Loss of boost and possible fuel cut?

It sounds like the turbo is going out or the exhaust system is partially clogged. I would use this guide to test the catalytic converter and we should run the codes as well. Also please remove the inlet or outlet of the turbo to check the turbo main shaft endplay.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

and

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

The turbo will boost at low RPM when it goes bad, also if there is engine oil inside the inlet tubes the turbo is bad. Here is how to replace the turbo and check the tubes and endplay. Check out the images (below). Please let us know what happens.

I was afraid that might be the case. I do have oil in my inlets and oil tends to get the MAP sensor very dirty and I have to clean it occasionally.

However, I do have to add that I have no engine codes for this or anything else.

Yep, it sounds like the turbo is no good. It may not have a code for this which is normal. Please check the turbo main shaft endplay which will tell you for sure.

Okay, so I am reviving this because I finally was able to replace my turbo with a known good OEM one and a brand new N75 valve too. The old turbo was definitely bad. It was praying oil everywhere, the shaft had a ton of play, and one of the tips of the compressor fins was even bent.

However, while the car drives 100% better, I still can't seem to go past 5k RPM at WOT without the boost dropping off completely and losing all power. I'm kind of at a loss here.

I can get up to redline at basically any boost level but if I try to do it at max, I can't seem to.

It sounds like you have a clogged exhaust system, plus have you run the codes?

Here are two guides to help us see what's going on:

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

and

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Please let us know what happens.

My exhaust is catless with a single resonator from the downpipe to the muffler. Everything is 100% clear, as I just finished checking it. However, I can't check the muffler without disassembling it or just buying a new one. Any suggestions? Could the muffler be clogged?

Quick update:

I removed the muffler, so it was just straight piped. The problem was still present. So, that rules out a clogged exhaust. Any other ideas as to what it could be?

A bad MAF sensor can cause this problem which can happen after a bad turbo, do you have any codes? I would try swapping the sensor out. Here is the location. Check out the images (below). Let us know how it goes.

I replaced the MAF sensor, and the problem is still there.

When you say it cuts boost after 5,000 RPMs, are you driving the car or just sitting still revving the engine? Also, are there any engine noises such as a rattle or ping? Can you do a live data scan so I can see the numbers?

I'm actually driving the car. If I go wide open throttle, I just lose power past 5,000. There are no misfires that happen when this occurs. Feels like the boost is cut and the timing is retarded. I do have a live data feed on my car watching the boost, timing, misfires and more. I can get a video and show you.

That would be great! This way I can see what's going on. Can you check the vacuum line going to the vacuum diaphragm? I am thinking the vacuum diaphragm could be bad as well, can you please re-run the trouble codes as well? Here is the valve so you can see what I am talking about. You might want to see if it holds vacuum. Check out the images (below). Please let us know what happens.

The only code I have is P0036, and it's for my rear O2 sensor. More specifically, it's a code I'll always have because the wires to that sensor are fried. They shorted on a 300 mile road trip in some deep snow. Otherwise, I have no codes active or pending.

As far as I can tell, the vacuum line is secure and fine. But it looks like the diagrams you gave me are for a MK6 GTI, where I have a MK6 Golf R. It doesn't have the same engine.

And I can go past 5k RPM just fine as long as I'm not trying to floor it. I can go all the way to the red line. I just can't get there by going WOT.

I'm attaching a video showing the live data readout while I'm flooring it.

You can hear where I punch it. When the sound stops, I'm not lifting off the throttle. I lift off about a second or so after it stops accelerating.

Thanks for the video. It seems like you have a restriction in the air intake of the engine, or the low-pressure fuel pump is getting weak. Is the power okay up to 5,000 RPMs?

Here is a guide to help and instructions for your car in the images below:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Delivery Pressure:

Test Conditions:
The fuses must be okay.
Battery voltage must be at least 12 volts.
All electrical consumers such as, for example, lights and rear window defogger, must be turned off.

