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Boost not building and fluctuating

Milehighgti

New member
Hey!

My 07 GTI has been having issues recently with boost. When it's around 15 in-hg it fluctuates from 15 down to 10 over and over and you can feel it and hear it. It stops as soon as you push past 15 in-hg. also doesnt hit boost as fast and only gets up to 8-10 psi if I really step on it. Obviously a boost leak somewhere but unsure where. it's unlikely it's charge piping because it was completely fine until it sat for about a month while I was away. I have replaced my diverter valve with a Rev d yesterday, and no change and it now makes a slight hissing/whistling noise that wasn't there before when spooling up. I have a ton of parts for it as I put a new motor in it and most of the parts from the old motor are still good. Going to try to swap out the PCV after work and see if that changes anything

Just wanted some opinions and suggestions before I just start pulling piping and resealing/replacing.

New here btw :)
 

ROH ECHT

K04 PLAY
Location
PDX OR
Car(s)
2007 MKV GTI
You can test the PCV to see if it leaks or will hold boost, you simply blow into it to see if it seals when in boost...here's my vid;

...also, at 15 in Hg, I assume that is at idle, correct? It should be more near 20 or 21 in Hg. You possibly have a vacuum leak and not a leak in the boost plumbing. This will cause a lack of boost as the vacuum holds the wastegate closed.

It was mentioned above, but you need to inspect all vacuum tubing attached to the intake manifold and the N75 boost controller that is mounted on the turbo. Smoke testing may be needed if you cannot find the cause. The N75 can be cracked and its resistance should be 25 ohm minimum. Unplug it and ohm test it...replace it if it is lower than 25 ohm. I replaced mine as it tested 22 ohm...my boost wouldn't reach peak and would drop and climb throughout a rev pull.
 
Last edited:

Milehighgti

New member
You can test the PCV to see if it leaks or will hold boost, you simply blow into it to see if it seals when in boost...here's my vid;

...also, at 15 in Hg, I assume that is at idle, correct? It should be more near 20 or 21 in Hg. You possibly have a vacuum leak and not a leak in the boost plumbing. This will cause a lack of boost as the vacuum holds the wastegate closed.

It was mentioned above, but you need to inspect all vacuum tubing attached to the intake manifold and the N75 boost controller that is mounted on the turbo. Smoke testing may be needed if you cannot find the cause. The N75 can be cracked and its resistance should be 25 ohm minimum. Unplug it and ohm test it...replace it if it is lower than 25 ohm. I replaced mine as it tested 22 ohm...my boost wouldn't reach peak and would drop and climb throughout a rev pull.

Hey sorry for the late reply I have been really busy. 15 in hg is not idle it idles around 20-22 in hg which is normal. I have already replaced the PCV and n75 with spares I had laying around which I know are good, and I can now build and hold boost after replacing n75 but the fluctuations are still there. I have noticed that it only happens when the car is up to operating Temp, and I start it the second time. For example I start the car and drive it and it runs completely normal, full boost and no fluctuations. Then I stop and turn off the car and restart it, boost fluctuations are back and I need to really step on it to get it to build 10 psi (20psi is my normal max). If I let the car sit and cool down below operating Temp and restart it, it is back to running normal. Really puzzled as to what else it could be.
 

Cellec

New member
Location
Rock springs wyoming
Car(s)
2007 vw GTI
Hey sorry for the late reply I have been really busy. 15 in hg is not idle it idles around 20-22 in hg which is normal. I have already replaced the PCV and n75 with spares I had laying around which I know are good, and I can now build and hold boost after replacing n75 but the fluctuations are still there. I have noticed that it only happens when the car is up to operating Temp, and I start it the second time. For example I start the car and drive it and it runs completely normal, full boost and no fluctuations. Then I stop and turn off the car and restart it, boost fluctuations are back and I need to really step on it to get it to build 10 psi (20psi is my normal max). If I let the car sit and cool down below operating Temp and restart it, it is back to running normal. Really puzzled as to what else it could be.
I’ve been having the same issue fresh start it hits upwards of 12 pounds of boost but after a sec it won’t pass 5 have you figured out what was causing your issue?
 

ROH ECHT

K04 PLAY
Location
PDX OR
Car(s)
2007 MKV GTI
I would want to confirm it has no leaks when it is warm. Perhaps while it heat soaks after shutting down, something swells and then leaks boost thereafter. I would smoke test it after warming it up and allowing it to sit a bit. Then, if it has no leaks and assuming the N75 and WG are OK, you should do a full scan of its stored error faults. Sensors can cause low boost if they are failing. Could be what the MAF or MAP sensors are reporting when they are hot. Looking at its codes and possibly looking at live data while operating it in boost, to see what all sensors are reporting to the ECU, are likely going to be required. Ghost problems can be difficult to isolate or are less obvious to see. Perhaps the WG valve is more open once warmed by heat soak. The DV may have an issue when warmed. Just because you know a part was installed, believing it to be good, this doesn't mean all is right. I've recently been told by one who installed a new DV__he thought it cannot be the DV as he just recently installed a new one__later, when trying to remove the DV just to inspect it and make sure the Newish DV is in fact OK, he discovered he had stripped one of the DV mounting bolts and this was the cause of a leak and whistle noise. You may need to go through the whole system and carefully inspect it all. Not skipping a thing for the assumption it is OK. Unfortunately, your issue may not be cut and dry. If you are on fb...you can pose your dilemma here. Being as it is more active, you might find one who can give you a direct fix for having gone through the exact situation; https://www.facebook.com/groups/529...2&notif_t=feedback_reaction_generic&ref=notif
 

Cellec

New member
Location
Rock springs wyoming
Car(s)
2007 vw GTI
i have good vaccum pressure when warm so it’s making me think I don’t have any leaks I do need to do a smoke test to ensure there are no leaks I have one code p0016 it’s 07 GTI fsi I’ve already been having issues with the timing before I replaced the variable timing solenoid and camshaft position sensor what’s a good way to check if there okay as soon as I replaced the camshaft sensor is when it started having boost issues which is weird to me since I didn’t touch anything else and it ran fine before I replaced it so could it be the sensor or could the sensor be fine and it’s just preventing boost since the timing is off I just need help figuring this out
 
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