GM 4.2L L6 Intermittent Misfire on Cold Start

[Falcon_LS]

Make this simple.

Vehicle: '05 Envoy 4.2L L6 (RPO LL8) with 200,800 km (125,500 mi) on the clock.

What: Rough idle, random misfire at times causing the exhaust to pop. Idles smoothly in Park/Neutral after a while, but vibrates in gear. Happens only when cold, goes away when engine is fully warmed up.

When: Cold engine, after it's been sitting overnight or when I fire it up after 9 hours at work.

Details: Engine chugs as if one or more cylinders are not running, sometimes you literally hear the exhaust pop intermittently like a gun. If you kill the ignition and restart it, it will sometimes go away, but until the engine fully warms up, you will sense vibrations with foot on brake in gear (goes away in neutral).

Maintenance Notes:

• VVT solenoid/actuator (OE ACDelco 126-15873) and camshaft position sensor (OE ACDelco 213-1557) replaced 81,022 km (50,639 mi) ago on March 6, 2014.
• Spark plugs (OE ACDelco 41-103) and oxygen sensors (OE ACDelco 213-1698 upstream and OE ACDelco 213-1574 downstream) were replaced 51,527 km (32,204 mi) ago on July 5, 2015. A set of injectors cleaned on an ASNU machine were installed at this point (I have two sets of injectors for each vehicle), and new intake manifold gaskets installed.

What I did:

• Checked for codes, nothing is stored in the PCM. Zero codes.
• Checked was the coolant temperature sensor, which operates within range. Key on/engine off reading matches ambient temperature when cold. Cylinder head temperature measured with infrared thermometer within 4 F of coolant temperature reading.
• Checked MAP sensor, which reads 14.4 PSI key on/engine off. According to the weather station, atmospheric pressure in Kuwait City is 29.92 "Hg, which is about 14.69 PSI. When driving however, if you let the truck roll forward on its own without touching the gas, this reading drops to the 4 - 6 PSI range respectively. Giving it some gas, the reading reads ~44 PSI under light throttle. Will look into this a bit more. This vehicle has no MAF sensor, only '06-'09 vehicles had it.
• Checked fuel pressure - 54 PSI key on/engine off.

Other items to check:

• Throttle body (electronic) - last time I cleaned it was 5,000 miles ago so it shouldn't be too dirty. I remove the throttle body to make sure the areas I cannot see are thoroughly cleaned as well, I don't just clean it on the engine. This morning with key on/engine off, I floored the gas and released it twice before cranking, just in case the valve is stuck/gummed up, and I did not have any rough idle issues. The vibrations were still there in gear. I will clean it out again regardless.
• Secondary Air Injection System (SAIS) - not sure how to check this, but if it's pumping unmetered air into the engine, I'm sure this would be an issue. Key on/engine off, you hear it run for a couple of seconds and then it goes off

Any input will be appreciated.

 

[Nick1994]

I had a misfire on my Camry, I texted my mechanic and he told me to put it in Drive, and push down the gas pedal and brake pedal, that it should set off the CEL and give me an idea of what's going on.

I honestly didn't believe it but decided to give it a try. Darn thing worked! Told me which cylinder was misfiring and found the bad spark plug wire.

In your case if it's a coil, you could swap that coil with another and see what happens. But without a CEL...

 

[Audios]

I had the same problem with a car like that, it was the headgasket. When it warmed up it sealed tighter and the misfire went away. Also dealt with the same issue on my X3, went nuts going over everything, and ended up being a tiny crack in the head causing a rough running for about 30 seconds on cold start, more towards fall/winter. Codes thrown were system too lean bank 1, even though both banks share intake manifold, and no bank has a specific vacuum line going to it that the other doesnt have, so no way theres only extra air getting in one bank. You may want to check compression, and pull the plugs to take a look at how they seem.
Edit: Also what threw me for a loop, was that the residual pressure in the system after shutting off forced the coolant through the crack, no proessure in the combustion chamber and pressure in the cooling system made that possible, hence only issues when sitting for a while.

 

[Snagglefoot]

If you haven't already, put in some fuel injector cleaner. If the coil pack is easily accessible, I would go ahead and change it on spec ( less than $100 ). I don't think you mentioned the spark plug wires. They are 13 years old if you haven't changed them yet. Might be time. Let us know how it turns out.

 

[ZX11Rebel]

GM Atlas engine bad valve seats?