Start the engine and run at idle speed.
Close the shut off valve "B" for a maximum of 15 seconds and read the pressure on the Pressure Tester (VAS 6551) :

Specified value .................... 0.5 bar (7.25 psi) minimum


Auxiliary Fuel Pump (In-line Fuel Pump) Pressure:

Test Conditions:
Battery voltage is at least 12.5 volts
The fuel filter is okay.
Fuel tank: Fuel level, min. reserve.
The ignition is off.

Connect the vehicle diagnostic tester and perform the guided function "Check Electric Fuel Pumps".

The fuel pump now runs for 30 seconds.
Let the fuel pump run until the highest fuel pressure is reached.

Specified value .................... at least 3.5 bar (50.7 psi)


Fuel Delivery Quantity:

Test Conditions:
The fuses must be okay.
Battery voltage is at least 12 volts

Specified value .................... at least 1000 ml/30 seconds

Check out the images (below). Please let us know what happens.

I haven't had time to test the low pressure fuel pump yet, but I doubt that would be much of an issue as it only has 20k on it. I bought a brand new one a while back.

The only thing I can think on the fueling side that might be failing may be the HPFP, it's an aftermarket APR unit that has about 90k on it. I'll need to test it though.

However, on the intake side, I removed the filter completely to see how it performed and I now can get reliable, full boost at WOT up to 6k RPM. Not quite to redline, which is 6.5k, but that's vastly improved. I'll be replacing that filter. It has over 100k miles on it, and I've been cleaning it at every oil change.

And to answer your question, the power is fantastic until it hits 5k (6k with a good filter/no filter now). I'll be doing a deeper clean on my current filter until I get a replacement.

I also wanted to ask if it's possible that my fuel injectors might need to be replaced? I'm not sure if there's a test for that or not. They're stock and have 201k on them.

Usually one will go out at a time, but if this is a performance application then the fuel volume may need to be increased. A weak HPFP can cause this issue as well.

Is it possible that my throttle body is the issue? The throttle body is stock, and has over 200,000 on it. I noticed that when it warmed up here (90°+) that I was able to take it to redline. The reduction in air density might have allowed me to do that. I wanted to field that as an idea since I don't think it's my HPFP. That only has 70,000 on it and it's an aftermarket APR unit.

Usually when the throttle body has a problem the car will go into limp mode and give a code, but you never know. You can give it a try and see what happens. I have seen the tune cause this problem, can you take the tune out and leave a stock tune in to see if the problem goes away?

I've changed the tune, gone to stock, a different stage 2 tune, back to what I have now, nothing changes.

I would rerun the codes to see if any new codes come up, also I have seen engine coolant temperature sensors cause strange problems like this as well. Here is the location if you want to try and swap it out. Check out the images (below). Let us know how it goes.

No new codes. Afaik the coolant temperature sensor is fine. Haven't had any issues with that or any odd temperature readings.

Quick update that could provide additional clues as to what may be going on. When I'm on a rough road with a lot of bumps, or a dirt road, I get occasional misfires. The engine runs noticeably rougher. As soon as I get on a smooth road it stops. Am I correct in thinking it might still be the throttle body? In my mind the butterfly valve is just bring shaken and is causing random misfires.

it does sound like you have a loose connection at a ground or power cable. I would clean the battery cables and check the ground connection at the engine block. Also, can we run the codes one more time to see if any are stored?

Okay, so I just got back from being out of town. I found a loose butt splice on the signal wire for my MAP sensor. I got that secured and replaced with a new one, and that solved the misfire over bumps issue. I ended up needing a new battery as well, so that's replaced and the terminals on that are all good. No engine codes pending or present. However, I do still have the loss of power at around 5k-6k RPM. I'm at a loss of what it might be still.

I have seen worn out MAF sensors cause low power problems and not trigger a check engine light or throw a code. Have you tried a new one?

Yes, I've tried a new MAF sensor and have cleaned the one I have now with MAF sensor cleaner. On your advice, I went ahead and replaced both coolant temperature sensors and that didn't help either.

So, to be clear the engine has less power in the upper RPMs, correct? The turbo is new and there are no codes?