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Loss of boost spike?

Whitedemon

Passed Driver's Ed
Location
Miami, FL
Hi guys, posting this to see if maybe someone can help me..so for some time now I’ve noticed my car hasn’t been hitting the same amount of boost it was hitting before.
My car is a 08 FSI with a K04 BorgWarner turbo and mapped with Revo “stage 3” file.
I used to spike/peak boost to about 25 psi and then hold steady around 22-23 psi but that spike in boost is gone, I can’t get to 25 anymore, my peak boost right now is 22.5 psi but it holds it there until almost redline (in 6th gear) that’s what makes me believe it might not be a leak? also it feels like boost builds steady just a little slower and less aggressive than before, it would get up to 22.5 psi and stay there as in before the needle in the boost gauge would jump really fast all the way to 25 and then slowly start to go down.
Anything you guys could suggest me?

I was doing some reading here:https://www.turbosmart.com/news/boost-spiking-explained/
It says something about the wastegate opening up too soon? Could that be it?
 
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ROH ECHT

K04 PLAY
Location
PDX OR
Car(s)
2007 MKV GTI
Sometimes boost just goes down in cooler weather, as less air is needed. If you've had the car through a number of cooler seasons and it never did this before....then yes, there may be something else going on.

It could be mechanical. For example; the wastegate may now be loose.
It may also be the N75 is failing. It should be 25 ohm with a ±5 ohm. VW spec actually states 20 ohm to 30 ohm. But they can begin acting negatively when off just 2 or 3 ohm. Mine was suddenly boosting 4 psi lower, would drop to 10 psi and build back to about 15 psi and it tested at 22 ohm. A new N75 fixed it.

The approach is up to you...I don't want to have you replacing bits for nothing. If you can wait to see if it boosts more when warmer, I would do that. If not, I would test the N75 ohm and replace if not 25 ohm. But it is possible the test was inaccurate (poor multi-meter) and you find a new N75 did nothing...then you are left with possibly needing a new turbo. You'll want to confirm there are no small leaks. And you may want to do this before testing the N75. So.....

One quick thing to check is the check valve for boost in the PCV if you have one. Disconnect the PCV tube from the intake manifold, wipe the oil from the disconnected end of the tube, and blow into it with some force. It should be 100% sealed. If you detect air passing through it, determine if the seals on the other end connected to the PCV are not leaking. If they are good, replace the PCV.

Another to test is the N80 EVAP purge valve. It can loose a couple of psi if leaking. Disconnect the hose from the N80 on the manifold side and blow into it. It also should be sealed. If not, replace the N80.
 

Whitedemon

Passed Driver's Ed
Location
Miami, FL
Sometimes boost just goes down in cooler weather, as less air is needed. If you've had the car through a number of cooler seasons and it never did this before....then yes, there may be something else going on.

It could be mechanical. For example; the wastegate may now be loose.
It may also be the N75 is failing. It should be 25 ohm with a ±5 ohm. VW spec actually states 20 ohm to 30 ohm. But they can begin acting negatively when off just 2 or 3 ohm. Mine was suddenly boosting 4 psi lower, would drop to 10 psi and build back to about 15 psi and it tested at 22 ohm. A new N75 fixed it.

The approach is up to you...I don't want to have you replacing bits for nothing. If you can wait to see if it boosts more when warmer, I would do that. If not, I would test the N75 ohm and replace if not 25 ohm. But it is possible the test was inaccurate (poor multi-meter) and you find a new N75 did nothing...then you are left with possibly needing a new turbo. You'll want to confirm there are no small leaks. And you may want to do this before testing the N75. So.....

One quick thing to check is the check valve for boost in the PCV if you have one. Disconnect the PCV tube from the intake manifold, wipe the oil from the disconnected end of the tube, and blow into it with some force. It should be 100% sealed. If you detect air passing through it, determine if the seals on the other end connected to the PCV are not leaking. If they are good, replace the PCV.

