2002 Trailblazer - Slightly rough idle, gradually drops RPM and stalls

[CakeBlazer]

2002 Trailblazer, 4.2L I4. 187,000 miles.

As noted in my most recent thread here I just replaced the ignition cylinder and got the vehicle running again. This was also after an exhaust manifold replacement.

After finally getting the exhaust manifold bolts adequately tight and eliminating the light exhaust leak we could hear the vehicle ran perfectly fine for a combined time of about ~40 minutes over the course of a few days. No uncharacteristic vibration, no odd sounds, no notable issues to speak of.

Now, out of the blue, it has a slightly rough idle in Park. There's a light vibration that can be felt throughout the vehicle and hooking up an OBDII code scanner shows a P0300 misfire code. A quick googling tells me that about a million things can be the root cause of this code.

There is no uncharacteristic noise however - it doesn't sound like an actual misfire or bad cylinder ignition. I've had one of the spark plugs go out while driving once and it knocked severely and threw up the "STOP ENGINE" warning which this isn't doing, it's just showing the Service Engine Soon light.

Additionally when at idle the RPM's will gradually drop until the vehicle stalls. Once the RPM drops to around 650 the vibration becomes much more noticeable.

I have also noticed that turning the AC on/off at idle caused it to "choke" and nearly stall. The idle will go very rough for a couple seconds immediately after the AC is turned on or off then spin back up and it'll run fine. I don't remember it doing this in the past.

Suggestions?

 

[budwich]

your misfire code will also have a corresponding count for a cylinder(s) that are impacted... there are associate PID to look to find the counts. That will tell you which / any cylinders that have issues. Go from there. further, on the idle, when was the last time that you cleaned the throttle body. A symptom of a "gummed body" is bad idle especially with AC.

 

[CakeBlazer]

your misfire code will also have a corresponding count for a cylinder(s) that are impacted... there are associate PID to look to find the counts. That will tell you which / any cylinders that have issues. Go from there. further, on the idle, when was the last time that you cleaned the throttle body. A symptom of a "gummed body" is bad idle especially with AC.

I have no idea if the throttle body has ever been cleaned. I've never done it myself but have put ~20,000 miles on the vehicle. Given the issues it was having cleaning the throttle body was already on the list of things to do.

See the photo for the OBDII readout. I'm admittedly not familiar with code readers beyond the fact they display malfunction codes.



The CAT, EVA, O2S, and HTR circles flash. I have no idea what that means, if anything.

 

[mrrsm]

The Primary Causes (..and the ones that are NOT involved) of P0300 Codes on the GM Atlas 4.2L LL8 Engines are as follows:

(1) Vacuum Leaks around the Intake Manifold:

Tightening the IM Fasteners up to 87 INCH Pounds and NO MORE is a good start. Over time... Strong Engine Vibrations along with the counterweight motion imposed on the Intake Manifold as the weight of the Throttle Body versus the weight of the PCM bolted onto the lower left side causes the IM to oscillate during the life of the motor.

This action tends to flatten out the Three "Figure OO Plump Rubber "O" Ring Sealing Gaskets and allows the ingestion of ambient, un-metered and un-measured air causing Random Misfires. Since these Fasteners are "Captured "... often, over-tightening to squeeze them down will simply create a bigger problem if they Snap Off after bottoming out in the side of the Intake ports in the Aluminum Engine Head, Replacing the IM Gasket Set is a PITA, but is often the necessary action to take if these will not re-seal.

(2) Malfunctioning Coils:

This might seem like the first place to start...but it is NOT. The odds of having All Six Coil Packs going sideways simultaneously is astronomical. Ignore this approach of "...just replacing them ALL".

(3) Failed or Cracked Spark Plugs due to Failed Coil Pack Seals allowing Engine Oil or Rain Water to invade the Circuit.

Same Answer as #2 ... THIS might be the place to look for the problemS with specific P0301-P0306 codes present again and again.

(3) Failed or Failing Fuel Pressure Regulator:

On the 2002 and 2003 Trailblazers and Envoys, the FPR failing will cause Random Lean Cylinder conditions and Random Misfires . Finding a Fuel Leak at the FPR signals the immdeiate need for an R&R of the FPR.

(4) Failed or Stuck SAIS Solenoid:

On 2005 and Later Model TBs & Envoys, this will allow un-metered air to recycle like an EGR Valve. P03XX Codes can also be involved along with a P0171 Code being present causing a Lean Cylinder Condition and cause Misfires. Not Your Problem Here.

