Very Slight Miss at Idle

[Mooseman]

Yesterday on the TB, at 117k km, saw the CEL come on and code came up for P0304. Fired up Torque and it was missing on cylinder #4 but only at idle or stopped at a light. It's not a big miss since I can't feel it at all and it's just a couple of misses per refresh cycle. While driving, nothing. Historical shows over 7000 misses while the others show only a few.

What I'll be doing this afternoon:
- replace the plugs (ACDelco 41-103)
- swap #4 coil with another (#1 had the fewest misses)
- clean the throttle body
- check the intake manifold bolts

I'll recheck after that to see if the miss follows the coil or is eliminated.

If anyone has any ideas, please chime in.

 

[littleblazer]

Is it 4 or 5 that can get rain water in it? I'd check that too.

 

[Mooseman]

It's 4. I'll know if it's the coil if the miss follows the coil.

 

[littleblazer]

Surprised that you cannot feel it if the CEL came on. My counts won't come up but I can feel an occasional miss on mine...

 

[Eric04]

I've only had the misfire code twice now, but both times it ended up being the coil. I followed the procedure you listed in order to cast a wider net anyway the first time around. The replacement coil failed a few months after installation so I only did the swap then and the code followed.

 

[mrrsm]

IIANM... It takes at least 30 Misfires within a certain time frame to trigger the P3XX Codes... thus eliminating 'Hitting the PCM Tripwire' ...too often.

This On- Topic TSB bears out @littleblazer 's suspicions and brings up a few other places to investigate as well:

Engine - Flashing MIL/SES/Misfire DTC's/Rough Idle
Bulletin No.: 06-06-04-048
Date: September 21, 2006

TECHNICAL

Subject:
4.2L LL8 Engine - Flashing and/or MIL/SES Light, Rough Idle, Misfire(s) DTC(s) P0300, P0301, P0302, P0303, P0304, P0305, P0306 (Install AIP Seal)

Models:
2004-2007 Buick Rainier
2002-2007 Chevrolet TrailBlazer
2002-2007 GMC Envoy
2002-2004 Oldsmobile Bravada
2005-2007 Saab 9-7X

With Vortec(TM) 4.2L Inline 6 Cylinder Engine (VIN S - RPO LL8)

Condition:

Some customers may comment that, after severe weather that includes large amounts of rain in a short period of time, the engine has a rough idle and/or flashing MIL/SES light. Upon further investigation, there may be the following DTC(s): P0300, P0301, P0302, P0303, P0304, P0305 or P0306.

This condition can be aggravated if the vehicle is parked nose down on an incline during this type of weather. The customer may also comment on repeat occurrences of this condition because water may be passing over the Air Intake Plenum (AIP).

Cause:

The dripping rain water onto the engine cam cover will collect at the coil(s) and may seep past the coil(s) into the spark plug(s) well of the cylinder head and may affect the operation of the spark plug(s) and coil(s), causing the misfire(s).

Correction:

Before replacing the AIP seal, refer to Misfire diagnostics in SI to determine if water intrusion was the cause of the misfire.

Diagnostic Aids for Misfire:

Refer to SI for Base Engine Misfire without Internal Engine Noises.

If no trouble found (NTF) using SI document on Base Engine Misfire without Internal Engine Noises, then refer to SI for Misfire DTC(s).

If Misfire diagnostic leads to the removal of the Coil(s) and Spark Plug(s), refer to the following SI Documents:

Air Cleaner Outlet Resonator Replacement
Ignition Coil Replacement Removal Procedure
Spark Plug Replacement

Coil(s) damage from water intrusion will have a film of white chalk build-up on the outside and inside of the spark plug boot to ignition coil(s) assembly. If present, remove the spark plug(s) and inspect for similar build-up on the outside of the spark plug(s). If NTF with the coil(s) and spark plug(s), continue on with the Misfire Diagnostic in SI.

Replace any coil and spark plug that has been diagnosed to be damaged from water intrusion. To prevent a re-occurrence, follow repair information outlined in this bulletin to replace the API seal.

