misfire cylnder #5

hi
gmc sierra 1500 2014 5.4 with 319k km

custmer compline missfire on #5

i sweep coil,plug and plug wire still same
control is ok
i do relative test with trigger on #5 (green))

i see #5 is higher than other
is this cuse misfire

still am not play with injector or it is ramp

That can definitely cause your miss. Given the engine, I'm betting you're looking at an damaged camshaft lobe or lifter. The #5 intake valve likely has insufficient lift/duration.

Go in-cylinder on #5 if possible. If not, do a running compression test on #5 and another known good cylinder for comparison. I'm betting running compression on #5 will be low.

i do relative test with trigger on #5 (green))

Is your Relative Compression done with Voltage, or Current?

i see #5 is higher than other
is this cuse misfire

Having higher current on one cylinder indicates MORE work on the Starter. Are you sure the misfire is on Cylinder #5? If the companion cylinder is coming up on the exhaust stroke, but the exhaust valve does not open, you will have TWO compression strokes, simultaneously, increasing the load on the starter.

Tyler wrote: I'm betting running compression on #5 will be low.

Did you notice that the .jpg is upside down? I'm curious if this is a Current Waveform, or Voltage waveform. If this is a Current Waveform, Compression on Cylinder 5 is HIGH, or companion cylinder exhaust valve failed.

Is it upside down, on purpose, to invert a voltage image? In that case, #5 would have Low Compression.

Hi guys,

Is that trigger on time?

I am seing that pulse advanced (or delayed depending on voltage or current waveform).

Considering the multiple humps on each compression stroke i would say to check valve adjustment. And stick a camera in the intake to see if carbon is building up on the valves also if the engine is vvt equipped disconnect the solinoids and do the relative compression test again. See if it changes.

Tyler wrote: Did you notice that the .jpg is upside down? I'm curious if this is a Current Waveform, or Voltage waveform. If this is a Current Waveform, Compression on Cylinder 5 is HIGH, or companion cylinder exhaust valve failed.

Is it upside down, on purpose, to invert a voltage image? In that case, #5 would have Low Compression.

I did notice that! I was struggling to make out the traces until I flipped the image:



That made it easier to see that the red trace is on a 200A scale. Sure, it still could be some kind of battery voltage trace, but the appearance of the waveform really said amp probe to me.

It sounds wacky, but I've seen wiped intake lobes on these engines cause higher cranking compression, but lower running compression. :silly: For example, a known good cylinder:



Then the bad:



I didn't expect this one to bury my 200 PSI scale. :blush: An added 30 PSI compression can certainly make an RC hump noticeably taller.

Tyler wrote: It sounds wacky, but I've seen wiped intake lobes on these engines cause higher cranking compression, but lower running compression. :silly:

:huh: I'm gonna have to chew on that one, for awhile. :ohmy:

Chad wrote: :huh: I'm gonna have to chew on that one, for awhile. :ohmy:

It still kinda hurts my head when I think about it too much. :lol: Here's a zoomed out shot of another vehicle with the same problem, during engine startup:

yes it is current clamp not voltage
i have no in-cylinder
but i will do compration test

tested #5 #2 #3 is near 180 psi #1 is near 150
now the misfire on #5 and #1

Tyler wrote:

Chad wrote: :huh: I'm gonna have to chew on that one, for awhile. :ohmy:

It still kinda hurts my head when I think about it too much. :lol:

It looks like the wiped lobe is causing the intake valve to open late and close early. Here is the valve timing for a 2.5 Jeep engine for reference. There's some overlap when the intake valve opens and the exhaust closes. The intake valve closes after BDC which'll dump the start of the compression stroke back into the intake.



The vacuum event after 360* has to be caused by the piston moving down with both valves closed. When the intake valve opens in cylinder pressure rises to normal so the valve is opening enough to get a full charge of air at cranking rpm. Compression stroke appears to be starting early due to the intake valve closing too soon. This would probably all look a lot clearer with degree markers on the trace.

tested #5 #2 #3 is near 180 psi #1 is near 150
now the misfire on #5 and #1

Did you happen to do running compression, too? 'Cause those numbers are way too high to be running. :lol:

Thank you, Matt. Knowing that the intake valve closes after BTC makes it easier to comprehend.

Matt T wrote: It looks like the wiped lobe is causing the intake valve to open late and close early.

I think that's exactly what was happening. The cylinder is still able to pull a decent charge of air through the intake valve at cranking speeds, but can't get enough air into the cylinder at idle speeds or higher.

Sorry, I should have put degree markers on the original post. I was just lazy. :blush: Here we go:





Really didn't mean to threadjack.