(5) Failing Fuel Pump:

If the Fuel Pump is incapable of delivering enough Fuel Pressure especially at IDLE... it can cause Random Lean Cylinder conditions resulting in Misfires.

(6) Clogged CPAS:

The Camshaft Position Actuator Solenoid can cause Random Misfires if the in-dwelling Stainless Steel Screens are either clogged or closed off with engine Oil Debris. Also...if the CPAS has Engine Oil invading the Electrical Harness Connector... it can likewise malfunction and cause Misfires of Unknown Origin due to the fact that the CPAS alerts the PCM using infinitely variable positioning via Oil Pressure and is NOT acting as a Sensor, so Random Misfires that do NOT throw any P03XX Codes may also result.

(7) CPS Failure:

The Camshaft Position Sensor communicates with the PCM as to the ACTUAL position of the Exhaust Camshaft in its relationship to the Crankshaft for purposes of EFI Spray and Ignition Spark Timing. If the CPS is malfunctioning... the Engine will Randomly Misfire.

There are many additional Threads covering the CPAS and CPS R&Rs here at GMTN.

 

[Mooseman]

After a while, the code should change to a P030x where x would give the affected cylinder. Sometimes it takes a couple of drives for it to decide.

If it really is a random misfire, then you will need a better way to get live data. An Elm327 bluetooth OBD adapter and the Torque app on Android would be a fairly cheap but good way of getting live misfire data.

What has the maintenance been like? Plugs replaced? Did you use GENUINE ACDelco 41-103 plugs? (there are counterfeits out there on Amazon and eBay).

 

[CakeBlazer]

After a while, the code should change to a P030x where x would give the affected cylinder. Sometimes it takes a couple of drives for it to decide.

If it really is a random misfire, then you will need a better way to get live data. An Elm327 bluetooth OBD adapter and the Torque app on Android would be a fairly cheap but good way of getting live misfire data.

What has the maintenance been like? Plugs replaced? Did you use GENUINE ACDelco 41-103 plugs? (there are counterfeits out there on Amazon and eBay).

Only one spark plug has been replaced while I've owned it and that was the one that went bad. Can't recall the brand.

Just odd that this started right after doing some work on the vehicle. I had also replaced the camshaft actuator solenoid, guess I might want to check that again.

 

[Mooseman]

I've never seen the CPAS cause a misfire but can cause a rough idle if stuck in the advanced position.

Start with the plugs. If one failed, they all should have been replaced. Also clean the throttle body. Be sure to disconnect the battery while cleaning it to reset the PCM.

 

[CakeBlazer]

I've never seen the CPAS cause a misfire but can cause a rough idle if stuck in the advanced position.

Start with the plugs. If one failed, they all should have been replaced. Also clean the throttle body. Be sure to disconnect the battery while cleaning it to reset the PCM.

Thing is the vibration gets worse only at low idle. Revving the engine doesn't make the vibration worse nor does it generate any uncharacteristic noise. It only gets worse as the RPM's go down, not up. As long as the RPM is above ~750 it's fairly consistent and just noticeable.

I'll check the plugs regardless.

 

[freddyboy61]

Engine vibration at idle or low RPM usually indicates a bad motor mount.

 

[Mooseman]

Engine vibration at idle or low RPM usually indicates a bad motor mount.

But would not cause an idle drop or stall. Normal idle is 650 rpm.

 

[Mike534x]

But would not cause an idle drop or stall. Normal idle is 650 rpm.

Not sure if it would help the OP, but when I had my first Envoy the CPS sensor had gone bad. It didn't trip any codes, but it did cause the engine to vibrate as if it was misfiring and it would idle down and stall out. After replacing it the problem went away. I'd imagine the worn spark plugs and 1 new one (perhaps the coil packs as well) are not helping with the vibration and idle issue they're experiencing.

 

[CakeBlazer]

Update -

So the first time the OBDII scanner was attached was almost immediately after this issue started and it wasn't attached for very long. I just now allowed the vehicle to run for a bit, revved the engine a few times, and reattached the scanner.

It now throws a P0301 Generic/Cylinder 1 Misfire Detected.

Guess I need to check Cylinder 1.

 

[Mooseman]

Try swapping coils 1 and 2 and see if the misfire follows.

 

[TequilaWarrior]

Also, unplug the connector at the coil while engine is running. If no change.... probably bad coil or bad spark plug.

 

[CakeBlazer]

Well it seems I've really entered sucksville now.