To prevent a repeat occurrence of the above condition, Do Not return the vehicle to the customer without replacing the API seal if water intrusion was determined to be the cause.

If water intrusion was determined to be the cause for the replacement of the spark plug(s) and/or coil(s), the AIP seal should be replaced.

Order and Installation of AIP Seal # 355 from WPC

To prevent a re-occurrence, the revised API seal will redirect the rain water flow away from the engine cam cover area. The following repair information outlined in this bulletin will assist technicians in the replacement and ordering of the revised AIP seal from the WPC.

Order WPC # 355 AIP seal for 2002 - 2007 Midsize Utilities (RPO LL8). Refer to the Warranty Parts Center (WPC) part request form at the end of this bulletin.

 

[Mooseman]

Yep. I'll be investigating this once I let the engine cool (or let it cool while I take everything else apart) as I am very much aware of removing the plugs only from a cool aluminum head.

The light went out today but came back on as I let it idle in the driveway. I'll probably also clean the MAF while I'm at it.

Edit: And no flashing CEL neither.

 

[littleblazer]

Eh. I've removed them hot.

 

[Redbeard]

Does it matter if the a/c is on? Mine will often do the same as yours, but only with the a/c on. I am beginning to lean toward the injectors getting a bit weak after 185, miles.
MRRSM ( I think) showed some injectors at amazon at a "reasonable" price. I believe the mixture is getting a bit weak and causing/showing a misfire.
YMMV.

 

[mrrsm]

When this thing goes sideways... The story evolves like this:

PLEASE TAKE NOTE:

If GM was NOT Very Serious about NOT attempting to Remove the Spark Plugs on anything other than an Ice Cold Engine Block... THEY WOULD NOT HAVE EMBOSSED A MESSAGE ABOUT THIS PROBLEM ALL DOWN THE FRONT OF THE UPPER VALVE COVER AS A PERMANENT WARNING!

(1) TB-Envoy-Ranier-SAAB Owners: Engine Gets Turned Off at Idle...
(2) Impatiently Waits only about 10-20 Minutes...
(3) The Aluminum Engine Head Cools Down a bit... but is NOT Stone Cold...
(4) The Spark Plugs are STILL Red Hot after removing COPs...
(5) Ratchet + Extension + 5/8" Spark Plug Socket + CCW Torque go to work prematurely...
(6) Spark Plugs feel a bit Snugger than usual... but they DO back out..
(7) Cylinder Head Spark Plug Threads remain stuck to the Spark Plug on the way out...
(8) Time for a TIME-SERT Spark Plug Sert Repair Kit...
(9) The Fix WORKS great... and will never fail again....
(10) Lesson on Having the Engine "Popsicle Cold" B4 Hand... Learned... BIG Sigh of Relief!

Roger THAT... @Redbeard... Here is the Link for the Injectors/Fuel Rail/Wiring Harness/FPR for right around $100.00 for the ENTIRE Rig:

https://www.amazon.com/dp/B072BZGRNW/?tag=gmtnation-20

 

[Mooseman]

Well f@*&!

Replaced plugs, cleaned throttle body (fuses 10 & 28 pulled for an hour), cleaned MAF (it was spotless), air cleaner was already replaced, intake manifold was tight-ish (that was a ). Plugs were OK, normal colour, nothing out of the ordinary.

Started and runs great, like it always did but the misfire on #4 at idle is still there. If anything, it's WORSE! Before, per refresh cycle, I was getting at most 20 misfires. Now it's 40.

Checked the fuel trims and O2 sensor. LTFT went up to 9.4 but STFT was hanging around 0-3. O2 sensor seems to be working OK.

Next: Compression test, injector balance test, delete SAIS (already tuned out but was still functional)

I'll edit this post with pics from my phone and a video of Torque screens of the misfires and trims.

Edit:

 

[Sparky]

I didn't see it mentioned but you also moved that coil right?

