L86 misfire, need opinions

[ryanvv355]

Hello, I daily a 2014 Sierra L86, 113k miles, no mods. For about a year and a half I have noticed a slight plum of blue smoke on "warm" startup. Hot restart or cold start, nothing. If it sits 1-3 hours after a drive up to normal heat soak, I get the smoke. Also, over that same period, I have noticed a slight studder at idle, very rhythmic. This has gotten progressively more noticeable over time.

Three weeks ago, I let the truck sit outside and idle for about 20 minutes. I came back to it and had the CEL on. 1,150 counts of P0302 and P0300. Bell rings in my head, there's your idle studder. Watching the scanner for misfire events, it seems like #2 misfires every combustion event. No other cylinder is showing misfires. I use the bilateral controls and shutdown #2, the idle studder gets slightly worse and in rhythm, shut down any other cylinder and I can tell a difference in rhythm. Reset STFT, bank 1 goes +6%, bank 2 -7%.

Ok, no problem, I got this.... swap the coil pack with #4, new wire, new plug. Studder still there, piss. Use the idle control in HPT, bump up to 800 RPM, zero misfires and absolutely smooth idle. Change idle to 790 RPM, studders and misfires occur at the same rate as 550 RPM. Change all idle settings to 800 RPM, drive like this for a week, aside from pulling hard on the brakes at a stop, truck runs so smooth.

Replaced injector in #2, matched the new injector p/n to the one previously installed, installed new seals on the other 3. Same studder, misfire, and 790 to 800 RPM idle "clear up".

Now I'm thinking F%^&, its the cam or lifter (non AFM hole). Pull the valve cover, watch the rockers move while cranking, they all move fine. #2 has similar travel to #4, but I didn't measure. Pulled the pushrods, they are not bent. Rockers "fulcrum" just fine. Put it all back together.

While doing the injectors, noticed a fair bit of carbon and junk in the intake runners, so I did the CRC upper engine cleaner process. Lots of smoke. Still studdering, even slightly worse now.

Start thinking loss of compression due to head gasket failure: Compression test, cold engine and TB closed #2 210 PSI, #4 220 PSI. Did'nt do any other. GM says a 30% difference is bad, I am at 4.7% on those two cylinders. Coolant is clean. Oil is clean. Tested an oil sample, no trace of coolant (Dex Cool).

I'm at a loss, and not really excited about pulling the head. I have SPS and a spare ECM, thought about doing that. Anyone got any ideas? I am quite familiar with Gen3/4 but this is my first Gen 5 issue. Thanks.

[grandam75]

Are you loosing any coolant? Read about a similar this issue a while ago. also on an engine with some mileage and issue was according to this TSB:
http://www.corvetteactioncenter.com/...rint-1364.html
Should be possible to check without removing the cylinder head.

[danmw2003]

Blue smoke is oil, Black smoke is too much fuel, white smoke is coolant. Id suspect bad valve seals or guides IMO

[Kyle_Customizer]

I'd recommend doing a running compression test. I've seen great static compression readings and have significant lower running compression due to valve issues. I'd suspect carbon buildup causing it to stick at low rpms.

[ryanvv355]

Thank you guys for the tips so far. New developments today, I was idling in my driveway and the service traction control light came on, then went off a short time later. I do not have access to a good scanner to retrieve that code though.

I noticed yesterday P135A and P135B where showing permanent, I thought for sure this was a clue. Today I programmed up a new ECM and installed. Did not fix the problem.

I found an TSB regarding terminal tension in the underhood fuse block, the fault condition is rough idle and service traction control system light, checked out the connector, looked fine, re-tensioned the terminals anyways. Did not fix the problem.

Will try the running compression test tomorrow. Going to the stealership Monday.

[ryanvv355]

Berger Chevrolet in Grand Rapids says the cam is wiped. $6,300.

[ryanvv355]

Traded the truck in on a 17 Silverado LTZ. For anyone who reads this later, the dealer used a dial indicator to measure the rocker travel and saw the the exhaust lobe was not within spec. This could have been only a lifter failing, I wasnt going to hang around to find out. The service advisor said this "Does happen". The new truck has a 10 year, million mile warranty on the powertrain.

[IARLLC]

That is a new one on me. There are a few TSBs on GEN4 and GEN5 DOD/AFM lifter failure and several of us have experienced it.

And dealers are known for their knowledge and honesty...