Strange Boost Issues

[kdubGTI]

Well where do I start. First of all, got a stage 1 code from Gavin Custom Code a few months ago. Car pulls well enough when the boost works.

So this morning (months after the ECU flash) I quickly installed a boost gauge. Started the car from cold, warmed it up to find that it boosts to 17psi dropping off to 15psi as it gets closer to the redline (stock turbo running out of puff for the engine me thinks).

Now the inherent problem I have had since day 1 I think.

If I drive the car for a period of time say greater than 10 minutes and park up for a while, say again greater than 10 mins. The car has no power after this. So today with the boost gauge I confirmed that this is the case with a lowly 4psi of boost (limp home mode equivalent).

Now when I first got the car there was a fault code on it relating to the N75 and the garage said that it was boosting to 9psi after resetting it (stock tune).

Right, so potentially faulty N75.

From there I disconnected the top hose (take the N75 out of the equation) and boosted to 17psi. I did quite a few 3rd and 4th gears runs getting braver to see where the boost would build to and never ever went over 17psi. Even with pretty hard presses on the accelerator for 5 seconds or more.



So first things first, would I be right in saying that the N75 is sticking or possibly going out of resistance spec when heat soaked with the car stopped.

Secondly, I think I must have something in the boost circuit limiting boost pressure as it never spiked above 17psi with the N75 disconnected.



Now, to take the N75 (delete) out of the circuit is it the right thing to just install a manual boost controller and set the relief to say 18psi for example and the ECU tune / remap will take care of everything else as long as I keep the N75 connected to the wiring harness.

[Bora Sport]

I remember with mine one of the two wires going to the N75 broke and obviously it went into limp mode as you suggest. However when it first started it would do it intermittently until eventually it broke completely. I surmised that it must touched enough sometimes and then other times it wouldn't. When I googled the fault code I found that it was apparently a fairly common problem. The wires are very thin. Just putting it out there as a possible reason. We simply resoldered the wire and it has been all good since.

[h100vw]

I'll read through this tomorrow. It can be fixed. No boost controllers.....

[h100vw]

It will be far better to fix the N75 issue. VW spent a small fortune developing the ECU with Bosch, it would be a big shame to bypass their efforts.

When the N75 fails, the turbo will run at actuator pressure. 9PSI is a bit high for that and 4 seems really low. Although I would prefer to see what the ECU is seeing as requested and actual boost. Using VCDS

When the N75 fails electrically, it will run the turbo at actuator pressure.

If something else was wrong the symptoms would be different I feel.

If you're handy with a multimeter, there's a few things I can talk you through to investigate.

You have my number?

[kdubGTI]

Hi Gav,

It's really weird. When starting from cold there is no issues. Only when the engine has been stopped for 10 mins plus and had a chance to heat soak then it becomes an issue.

If you don't stop the car its fine for as long as you drive it. Just drove an hour and no issues. Checking boost all the way home and peaking at 17psi and dropping off to 15psi.

Just about to eat, if I went out in 10 mins or so, 4 psi would be all I would get. Leave it for an hour and back to full boost. Has me stumped hence why I was thinking a sticky waste gate or N75.

Unfortunately no VCDS available without paying 150 bucks WA rate for a quick scan. Wish I could find a reasonably cheap hand held diagnostic tool without going down the Ross tech route as I have no windows based PC's/laptops in the house anymore. We are all Mac / ipads now.

[h100vw]

Mac, pffft. �� Anything with XP would be OK. Cheap eBay cable and a proper license for 99usd.

[seangti]

The intermittent fault you've described sounds exactly how my n75 died. Take off at lights with boost, next set of lights, nothing. Can you borrow an n75 from another member for the day to validate?

You mentioned you had disconnected "pipes" to check. You should just pull the electronic connector from the n75, leave all pipes in place. That'll give somewhere around 4-7psi. If you pull the vacuum line that runs onto the wastegate, that'll boost ~25psi. I wouldn't recommend this later approach.

[sambb]

in the mean time plumb everything back up, reseat the N75 plug with some CRC to clean the terminals and while you're at it check that the N75 plug wires look ok ( no cracks, creases or breaks). It does sound like an N75 issue. When the N75 open circuits the peak boost will drop to actuator pressure. That you are getting only 4psi at that stage sounds like you may possibly have a turd actuator ie rod tension too slack and holding the flapper open?