Another to test is the N80 EVAP purge valve. It can loose a couple of psi if leaking. Disconnect the hose from the N80 on the manifold side and blow into it. It also should be sealed. If not, replace the N80.
Thanks for the reply.
I live in south Florida, it never gets cold down here lol it’s actually been like this for months now, also I’ve had the car for 6 years and been through every season, it’s never done this before so I know for sure it’s not the weather.
Also, I don’t have a PCV valve, I replaced that already with an oil catch can (issue has been happening even before I did the replacement).
the n80 valve I replaced not so long ago, I remember it was giving me a code when it was bad so I don’t think it’s that either.
Also replaced DV valve and MAP sensor.
The only thing I haven’t checked/replaced yet is the N75, well I did replace it once but it was almost 3 years ago when I swapped the turbo.
What would be the best way to check for small leaks? I did check all my hoses and visibly at least I can’t see any leaks, I made sure all clamps were tightened up and all connections were good, so if it is a leak it might be a really small one.
 
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ROH ECHT

K04 PLAY
Location
PDX OR
Car(s)
2007 MKV GTI
Sounds like the only possibilities left are the N75 or a very small leak. N75 test is very easy to do with a good multi-meter. Unplug it and test the ohms across the two tabs of the N75.
Testing for a leak would be best done with smoke or to pressurize the air inlet and spray all of the air inlet with soapy water while pressurized. You can actually use a shop-vac with the hose on the blower side...just do not want to over pressurize using an air compressor. Check everything between the turbo outlet and intake manifold obviously. I would imagine if it is a leak, it would be small given it is losing just a couple of psi. Have you looked at the boost data? Both actual and specified? Just to be sure it is well below specified. WGDC may be higher than normal while trying to make up for the deficiency...if you've logged boost data in the past to compare it to.
 
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Whitedemon

Passed Driver's Ed
Location
Miami, FL
Ok so quick update.

I decided to go for a test drive and started playing with the throttle, something interesting just happened, I just found out that in the lower gears (3rd and 4th) if I jump on the throttle and let go quickly from high RPMs (3000-4000) I’m able to get the boost spike again! up to 24-25 psi. But somehow, if I just accelerate normally and try to build boost in a steady way from low RPMS it still doesn’t get past 22.5 psi. What could be causing this? I remember before no matter what gear I was or RPMs I would always get to 25 psi, weird...maybe a pre-turbo exhaust leak? 🤔

i have logged actual vs specified before, they match, it just feels like the spool is slower than before so it takes longer than usual for actual to meet specified
 
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ROH ECHT

K04 PLAY
Location
PDX OR
Car(s)
2007 MKV GTI
Still thinking the N75 needs a quick test. If you suspect an exhaust manifold leak, you can do the same pressure test using a shop-vac on the blower side. Attach it to one exhaust outlet and plug the other/s. Get under there with soapy water in a spray bottle and spray all over the manifold and see if it makes bubbles.
 

Whitedemon

Passed Driver's Ed
Location
Miami, FL
Ok, I will test the N75 first, if that’s good then I’ll do the exhaust manifold pressurized test, what about the wastegate getting stuck? Could that be considered as well? I’m definitely thinking it gotta be one of these: n75, wastegate actuator, or pre-turbo leak.

btw I have a multimeter but I haven’t use it to test sensors before, do I have to keep the ignition switch on?
 

ROH ECHT

K04 PLAY
Location
PDX OR
Car(s)
2007 MKV GTI
To test if the Wastegate is stuck closed; remove the nut on the end of the actuator rod. See if the WG linkage can be moved freely.

The N75 test; No. The ignition is off when you test it. Just unplug the wire connector to the N75. You'll see two tabs on the N75. You just test the ohms across those two tabs with the meter leads.
 

ROH ECHT

K04 PLAY
Location
PDX OR
Car(s)
2007 MKV GTI
OK. I would let that go until you see more abnormality in the boost. Just showing it a couple of PSI below normal wouldn't convince me to change it.
I doubt it would be a stuck or closed WG, because this would make it spike the boost higher.

All you can really do now is test for small leaks. But if the boost data shows actual matching specified...then it is where it wants to be. Something else could be causing a lower specified boost right now...but does specified now match specified when you checked it previously?
 

