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apollo18

6.2l high km's misfires at idle but not when driving

Question

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 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.

 

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 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 94% so he said the engine is still strong, he changed the gasket on the intake manifold i believe but hes stumped otherwise

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

 

thx!!

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also mechanic said he did a compression test too and was getting 200+ each cylinder, he doesnt remember off the top of his head but he doesnt think compression is a problem

 

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Getting a live data scanner that can watch each cylinder is the best way to watch what is going on and track your progress. See if anything changes or if other cylinders are reporting misfires too.

 

Try swapping the injectors around. Toss some injector cleaner or a full bottle of seafoam in the tank. Hopefully you're running 91+ octane like ask.

 

Check for broken exhaust manifold bolts too, a leak at the flange could cause issues.

 

Then beat the shit outta the engine, get everything worked up and hot.

 

I can't comment on the VVT or a worn camshaft without seeing the truck or tearing the engine down.

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1 hour ago, apollo18 said:

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 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.

 

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 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 94% so he said the engine is still strong, he changed the gasket on the intake manifold i believe but hes stumped otherwise

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

 

thx!!

 do you have a live scan tool ?  mine has a misfire at idle  

No CEL codes on mine

 

I can see the MAF drop by 0.2 G/S  when it misfires    it worked fine for 250 miles after changing the MAP sensor  then i got the idle misfire... i'll try cleaning the MAF again  my self 

 

90k miles  6.0 vortec  I use 91 octane from Chevron  or mobil  they have the HIGHEST Quality fuel for you GM engine..

 

 chevron is #1 though

Edited by bg1988

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I’ll try a live scan tool, never used one before so I’ll see what’s up! Thanks!

 

never swapped injectors before, is it relatively easy and do I need some more tools?

 

and yeah I fill 91 to 94 octane lately hoping to stop the misfire but nothing :(

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Higher octane isn't going to stop any misfires. Higher octane means a slower burn - when you have a high compression engine, or forced induction of any kind, or are advancing the timing farther than spec (not possible on a stock vehicle today), then you would require higher octane fuel to prevent pre-ignition, or "knock" . You'll hear spark knock at high engine loads. Letting up on the pedal a bit will make the knock stop. That's pre-ignition. Detonation will make the same sound, but it sometimes has a different cause, like glowing carbon chunks, or part of a head gasket.

 

A blinking check engine light is a complete dead miss - not a partial miss. Should be easy to narrow down. Spark, air, fuel, compression - it's one of the 4. See if you have injector pulse on the misfiring cylinder. If you do, try swapping the injector with another good one as was already suggested.

 

If you have 94% leak down on ANY cylinder, that's your problem! Did you mean 6% leak down?

 

 

EDIT just to add, don't be fooled by a lack of injector pulse on a severely misfiring cylinder - the computer will cut injector pulse on certain vehicles if there's a severe, constant misfire. Check for pulse after a key-off event, and pay attention to it immediately after start up. It will take several seconds for the computer to cut the pulse.

 

If any of this is above your skill level, take it to a local garage (NOT the DEALER !!!) for diagnosis.

 

Given your symptoms, I'm betting on a vacuum leak. Either a nearby hose, or the intake gasket.

Edited by Jsdirt

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8 hours ago, Jsdirt said:

Higher octane isn't going to stop any misfires. Higher octane means a slower burn - when you have a high compression engine, or forced induction of any kind, or are advancing the timing farther than spec (not possible on a stock vehicle today), then you would require higher octane fuel to prevent pre-ignition, or "knock" . You'll hear spark knock at high engine loads. Letting up on the pedal a bit will make the knock stop. That's pre-ignition. Detonation will make the same sound, but it sometimes has a different cause, like glowing carbon chunks, or part of a head gasket.

 

A blinking check engine light is a complete dead miss - not a partial miss. Should be easy to narrow down. Spark, air, fuel, compression - it's one of the 4. See if you have injector pulse on the misfiring cylinder. If you do, try swapping the injector with another good one as was already suggested.

 

If you have 94% leak down on ANY cylinder, that's your problem! Did you mean 6% leak down?

 

 

EDIT just to add, don't be fooled by a lack of injector pulse on a severely misfiring cylinder - the computer will cut injector pulse on certain vehicles if there's a severe, constant misfire. Check for pulse after a key-off event, and pay attention to it immediately after start up. It will take several seconds for the computer to cut the pulse.

 

If any of this is above your skill level, take it to a local garage (NOT the DEALER !!!) for diagnosis.

 

Given your symptoms, I'm betting on a vacuum leak. Either a nearby hose, or the intake gasket.

 

hey thanks a ton for your reply, yeah sorry i meant 6% leak down aha.

perfect ill take a look for injector pulse and also the vacuum leaks !

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