SOLVED: The Case of the Never Ending Misfire

[therealsethallen]

Here's a curiosity for y'all. Occasionally at highway speeds, I've had one a random misfire, no codes and no flashing engine light. My exhaust freaking reeks, not like sulfur (rotten eggs) but of a somewhat fuel-ish smell, and my MPGs have gone from a combined 17.6 to a combined 15.9 Leading me to think that it may the catalytic converter, a fuel delivery issue or a wonky coil. The plugs are new a week ago, it's happened less but only once since. Usually uphill and acceleration. I'm not sure what else to think at this point. I know my water pump is failing, the fan clutch too. But could this cause a misfire? I'll check the fuel pressure today. I ordered Delco 41-103s and new coils, as it's been hard starting lately. What do you GMTNation folks think, I'm curious.

 

[Ziggy]

Exhaust that smells of fuel indicates a very rich air fuel mixture. Possible bad injector flooding a cylinder and washing the spark out. Combustion requires that the fuel be properly atomized in order to ignite under spark. If you have a clogged injector that is preventing the injector from firing properly, you could have a case where the fuel in that cylinder doesn't ignite properly, that cylinder may not fire.

 

[Mooseman]

But in that case, you would think he'd get a CEL/code for a rich mixture. Maybe reading the plugs would tell you something (i.e. if one is black and the others are normal brown).

So are you saying you first used other than ACDelco plugs? If so, that is a very real possibility as these engines are very picky on plugs.

What's the age of the O2 sensor? Maybe it's getting lazy and needs to be replaced.

 

[Ziggy]

Moose, I would generally think you're right, a fuel mix rich enough to smell like gas from the exhaust should trip a check engine light. I wonder though if it might be just at the very top of the computer's acceptable range. It doesn't take much fuel to make the exhaust stink, and if it only misfires under specific conditions, at random times, it may not be enough of a misfire to set a code. Just spitballin' here... I imagine if left unchecked, the codes will come in short order.

Definitely recommend pull and read the plugs. It's funny how certain vehicles are funny about some components. I've read enough times on here that our trucks prefer ACDelco plugs, if you used something else, go ahead and swap 'em out.

 

[mrrsm]

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

 

[therealsethallen]

I actually put Delcos in it, and they were more problematic. I run NGK plugs, brand new two weeks ago. I don't suspect they are the root of my issue. Although when I run a bit of fuel system cleaner through, they seem to eliminate that issue. But it still reeks. I'm at slightly under 130k miles. I think I'll start by cleaning up the CPAS and CPS today.

 

[Mooseman]

Apparently NGK makes the plugs for ACDelco, just like they don't have their own refinery for oil and lubes.

 

[mrrsm]

//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

 

[therealsethallen]

I had heard that, I've run these in everything I've owned. I did clean the CPS and CPAS today, both of them were pretty bad. However, I'm still going to think it's the cat.

Apparently NGK makes the plugs for ACDelco, just like they don't have their own refinery for oil and lubes.

 

[mrrsm]

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

 

[therealsethallen]

Did you change your Oil and Oil Filter at the same tie you cleaned the CPAS? If not... whatever residual garbage that was left in the oil pressure stream will re-infiltrate the screens of the CPAS and after clogging it right back up... it will cause the same stumble symptoms to recur.

I did not. However, today I got a P0306. Changed an NGK with another one.
I do think it's a fuel delivery issue, as the shudder at idle has stopped since I put in some injector cleaner.

 

[mrrsm]

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

 

[therealsethallen]

I'll be grabbing myself a fuel rail tomorrow in the AM.

 

[therealsethallen]

My tides have changed slightly. So, now when I leave the truck warm, and go start it after sitting more than about ten minutes, it takes about four seconds of cranking to get started. No power loss and no drivability issues. Idles decent, not well but good enough. I did manage to reach down and tighten a couple of the manifold bolts and it did help just a tiny bit. Any other suggestions before I just throw parts at it?

 

[mrrsm]

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

 

[therealsethallen]

Intake Manifold Bolts....? Or... Exhaust?

That'd be intake.

 

[mrrsm]

//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

 

[therealsethallen]

...and what happened with the smell of fuel and noxious odors problem?

They seem to be gone, but the mild rough idle and horrible starting are still there.

 

[mrrsm]

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

 

[therealsethallen]

My money is on what @Mooseman mentioned at the beginning of this RFH (Request For Help) and that is to consider replacing your Upstream O2 Sensor...

Well, today I got a P0306 (misfire in 6), replaced the plug and the coil. It's undoubtedly a bad injector, at this point I can't think of anything different. Hard starts and an idle like this. It's got to be a leaky injector.

 

[mrrsm]

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

 

[djthumper]

Well, today I got a P0306 (misfire in 6), replaced the plug and the coil. It's undoubtedly a bad injector, at this point I can't think of anything different. Hard starts and an idle like this. It's got to be a leaky injector.

Did you move the coil to another plug? Every things is pointing to an O2 sensor.

 

[Mooseman]

Well, if you put a new coil and plug on #6 and the code returns after clearing it, three things to check I'd say:
- intake manifold and throttle body bolts and seals for leaks.
- check for vacuum leaks
- check compression
- check for faulty injector

These are in the order of difficulty/cost. To check for intake and vacuum leaks, use the method in this video.

 

[therealsethallen]

Well, if you put a new coil and plug on #6 and the code returns after clearing it, three things to check I'd say:
- intake manifold and throttle body bolts and seals for leaks.
- check for vacuum leaks
- check compression
- check for faulty injector

These are in the order of difficulty/cost. To check for intake and vacuum leaks, use the method in this video.

Checked for a manifold leak (using that method)
Compression is good.
The injector however is not.

Putting in a new one tomorrow.