Whitedemon

Passed Driver's Ed
Location
Miami, FL
OK. I would let that go until you see more abnormality in the boost. Just showing it a couple of PSI below normal wouldn't convince me to change it.
I doubt it would be a stuck or closed WG, because this would make it spike the boost higher.

All you can really do now is test for small leaks. But if the boost data shows actual matching specified...then it is where it wants to be. Something else could be causing a lower specified boost right now...but does specified now match specified when you checked it previously?
Ok but isn’t it supposed to be at 25 ohm at least? If it’s below 25 it means it’s probably making the wg open up early, right?
I read VW specs and it says that it needs to be in the 25-35 ohm range.

regarding the datalog, I don’t remember now I’ll have to look and see if I find any old logs in my pc of when car was running fine but I’m not sure if I have any saved.
For sure I know that the spool is slower than before. Boost always meet specified, but it feels like taking longer than before to get there.
 
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ROH ECHT

K04 PLAY
Location
PDX OR
Car(s)
2007 MKV GTI
I was sent a document from a tech which showed VW spec is 20 to 30 ohm...but the tech stated it's actually 25 ohm ±5 ohm....and he said to replace it if not 25 ohm because they can act oddly when off just a couple of ohm.

Right, you could change the N75 first. I was only suggesting letting it go until you've ruled out any small leaks.
When mine was at 22 ohm...the boost would take time to build and only reach about 18 psi. Then it would drop instantly to below 10 psi....maybe 8 or 9 sometimes. And would start building boost again but would only reach 14 or 15 psi.

If you can compare...look at data logs where you begin the log at the same rpm. Then compare to when you went full throttle to when actual boost meets specified. How many rpm did it take to do so. This is why I always do my data pulls beginning at the same rpm. So whether it's fuel, boost, or whatever I am logging....I can compare what was going on at a specific rpm. Temps and all factor in, but it does help sometimes.
 

Whitedemon

Passed Driver's Ed
Location
Miami, FL
I don’t know but I have a strong feeling it’s going to be the n75, I was reading different posts on other forums and people describing the exact same symptoms mine is having (2-3 psi loss and not reaching peak boost until really late in the rpms) they were able to fix it by replacing the n75....so I guess I’ll give it a shot and see what happens. Valve is only $19 at FCP euro so I think it’s worth the try. At least so I could rule that out.
Also since I’ll have to take the turbo out anyways I might replace the gasket that goes onto the manifold, today while I was inspecting it I noticed there was a little bit of oil accumulated at the bottom/back of turbo, so I’m guessing gasket must be bad...it was just a tiny bit of oil...not sure if that would have any effect on the performance though...
 

ROH ECHT

K04 PLAY
Location
PDX OR
Car(s)
2007 MKV GTI
I don’t know but I have a strong feeling it’s going to be the n75, I was reading different posts on other forums and people describing the exact same symptoms mine is having (2-3 psi loss and not reaching peak boost until really late in the rpms) they were able to fix it by replacing the n75....so I guess I’ll give it a shot and see what happens. Valve is only $19 at FCP euro so I think it’s worth the try. At least so I could rule that out.
Also since I’ll have to take the turbo out anyways I might replace the gasket that goes onto the manifold, today while I was inspecting it I noticed there was a little bit of oil accumulated at the bottom/back of turbo, so I’m guessing gasket must be bad...it was just a tiny bit of oil...not sure if that would have any effect on the performance though...
It could be the N75. I may have just dismissed it or missed it all together when mine was just beginning to fail. It seemed all of a sudden when I noticed boost was down and how far it dropped. I just considered the WG preload variances would have an effect as well and skew the symptoms from one to another.
 

Whitedemon

Passed Driver's Ed
Location
Miami, FL
Ok so bad news I guess... I was able to get my hands on a brand new n75 and tested the resistance and got 23.9 ohms as well...so now I’m thinking it’s my multimeter that’s not reading it right? Maybe the n75 wasn’t bad after all...

just find it odd that both would give me the exact same measurement....by the way this is an almost brand new multimeter
Not sure if I should replace the n75 anymore..i think I will try to do the leak tests first
 
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