Low boost diagnosis help

[Mad Cow]

I've got what I thought was a diverter valve leak, replaced it with a new 710P and it's still there but not as bad. The old one was leaking through the diaphragm badly, the new one tests fine with the finger over vacuum port method and holds pressure no problem with the vacuum port pressurized. Under boost I can hear a pretty loud whooshing sound, but a boost leak test didn't expose anything. I'm supposed to be peaking at 17ish psi but I'm only getting 7 at most, with the N75 unplugged electrically I get 2.5-3ish, pulling the wastegate vacuum line completely gave just over 7 psi. I did some logging and it is requesting full boost, WGDC is nearly 100% on spool. The weird thing is AFRs are pretty solid, it goes a bit richer than requested at some points but not the amount I'd expect from a boost leak big enough to more than halve the pressure. I did find a few tears in the line that goes from the N75 to the nipple on the turbo, and pressurizing it did reveal some pretty serious leaks but I temporarily replaced it with a similar size vacuum line and it didn't help.

I'm stumped. I wanna say it's the wastegate not closing properly, but that wouldn't explain the whooshing noise. The only thing I can think of is it all started the day I got a resonator installed but I can't see how that would affect anything. Is there anything I could've overlooked?

[ECS Tuning-Audi]

Have you done a complete boost leak test? Seems you checked some problematic areas but didn't completely pressure test your whole intake plumbing.

I would do this first and if it checks out OK move on to other areas.

For all you know you could have a hole in your intercooler because a rock flew in the ducting and cracked the plastic end tank which is where most of your boost is escaping.

Good luck!

Jason

[Mad Cow]

I tested up to 12ish psi through the TIP with the PRV port plugged up, only got a little air going through the breather pipe. I can hear the whooshing as soon as boost starts building, it'll even do it a bit before my boost gauge registers positive pressure.

[ECS Tuning-Audi]

I tested up to 12ish psi through the TIP with the PRV port plugged up, only got a little air going through the breather pipe. I can hear the whooshing as soon as boost starts building, it'll even do it a bit before my boost gauge registers positive pressure.

Typically you want to pressurize the intake plumbing to a level which your vehicle sees regularly. I would pressurize it to at least 17psi FYI.

If you can hear noise at 12psi you obviously have a leak somewhere. You should be able to pressurize your system to your typical boost level and it should hold. If it cannot sustain pressure start searching around the area you hear the leak. You can also smoke test it help give you an exact location.

Jason

[Mad Cow]

The thing is it doesn't even hit 5 psi with the n75 unplugged electrically, if I can pressurize it to 12 psi and only get a tiny amount of leakage I'd say the problem is elsewhere.

[ECS Tuning-Audi]

The thing is it doesn't even hit 5 psi with the n75 unplugged electrically, if I can pressurize it to 12 psi and only get a tiny amount of leakage I'd say the problem is elsewhere.

You shouldn't have any amount of leakage. Your intake plumbing should be sealed and be able to hold pressure without it bleeding off quickly. A few psi drop over an extended period of time like say 30min to an hour is one thing, but you shouldn't be having to keep re-pressurize it while testing. If you are there's a significant leak you need to track down.

Jason

[Wrath And Tears]

The thing is it doesn't even hit 5 psi with the n75 unplugged electrically, if I can pressurize it to 12 psi and only get a tiny amount of leakage I'd say the problem is elsewhere.

When you unplug the n75 valve you are making the wastegate open at its spring rate which is about 5 PSI. The fact you aren't getting 5 PSI with the n75 valve unplugged means you have a boost leak, and that the n75 valve, and the wastegate are working properly.

How is your vacuum at idle? should be between 19 - 24 inHG. If not, or if you have a high / wondering idle then you have a vacuum leak which could turn into a boost leak when the car builds boost.

Do a full Boost test / smoke test / check all of the check valves for proper orientation and function.

By AFR's being correct are you just looking at the wideband sensor from your ATW swap? Are your short and long term fuel trims between -10 and 10%?

If you are logging requested boost I assume you can see actual boost through VAGCOM correct? When you installed your intercooler you got one with a MAP sensor for the ATW ECU/Wiring Harness? Sorry to ask a silly question but I want to rule out dumb stuff.

Further on the path of silly stuff, have you check all your boost gauge fittings and hoses and stuff. When I first installed my boost gauge I hadn't properly tightened the fitting into the gauge and I could hear the air rushing out pretty loudly.

[Mad Cow]

When you unplug the n75 valve you are making the wastegate open at its spring rate which is about 5 PSI. The fact you aren't getting 5 PSI with the n75 valve unplugged means you have a boost leak, and that the n75 valve, and the wastegate are working properly.

How is your vacuum at idle? should be between 19 - 24 inHG. If not, or if you have a high / wondering idle then you have a vacuum leak which could turn into a boost leak when the car builds boost.

Do a full Boost test / smoke test / check all of the check valves for proper orientation and function.

By AFR's being correct are you just looking at the wideband sensor from your ATW swap? Are your short and long term fuel trims between -10 and 10%?

If you are logging requested boost I assume you can see actual boost through VAGCOM correct? When you installed your intercooler you got one with a MAP sensor for the ATW ECU/Wiring Harness? Sorry to ask a silly question but I want to rule out dumb stuff.

Further on the path of silly stuff, have you check all your boost gauge fittings and hoses and stuff. When I first installed my boost gauge I hadn't properly tightened the fitting into the gauge and I could hear the air rushing out pretty loudly.