[kdubGTI]

The intermittent fault you've described sounds exactly how my n75 died. Take off at lights with boost, next set of lights, nothing. Can you borrow an n75 from another member for the day to validate?

You mentioned you had disconnected "pipes" to check. You should just pull the electronic connector from the n75, leave all pipes in place. That'll give somewhere around 4-7psi. If you pull the vacuum line that runs onto the wastegate, that'll boost ~25psi. I wouldn't recommend this later approach.

With the vacuum pipe unplugged (the top one) it only reaches 17psi with careful application of the throttle. Even with a stab when sitting at 14psi for example peak is always 17psi. Which makes me think I either have a leak at that pressure or the waste gate isn't holding beyond this point.

Maybe Gav, you can help here mate. With the CC stage 1. What should it spike to an then settle at. I was thinking 20-22psi spike and 17-19psi steady tapering off as the engine eats up the boost further up in the rev range.

[kdubGTI]

in the mean time plumb everything back up, reseat the N75 plug with some CRC to clean the terminals and while you're at it check that the N75 plug wires look ok ( no cracks, creases or breaks). It does sound like an N75 issue. When the N75 open circuits the peak boost will drop to actuator pressure. That you are getting only 4psi at that stage sounds like you may possibly have a turd actuator ie rod tension too slack and holding the flapper open?

Will have a check at around the plug tonight. It's a strange one. If I can't find a break or bad connection I think a new N75 wouldn't be a bad thing. Even if it turns out not to be a problem, at least it will be good for years to come once replaced.


I have a feeling too that my waste gate is not quite right. Seems to be in my eyes missing 2-3psi when the N75 isn't working right once heat soaked but only after switching the car off. Will not spike above 4psi even in cold dense air like this morning (raining and 17.5C).

[h100vw]

If the N75 is failed electrically, it won't spike at all.

VCDS will get to the bottom of this quickly. Please don't buy any parts, just because. It can be a few things.

An output test of the N75 hot and cold, using VCDS will help. Check it cold. Then when it fails, check again without cycling the ignition.

Gavin

[kdubGTI]

It doesn't fail unless you switch off the ignition and leave it for 10 mins.

Is VCDS the only way of monitoring the fault codes/engine etc. Would much prefer to have a non laptop hand held device. Refuse to have a windows machine in the house and at 150-180 per shot at all the so called specialists here in Perth is not happening either. Been to a couple of them before xmas to be told fault on the N75, its reset and all good. For it to return as soon as you switch the car off.

Let it cool down and it disappears.

Which to me pushes me in the direction of a faulty coil being subjected to heat (windings breaking down) or something sticking in a position.

Is the N75 failed open / energised closed. Because the minute it goes to 4psi if I disconnect the vacuum hose I get exactly 17psi which is identical to normal operation when everything seems to work fine.

[sambb]

if the N75 or its wiring/plug connections fail electrically it would cause the valve to be in its open position. Its designed this way as a fail safe so that all the available boost pressure in the boosted side of the inlet pushes through the valve straight to the actuator = a max boost of 6-7psi which is actuator spring pressure. Now if the N75 was mechanically sticking you could get boost anywhere from massive spikes if it stuck closed, to 6-7psi if it stuck open That you saw 4psi tells me maybe your wastegate flapper is loose as that's the only way I can think of that would give you boost below spring pressure - but then you said it can come good and boost again which sort of flies in the face of that. Whatever fault you have could be openeing the N75 but also throttle limiting you too which could explain the very low boost. So you see it could really be anything so like Gav said you really need to scan it. if your service guys are turd and charging big time for it, then try to find a VAG enthusiast near you that can do a log and fault scan and post the results here. Don't stress, you will get it sorted but it kind of starts with some data.

[sambb]

just to rule it out look up VAG 1.8T diverter valve testing on youtube and do the same on yours to check its ok. Also check/spray the plug connection on the other solenoid on the cam cover that has pipes running from it to the vacuum canister (black).

[kdubGTI]

Ordered diagnostic gear today. Hopefully be here in the coming weeks and the saga can continue lol.