Another Stage 3 Boost Issues Thread

[skree25]

Trying to tie up my Stage 3 build and slow boost issues are keeping me from finishing my tune.

Car is running RS6-RS turbos with OEM hotsides that had CHRA's replaced with PB5 RS6-X setup. Car is boosting very slow, not building full boost until ~6,000 RPM and peaking mid 20's. Fueling is also very rich in the low-mid 10's at WOT. N75 duty cycle at 100%

Everything screams boost leak, but I've done a quality boost leak test half a dozen times now and can't find anything. Also tried charging the IM and replaced all check valves. Bypassed the n249 valve, replaced the DV's. Mityvac tested the wastegates control and charge lines. Tried capping off the wastegate charge line rule out a N75 leak with no change.

I'm at a loss of what to check next. What have I missed that could be causing this?

[FlyboyS4]

What does the boost curve look like with just the wastegate preload?

[vavJETTAw36]

Did you replace your non return valve for the pcv?


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[skree25]

What does the boost curve look like with just the wastegate preload?

You mean with n75 disconnected? I can log and post tonight...


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[skree25]

Did you replace your non return valve for the pcv?


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I deleted the while pcv system with IM capped along with ypipe...

The IM is hard to pressure test, but I’ve tried to be thorough with testing all the individual connections and check valves with no luck finding a leak


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[vavJETTAw36]

Going out on a limb here. Perhaps the throttle plate is controlling boost. What’s that look like in your log?


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[skree25]

Going out on a limb here. Perhaps the throttle plate is controlling boost. What’s that look like in your log?


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That’s a good thought. Looks though like throttle plate angle at 100% throughout...

551M 5120 AEM_20180411_161749.JPG




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[vavJETTAw36]

What is your WG cracking pressure?


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[skree25]

Set at 14psi


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[CELison]

Make sure your wastegate nuts didn’t back off at all. It’s also possible the springs lost a little preload even if the nuts haven’t backed off. My K24 wgs settled in after about 500 miles and needed a little more preload.

[skree25]

Make sure your wastegate nuts didn’t back off at all. It’s also possible the springs lost a little preload even if the nuts haven’t backed off. My K24 wgs settled in after about 500 miles and needed a little more preload.

These wg’s aren’t brand new. Have about 30k miles on them. Still have the factory retainers so I don’t think they’ve backed off and both are still cracking at 14psi measured with the mityvac


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[Corradovolksb]

What does your wastegate duty cycle look like?


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[skree25]

What does the boost curve look like with just the wastegate preload?

Here’s a log with n75 unplugged

551M 5120 AEM_20180417_082234.JPG

Building boost to 12psi by 3,750. Does the fact that the preload should be 14psi but it only reaches 12psi indicate a boost leak somewhere?


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[vavJETTAw36]

What are you using for checking boost leaks? Describe your setup. Just want to make sure no stone is left unturned.


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[CELison]

You should see 14psi. Sounds like a wastegate issue if you don’t have any boost leaks.

[skree25]

What does your wastegate duty cycle look like?


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Here’s what the original logs looks like with the duty cycle.

551M 5120 AEM_20180411_161749.JPG

Is it funny that n75 is working mid pull at 3.5-4K rpm? Shouldn’t it be close to 100% until requested vs actual meet?


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[skree25]

What are you using for checking boost leaks? Describe your setup. Just want to make sure no stone is left unturned.


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I'm charging directly through the Y-Pipe with a regulator and shutoff valve so I can see how fast the discharge is. Charge to ~25psi and holds pressure for 15-20 sec (purge valve on n75 vents and get bubbling in the oil pan). I spray all hoses/connections with soapy water to see if any bubbles show.

I've checked TBB, DV's, lobster claws, inlet pipes, intercooler hoses and welds, and bi-pipes--all hold well. Mityvac tested the control lines and smoke tested the charge lines for the wastegate/n75. Smoke tested the IM and tried to pressure test it best I could. Bypassed the n249 and checked the check valves for brake booster and n80 valve. Capped the IM where the PCV piping ran.

What else have I forgotten or missed?

[CELison]

12psi with the n75 unplugged makes me think a wastegate issue. I’ve lost preload on both sets of my frankenturbos. Late spool and drop in peak boost, even with the gates cranked all the way.

