martin