Cylinder 5 misfire - sorry, long post... but stumped

mckpaul · June 12, 2018

(I posted this on another board, if you've seen it there and replied then disregard. )

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.

mckpaul · June 12, 2018

I have a question for some of you GM dealer techs. As an end user, we can test and troubleshoot and diagnose problems with the tools available to the end user, and sometimes it's still a maybe on what we think the problem is.

From the outside, it's easy to imagine that a GM dealer shop has high end diagnostic tools and software and they can magically pinpoint the exact problem with the click of a mouse.

So I have questions. Can they plug into an ECM and run some kind of test program that tests all the inputs and outputs?...basically check the 'health' of the ECM?

I've read where the outputs, say for coils or injectors, are transistor driven outputs, might have 4 transistors on an output for example, and it's possible for a couple of transistors to crap out leaving a functional but weak output.

As for misfires (in my case for example) can they identify the exact cause of a misfire more efficiently than disassembling and looking or old school trial and error, and know without a doubt the cause of a misfire before making the repair? Can the confidently say "(This) is the cause and it will cost $X for the repair"

Here's my dilemma..

I have a cylinder 5 misfire (P0305) that I've gone full circle on (new plugs & wires, rotating out plugs, wires, coils and injectors with other cylinders, new intake gasket and new injectors, watched rockers and valves open and close, watched oil come out of pushrods, steady flashing noid light on injector, steady flashing spark indicator light on plug, no apparent reason for misfire)

The ONLY thing I can find out of order is while listening to the rocker arm bolt heads with a stethoscope is the exhaust valve on cylinder 5 didn't sound smooth and machined like the other 7, sounded a little raspy or rattley, but not grinding rough or loud in the stethoscope, just a little louder and not as smooth as the others. I would think a rotated lifter would really make some noise and cause major knocking.

Before I realized it was cylinder specific (with the P0305 code) and it was just a rough idle, I changed the throttle body w/position sensor but, found a TSB saying 2008-2010 Tahoes require an ECM reflash with latest software (not required for '11 and up) if TPS is changed (mine is a 2010 Tahoe 5.3). For now, put the old throttle body/TPS assembly back on.

Now, the misfire may be doing damage to the cats, or may have already plugged them enough for the engine to run at full efficiency even if the misfire were fixed.

So I need an ECM reflash if I want to put the new throttle body back on, I may need cats, and I need the misfire diagnosed. The ECM may have output issues (probably very slim chance) but if I get re-flashed and still have the misfire, that would still have to be diagnosed. If the cats are partially plugged, that may keep them from properly diagnosing the misfire. If it needs cats (y-pipe) I would prefer to do that myself and not have it done at a dealership. If I replace the y-pipe/cats, reinstall the new throttle body and take it in for a reflash and still have the misfire, I'm out the cost of the cats and shop charge for re-flash and still have a truck I can't drive without damaging the new cats. If the misfire turns out to be a lifter/cam issue, I'm looking at a costly repair, especially if I get it done at the dealership instead of tackling it myself.

That brings me back to what are the diagnostic capabilities of a GM dealership? Should I just drop it off for a diagnosis before doing anything, and if so, can they pinpoint the cause of the misfire? If it's gonna need lifters and cam I may just want to find a lower mileage engine and just swap them out (mine has129,000...for a Tahoe that's not terrible, my old one had 185,000 and was still running strong when I traded it in) If it's a faulty ECM I don't want to pay to flash it with new software.

hollywoodsle · June 12, 2018

No there is no magical diagnostic machine. Something like that will most likely involve some sort of tear down

mckpaul · June 12, 2018

Thanks. In hind site I just stumbled across a section for Ask as GM Tech, this post probably should have gone there. Just joined yesterday, still figuring out where everything is.

newdude · June 12, 2018

The GM Tech 2 system that would work on the 2010 you have pretty much works along the lines of a really good Snap-On/Mac/etc. scan tool. Dealers do also posses some really neat fuel injector tester called the AFIT (Active Fuel Injector Tester) that can do an in depth injector test (balance, flow, etc.) and can pinpoint a weak fuel injector.

