What can this be?

[brokeasseuro]

I have searched, but to no avail.

Recently I have installed a forge boost bleeder valve in line with my stock n75. The first day I had the setup it was fine, but I was having boost spikes so I installed a 4.7v diode which helped for a bit. Now before you tell me to go get a tune, I know, I'm waiting for h2o. Now I have had this setup for about a week, now it seems as though something has happened and I don't have a clue as to what it is.

What it's doing is, I will be able to boost to about 17psi WOT, but partial throttle with anything over 5psi it surges like a b*tch. It only seems to like WOT. But alot of the time when I shift into the next gear, I get a very loud backfire which was not there before. It's almost as if it's dumping raw fuel perhaps? Also, while in partial throttle, the boost seems to flutter almost with what feels like very dull thuds. I'm not sure if that is part of the boost surging or what. I'm lost at this point, and I know alot of people may see these mods as unsafe to the motor, but I have other mods that should help.

Any help would be appreciated. Thanks

[Weehe]

Check all the lines from the wastegate to the MBC and N75. Since that is the last thing you altered it is probably there. Did you replace it with all new lines? If not it's probably a good idea to.

[brokeasseuro]

I'll check them again but everything looked okay, would a cranked waste gate have anything to do with it?

[brokeasseuro]

bump, now it seems to be doing it on WOT and while its under any sort of boost. any advice is helpful!

[Scigano]

When you say you installed the boost bleeder valve in-line with the N75, do you mean you installed it in between the waste gate and the N75 — increasing the pressure necessary for triggering the N75 to open for more peak boost?

To the best of my understanding, the N75 valve is effectively changing your waste gate setting constantly. The waste gate itself is set to 5 psi or thereabouts (I think it's that because that's what limp mode is and limp mode is likely achieved by the ECU not requesting the N75 to keep pressure from reaching the waste gate, leaving it directly open).

When you need full throttle boost, the ECU will not open the N75 until pressure reaches what the ECU was told waste gate pressure is + whatever it has to divert through the N75 until the total requested boost is reached (then it sends air to the waste gate to open it).

If you have the boost controller before the N75 valve, even more boost pressure than what the N75 is actually seeing is being pumped out of the turbo (correct me if I'm wrong here anyone, really) — so more air is being sucked in too, right?

All your mods are designed to try tricking the ECU a bit into pushing the motor a bit more (n' make more power). The adaptive ability of the ECU allows for this some, but that ability is marginal in comparison to a real re-writing of parameters in the ECU's software like you get in a tune.

I'd imagine that when you exceed the ability of the ECU to adapt to the trickery, it does its best to keep the engine both running and from blowing up.

This engine and ECU loves the N75. While many people here curse it when it goes bad and begins letting boost go all jittery, a properly functioning N75 with a well written tune actuating it properly is the best way to get smooth performance from the spike-happy, quick-spooling K-series small frame turbos.

When I was struggling with boost surging and even worse issues (which all turned out to be crappy tune which I changed for Unitronic), I tried cutting the N75 out completely in favor of a boost controller. This meant my right foot had to be the "N75", at part throttle — not requesting too much boost at part throttle.

If I did at the wrong time, the engine would bog down, performance was poor, and basically, since the ECU wasn't reading boost through the N75 as it was meant to be, it tried to "guess" how much fuel to add to go with the air that it wasn't entirely sure it was getting.

Yes — air is measured through the MAP, but unless you've gotten big turbo, N75 and MAF eliminating software, the ECU relies on all those sensors for the best idea of fuel and timing to add with the air it thinks it's ingesting.

I'd get boost dancing up and down (when I was being careful) and the engine cutting out completely because it thought it was getting way to lean for its own safety (when I wasn't).

I don't think I'm Lord of the VAG (not even in the same galaxy), but I have had boost tuning woes — n' I've had some of this when I kept trying to fool the ECU a bit instead of fixing the problem itself: the tune wasn't right for what I was trying to get the car to do — be it poorly written software or trying to do chip-like performance on stock-ECU parameters.

Do check your boost lines to make sure you haven't got a leak — a leak of any sort can definitely ruin your day something fierce. If you find all the hoses you tapped to fit the bleed valve to be tight, I'd have to guess your car doesn't like trying to sneak some extra boost past the N75 terribly and 'that dog won't hunt'.

You can leave the diode in, if installed properly, that shouldn't mess anything up on its own — but you may want to try taking the controller out and leaving the N75 to its own devices. See how it runs like that and if boost is again controlled smoothly and progressively. It should be able to regulate boost for any load (throttle application) and RPM situation in any gear you find yourself in.

If you have a boost gauge (very advisable if your trying to adjust boost levels), at part throttle — boost should spike a bit and then settle quickly at the optimum level of pressure for smooth power at that RPM and load.

Think of boost as progressive, if you're bleeding some pressure before it gets to the N75, or you've cranked the waste gate, peak boost will be higher, but boost at part throttle will be higher than it was before. The N75 diverts pressure from the waste gate until it thinks it has achieved the necessary level of pressure to add to the the stock waste gate pressure in order to get the requested boost the ECU is asking to see. If any of those are changed without the ECU being told to expect that, it can only adapt so much for those new conditions before things are just too much out of wack for it to work properly.

All the N75 is doing is trying to add in a Y after it was told X to reach a requested boost level. Throw in another variable that wasn't there before (an additional pressure diversion like the controller) or change an existing variable (like a cranked gate) and it can piss off the computer (and I've been told no two 1.8Ts are exactly alike, n' some handle jury-rigging better than others).

[brokeasseuro]

Thank you for the lengthy response. There was alot of good info in there and it should help me better diagnose the issues. I will start with eliminating some variables and see where I stand