gm 6.2l cylinder 8 misfiring only at idle

[jalikaria24]

hey guys, got a 07 escalade with the 6.2l v8. its got 335,000km on it and has been running great, never had any engine issues other then the piston slap and noisy lifters so i have noticed that over the past 50,000 km the truck has had this problem but back in the day it would only do it if i let the truck idle for 4-5 minutes (which i rarely would ever do) and it just gradullay got worse and worse and now it "misfires" at idle within 15 seconds so the thing is whenever i come to a stoplight and stay stopped for like 15-20 seconds, ill get the message that says " stabilitrak off service traction control" and then the tone of the engine will change and i guess it will sound rougher with a shake, but right when i start driving agian it goes back to normal and then message clears. if i go on the highway, it drives perfect, smooth, good power good revs and never triggers a messsage or anything, but when i come a stop light or any stop, if i wait long enough it will do its thing again.. rough idle, stabilitrak message, and also if i wait like a minute or 2, then the check engine light starts flashing as well, i believe this is because the truck thinks its misfiring or is misfiring and thats why the check engine light flashes. firstly, the codes i get are p0308 so i know there is a misfire P0036 HEATER CONTROL CIRCUIT BANK 1 SENSOR 2 P0056 HEATER CONTROL CIRCUIT BANK 2 SENSOR 2 P0449 EVAP EMISSION SYSTEM VENT VALVE/SOLENOID CIRCUIT P0452 EVAP PRESSURE SENSOR/SWITCH LOW P0308 ENGINE MISFIRE CYLINDER 8 i also do have some evap codes but have had those forever so im not a mechanic on my own but i swapped coils and tried to see if the misfire stops and it doesnt. i also swapped the plugs and wires around and still has the same problem. the plugs and wires are relatively new only 50,000 km on them. i used my code reader and traced it as a p0308 so i swapped a brand new spark plug into that cylinder and still has the same problem. but have showed it to a local indie mechanic and he did a leakdown test got 6% so he said the engine is still strong, he said compression test was above 200+ for all cylinders, lastly i believe he said he changed the intake manifold gasket but it still has the same problem. i am assuming he checked the fuel injectors but if not, i guess thats the first thing i should try? anyone have any luck? i was reading more and someone said when the engines get gunked up and old, the vvt goes wonky and ends up causing this? but not sure if thats the truth

 

[Olas]

if you get the Torque Pro app on your phone then you can view live data. this will show you if its spark or fuel and you will know where to go from there,

 

[mk378]

Air leaks will cause idle problems. This can be seen in the fuel trims, assuming the sensor 1 oxygen sensors are still working.

 

[dbias]

Not familiar with the engine but if its coil on plug, move the coil to see if misfire moves with coil, if misfire stays on 8 then check the plug if it doesn't follow the coil.

 

[mattd]

Did your mechanic run a dynamic compression test?

 

[vavavroom]

It does sound like an air leak to me.

 

[P10crew]

Intake gasket. Easily proven

 

[antonmnster]

Air leak or injector.

 

[clinebarger]

Originally Posted by mattd

Did your mechanic run a dynamic compression test?

X2.....But with his mechanic getting 200+ psi on a compression test makes me question his diagnostic skills or his equipment, Should be around 175 max with correct valve timing. The fuel injectors can be balance tested with a scan tool & a fuel pressure gauge. I wouldn't be surprised if it was a lifter, You say you have noisy lifters? This is likely from the Valley Cover AFM Galley Plug Seals leaking. An L92 doesn't have AFM but the AFM Activation Galleys have to be plugged. Over time the seals will compress & no longer seal, Causing internal oil leaks & noisy lifters. The good thing about Gen IV blocks is you can remove the Intake/Valley Cover & visually inspect #8 cam lobes. Unfortunately the AFM seals are not serviced separately so you have to buy cover.....GM part# 12598832, Cost @ $40. #8 lifters can be changed without pulling the head/s once the cam is removed.....But it's NOT an easy task to fish the lifters out & in from the bottom of their bores through a little hole!!

 

[jalikaria24]

i moved coil and got a new plug as well and even swapped wires, the misfire stays on 8. my mechanic did a leakdown test and said the results were good and only had 6% loss. i also took it to gm dealership and this is what they said " PO300 MISFIRE. MISFIRE IS ON CYLINDER 8. THE IGN. COIL AND WIRE ARE GOOD. COMPRESSION IS OK. HOOKED UP INJECTOR BALANCE TEST FIXTURE. ALL INJECTORS ARE GOOD. SUSPECT CAMSHAFT ISSUES DUE TO NOISE IN RIGHT REAR BANK AT HIGHER RPM. WOULD NEED ENGINE DISASSEMBLY TO FIND " where do you think i should go with this? is it definately something internal meaning wont be fixed unless i dissasemble? i also wanted to add some more info maybe this helps, i didnt drive this truck at all for 3-4 weeks a few months ago (i left for vacation) and when i came back home and started driving it again, the misfire at idle completely dissapeared for 3 days straight. if i waited at long lights or even let it idle a bit longer, it didnt misfire at all or get shaky. but then after 3 days it started again like its normal habits. does this help anything?

 

[clinebarger]

Pull the Valley Cover & inspect the #8 camshaft lobes. Takes about 20 minutes for a seasoned mechanic to pull the intake & valley cover, Less if you've done a thousand of them. The sad thing is, You've spent all this money for people to guess!! Mattd mentioned a dynamic compression test.....I love diagnostic shortcuts, But I have problems with inconclusive results using this particular method & question how long it takes compared to visual inspection on this particular engine knowing that I can visually inspect #8 cam lobes in <15 minutes. .

 

[kc8adu]

not to mention you swap out the other possibility at the same time. the intake gasket.

Originally Posted by clinebarger

Pull the Valley Cover & inspect the #8 camshaft lobes. Takes about 20 minutes for a seasoned mechanic to pull the intake & valley cover, Less if you've done a thousand of them. The sad thing is, You've spent all this money for people to guess!! Mattd mentioned a dynamic compression test.....I love diagnostic shortcuts, But I have problems with inconclusive results using this particular method & question how long it takes compared to visual inspection on this particular engine knowing that I can visually inspect #8 cam lobes in <15 minutes. .

 

[mattd]

The reason I mentioned DYNAMIC compression is because it tells a much more detailed story of cylinder health than just static compression, which it sounds like what your mechanic did. It's a little time consuming to do that test. however, with the associated valve train noise it could very well be a flat/worn cam lobe or a failed lifter. Clinebarger's suggestion would be the fastest in inspection of the #8 cam lobes/lifters.

 

[jalikaria24]

thx for the replies everyone! i will for sure do that the weather randomly got way worse so i wont be able to check and diagnose until next month. is it really bad if i keep driving it like this? or will it not really make a difference to the engine, it only does the "misfire at red lights/idle