Do you have just the P0305? If you had a cat issue, it would set a P0420 or P0430. With an awful lot of new parts thrown at it though and it still makes noise and misfires, I'd want to check the lifter and cam.

aseibel · June 12, 2018

I had a misfire on old GM 3.4 liter V-6 (2004 Grand Am)

Took it to dealer where I purchased twice, first time they found nothing. Went back a couple days later and left it so they could do a cold start, they claimed I had a bad injector. Well, I paid for them to replace that.

Next morning, same misfire. Finally I had a local (non GM) mechanic look at it and he figured out cracked head was letting coolant into that cylinder, hence the misfires in the morning. I paid him cash to replace the head with a junkyard part. Engine has been running problem free for several years since then.

That experience really soured me to GM diagnostics. I wish they knew how to fix everything, but I'm telling you from experience, sometimes its just trial an error. Gotta start with the easiest/cheapest fix. it sucks.

mckpaul · July 25, 2018

Well, I gave up and dropped the truck off at a GM certified shop near me. After a couple of days messing with it he called and said he was pretty sure it was a lifter so I told him to go ahead and dig in. He found the exhaust lifter on #5 to have a roller that was kind of chewed up and the cam was damaged as well. I attached a pic of the lifter, I hope it's visible. It's frustrating because No. 5 not even an AFM cylinder. He said the plastic lifter tray wasn't cracked and it didn't appear the lifter had rotated, not sure why it did what it did. However I'm not the original owner, it was a trade in vehicle at a Ford dealership when I purchased it back in November. It may have had repairs prior to me buying it that were done poorly just to make it run to trade it in.

So of course that meant new cam and all new lifters. I made sure he got the newest updated GM lifters, replace the plastic lifter trays, and had him change the VLOM to the newer updated version. Knowing he was going to have the timing chain cover off and would be looking at the oil pump, I had him replace that too.

The truck now idles quietly and smoothly, no misfires, and has the throttle response of a new truck.

Just wanted to say thanks to all who shared their thoughts, and post the solution to the P0305 misfire in hopes it helps someone who finds this discussion later.

clemson3479 · September 17, 2019

On 7/25/2018 at 10:49 AM, mckpaul said:

Well, I gave up and dropped the truck off at a GM certified shop near me. After a couple of days messing with it he called and said he was pretty sure it was a lifter so I told him to go ahead and dig in. He found the exhaust lifter on #5 to have a roller that was kind of chewed up and the cam was damaged as well. I attached a pic of the lifter, I hope it's visible. It's frustrating because No. 5 not even an AFM cylinder. He said the plastic lifter tray wasn't cracked and it didn't appear the lifter had rotated, not sure why it did what it did. However I'm not the original owner, it was a trade in vehicle at a Ford dealership when I purchased it back in November. It may have had repairs prior to me buying it that were done poorly just to make it run to trade it in.

So of course that meant new cam and all new lifters. I made sure he got the newest updated GM lifters, replace the plastic lifter trays, and had him change the VLOM to the newer updated version. Knowing he was going to have the timing chain cover off and would be looking at the oil pump, I had him replace that too.

The truck now idles quietly and smoothly, no misfires, and has the throttle response of a new truck.

Just wanted to say thanks to all who shared their thoughts, and post the solution to the P0305 misfire in hopes it helps someone who finds this discussion later.