 

[dogememe]

Originally Posted by ZX11Rebel
GM Atlas engine bad valve seats?



Yup, either that or a bad coil.

 

[JTK]

Falcon, does your 4.2L use a mass airflow sensor? If I remember correctly, my 2005 Trailblazer I6 had one. I don't recall when the exact changeover was, but the earlier Atlas inline 6's didn't use a MAF, but relied on programming/mapping for that portion of air/fuel management. My point is, check the MAF.

 

[Falcon_LS]

Thanks for the input!

Just an update; I went ahead and pulled the throttle body off. It wasn't filthy, but it wasn't clean either. Liberally blasted it with CRC intake cleaner, and all I can say is, it left quite a mark on the asphalt. Had the battery disconnected while I was at it, now the idle is smooth as glass. It revs smoother too, but the real test will be firing her up tomorrow morning to see if I get the same symptoms. Will post an update.

 

[BrianF]

No wires to check, direct coil on plug. I was going to yell out the Tb clean but you are on top of that. See if the SAIS solenoid is not sucking air back into the exhaust.

 

[Falcon_LS]

Update - no changes with a stone cold engine.

Given this happens when the engine is cold, and that fact that the engine operates in open loop mode at that point, I'm going to rule out sensors for now and turn my attention to the injectors and the SAIS:

Originally Posted by BrianF
See if the SAIS solenoid is not sucking air back into the exhaust.


Yup, the solenoid is easily accessible and I will just pull it off to make sure it's clean and not corroded like some examples I have seen online. If it looks bad, a brand new GM Original Equipment unit (ACDelco 214-2222) is only $70 or so. Since there are no codes stored, particularly the infamous P0410, I have a feeling I may be looking at injectors.

Starting in '05, GM decided to do away with the inline fuel filter, and despite using Euro V gasoline, I have sent some horrible looking injectors to Trav for cleaning. I usually replace spark plugs, COP boots, oxygen sensors, injectors and intake manifold gaskets in one shot, and was hoping I could get 100,000 km (62,500 mi) out of them, but it looks like 50,000 km (31,250 mi) might just be the limit, depending on how these injectors look when I pull them out. I'll start with a decent dose of Techron and see if that makes a difference, if it does, I know what I'm looking at.

Originally Posted by JTK
Falcon, does your 4.2L use a mass airflow sensor? If I remember correctly, my 2005 Trailblazer I6 had one. I don't recall when the exact changeover was, but the earlier Atlas inline 6's didn't use a MAF, but relied on programming/mapping for that portion of air/fuel management. My point is, check the MAF.


Sorry, I missed this. '02-'05 did not come with a MAF sensor, only MAP. '06-'09 vehicles are equipped with MAF sensors.

 

[tiger862]

Worn Valve seats

 

[Falcon_LS]

Update - it's time for a tune up.

I filled up the tank last night and poured in two 355 ml (12 oz.) bottles of Techron - the instructions said one bottle for 45 liters (~12 gallons), my tank is 90 liters (24 gallons) IIRC, so I added two.

I drove about 80 km (50 miles) last night, and noticed partial acceleration was a bit better. Fired her up this morning, not a single problem. No misfires, no popping exhaust. The vibrations in gear are still there, but I'm thinking that may be the oil filled engine mounts.

I guess 50,000 km (31,250 mi) is the limit for a tune up and I will have to get to it - plugs, coil boots, my set of cleaned injectors, intake manifold gaskets and oxygen sensors.

Fuel quality, operating conditions, etc. all have an impact, and although I may be paying for Euro V, I have a feeling the tanks at the station are not as clean as they should be. I'll have to take a look into replacing the fuel lines from an '02-'04 with the inline fuel filter.

 

[tiger862]

My Saturn needed plugs every 30k do to carbon buildup symptoms were as follows. Repeated rough idles so repeated throttle clean

Miss at interstate speeds random as felt like plugs.
Stall at times on hard acceleration.
Plus unleaded fixed or shall I say made better.
Lucas fuel injector cleaner made smooth again.
Tried everything except head work but that was coming but other things were going out so I traded it in.
I don't like to seafoam a carboned up engine as with lots carbon will either soak up cleaner or lock up engine. Lightly carbon is ok not severe carbon. Did the floor interstate runs with black smoke but fouled plugs so I knew head needed cleaning manually.