Re:HELP! P0305 Single Cylinder GM 3800 Series 3

Update! The failure discussed here ended up being a broken exhaust valve spring. I've attached a photo for reference. Car was still able to run but the most telling tale of the problem is the running in cylinder pressure test. Intermittent pressure drop on exhaust stroke.

As an aside I was introduced to a company named Rotkee that has a boatload of inexpensive scope tools for beginners like me.

Thanks for the help and good luck out there!






Hi All!
Thanks for the videos and the information I think this site is fantastic! I've tried some work on my own and have hit a point where I feel I could use some help from the community. Any info is greatly appreciated.

Vehicle: 2007 Buick Lacrosse 3.8L - GM 3800 Series 3
Problem: Misfire Cylinder 5 P0305
Background: Neighbor spun rod bearing last summer. I helped by putting in a junkyard motor with new seals and gaskets. Heads and intake did not come off engines. Engine ran fine for 2 months. Early December failsafe thermostat (new) stuck in the open position and vehicle was driven 400 miles back home not coming up to temp with #5 misfire. Thermostat fixed misfire remains.

Secondary igntion waveform for cyl 5 (the only remarkable out of set)


Compression on all 6 appx. 150 psi
Cylinder balance 100-100-95-85(#5)-100-93
Leakdown test is less than 5% on Cyl #5

Diagnostic attempts:
Tested (all good) but still swapped spark plugs, spark plug wires, ignition module, ignitions coils, injectors, injector rail, (off spare motor). I should have started scoping first...

Smoke test failed due to bad injector o-rings on #2 and #5, replaced and passed with no smoke in hood.
EGR was removed to verify not stuck open. Moves freely. No indication of smoke on during smoke test.
O2 sensor Bank 1 appears responsive ( Short term <5% and Long Term currently at 7%). O2 sensor bank 2 is 98% fuel trim add ( I believe this is by intent)

Injector resistance is 12.2 ohms and within .1 of each other
Injectors were unplugged and swapped
Injector passes noid light visual

Cyl #5 injector waveform


Cranking compression amperage

I suspect you have valve leakage. The firing KV seems low and the saw-tooth hash marks in the second half of the spark line is an indication of turbulence, cause by compression leakage. I would like to see a good cylinder, to compare firing KV.

Your injector waveforms look good.

I would smack the RC waveform with about 500 hz of filter. That will take most of the hash out, and make the waveform easier to read.

If you have a pulse sensor, or two, a cranking waveform of intake and exhaust might help prove, or disprove, my suspicions.

Thank you Chad! I cleaned up the cranking test and it looks much better. Definitely shows one of the cylinders a shade lower than the others just as the cylinder balance test reflected. I have a pulse sensor on order but it just isn't here yet.

Is it possible that that I have a hanging intake valve (bad spring) that was allowed to push and seal during the leak down testing? I'm really pushing toward this and removing the valve cover to begin investigation.

Here are the remaining secondary ignition waveforms as requested. Does the kv seem low?

lvkeith wrote:

Is it possible that that I have a hanging intake valve (bad spring) that was allowed to push and seal during the leak down testing? I'm really pushing toward this and removing the valve cover to begin investigation.

If you have a pressure transducer that you can put in the spark plug hole, set your scope to a 20 second screen, disable fuel, and give the engine a good, long crank. All pulses of the compression strokes should be the same. If the peak compression of each pulse is different, or if there is even just an occasional low pulse here and there, that is an indication of inconsistent valve sealing.

Here are the remaining secondary ignition waveforms as requested. Does the kv seem low?

Relatively, they look okay. If you want to know if the Firing KV is good or bad, look at the Spark-line burn time. Burn time, as rule of thumb, should be 1 - 1.5 ms.The higher the KV demand, the shorter the burn time. And vice-versa. We don't, really, care about the Firing KV demand, if the burn time is good. Your burn times look good, at 1.3 ms. With the exception of Cylinder #3 is a little short on burn time and it looks to be a little lean.

This is the in cylinder cranking pressure test. I did not have the WPS 500 but I used a combination of a 500 psi transducer I had. Still working out the scaling on my pt in pico software. The peak compression for both cyl 3 and cyl 5 appear to be consistent with a little drift towards the end. The scaling you see is with the pico WPS500 scaling. Sorry. I had to use a 10hz filter here because it was very noisy

let me know if I should pull these back and include the 720 degree phasing lines or if I need to set the x axis to 20 seconds, the software autostopped and started back on me.

This is problem cylinder #5. This measurement was taken while the vehicle was cold and cranking. I read that some do this with the vehicle idling?




The images below are of the non-misfiring cylinder #3

Hi all. Still struggling with the diagnosis here. Finally learned to do a running in-cylinder pressure test here and I found something showing up in the misfiring cylinder. It looks like a pressure drop during the exhaust stroke and is unique to this cylinder. Can you please give me your interpretation of what could cause this waveform? It seems like a stretch that a leaky valve guide and seat could have this much affect on the exhaust stroke

@chad your diagnosis was spot on.

Just to follow up here the culprit was a broken exhaust valve. I've attached a photo and will leave the information up for any future reference.


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Good Job!

My apologies for the lack of response to your follow-up posts. You slipped through the time cracks on my "recent topics" list. It appears that you didn't, really, need anymore help, anyway. Well done.

As I was scrolling through your In-cylinder waveforms, I was thinking that those waveforms were un-useable due to the low resolution/response time. The later waveforms are not as bad.

I would like to see the picture. I am struggling with a broken exhaust valve, on cylinder #5, creating the exhaust waveform that you have on cylinder #5.

During the exhaust stroke, the piston is traveling upward with the exhaust valve open. The exhaust stroke of the waveform should be a, fairly even, plateau of near zero/atmosperic psi. If the exhaust valve were to close as the piston was traveling upward, pressure in the cylinder would rise. That is not what we are seeing. The exhaust plateau is falling into vacuum. This makes believe that it may be the intake valve falling open?

I think the same case was presented on DN where Dimitry pointed out that most probably the substandard sensor was adversely affected by electromagnetic interference causing this unusual waveform....it would be difficult to base a diagnostics on it.

The secondary ignition waveform on the other hand is much more valuable as it clearly shows turbulance within the cylinder.

Chad wrote:

I would like to see the picture. I am struggling with a broken exhaust valve, on cylinder #5, creating the exhaust waveform that you have on cylinder #5.

I'm struggling with anything mechanical causing that abrupt exhaust stroke dropout and recovery?? It's in about the right place to be caused by #3 plug firing. And even wierder it recovers about the time #2, which is #5s companion cylinder, fires. Noise in the good exhaust strokes to the right might also be #2 firing??

Copy and paste of the fotoshoped image...

I think juergen.scholl has a rock solid theory about the occurance of the dip in the exhaust plateau.

Is this occurance repetitive, does it occur every second, third cyle? Or is it random?

Sorry I didn't follow up sooner. The capture was correct and it was a broken exhaust valve spring.


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