Please help with misfire - I'm stumped


New Member
Jun 7, 2018
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Houston, TX
Hey folks, I’m new here. I’ve been reading post for a while, my vehicle before the 2010 Tahoe I now have was an ’03 Tahoe.

I’m looking for input on a problem I can’t wrap my mind around. A quick bit of info about me, I’m one of those older guys from the days of points and condensers and manually setting your timing (1970’s era) etc. I never built ‘hot rods’ so I never got deep into the technical side of things, but I have rebuilt 3 or 4 engines over the years and usually do all my own repairs. I’m not into engine modifications, pretty much an OEM spec kind of guy. I do commercial low voltage electronic systems for a living, so I have no problem with automotive electrical, and have a decent understanding of module control of inputs and outputs. I’m not a pro at this by any means, but I’m also not a newbe. No doubt you can get over my head if you happen to be a pro and have some input, but hopefully you won’t have to candy coat it too much.

So back in November, I traded my ’03 Tahoe in for a 2010 LT Z71 Tahoe with a 5.3 and 120,000 miles. I plugged in my cheap code reader and there were no codes and all the monitors were ready, and it ran fine, so, no need to fix what’s not broken. I wasn’t real pleased with the mileage, it averaged a little better than 13.5 mpg, but figured I’d work on that as time went on. I did put new plugs and plug wires in the first month.

Back in April it started idling rough, but didn’t throw any codes. Knowing it’s 8 years old with over 120,000 miles, and not knowing the history, I don’t have a problem replacing parts so I’ll know what I have, especially anything that will help gas mileage. I replaced the throttle body and position sensor (along with the accelerator pedal w/position senor), and once the idle speed settled down it was still rough, and threw a P0305 code (cylinder 5 misfire). I’ve been fighting that code ever since. I have a scan tool, I can see the misfire counter add up on cylinder 5, and it seems to be only at an idle. I moved plugs, plug wires, coils and injectors to other cylinders, but the misfire at idle stays on cylinder 5. I researched the hell out of Active Fuel Management, which I have, and I know all the lifter issues and problems they can have, but AFM is on cylinders 1, 4, 6 and 7, and my misfire is on cylinder 5. It’s not consuming oil like the ones with the AFM lifter problem. I’ve replaced the Mass Air Flow sensor, MAP sensor, water temp sensor was replaced when I changed the water pump back in February (shaft bearing was starting to squalk), I changed the oil pressure sensor (and the VLOM screen underneath it) just to be sure the oil pressure reading I was seeing was accurate. I changed the vapor canister purge valve and vapor canister vent valve (problem with gas pump handle disengaging while filling the tank), I changed both the crank and cam position sensors (and did the crank position relearn), still have the misfire.

I’ve checked compression with all the plugs out, over 180 pounds on every cylinder, including cylinder 5. I checked vacuum (inserting a ‘T’ at the brake booster vacuum connection) 19 inches at idle, goes to 17 inches if the AC is on.

I have a noticeable loss in power, most of the time it feels like I’m towing a pontoon boat. My gas milage is down around 12 mpg or less.

I noticed on my scan tool the long term fuel trim on bank 2 was running about +12 at idle, but would go down to about +1 while driving, while not the same bank as cylinder, assumed I had a vacuum leak on bank 2. I also noticed O2 sensor Bank 1 Sensor 2 constantly stayed around 810 mV regardless of what the others were doing. Thinking the O2 sensors may be the original ones I went ahead and changed them. I suspected that if they were getting old and slow and sending bad info to the ECM, that when it was open loop (with a vacuum leak) and then going closed loop and seeing bogus info from the O2 sensors and making fuel adjustments for that that it was constantly changing fuel mixture as it bounced in and out of open and closed loop mode, which I thought may be why it was so sluggish.

I decided to change the intake gaskets, went with Felpro. While I had the intake off I went ahead and changed the fuel injectors. The long term fuel trim is back down low again so apparently I did have some vacuum leaks. And now I feel better about the fuel injectors actually delivering what the ECM thinks they are delivering. But, still have the misfire on cylinder 5 and recurring P0305 within two drive cycles.

I can pull the injector electrical connector off cylinder 5 while idling and it really doesn’t change the idle speed. At that point I was suspecting maybe the ECM is stuttering on the pulses to the cylinder 5 injector. I put a noid light on the injector connector for cylinder 5 and at an idle (when the scan tool is steadily counting misfires on cylinder 5) I’m seeing a nice steady light pulse not missing a beat. I put a spark indicator on the spark plug, same thing, steady light pulse, not missing a beat.

Now, off with the valve cover. Cranking the engine over I can see both valves on cylinder 5 open and close as they should. I put #5 at TDC and removed the rocker arms, no visible damage, no cracks, no wear. Pulled the push rods, not plugged, you can clearly see light thru them and their walls just like looking through a new straw. Both valve springs look fine, neither is broken, both seem to be keeping the valve closed tightly. That didn’t satisfy me, so (after reinstalling the rockers) I hung the coil pack with some string, reconnected the harness and plugs and started the motor to watch the rockers in action. Neither valve missed a beat, both opening and closing the same amount as the other valves by just watching them (I’m sure there’s probably a scientific way to measure that, but with the naked eye they appear to be working just fine) Both pushrods are pushing out oil on top of the rock arm, not tapping any louder than the other, so I’m thinking the lifters are. I got my stethoscope and listened to the fastening nut on all 8 rocker arms. I will say this. On seven of the eight rocker arms you can hear that whirring sound of the engine and a slight tapping sound as the valve operates, but the exhaust valve of cylinder 5 was a little louder and sounded kind of rattley in between taps. Not extremely louder than the others with the stethoscope, but enough to notice it was not as smooth and quite as the others. BUT, still opening and closing the valve and pumping oil. And with the naked ear, it’s no louder than any of the others.

I found a TSB (#11-06-04-007B) about changing the throttle position sensor that said for Tahoes from ’08-’10 if you put a new throttle position sensor that you MUST reflash the ECM with he latest software, but from ’11 up the reflash was not required. Mine is a 10 and I have changed the throttle body/throttle position sensor as and assembly, so I put the old one back on for now just to be sure that’s not causing an issue.

So I’m at a loss. I’ve used all ACDelco parts (except for the Felpro intake gaskets) I’ve eliminated every external thing I can think of that could cause a misfire, I have compression, vacuum, fuel pressure running 58-60 psi key on and engine off, and stays around 58 psi at idle, no vacuum leaks. The one and only thing I’ve found wrong with cylinder 5 is a little extra noise on the exhaust rocker are when listening with a stethoscope. I may be wrong, but I can’t believe THAT is enough to cause a constant misfire since the valve is opening and closing as expected.

The only thing I haven’t changed is the knock sensors. Starting in ’07, they moved them from underneath the intake to down on each side of the of the lower block. Easy to get to now, and ironically the one on the left side is about where the #5 cylinder is. The scan tool shows no knock on any cylinder. I really don’t thing a bad knock sensor would cause a cylinder specific misfire, but they are less than $8 each at Rockauto, so I ordered a couple of them.

On a side note, I’ve also ran some Seafoam in the oil for about 200 miles thinking if the problem was a lifter it would loosen up. I haven’t tried Marvel Mystery Oil yet.

If you have any input that would help I would appreciate it. I’ve seen posts about misfires on discussion boards, but most of the time there is no follow up post if they fixed the problem. If I find a solution I will absolutely post what I did to resolve it.

Thanks for reading if you got this far, sorry it’s so long.
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New Member
Oct 13, 2022
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Perform a leak down test and make sure that cylinder is sealed. I have seen carbon on valves cause this.