 

[Mooseman]

Yup. Not the coil unfortunately.

 

[mrrsm]

If you are still using the OEM Stock Air Box.... and THIS PITA PCV Hose gets real loose or cracked... it can allow un-metered air to enter the Air Stream... perhaps just enough to throw things off on STFT when at idle:

https://www.ebay.com/itm/PCV-Valve-...RbI08G:sc:Other!34684!US!-1:rk:3:pf:0&vxp=mtr

 

[Mooseman]

Although I didn't check this hose specifically, I know it's good because I had to take the resonator off again because I forgot to connect it to the valve cover the first time. It had good resistance slipping it on. And if it were that, wouldn't all the cylinders would be affected?

I'll add to my list checking for vacuum leaks.

 

[mrrsm]

Ordinarily... with any un-metered source for incoming air... you would expect to see a combination of P0300 Code for AND a P0171 Code for the SAIS Solenoid if it were involved for any random misfires ...with a rise in RPM to match the excess un-metered air being fed more Fuel via the O2 Sensor striving for what the Preamble in our U.S. Constitution calls... "A More Perfect Union" of Fuel and Air.

But not necessarily in this case... if the vacuum leak is VERY small... BUT the EFI parked in the Intake side of the Head at the #4 Cylinder... is likewise a bit weak on Fuel Delivery Performance. Between the two of them... they could make for a lean condition every so often... and still not universally affect the other five cylinders... or... happen often enough to trigger a P0304 Code.

@Redbeard's idea about having a sketchy EFI in Cylinder #4 begins to make sense... either that... or the Harness Wiring feeding it juice is either loose, broken or could be failing at the EFI Connector and thus... only occasionally allowing for the occurrence of a misfire at idle.

 

[littleblazer]

Balance rate with the tech 2 and pressure gauge?

 

[Mooseman]

The injector balance test will be in the next round of testing, hopefully this afternoon, along with the compression test and, depending on those results, delete SAIS.

The high'ish LTFT leads me to a vacuum leak. I know that anything under 10 is supposed to be OK but this is just too close to dismiss it. I usually look more closely at around 6.

Please pray to the combustion gods for me that it's not a compression issue like a burnt valve or rings

 

[littleblazer]

If it's a valve you'd feel it. Tissue by the muffler to see if its sucking it back in or not for exhaust.

Rings you could pull the oil cap and see if its puffing out. If it is that bad you should easily see it.

 

[Mooseman]

Checked for vacuum leaks with carb cleaner, nada. Injector balance passed. All went from 55 to 20 psi. I also swapped back my stock PCM. Same misfire at idle on 4.

Next. Compression test after lunch.

 

[Mooseman]

Compression test:
1: 249
2: 245.5
3: 237.5
4: 230.5
5: 247
6: 240

Unfortunately, didn't think of doing a wet test. So it's about 8% between the highest and lowest. Since 4 is the lowest and the one misfiring, I wonder if this is the real reason. Maybe not low enough for a bad misfire but only a little at idle.

Also deleted SAIS. Still slight misfire on 4.

For another day, soak it with Top Engine Cleaner and replace the O2 sensor.

Grasping at straws here.

 

[littleblazer]

That looks pretty good....

 

[Eric04]

Did you give the coil harness the stink eye? Given your somewhat backward problem(idle skip but not under load) I'm wondering if maybe it's got some janky and able to arc past it under load. I know nothing but since the straw box is out.

 

[mrrsm]

It would be hard to ignore that of all the Cylinders... the #4 location turns out to have the lowest compression on the list... even if within acceptable margins... and that prompts the (probably tiresome) suggestion to Pour some ACDelco Top Engine Cleaner down inside the #4 Cylinder as the Piston approaches TDC until the Spark Plug Hole is literally over-filled and see if it will hold that level as the stuff dissolves the carbon on the Piston Top, Compression Rings and from around the Four Valves inside the upper combustion chamber.

