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Discussion Starter #1 (Edited)
So this is one of those things that was minor enough I never really put much thought into it until now, but I'm curious.

I have a 2009 GXP with the GMPP Stage II tune that was professionally installed. It's all good. When I first got the car back in 2015, it was a bat out of hell and it wasn't terribly difficult to hit 22psi of boost. I think I even hit 23. I don't know if it was overnight, because I don't run it that hard often, but about a year or so later, no matter how hard I push it in 3rd or 4th gear, I cannot break 18 or 19psi, and the car just feels like it's missing that edge it had when I first got it. Aside from general maintenance (water and fuel pumps, cam solenoids, tires, oil, etc) and a stereo system install... I'm not sure I touched anything that would cause this.

Is it possible for a tune to revert to stock? There are several time I had the battery disconnected for days, but I can't imagine that would cause it. Is there a sensor or sensor that could go bad to do this? Any other things I should look into to see why I am being robbed of boost and HP? I cleaned the MAF and air filter, but that didn't change anything. Any help would be appreciated — I want to exhaust all reasonable options before get getting a Trifecta tune and see if that takes care of it. I was absolutely happy with the GMPP tune.

Also, possibly related... possibly not... but my gas mileage isn't as good as it was early on, either. Probably about 40-60 miles less per tank than I was in 2015 with similar driving habits.
 

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First thing I thought of was the learn down feature, but the GMPP removes that, next thing would be vacuum leak, but you should be getting codes for that. I also have the GMPP but had it installed before the connector had been developed, and I've always wondered if crimping affects boost over the long haul. Then again, it's been so long since I've floored the pedal that I don't even know what it reads now. Good luck on figuring it out.
 

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How many miles? Have you pulled the plugs and looked at the intake for carbon buildup? That is known to restrict flow and power.

Have you taken a look at the HPFP? They have been known to "fail" and still function. they can leak fuel into the oil or just not put out as much as they are rated for. Reduced fuel delivery will result in the ECM detuning the car. While you are at it, consider putting a gauge on the fuel hose from the tank. Make sure the in tank fuel pump is working to specs.

Have you looked at the throttle body? They can accumulate dirt over time and that can restrict the function.

Have you checked the PCV nipple on the hose to the air filter? Sometimes these break, then leak and affect the tune.

Check the connectors to the intake sensors. Never know.

Cleaned the MAF lately or ever?
Good luck
 

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I would add that you might want to look at all coupler clamps on your intake path. Very weird but GM put T-clamps on the couplers for the pipes that go into the intercooler, but they used a ****ty c-clamp (I think that's the name if not it's the clamp that you use a straight head screwdriver to tighten) for the coupler that goes into the throttle body. That clamp/coupler have been known to loosen over time and cause small boost leaks...
 

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I don't know about fuel mileage, but look through the front grilles and check if your intercooler is ballooning. All of the horizontal tubes should be the exact same height.

Mine aren't (pic attached) and I have noticed the same boost numbers as you. About a year ago it was 19 to 21, and now it's only 17 or 18. I also threw a CEL for some code I don't remember and was limited to 10psi until I cleared it. Once I have the money and time to install a better aftermarket intercooler I'm hopeful it'll go back to 20 and the random CEL will stop, as confirmed by a few older threads I found a while back.
 

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You need a HPFP. It's leaking. Is the tick still regular or is it random? These are all the same issues I had. Our cars "pend" codes (CEL). In other words, it hasn't quite set the CEL yet because it has to see the same issue again before it will set the CEL. Almost all here don't know that!! When they pend, they only cut a portion of the power until you have 5? key cycles (or 30 minutes of driving without another issue, I can't remember the exact time) or the you actually set the CEL. Gaurantee if you hooked up a code reader, you would have a pending code of p2178 (or 2188, can't remember). I've been there, done that.....
 

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So this is one of those things that was minor enough I never really put much thought into it until now, but I'm curious.

