To confirm, your idle related shuddering was not affected by ambient temps and ONLY occurred while stationary?

 

i still have not checked for this noise while running. I will say that there should not be gas on the gas cap. In my experience pulling the tank, the filler hose, fuel filter breather, and charcoal canister breather are the only things attached to the rear of the tank. Knowing that, I would look at the filler hose and filter breather. These are the only things that could influence the gas cap. Narrowing it down further, I do not see any possible way for gas to make its way up the filler tube to drench the cap. This leaves us with the filter breather. It would appear that gas is being forced up the breather. As the pump feeds through the filter, that tells me that it may be breaching that wall and you may have a broken filter assembly.

the noise could be attributed to the fuel pump. It sits very close the the filler hose. It may have become loose from its mount (it's a twist-lock set up).

the rubber gasket of the gas cap does degrade over time. I would purchase a new one and see if that fixes it; before undertaking a fuel tank drop.

 

I am not sure if it's affected by ambient temp. It seems that it bogs more after warm starts, and once the engine is fully warmed up. The bog/vibration is more noticeable at idle, but I may have had some hesitation driving as well. It's hard to say. I chased a drivetrain vibration at partial throttle (felt in steering wheel), but chalked it up to the tires.

anything thing I noticed is that if I allow start stop to execute, it bogs a little at restart. Correlating this, when i turn it off, it bogs as a time when the engine would normally shut off--could be me making a connection where none exists.

another thought: there is a TSB that I intend to execute whereby the TCM software is updated to take into account idle. It is characterized by harsh engagement of the transmission when shifting from park to drive. I notice this if I try to immediately shift into drive after starting (and the idle is still high). If I wait for the idle to drop (seemingly after the cats are warmed up from the increased idle) it shifts just fine.

maybe the software is cause an idle bog when stopped st a traffic light.

again, I still need to keep monitoring it. I think by this weekend I should be able to confirm the repair.

 

Have you ruled out the purge valve? As I understand it, the purge valve should be normally closed. It only opens to vent the canister under a specific set of circumstances. The vent valve on the tank side of the charcoal canister is normally open and closes only when running an evap leak test. It seems that the purge valve, if stuck open or is just not sealing, would pull a vacuum in the tank when the engine is at idle (maybe causing a gurgling/rumbling noise that is more pronounced with less fuel in the tank.

If the purge valve is not sealing, removing the gas cap would perhaps allow air into the system at the throttle body resembling a major vacuum leak and causing the engine surge.

You could pull the purge line off the throttle body (plug the throttle body connection) and see if the gurgling goes away and if it prevents the surging that starts when you remove the gas cap.

 

Ok, well here is where I am. The filler neck is definitely making a LOUD gurgling, rumbling, hissing, chuffing noise at 1/4, 1/2, 3/4 tank, not certain with a full tank. It makes the noise whether the cap is on or off and the cap seal appears it be in perfect condition. It makes this noise if the idle is rough or smooth. After adding 5 gallons of gas, the noise emanating from the filler went away BUT the 100 RPM rhythmic idle drop returned.

And the rigid vacuum line on the left side still makes the intermittent ticking noise.

I agree what I am experiencing feels like a vacuum leak.

I have not tried removing the small vacuum line next to the part load breather from the top of the throttle body yet.

Any other suggestions for testing/eliminating the purge valves variable, etc?


EDIT: Disconnected the purge line and started the car. no issues, no ticking for a few minutes, then from the left side (drivers) I hear a rumbling noise. Checked the filler neck and confirmed the noises matched. What this means, I have no idea...outside of my expertise.

 

The noise reduction makes sense if it's the pump. The pump attaches to the bottom of the tank. If adding gas reduces the sound, the gasoline is covering the pump and thereby dampening it's vibrations.

does your DMTL still run after shutting off the vehicle?

smoke test is the best way to tell. There are only four main gateways to engine vacuum: 2 at the throttle body, and the MAP sensor and vacuum line at the back of the supercharger. There is also the brake booster line and the crankcase lines...

can you hear the noise through the wheel well? This would indicate an exhaust leak. That purge line comes from the evap canister, not the tank.

 

With half a tank of gas, is the pump still not submerged?

How can I tell if the DMTL is running?

The brake booster line is not leaking, the throttle body lines are not leaking at the throttle body, the purge valve noise is on the top side of the engine, not underneath or near the catalytics.

EDIT: Also it still makes the noise with a full tank of gas.

 

Did disconnecting the purge line (and plugging the connection hole at the throttle body) eliminate the idle issue (or make any difference in any symptom - tank noise, etc.)? Or did it not make any difference? Where is the rumbling from the driver’s side heard? Drivers side rear or drivers side of the engine compartment?

 

with the engine off, you will hear a whirring sound from behind the right rear wheel. I noticed it after driving home from work and parking in the garage. Since pump and filter change, I have not heard it since. Knock on wood.

Start the truck and put it in off road height. Start the vehicle and climb underneath. Start listening to see if you can narrow the sound down. Also use an automotive stethoscope or long handled screwdriver up to your ear to find vibration.

 

The rumbling, gurgling, hissing, chuffing is coming from the filler neck and is definitely related in some way to the matched rhythm of the ticking and rumbling on the left side of the engine (where the purge vacuum lines are located). When I disconnect the vacuum line, I see brown soot but even plugging the hole with the engine running has no impact on the low idle. Also with the vacuum line disconnected, the purge ticking is still there however the filler neck noises cease. As soon as the line is reconnected, the filler noises return. I do not hear any noise from the gas tank or neck when the engine is turned off so I suspect no DMTL.