If the ACDelco TEC is allowed to well up into the bottom of the COP Tube area of the Head.... this would guarantee a very good exposure to everything below that point. After an overnight soak... plug that area with Blue Scott towels and expel the "Black Carbon Mung" as you turn the motor over a few times and avoid hydro-locking before installing the #4 Spark Plug. I would bet this will improve Valve Seat Sealing and "Springier" Top Compression Ring(s).

You WILL have to Change the Motor Oil and Oil Filter once the ACDelco TEC bleeds down into the Crankcase... lest the stuff do damage to the Front and Rear PTFE Crankshaft Seals and the Valve Guide Seals in the Engine Head if exposed to this Gnarly Stuff by ruining Motor Oil Lubricity as well.

 

[Mooseman]

Did you give the coil harness the stink eye? Given your somewhat backward problem(idle skip but not under load) I'm wondering if maybe it's got some janky and able to arc past it under load. I know nothing but since the straw box is out.

That`s a pretty thin straw. I wouldn`t even know how to test for that.

It would be hard to ignore that of all the Cylinders... the #4 location turns out to have the lowest compression on the list... even if within acceptable margins... and that prompts the (probably tiresome) suggestion to Pour some ACDelco Top Engine Cleaner down inside the #4 Cylinder as the Piston approaches TDC until the Spark Plug Hole is literally over-filled and see if it will hold that level as the stuff dissolves the carbon on the Piston Top, Compression Rings and from around the Four Valves inside the upper combustion chamber.

If the ACDelco TEC is allowed to well up into the bottom of the COP Tube area of the Head.... this would guarantee a very good exposure to everything below that point. After an overnight soak... plug that area with Blue Scott towels and expel the "Black Carbon Mung" as you turn the motor over a few times and avoid hydro-locking before installing the #4 Spark Plug. I would bet this will improve Valve Seat Sealing and "Springier" Top Compression Ring(s).

You WILL have to Change the Motor Oil and Oil Filter once the ACDelco TEC bleeds down into the Crankcase... lest the stuff do damage to the Front and Rear PTFE Crankshaft Seals and the Valve Guide Seals in the Engine Head if exposed to this Gnarly Stuff by ruining Motor Oil Lubricity as well.

Was thinking the same thing. It just sucks that I will lose that oil as it is still pristine. This thing doesn't dirty its oil. I don't know what the OLM % is since this is an LS, I can get it with the Tech 2.

Anyway, the rest of this week is a blowout for me. Working two nights, a day off kinda, and then two days of first aid training. Yay! I'll keep you all posted and keep the ideas coming, if any.

 

[Mooseman]

So coming home from working a night shift, I notice a fuel smell. The Shrader valve for the test port is leaking . I hope I won't have to replace it as it is in a b!@tch of a spot. Don't even know if you can get these separately. I'll look at it after a good day's sleep.

 

[mrrsm]

Tiny h/j on the subject of the OLM % ( ...designated as OLS by GM... )This link is worth a "Page Save" for those who want to bone up on the topic ...and get ready for the coming Cold Months:

https://www.mainmotor.com/Certified-Service_Oil-Change

 

[Mooseman]

Well, I'm hooped. It's not the valve but the fuel rail itself. It cracked right at the base of the test port and is seeping. It's in such a tight spot, the tester probably put some pressure on it sideways with its hose bent. Dammit!

I now have to take the Saab back out until I get that fuel rail from Amazon. It has problems itself and I have to re-activate the insurance. Decided to order that one which comes with the injectors just to be sure it's not a marginal injector causing the misfire. Also ordered two cans of the spray type TEC. I will have to pick them up at my US address so this be till next week.

 

[mrrsm]

This will sound a bit unorthodox... But there really is nothing preventing you from obtaining some Cheap Motor Oil and a Cheap Oil Filter in advance of the arrival of the Fuel Rail... and draining out your 'Fairly Fresh and Expensive' Motor Oil and likewise carefully unscrewing the Quality Oil Filter into a Very Clean Drain Pan... and Invert The Oil Filter to salvage its contents inside of a New Zip-Lock Bag.