I have a 2009 GXP with the GMPP Stage II tune that was professionally installed. It's all good. When I first got the car back in 2015, it was a bat out of hell and it wasn't terribly difficult to hit 22psi of boost. I think I even hit 23. I don't know if it was overnight, because I don't run it that hard often, but about a year or so later, no matter how hard I push it in 3rd or 4th gear, I cannot break 18 or 19psi, and the car just feels like it's missing that edge it had when I first got it. Aside from general maintenance (water and fuel pumps, cam solenoids, tires, oil, etc) and a stereo system install... I'm not sure I touched anything that would cause this.

Is it possible for a tune to revert to stock? ...
...
interesting... my son drives my yellow coupe, and he is saying the same thing... the "feel" is different...

following this thread...
 

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I don't know about fuel mileage, but look through the front grilles and check if your intercooler is ballooning. All of the horizontal tubes should be the exact same height.

Mine aren't (pic attached) and I have noticed the same boost numbers as you. About a year ago it was 19 to 21, and now it's only 17 or 18. I also threw a CEL for some code I don't remember and was limited to 10psi until I cleared it. Once I have the money and time to install a better aftermarket intercooler I'm hopeful it'll go back to 20 and the random CEL will stop, as confirmed by a few older threads I found a while back.
can you please update the photo to point to where you think there is a problem? I am not seeing a difference...
 

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It's right under the bolt head where the photo is slightly washed out. Wish I had a better picture but it's tough to get a good shot while laying on your side and holding both my phone and a flashlight.

If you look closely, the horizontal tube is almost twice the height as the tubes further down and to the left. While this may not be your issue, pretty much all the stock intercoolers do this eventually with anything above the stock 260hp tune, whether it be GMPP or aftermarket.

Here's a google image result of an extreme case:
 

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Seeing pictures of deformed stock ICs made me include a WERKS IC as part of my intake upgrades...
 

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Okay. Can you please try to read codes? If you have an intercooler issue, YOU WILL HAVE A CEL because the issue doesn't just go away like a HPFP issue!!!! If you have a faulty HPFP, you will have codes pending. An intercooler will set a CEL. A HPFP will not. As I stated earlier, the HPFP will pend a code (p2188 or p2178, one is too rich at idle, the other is too lean at idle). What happens is that the car, when it pends these codes, as I stated earlier, somewhat reduces a bit of power. If you have a hole or ballooned IC, it will limit you to 5psi and you'll get a CEL. With the HPFP, you have to meet certain criteria 5 times within so many minutes to get a CEL and be limited to the 5psi. Until that point, the code "pends". If the code has pended, it only moderately limits your performance. Please....check for a "pending" code.

And for the record, I have a stock IC. I have a 283hp tune (which equates to about 330hp at the crank)...I get 24psi of boost.... I have no issue with my stock IC. The GMPP tunes are 290hp at the crank or 290bhp before anyone asks....
 

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Discussion Starter #12 (Edited)
always wondered if crimping affects boost over the long haul.
Mine is soldered, so that wouldn't be a problem here, fortunately.

How many miles? Have you pulled the plugs... Have you taken a look at the HPFP? Make sure the in tank fuel pump is working to specs. Have you looked at the throttle body? Have you checked the PCV nipple on the hose to the air filter? Check the connectors to the intake sensors. Cleaned the MAF lately or ever?
Mileage is just under 50k. I have not checked the plugs but will do so. I replaced the HPFP just over a year ago. I have not checked the throttle body. PCV nipple is good. I need to do a connection check, including the fuel pressure sensor. I cleaned the MAF last year, but I'm tempted to just replace it along with the upstream O2 sensor for good measure.

I would add that you might want to look at all coupler clamps on your intake path.
Good advice. Thanks!

I don't know about fuel mileage, but look through the front grilles and check if your intercooler is ballooning.
I had never heard of this, and I thought I knew everything. ;-) Looks good though. I'll keep an eye on it.

You need a HPFP. It's leaking.
I just changed the HPFP about 18 months ago (because my oil had gas in it and the cam solenoids went bad) and it is ticking regularly and as expected. I do my own code checking and there are no other codes, pending or otherwise. Also, my one code I do get (P0089) will go away on its own after a couple cycles, so it's minor — pretty confident it's the fuel rail pressure sensor since 99% of the time the HPFP tends to throw *both* the P0089 and P0087.
 
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