[skree25]

You should see 14psi. Sounds like a wastegate issue if you don’t have any boost leaks.

What could be the issue? Sticking wastegate or set unevenly? If I'm topping out at 12psi, does that just mean the wastegate preload is a little less than what I tested it at?

[skree25]

12psi with the n75 unplugged makes me think a wastegate issue. I’ve lost preload on both sets of my frankenturbos. Late spool and drop in peak boost, even with the gates cranked all the way.

So just tighten them back up to 14psi? I thought 12psi is still plenty of preload for this setup...

[CELison]

What could be the issue? Sticking wastegate or set unevenly? If I'm topping out at 12psi, does that just mean the wastegate preload is a little less than what I tested it at?

The springs weaken and the gates can't hold boost. My FTs would still crack at 10psi like they should, but not matter how tight I cranked them they couldn't produce more than 22 psi.

[skree25]

The springs weaken and the gates can't hold boost. My FTs would still crack at 10psi like they should, but not matter how tight I cranked them they couldn't produce more than 22 psi.

Makes sense. I hope that isn't the case. How were you able to determine that was the issue vs. a boost leak elsewhere?

What could I do to test that? Would a normal pressure test be pushing against the wastegate and I could test that way? Or may I should add an assist spring and see if my spool-up profile changes? Just brainstorming here...

[CELison]

I made sure there were no leaks and my n75 was working fine. My tuner upped the duty cycle and it still wouldn’t produce more boost. Cranked he gates and still nothing. All things pointed to weak gates and that’s what it ended up being.

[skree25]

I made sure there were no leaks and my n75 was working fine. My tuner upped the duty cycle and it still wouldn’t produce more boost. Cranked he gates and still nothing. All things pointed to weak gates and that’s what it ended up being.

How can I test my N75 to rule that out 100%? I guess what I don't understand is that the only boost the wastegates should be feeling is coming from what the N75 is sending its way, right? (I'm not getting any pressure from the exiting exhaust gasses obviously). So if wastegates are cracking at 12psi as preload log above shows, then isn't my boost leak happening because the N75 is sending 12+psi of control pressure to the wastegates (which it shouldn't be doing until requested boost vs. actual boost are close to meeting)?

Are the wastegates getting pushed from boost in another way? If not, doesn't that mean my N75 or tune is to blame (if the wastegates are where I'm leaking)?

[Corradovolksb]

Your wgdc goes from 95% and drops to 85% that is a little strange since your actual is not close to your requested so your boost control should still be requesting 95 until requested is closer to actual. If tighten your WG like everyone is saying and see how it does. It’s looking like your at the max boost your WG are capable of sustaining. My k24s with 15psi cracking pressure will hit 23psi by 4300. Who is the tuner? What do they say about it?


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[skree25]

Your wgdc goes from 95% and drops to 85% that is a little strange since your actual is not close to your requested so your boost control should still be requesting 95 until requested is closer to actual. If tighten your WG like everyone is saying and see how it does. It’s looking like your at the max boost your WG are capable of sustaining. My k24s with 15psi cracking pressure will hit 23psi by 4300. Who is the tuner? What do they say about it?


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Someone needs to explain to me how wastegate actuation works. Why does it matter what cracking pressure is set to when the N75 should be controlling what psi is getting sent to the wg's? Whether the cracking pressure is set at 5psi or 15psi, the wastegates shouldn't be opening until the N75 decides to send pressure to the wastegates, right? Unless the N75 is incapable of not letting air by (e.g., if boost pressure in charge line is 23psi and N75 isn't able to keep control pressure below 12psi, then wastegates would start opening even if N75 is still at 95%+ duty cycle), then I could see benefit in tightening wastegates further.

In theory, I should be able to test if the leak is wastegates by capping off the charge line and eliminating any possibility of wastegates opening and see if spool up improves (obviously being careful not to overboost)

[Corradovolksb]

When the n75 is at 95% duty cycle that means no boost pressure is getting to the WGs and they will stay closed to build boost. If your cracking pressure is set to 12psi it will build 12psi of boost and continue to build boost but at the same time the exhaust pressure will start opening the wg flap a little due to exhaust back pressure so if you increase the wg spring tension to raise the cracking pressure to 14-15 the wg flap will stay closed longer in turn allowing the turbo to spool a little quicker and build more boost over all. That’s the fast answer to your question without getting too technical. The boost pid is what the ecu uses to control boost via the n75 duty cycle and if the boost pid is not correctly setup in the tune that will affect how boost builds and how it’s controlled.