I've spent a lot of time chasing vacuum leaks earlier this year, everything on the intake manifold side of the motor is solid, lots of new check valves, hoses, fittings, not many original parts left. Everything's been fine for the 2 years I've been driving since the swap, it's had trouble holding boost for the past couple of months but that was from the fubared DV which I replaced. I'm logging with ME7Logger, so I have access to basically all variables, forgot to check fuel trims though. Here's requested vs. actual AFR and boost:


Like I said earlier, I pressurized up to 12 psi and the only air moving was through the metal breather tube that goes around the VC, if I plugged that the crankcase would pressurize and i'd hear air coming out of the dipstick. That's normal though, right? I'll try more pressure tomorrow, but I don't think it'll be anything conclusive.

[MetalMan]

Like I said earlier, I pressurized up to 12 psi and the only air moving was through the metal breather tube that goes around the VC, if I plugged that the crankcase would pressurize and i'd hear air coming out of the dipstick. That's normal though, right? I'll try more pressure tomorrow, but I don't think it'll be anything conclusive.

There shouldn't be a bunch of air coming out through the metal breather tube to the PRV valve, but some is normal. Any air that comes out from there has 2 sources: turbo oil feed/drain lines and PCV valve.

Is it a safe guess that you replaced the PCV valve as part of your under-intake-manifold parts replacement?

I've been wondering other possible causes, but hadn't yet posted anything. It seems that you'd need a fairly substantial leak to lose 2 psi with the N75 unplugged (5psi wastegate pressure), and I think the turbo should have no problem making up for that 2 psi since it basically acts like a fixed MBC with N75 unplugged.

Therefore it's either a HUGE boost leak, DV problem, or wastegate problem (IMO).
Do you have the N249 operational? If so, maybe try the DV hooked directly to vacuum. Wouldn't be the first time a N249 went bad.

[Mad Cow]

PCV valve was replaced with OEM earlier this year, I had an aftermarket one for a while but it leaked. It's not a bunch of air coming out the tube, just enough to be able to feel it with my hand right up against it. When I cover it pressure builds up really fast and within a few seconds I hear air coming out the dipstick tube. Tried bypassing the N249, didn't help. I'm beginning to think WG problem too, but that whooshing sound confuses me. It really does sound like a boost leak, is it possible to be leaking internally to the turbo somehow? Regardless I'm probably going to be taking off my test pipe later today to inspect, that's really the only option I have left.

[Mad Cow]

Tested the wastegate actuator, it begins opening at 3 - 3.5 psi and is fully open at 8.5 - 9ish. I've never dealt with a wastegate before, but from what I understand it needs to begin opening at 5, right?

[MetalMan]

I think the 5psi is called "cracking pressure", so I believe your understanding of it is correct.

When you test for boost leaks, can you pinch the line that goes to PCV valve? At least that can eliminate it as a possible source of pressure in the crankcase during the test.

[Mad Cow]

I'll do that during the next boost leak test. Another question, should the DV spring be able to hold pressure? Even a little bit? The diaphragm is good and it holds pressure with pressure applied to the vacuum line nipple, but it doesn't hold anything with the port open to air.

[MetalMan]

Since nobody else has answered, I can provide some potentially useful information: I think it's supposed to have a bit of spring pressure to keep it closed, but if boost will hold it shut then I think there's no problem.

When the turbo is not in boost, I think the DV may be open due to vacuum holding it open. This is a good thing because the DV would allow intake air to bypass the turbo, where the turbo would otherwise only serve as a restriction to the intake tract. Therefore the DV could offer the path of least resistance for fresh air. This could just be speculation, but that's the conclusion the mechanical engineer in me suggests.

If you're worried about the DV, how about temporarily deleting it? Would only require two 1" plugs, and of course you'd want to keep it smooth coming off throttle to minimize surge due to the throttle body closing.

[Mad Cow]

Adjusted the wastegate for the hell of it, gave it 1.5 turns of preload, from what I've read it's normal that it cracks open at 3.5 psi. That didn't help. Did some boost leak tests again with the SJP hose blocked off, still get a little air through the breather but it's miniscule. I tested through the TIP, the turbo outlet hose, and the intercooler outlet hose, same results. I did notice that the small hose going from the TIP to the evap hardline was leaking, but that doesn't see boost so it can't be that.

I'm beginning to think it has something to do with the resonator I had installed. It's chambered and the same size and shape as a standard resonator, but the pipe inside is tiny, probably just over 1" in diameter. I assumed it would be fine since the guy at the shop recommended it, but now I'm thinking it's creating too much backpressure. Would too much backpressure manifest itself this way? Would it create a hard limit to boost (say 6 psi) or will it lower efficiency as in a 50% reduction in boost pressure?

[Mad Cow]

Turns out it was the resonator. Went for a really loud drive with the exhaust after the flex pipe disconnected and hit 17 psi for a second, then I decided to go home before I piss off more pedestrians. Sounded mean as hell though, so that was a plus I guess.

[MetalMan]

Glad you at least figured out the problem. Time for a Magnaflow or Vibrant (something with better quality) resonator, and maybe a new choice in exhaust shop?

[Mad Cow]

I'm at the shop now, they've never done me wrong and are currently swapping it out for a more expensive freer flowing resonator free of charge. It's big shop with lots of techs, the one that's doing it now instantly saw that the resonator the other tech installed was too restrictive and shouldn't have been installed in the first place.