Engine Shaking - P0304 Misfire Code

DustyDee · February 5, 2015

I need some help!

I have a 2009 Chevy Hybrid, 6.0L V8 engine, about 150,000 KM on it.

While driving home, the whole vehicle started to shake violently, and the engine light came on (flashing). I checked the code and it is a P0304 cylinder 4 misfire. I drove it home to the garage and have been troubleshooting the problem since then. I am stumped.

I have changed the spark plugs, all of them. I have confirmed spark using an inline light style of tester.

I checked compression and didn't have any on that cylinder, all the rest were normal. I pulled off the head and cleaned up the valves and valve seats. Cyl 4 was pretty dirty with lots of excess carbon build up. I took the head to the machine shop to resurface it. New gaskets all around; everything back together. I now have good compression on that cylinder. But I still get the misfire code and the engine shakes.

I checked the signal to the fuel injector with a noid light and I get proper signal. I replaced the fuel injector. I took it for a run that evening and the code didn't show up, but the engine still shook. The next day the misfire code showed up again.

When the engine shakes, it shakes the entire truck. When it first happened I thought I threw a wheel weight. The shaking is most prevalent when at low RPM, with a low load. When I get the RPM's up there, it is still slightly noticeable, but not as significant. When I have the truck in park or neutral, it still shakes. When I am coasting down a hill, it doesn't shake, and often the flashing MIL will go away. The shaking is the worst when I am slowly pulling into the garage, or other parking spot.

Any brilliant ideas on what to look at next?

Thanks,
Dusty

fondupot · February 5, 2015

What about coil packs and all the wires?

Edited February 5, 2015 by fondupot

DustyDee · February 5, 2015

I have an inline spark tester. It lights up like it is supposed to. I don't see the reason for changing wires and coil packs.

POS VETT · February 5, 2015

If you swap the wire #4 with one from another cylinder, does the code follow the wire #4?

Spark will still jump if the wire is "leaky", however the spark plug won't ignite.

DustyDee · February 7, 2015

Nope. I swapped the #4 wire with #2, and the code did not follow.

I'm working on swapping the coil packs to see if that makes a difference. Should be able to get that done tomorrow. All the Hybrid gear in there makes it tight to take the coil pack off the passenger side.....

oldscsc · February 7, 2015

After this work, you have:

-Eliminated spark concerns (spark plug testing good = plug, wire, and coil are ok.

-Eliminated fueling concerns (confirmed injector pulse signal = PCM knows when to fire cylinder #4, replaced #4 injector)

The fact that the misfire is concentrated on a single cylinder means timing isn't the issue, so we can rule that out.

Short of some freak electrical problem, this diagnostic path seems to be leading us down the mechanical path.

What is the compression like on #4 compared to the other cylinders?

A flat cam lobe can prevent the valves from opening or sealing properly. Did you inspect the cam lobes when you had everything apart?

A cracked valve spring may not have the amount of spring pressure required to properly shut the valves and seal the cylinder. Did you inspect the valve springs on #4 when you had the head off?

Also, just out of curiosity, is this engine a fixed Atkinson cycle design or variable? (can switch between Otto and Atkinson)

Edited February 7, 2015 by oldscsc

POS VETT · February 7, 2015

If it was a coil, your inline spark tester wouldn't have lit up.

DustyDee · February 8, 2015

Compression on cylinder #4 is 100 PSI, the other cylinders varied from 105 PSI to 120 PSI.

I did not inspect the valve lifters or the cam lobes when I had it all apart. I am kicking myself for not doing that...

I am pretty sure that it is not an Atkinson engine. It is the LFA engine code: 6.0L, V8, sequential fuel injection.

That's a video clip that I took to show the noise that the engine is making. Hopefully the sound comes through okay.

Thanks,

Dusty

oldscsc · February 8, 2015

Compression on cylinder #4 is 100 PSI, the other cylinders varied from 105 PSI to 120 PSI.

I did not inspect the valve lifters or the cam lobes when I had it all apart. I am kicking myself for not doing that...

I am pretty sure that it is not an Atkinson engine. It is the LFA engine code: 6.0L, V8, sequential fuel injection.

That's a video clip that I took to show the noise that the engine is making. Hopefully the sound comes through okay.

Thanks,

Dusty

Thanks for the video. That is a dead miss...absolutely no contribution from that cylinder.

Did some research and confirmed that the hybrid rigs did indeed get Atkinson cycle engines. They also have AFM.

If that cylinder is one that the AFM deactivates, I'd inspect #4's lifters to ensure they aren't permanently collapsed. The AFM lifters are designed to collapse under normal operation, but they may fail to where that cylinder is essentially permanently deactivated.

DustyDee · February 8, 2015

Thanks for the info!

If I'm reading the repair manual correctly, that means I need to take the head off again to get to the lifters... new head bolts, new gasket, more $$. Crap, why didn't I think of this earlier.

Anything else to check before I take the head off again?

Edit: when I had the valve cover off last time, I cycled the engine through a few rotations using a wrench on the crank shaft; the valves on that cylinder lifted and closed like they are supposed to. What deactivates the lifters? Maybe that is where the problem lies.

Edited February 8, 2015 by Dusty Dee

kstruckcountry · February 8, 2015

It is not a "real" Atkinson engine, basically just has an extra long intake stroke that effectively lowers the compression I believe.

Not a motor I'd want to work on.

oldscsc · February 8, 2015

Quick engine stats:

VORTEC 6.0L Gen IV V-8 (LFA) HYBRID TRUCK ENGINE

2009 Model Year Summary

· New Engine for 2009 Chevrolet Silverado Hybrid and GMC Sierra Hybrid. Continues from 2008 model year as carryover model for Chevrolet Tahoe Hybrid, GMC Yukon Hybrid, Cadillac Escalade. Hybrid.

· Variable Valve Timing

· Active Fuel Management

· Variable Displacement Oil Pump

· Returnless Fuel Injection with Stainless Steel Fuel Rail

· Advanced Electronic Throttle Control

· E67A Engine Control Module

· 58X Ignition System

· Enhanced Noise, Vibration and Harshness Control

· Late Intake Valve Closing

oldscsc · February 8, 2015

The AFM lifters are controlled by oil pressure I believe, but I'm no expert.

I don't want to guide you down the wrong path. I'm happy to give you my opinion, but don't want my opinion to cost you a bunch of money. I'm just throwing the AFM lifter out as a possibility.

Without tearing it down again and inspecting the valvetrain and valvetrain control circuits (AFM lifter namely), I'm out of ideas.

DustyDee · February 8, 2015

Thanks. I fully understand that diagnosing engine problems solely based on information from an internet forum is not the best way to do things. I should probably just take it to the shop and get it fixed there, but I enjoy the challenge too much.

Well, time to tear it down again. Wish me luck...

DustyDee · February 12, 2015

Looks like the plastic lifter holder failed. It allowed the lifter to spin sideways so it was no longer rolling on the cam shaft. Cam is damaged and will need to be replaced also.

This just keeps getting better... Time to pull the radiator off.