I am having the same issue with my 2011 suburban. Constant misfire on cylinder 5 which had me dismiss AFM lifter issue. What was the $ damage to have all that done?

mckpaul · September 17, 2019

Sorry to hear you're having this problem. If I remember correctly, my repair was around $4200. The shop was a GM certified shop so I'm sure the parts costs more since he ordered most of them directly from his GM supplier, plus I had him replace some items that would not have normally been replaced with a repair by the books, like the oil pump and the VLOM (valve lifter oil manifold?...it's the manifold between the heads that sits underneath the intake manifold, has a wiring harness connections, has solenoids in it that control the oil that turns the AFM lifters on and off). I figured it was mostly just parts cost since he was in there anyway and gave me peace of mind to know I had good oil pressure and control of the new AFM lifters so hopefully this won't happen again. There have been some improvements made to the AFM lifters since the original ones were installed back when they were new and one advantage of him ordering from GM he was able to verify the lifters he got were the newer versions.

All that AFM stuff could have been deleted and everything replaced with regular lifters, but it would have required some 'aftermarket' programming to the ECM . He said being a GM certified ship that his shop couldn't do anything but OEM type work, and I'm a trade in kind of guy and kind of like to keep things OEM anyway so I didn't have a problem leaving all that AFM stuff in play. It's been over a year now and all is well. I had hoped for improved fuel economy, but since most of my driving is around town I still average about 12 mpg. But, on the highway with cruise set I'll get about 16-17 average. It's my lead foot when I get off the highway that brings it down.

If your problem really is the same, you should have a P0305 code that keeps coming back if cleared. Also on mine, listening to the rocker arms with a mechanics stethoscope, that rocker did have a gravelly sound compared to the rest. I thought about doing the repair myself, but the cam has to come out, so the radiator and AC condenser coils have to come out. This time it was just worth spending more and having it done in probably 1/3 of the time it would have taken me, and let him deal the AC work, not to mention having a two year warranty. It made my butt pucker to pay the bill, but I guess it was worth it.

swathdiver · September 17, 2019

Replacing the VLOM is SOP with an AFM repair.

Avery Gentry · November 8, 2022

On 9/17/2019 at 1:43 PM, mckpaul said:

Sorry to hear you're having this problem. If I remember correctly, my repair was around $4200. The shop was a GM certified shop so I'm sure the parts costs more since he ordered most of them directly from his GM supplier, plus I had him replace some items that would not have normally been replaced with a repair by the books, like the oil pump and the VLOM (valve lifter oil manifold?...it's the manifold between the heads that sits underneath the intake manifold, has a wiring harness connections, has solenoids in it that control the oil that turns the AFM lifters on and off). I figured it was mostly just parts cost since he was in there anyway and gave me peace of mind to know I had good oil pressure and control of the new AFM lifters so hopefully this won't happen again. There have been some improvements made to the AFM lifters since the original ones were installed back when they were new and one advantage of him ordering from GM he was able to verify the lifters he got were the newer versions.

All that AFM stuff could have been deleted and everything replaced with regular lifters, but it would have required some 'aftermarket' programming to the ECM . He said being a GM certified ship that his shop couldn't do anything but OEM type work, and I'm a trade in kind of guy and kind of like to keep things OEM anyway so I didn't have a problem leaving all that AFM stuff in play. It's been over a year now and all is well. I had hoped for improved fuel economy, but since most of my driving is around town I still average about 12 mpg. But, on the highway with cruise set I'll get about 16-17 average. It's my lead foot when I get off the highway that brings it down.

If your problem really is the same, you should have a P0305 code that keeps coming back if cleared. Also on mine, listening to the rocker arms with a mechanics stethoscope, that rocker did have a gravelly sound compared to the rest. I thought about doing the repair myself, but the cam has to come out, so the radiator and AC condenser coils have to come out. This time it was just worth spending more and having it done in probably 1/3 of the time it would have taken me, and let him deal the AC work, not to mention having a two year warranty. It made my butt pucker to pay the bill, but I guess it was worth it.

Hey I was looking For your messenger, I just wanted to ask a question about this lifter. Was the Engine making any noise? Ticking or knocking? Was it idling ok and running down the road ok? Was the cylinder down on compression? Seeing you checked the valves out so well and said the valves were seated and operational. I think I have same issue. Can you please email me [email protected] thanks or averygentry is my FB message on Messenger. I would really appreciate it. Info is priceless.