There is what appears to be oil in the Cylinder 1 coil socket. See photos below.

Cylinder 1 coil socket -



Cylinder 2 coil socket - Much less residue here. The coil itself was clean.




Coil from Cylinder 1 - oily residue noticeable.



So. I guess the valve cover gasket is going to have to come off, yes?

 

[TequilaWarrior]

"Yes".... but, no - maybe.
Soak it up and watch it. If it takes a loooong time to pool again, maybe not worth the effort to replace the gasket(s). First try torquing the valve cover bolts again. I'm not sure what the torque for them is however.
To change the gasket(s) you have to remove a whole bunch of stuff.... throttlebody, intake, coils at a minimum. Intake is best removed after removing engine lift bracket and alternator, which of course means removing the belt. etc... etc.... etc....

 

[TequilaWarrior]

Forgot to mention.... there is a flaw that sometimes allows water to collect in the plug well. It's "common" but obviously not ideal for there to be fluid in the plug well. You definitely want it to be clean/dry. Personally, I'd pull all the coils, clean/dry the plug wells, lubricate the gaskets on the coils with something like vaseline and reinstall them. It will keep water out. I'd also retorque the valve cover. Any accumulation of oil in the plug well may be slow enough that you only have to clean it once a year or less.... I had to do mine ONCE in 5 years but basically it looked just like yours. I DID just do the valve cover gaskets but I was already in the neighborhood so it was worth it to me. The "flaw" is in the hood to cowl gasket. There was a recall for it.

 

[CakeBlazer]

Forgot to mention.... there is a flaw that sometimes allows water to collect in the plug well. It's "common" but obviously not ideal for there to be fluid in the plug well. You definitely want it to be clean/dry. Personally, I'd pull all the coils, clean/dry the plug wells, lubricate the gaskets on the coils with something like vaseline and reinstall them. It will keep water out. I'd also retorque the valve cover. Any accumulation of oil in the plug well may be slow enough that you only have to clean it once a year or less.... I had to do mine ONCE in 5 years but basically it looked just like yours. I DID just do the valve cover gaskets but I was already in the neighborhood so it was worth it to me. The "flaw" is in the hood to cowl gasket. There was a recall for it.

There was no water or actual liquid or any kind, it was only stuck on residue so it appears the leak isn't too bad, at least not now.

 

[TequilaWarrior]

There was no water or actual liquid or any kind, it was only stuck on residue so it appears the leak isn't too bad, at least not now.

Then I wouldn't sweat it. Get your misfire sorted out and put some miles on it.

 

[Sparky]

Don't worry about that bit of oil down there. Not uncommon on DOHC engines. Mine had over an inch of standing oil in one well before. Sop it up and move on.

 

[gmcman]

Very curious to see the condition of the plugs. Also, might be a good idea to change the fuel filter if you don't know how long its been in use. Low fuel pressure could cause issues at idle but since you have a Cyl #1 misfire I agree to start there.

Sounds like all the plugs may need replacement. Your 2002 likely has the platinum plugs if they are original.

 

[freddyboy61]

But would not cause an idle drop or stall. Normal idle is 650 rpm.

My poor choice of wording, didn't mean that motor mounts would cause idle drop or stall. Only that engine vibration at idle is sometimes an indication of worn motor mounts.

 

[CakeBlazer]

Well I replaced all the spark plugs, not that the originals looked bad. All plugs are ACDelco Iridiums. Also cleaned the throttle body. No more stalling when turning the AC on/off, so there's that.

However, I swapped coils 1 and 2 and now we're back to a P0300 Generic Misfire code. Nothing else seems to have changed.

 

[Mektek]

Once you swap coils it will take a while for the pcm to set the specific misfire code again. It's good to go for a test drive and watch the misfire count on each cylinder. But your code reader will not show that

 

[gmcman]

If you have Torque Pro, this is the layout I use to monitor any misfires.

Cyl 1-6

Injector pulse width (not really needed for misfire count but just helpful overall)

Cylinder misfire current

Cylinder misfire history

 

[CakeBlazer]

Ah, there we go. After letting it sit for over 48 hours now with the battery connected the code scanner threw a P0302 Cylinder 2 Misfire code shortly after starting the vehicle today so the misfire followed from 1 to 2 with a coil swap.

Sounds like that coil is bad, yes?

I should also note that I mixed up spark plug and ignition coil in my earlier posts. It was an ignition coil that went out while driving and was replaced, not a spark plug. None of the spark plugs in the engine were ever changed by me.