Need help on P0300, seeing missfire data on cylinder 4 and 5

stagrlee · January 12, 2016

I'm getting a P0300 thrown every 10 days or so and I'm trying to track it down. Haven't thrown any money at it yet. This is on my 2004 Tahoe Z71 5.3L Flex engine with 161K miles. The image is the missfire data from my Maxidas code reader. I don't know why cylinder 6 and 7 are red (maybe the Maxidas software was written for the 6 cylinder?). The missfire data is almost always at idle. Over 1000 RPMs, missfires are rare. The photo shows 1400 RPMs, because I was goosing the throttle and the misfire is a counter over 15 seconds. If you leave the engine speed high, the misfires go almost to zero. Truck has plenty of power, decent idle.

Tests I've tried...

Moved fuel injector from cylinder 5 to 2
Moved ignition wire from cylinder 5 to 2
Moved coil pack from cylinder 5 to 2
Moved spark plug from cylinder 5 to 2
Measure 19 In.Hg vacuum at the manifold
Ran a crank position relearn from my Maxidas
Could not confirm the misfire with a timing light, looked like good strobe

No changes noticed on the missfire data. Cylinders 4 missing somewhat at idle. Cylinder 5 missing a lot more.

Intake manifold gaskets are about 20K miles. Plugs looked fine. O2 sensors (Bosch), injectors, plugs, wires are about 60k miles. Coil packs and cam position sensor are original.

The truck was recently throwing P2135 codes and I replaced the throttle body and pedal position sensors as well as the throttle body eletrical connector/wiring. I believe that issue is fixed.

Thinking the next test is a compression test. I'm not afraid of doing a cam position sensor replacement as I've had the intake off a few times and it isn't too bad of a hassle. Anyone run into anything like this? Any ideas?

stagrlee · January 13, 2016

Compression numbers are about perfect.

1 - 183 2 - 184

3 - 175 4 - 179

5 - 185 6 - 184

7 - 180 8 - 179

All plugs look nearly identical. I'm thinking its either an O2 sensor or a cam position sensor. Since the cam position is original, I'm gonna spring for that next week. The truck still drives fine.

tbarn · January 13, 2016

Seems likely it's a vacuum leak. One thing I have run into is a lifter not compressing. Causes the valve to hang open slightly. It will pass a compression test and cylinder leak down test. Remove the rocker cover and loosen the rockers a couple of turns on the suspect cylinder. Reattach the coils with the cover off and run the engine. Slowly adjust the rockers up or down to see if the miss comes and goes.

stagrlee · January 13, 2016

Reattach the coils with the cover off and run the engine.

Wow! Never thought of running wet like that. How much oil sprays around? My lifters are original.

tbarn · January 13, 2016

Usually not much at all but sometimes you get a shooter. Throw a towel over the head before you start it to be safe if you are worried.

stagrlee · January 13, 2016

I guess I'm remembering how much oil gets flung off overhead cam heads. Rocker motors don't fling oil like that.

Been driving it post testing/relearn/compression work and it runs perfect. Probably the sensor signal response drifted a bit and the engine went rough. Seems "fixed" right now.

stagrlee · March 5, 2016

The problem is still happening. I found small vacuum leaks at 2 injectors and the MAP sensor so the ACDelco 217-3361 and GM 16194007 O-rings slowed down the misfire rate a bit. Still getting occasional P0300. Haven't done the sticky lifter test yet. Want to replace all the vacuum lines but having part number problems that I'll post in another thread.

http://www.amazon.com/ACDelco-217-3361-Original-Equipment-Multi-Port/dp/B00556TP0A

http://www.jegs.com/i/Chevrolet+Performance/809/16194007/10002/-1