Boost issues
07-10-2018, 07:09 PM
#1
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Boost issues
I have been experiencing boost issues on my 01 Turbo (manual) lately. I first had the occasional overboost issue with the car going in limp mode (max 0.4 bar - no CEL) as a consequence.
Went to my OPC and they changed both DV valves (one shot) and a vacuum hose as there was a small leak as well.
Got the car back and I have been driving 300 miles, and today i experienced overboost again (although no limp mode).
On today's drive I noticed 1.4 bar for a second, and several times 1.0 bar. Did a 3rd gear pull two times before parking and then I saw 0.8 bar (as I normally do) both times.
According to the OPC the actuators are both fine (they checked when they changed the DV valves). I am thinking about the N75 valve - but my understanding is that this will cause a lack of boost (not overboost).
Any suggestions where to start looking into this issue ?
Went to my OPC and they changed both DV valves (one shot) and a vacuum hose as there was a small leak as well.
Got the car back and I have been driving 300 miles, and today i experienced overboost again (although no limp mode).
On today's drive I noticed 1.4 bar for a second, and several times 1.0 bar. Did a 3rd gear pull two times before parking and then I saw 0.8 bar (as I normally do) both times.
According to the OPC the actuators are both fine (they checked when they changed the DV valves). I am thinking about the N75 valve - but my understanding is that this will cause a lack of boost (not overboost).
Any suggestions where to start looking into this issue ?
07-10-2018, 07:47 PM
#2
N75 valve is probably the problem. I had the same issue boosting to 1.4 bar and then the car would pull timing for a while and be down on power to protect itself. They are cheap, get a new one. https://www.suncoastparts.com/product/SKUN75.html
07-10-2018, 08:01 PM
#3
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Funny, we were having a similar chat in another thread...
I was under the impression, from the other thread, that a buggered N75 valve would do the opposite, ie, not provide any boost and revert to the mechanical springs on the WGs to regulate boost...
I was under the impression, from the other thread, that a buggered N75 valve would do the opposite, ie, not provide any boost and revert to the mechanical springs on the WGs to regulate boost...
07-10-2018, 08:03 PM
#5
Here you go. There are other threads about this. https://www.6speedonline.com/forums/...rging-fix.html
By the way my mechanic managed to swap it in about 2 minutes, without removing anything else in the engine bay.
By the way my mechanic managed to swap it in about 2 minutes, without removing anything else in the engine bay.
07-10-2018, 08:08 PM
#6
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From the other post:As I understand this comment from pfbz, a faulty N75 results in 100% vacuum, not boost, or I could have this completely wrong...?
Quote:
Originally Posted by pfbz 
If the wastegates were just connected to the intake manifold (actually Y pipe) post turbos, boost pressure would be controlled solely by the mechanical wastegates, opening when the boost pressure starts to overcome the mechanical spring pressure in the wastegates...
The N75 valve (aka frequency valve), controlled by the ECU, "switches" the line going to both wastegates between post-turbo manifold pressure and pre-turbo intake vacuum and allows the wastegates to hold higher boost pressure than a simple wastegate connected to post-turbo pressure. The higher the 'frequency" of switching to vacuum instead of boost, the higher the pressure the turbos will produce.
I think higher pressure 1 bar wastegates begin to be useful when you really start to crank up the boost, definitely past the 1.1 bar you are seeing. Standard Turbo (non X-50) wastegates start to crack at about 6 psi but they still have no problem holding 1.1 bar (16 psi) with the N75 valve controlling them on a tuned ECU.
If the wastegates were connected to vacuum 100% of the time (say through a faulty N75 or intentionally mis-plumbed), they would still eventually be forced open just by the pressure on the turbo side of the wastegate, and i think this is one reason stiffer springs (higher psi wastegates) are sometimes used on highly tuned cars.
If the wastegates were just connected to the intake manifold (actually Y pipe) post turbos, boost pressure would be controlled solely by the mechanical wastegates, opening when the boost pressure starts to overcome the mechanical spring pressure in the wastegates...The N75 valve (aka frequency valve), controlled by the ECU, "switches" the line going to both wastegates between post-turbo manifold pressure and pre-turbo intake vacuum and allows the wastegates to hold higher boost pressure than a simple wastegate connected to post-turbo pressure. The higher the 'frequency" of switching to vacuum instead of boost, the higher the pressure the turbos will produce.
I think higher pressure 1 bar wastegates begin to be useful when you really start to crank up the boost, definitely past the 1.1 bar you are seeing. Standard Turbo (non X-50) wastegates start to crack at about 6 psi but they still have no problem holding 1.1 bar (16 psi) with the N75 valve controlling them on a tuned ECU.
If the wastegates were connected to vacuum 100% of the time (say through a faulty N75 or intentionally mis-plumbed), they would still eventually be forced open just by the pressure on the turbo side of the wastegate, and i think this is one reason stiffer springs (higher psi wastegates) are sometimes used on highly tuned cars.
07-10-2018, 08:11 PM
#7
As I said...I had the same issue and checked all else before swapping a new N75 valve which fixed the over boosting to 1.4 bar.
Edit: Could also be a split tube connecting the N75 valve.
Here's a useful link for other common 996TT issues. https://www.renntech.org/forums/tuto...ential-issues/
Edit: Could also be a split tube connecting the N75 valve.
Here's a useful link for other common 996TT issues. https://www.renntech.org/forums/tuto...ential-issues/
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07-11-2018, 02:40 AM
#8
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I was a bit puzzled how a "bad" N75 valve might cause overboost...
.
Looking at the thread linked above, it seems the answer is much simpler... The N75 "failure" in the overboost situation had nothing to do with the switching solenoid, it was simply a crack in the connectors that allowed pressure to bleed out, no different than if you somehow developed a hole or crack in the tubing line running down to the turbo itself.
Yes, technically a failure with the N75 valve, but nothing to do with the switching operation, simply a crack on the hose nipple that bled some pressure out before it got to the wastegates!
.
- When plumbed properly and without power applied (or if unplugged, or as sometimes happens with a complete failure of the valve), it always feeds the boosted air pressure to the wastegates which would produce an *underboost* situation as the wastegates would open at the wastegate spring pressure without the valve tweaking higher boost pressure.
- With power applied, it switches to non-boosted pressure, keeping the wastegates closed longer.
Looking at the thread linked above, it seems the answer is much simpler... The N75 "failure" in the overboost situation had nothing to do with the switching solenoid, it was simply a crack in the connectors that allowed pressure to bleed out, no different than if you somehow developed a hole or crack in the tubing line running down to the turbo itself.
Yes, technically a failure with the N75 valve, but nothing to do with the switching operation, simply a crack on the hose nipple that bled some pressure out before it got to the wastegates!
Last edited by pfbz; 07-11-2018 at 02:56 AM.
07-11-2018, 03:03 PM
#11
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07-11-2018, 03:13 PM
#13
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I don't have children, and, I don't share my cigarettes...
07-11-2018, 03:18 PM
#14
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#2 in the Urban Dictionary for OPC is "Off Premises Contact". That might be stretched into fitting in with what he was talking about, but I'm thinking that's not what he meant.