My07C6

I have a new 2009 Z06 with 322 miles on it(mfg 9/23/08). MIL with random misfires on cylinder #1. Dealer has had it since Saturday the 4th. Came on while car was warm and idling in driveway. Not driven hard yet, rpms kept below 4000.

Details
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Saturday
Swap cylinder 1 plug with another cylinder - misfire still detected on cylinder 1
Swap cylinder 1 plug wire with another cylinder - misfire still detected on cylinder 1

Monday
Swap cylinder 1 coil with another cylinder - misfire still detected on cylinder 1 Suspect bad injector - order new injector air freight

Tuesday
Install new injector on cylinder 1 - misfire still detected on cylinder
1
Remove valve cover inspect rocker arms, valve springs & retainers - All ok
Spark plug in cyinder 1 somewhat sooty.
Compression check cylinder 1 - 130psi, others 160psi

Wendsday
Pulled head to investigate lifters, and valve & seats further.
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Questions
1. How does the ECM detect a misfire? I suspect it's from the O2 sensor threshold either rich or lean.

2. If the spark plug is somewhat sooty, wouldn't that suggest a rich condition, thus pointing to spark?

3. I'm having a hard time understanding how a leaky valve could cause a misfire.

I suspect that after they put the head back on it will still show a misfire on cylinder #1. I insisted they doouble check the connections and circuits from the ECM to the coil & injector before tearing down my engine. They said they have double checked and everything was ok. My gut says it's a bad connection from the ECM to the coil.

Any thoughts?

s ladd

Well if the compression is low, why wouldn't you want them to check the engine out? That should be a sure sign that something is wrong with the engine. 130 is low... even if the engine is cold.

Xwet

I had a issue on my 03 Z06. It was the crank trigger sensor. Got misfires above 2000 RPM.

My07C6

UPDATE

The cylinder head has imperfection on the valve seat for the exhaust valve. They are ordering a new head.

patton

thanks for the getback

Vito.A

I am glad you found the problem.
Trouble shooting mysteries like this are better done with a leak down test rather than a compression test. A leak down test is performed with the piston at TDC. An adapter is installed in the spark plug hole and regulated air is applied. Some testers have two gauges and you read the difference between the two. Newer models have a single gauge and you can read the cylinder leakage directly. A new engine like this should only leak 2-4%. I suspect yours was over 30%.

HANNY

a leaking valve can absolutely cause a missfire, so they could be right there (it would have to be an exhaust valve, if it were intake it would "pop" back thru the intake). only thing else they havent looked at is ECM failure (not firing that coil) or a connection issue to that coil. b. good luckoth of those things would probably set anoher code tho?
To answer your "how does it detect a missfire" ? the ecm is watching 360 degrees of rotation via the crank sensor. It knows where and when #1 is firing and watches Degrees of rotation. It compares that to other cylinders, weeak cylinder equals missfire.
the first thing i would have done is to try and clean off the back of the valves with intake cleaner (kinda like a polish tune up)
a weird issue on a new engine IMO. good luck

tjwong

The engine control module (ECM) uses information from the crankshaft position (CKP) sensor in order to determine when an engine misfire is occurring, it also uses information from the camshaft position (CMP) sensor in order to determine which cylinder is misfiring. By monitoring variations in the crankshaft rotation speed for each cylinder, the ECM is able to detect individual misfire events. If the ECM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 sets. Under certain driving conditions, a misfire rate can be high enough to cause the catalytic converter to overheat, possibly damaging the converter.

My07C6

Thanks for the explanation, never thought about it using the CKP and CMP position sensors vs speed to detect misfires. It makes alot more sense to me now that I know HOW it detects a misfire.

I've been meaning to get HpTuners to learn more about how the ECM works, programming wise. Coming from > 24 yrs experience design/build/troubleshooting industrial control and automation systems, I am very familiar with how controls work. I wish there was a tool to hack the rest of the modules like the BCM and others. Then you could really customize your ride.

patton

why didnt they grind the valve and seat ? testers like tech 11 and snapon can help

tjwong

Wow! It is a small world. I spent 25+ years as a EE specializing in process controls for pulp and paper mills all over the world. I spent the last 2 years of my career in Southern Brazil working on a large pulp mill expansion project, I came home realizing that I should stay closer to home as my parents and siblings along with all the new kids in our family were getting older and growing up. So I decided to quite the engineering work and started up my dyno tuning shop which was a childhood dream to have my own speed shop. I have survived now for this is my 5th year in business, it isn't nearly the same kind of money I made as a EE but it works plus at 52 I got a baby boy coming in December and I now have a beautiful wife 22 years younger than me, and I got a new Z06 last summer, so life is good

The BCM is another sore spot for me, I can tweak it with the GM TECH2 scan tool. But I am only allowed to make the changes that the tool can access. In the old days we were allowed to do much more, but since the BCM now controls ALL of the body functions GM has limited us to what we can access. Case in point, I installed Euro tail lamps in my new Z. It took me hours to figure out how to manipulate the BCM to make it do what I wanted it to do. All my Euro lamps work as designed, but I lost my tire monitoring system. If I make the tire monitor system work and keep my Euro lamps, then I loose my satellite radio. So I choose to keep the satellite radio and loose the TPMS system. If it wasn't for having a 2 year subscription to SIRIUS I would have went the other way

I forgot to mention, if you should ever cam up your Z, you will have to use HPT to tune the car, and part of the tuning will involve tuning the misfire detection tables. The tables represents time between events or pulses that the PCM sees from the crank sensor to calculate misfires. With a larger camshaft, the idle becomes erratic so the variations in the time between the pulses is also eratic thus indicating a misfire to the PCM. So in this case you would be detuning the tables to desensitize the PCM for the camshaft.

My07C6

I don't know, I haven't seen it myself. I suspect the seat must be bad enough not to be able to regrind it. Or maybe they didn't want to risk repairing the head on a new motor they still have to warranty for the next 5yrs 100k miles.

I like the HpTuners because I can log the data and review it later. Also be able to tweak the tunes on my cars if I feel like killing my warranty.

I just want it back before the weather starts turning. It will be in the shop longer then it's been in my garage by the time it's all said and done.

KB9GKC

Spark plug in cyinder 1 somewhat sooty.
Compression check cylinder 1 - 130psi, others 160psi


Hello,

General Rule is that from highest to lowest cylinders compression redings need to be within 10% of each other. If the highest was 160 then the lowest acceptable would be 160psi - 16 = 144PSI.

To find out if the problem is in the valves or the rings you would squirt some oil in the spark plug hole and then retest compression, if the problem cylinder gains a lot of compression then the rings are at fault, if compression reading stays close then you have a valve problem.

Good Luck,
Douglas in Green Bay