HELP!!! 2004 GMC Yukon Xl Denali 6.0 intake noise

[kaysha241]

So my Yukon has had a little tick that has been there since 90k. Not really noticeable unless you floored it and then you could barely hear it. It now has 250k. For the last couple of weeks whenever it would downshift, and RPM increase, you could hear it a little more and the CEL would flash a little. A couple of days ago, when it downshifted it did the same thing and then started to run crapy. As I got off the freeway, waiting at the light, it would barely idle. A few times over the last few days it stalls, continuously poor idle, and rough running along with CEL light on and flashing once i get over 30mph.
1st thought is PCV hose so I checked it, but it is on and ok. I can hear a constant PSST, PSST, PSST, PSST, as it idles, coming from the intake manifold.
I have inspected it, sprayed carb cleaner around the base, and checked all suggestions I could find for potential vacuum leaks without removing anything.
The only other thing I can think of is to replace the intake manifold gaskets, thinking that maybe when it downshifted and drove up the RPM, it blew the gasket. I intend to do the valley gasket, knock sensor, and harness while I'm in there.

Is there anything I am missing that may be causing the rough idle, poor performance, and PSST, PSST, PSST, PSST vacuum noise that I can check before tearing into it???

Thanks - your experience and knowledge is appreciated!

 

[gooffeyguy]

Have you hooked a code scanner to it and see if it has any trouble codes? Usually when the engine light comes on it will store a code.

 

[kaysha241]

Yes… po300, 101, 220.

 

[kaysha241]

and the live data numbers

 

[iamdub]

So my Yukon has had a little tick that has been there since 90k. Not really noticeable unless you floored it and then you could barely hear it. It now has 250k. For the last couple of weeks whenever it would downshift, and RPM increase, you could hear it a little more and the CEL would flash a little. A couple of days ago, when it downshifted it did the same thing and then started to run crapy. As I got off the freeway, waiting at the light, it would barely idle. A few times over the last few days it stalls, continuously poor idle, and rough running along with CEL light on and flashing once i get over 30mph.
1st thought is PCV hose so I checked it, but it is on and ok. I can hear a constant PSST, PSST, PSST, PSST, as it idles, coming from the intake manifold.
I have inspected it, sprayed carb cleaner around the base, and checked all suggestions I could find for potential vacuum leaks without removing anything.
The only other thing I can think of is to replace the intake manifold gaskets, thinking that maybe when it downshifted and drove up the RPM, it blew the gasket. I intend to do the valley gasket, knock sensor, and harness while I'm in there.

Is there anything I am missing that may be causing the rough idle, poor performance, and PSST, PSST, PSST, PSST vacuum noise that I can check before tearing into it???

Thanks - your experience and knowledge is appreciated!

Flying leap/let's-play-Dr. House guess: Small exhaust leak over time threw off the O2 sensor(s) making it run a little rich. The extra fuel eventually clogged up a cat. It wasn't a problem at lower RPM do to the minimal air volume. At WOT, it was a restriction causing intermittent misfires and the flashing CEL. Maybe, more recently when you floored it, the catalyst overheated, cracked and instantly became a worse clog. The clog suddenly stopped enough air flow through the engine that the weak point in the exhaust leak blew out and is now more audible. The sudden stall in flow threw off the MAF enough to cause the P0101.

 

[OR VietVet]

@iamdub has the answer for you. If you can afford it, do those knock sensors and harnesses too and use GM parts all around. Bet the plugs show bad too.

 

[kaysha241]

Flying leap/let's-play-Dr. House guess: Small exhaust leak over time threw off the O2 sensor(s) making it run a little rich. The extra fuel eventually clogged up a cat. It wasn't a problem at lower RPM do to the minimal air volume. At WOT, it was a restriction causing intermittent misfires and the flashing CEL. Maybe, more recently when you floored it, the catalyst overheated, cracked and instantly became a worse clog. The clog suddenly stopped enough air flow through the engine that the weak point in the exhaust leak blew out and is now more audible. The sudden stall in flow threw off the MAF enough to cause the P0101.

Great theory! However, The cats and 02 sensors have less than 5000 miles on them, and the psst psst psst is coming from the top center of the intake manifold as opposed to the lower side where the exhaust manifold is.

I read on one thread where someone had broken the caps as shown in the picture here, but it doesn't appear to be leaning around any of these caps.

 

[OR VietVet]

Back to the drawing board then. Did you spray that carb cleaner near the throttle body? Can you post a video of the noise?

 

[kaysha241]

@iamdub has the answer for you. If you can afford it, do those knock sensors and harnesses too and use GM parts all around. Bet the plugs show bad too.

Yes, if I end up needing to do the intake gaskets, I will do the valley gasket, knock sensors, and harness. I ordered AC Delco knock sensors/harness. Also, there is a TSB to put silicon around the rubber seals on the harness to prevent potential corrosion.

 

[Fless]

The MAF malfunction could be the cause of the extremely negative fuel trims -- those are way out of whack (technical term). The sum of your LTFT and STFT for any given bank should be less than 10% either way, but the closer to 0 the better. https://www.mechanic.com.au/news/understanding-short-term-and-long-term-fuel-trims1

Clean the MAF or replace it with a known good one. Do a standard vacuum test with a vacuum gauge. Graph the upstream O2 sensors' activity to make sure one's not lazy or just plain dead. A vacuum leak should cause fuel trims to go way positive.