Then you can slip the whole lot inside of a Large Plastic Garbage Bag to set it all aside until after the thorough job of doing the TEC Treatment is concluded. There would be a very minimal loss of the HQ Oil in this way... and after Draining out the TEC contaminated Cheap Oil and Cheap Oil Filter... the Original HQ Oil/Filter could be carefully poured into Quart sized Bottles and re-introduced into the LL8 Engine. This extra effort seems worth doing to prevent the loss of the $60-$100 you would save by not having to completely replace all of that Pristine Liquid Gold.

 

[Mooseman]

Now that's a good idea!

 

[Mooseman]

Update. Replaced the fuel rail with new injectors. Job went fairly smoothly. That's the good news. Bad news is the slight misfire at idle in #4 is still there :sad: .

For another day, I'll try soaking #4 cylinder with TEC. Might wait till the oil is due for a change. At least I don't need an e-test as the registration is good till 2020 and e-tests will be a thing of the past by then.

 

[mrrsm]

I know how organized your thinking is and how you approach these issues so methodically....and this prompt me to suggest one last possibility:

When you Pulled all the Spark Plugs to Check the Compression Balance between cylinders... did you replace those Plugs in exactly the same Cylinder Positions? Every once in a while.. there will be a defective ceramic Insulator or Cathode-Anode damage to the plug that was accidentally dropped and might present itself later on if someone purchased, damaged and then returned some "New" Spark Plugs to the Auto Parts Store just beofre you walked in to buy yours.

I know this is a long shot... but besides having a sketchy set of separating wires with Hidden Damage in the Ignition Harness serving that #4 Position Coil On Plug Connector... nothing else comes to mind. When it comes to "Counterman Shenanigans" I would not trust them as far as I could throw them.

 

[Mooseman]

Hmmmm. I had a miss on #4 when all this started with the old plugs and continued with the new plugs. Swapped the coils too. And now new fuel injectors (not by choice). So I don't think it's anything ignition related.

 

[mrrsm]

ScannerDanner ran a Broken Wire to its source for a Misfire on a Fuel Injector Harness trace that ...for no other reason than it might still be a possible source of this problem... it provides an opportunity to observe how much unwrapping of the harness(es) that he had to do in a portion of the Harness using an Amp-Probe over an area that you would not imagine an electrical problem to develop:

 

[Mooseman]

That's the problem, I don't have a steady miss, only slightly at idle. And it also shows a few misses here and there on other cylinders. My old '02 with over 300k km never showed any, ever, except when a coil failed and it was the classic miss and flashing CEL.

I'll be concentrating on the compression and then look at the O2 sensor as I think that's what reports misfires.

Another possibility, although slim, is water in the tank. My son, either from condensation or a bad batch of gas, had a ton of water in his tank. I gassed my truck at the same place but it runs fine compared to his. He had misses everywhere and ran like crap. We pumped it out and water was very evident. I might run it down to near empty and get premium fuel without ethanol just to see if that has any impact.

 

[littleblazer]

I still feel mine missing but can't log an actual miss count. Mine feels VVT related though. It couldn't be a lifter pump up issue could it?

 

[Mooseman]

Maybe it's your motor mounts if you feel something but not getting anything.

 

[littleblazer]

Maybe it's your motor mounts if you feel something but not getting anything.

Did them with delcos not 20k ago. It's a very different feeling than mounts. More of a swaying than a shake.

 

[m.mcmillen]

'll be concentrating on the compression and then look at the O2 sensor as I think that's what reports misfires.

Misfires are detrimined by the crank sensor. When there’s a misfire, the velocity of the crankshaft changes and the PCM picks that up and puts out the appropriate code.

 

[Mooseman]

Good to know. I had a direct line of sight of it when I was doing the fuel rail and was wondering about it. Wonder if it's got issues? Maybe a fresh CASE relearn might help? I think I'll just give that a try. I might even put in a new crank sensor.