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[vavJETTAw36]

Makes sense. I hope that isn't the case. How were you able to determine that was the issue vs. a boost leak elsewhere?

What could I do to test that? Would a normal pressure test be pushing against the wastegate and I could test that way? Or may I should add an assist spring and see if my spool-up profile changes? Just brainstorming here...

If you find that it is indeed a weakened actuator, I wouldnt worry too much about it. These can be swapped engine in. Still painful, but not "pull motor" painful.

[skree25]

Wow I guess I didn’t think that exhaust backpressure was high enough to crack wastegates...


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[skree25]

So I capped the charge line to eliminate any possibility of actuator opening the wastegates and things didn’t change much

551M 5120 AEM_20180417_125434.JPG

Peak boost is higher which makes sense because the n75 was controlling that in previous logs. Spool up is pretty much the same. Would this suggest exhaust backpressure prob not the culprit since I’m losing performance in the lower rpm range?

I would think this tells me boost leak elsewhere besides wastegates...I just can’t figure out where else it could be coming from that I haven’t checked


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[CELison]

You don’t have a boost leak. You have failing wastegates so it seems by the info you provided. If you had a boost leak big enough to cause these issues you would probably hear it while getting in boost and for sure wouldn't miss it in a pressure test. If you don't catch it during the test, it's not there. I've seen this happen many times with Chinese wastegates. They weaken causing slow spool and loss of peak boost. Those are your symptoms and you've ruled out pretty much everything besides the wastegates.

[FlyboyS4]

Are the wastegate actuators the ones that came with the RS6-RS or PB5 RS6-X, or something else?

[skree25]

You don’t have a boost leak. You have failing wastegates so it seems by the info you provided. If you had a boost leak big enough to cause these issues you would probably hear it while getting in boost and for sure wouldn't miss it in a pressure test. If you don't catch it during the test, it's not there. I've seen this happen many times with Chinese wastegates. They weaken causing slow spool and loss of peak boost. Those are your symptoms and you've ruled out pretty much everything besides the wastegates.

So you’re saying it’s backpressure from the exhaust that’s pushing the gates open?

If the spool up profile is the same whether or not the wg lines are connected then exhaust backpressure is the only other force that could cause wg’s to open...if I’m understanding you correctly


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[CELison]

Exhaust gases are blowing the flapper open, yes.

[skree25]

Are the wastegate actuators the ones that came with the RS6-RS or PB5 RS6-X, or something else?

Actuators are the ones from JHM's RS6-RS. Were originally set at ~5psi, but tightened them to 14psi when turbos were rebuilt.

However the fact that my spool up profile is still very lazy even with the wastegate charge line capped I think would mean that the wastegates are not the culprit. (Unless as CELison suggests exhaust backpressure is pushing them open, but I feel like if exhaust backpressure was that high, I'd have even bigger problems)

Tuner thinks it's still a big boost leak in the piping, DV's, or check valves up top. But I just bought new DV's and put the stiffest springs in, checked the check valves, and done half a dozen pressure tests...

One place I haven't been able to really test is the wastegate charge line to the turbo. It's the funky hardline with banjo bolt and I can't find a good way to get access well enough to verify. Certainly don't hear anything though...and to be leaking that much I think I'd be able to tell.

[Dr Awesome]

Is it possible your wastegate bushing is loose? It connects the wastegate flapper to the waste gate actuator arm. If the bushing is loose, the wastegate can slide open inside the turbo and slow spool up and lower total boost ceiling. I've heard tack welding the bushing for the wastegate flapper can prevent something like this from happening.

[skree25]

Is it possible your wastegate bushing is loose? It connects the wastegate flapper to the waste gate actuator arm. If the bushing is loose, the wastegate can slide open inside the turbo and slow spool up and lower total boost ceiling. I've heard tack welding the bushing for the wastegate flapper can prevent something like this from happening.

I checked them when I put them in 500 miles ago. Hot sides and flapper arms are genuine BW and they looked good when installed. Only way to check that would be to remove DPs correct and physically check, right?


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[CELison]

It’s not backpressure, it’s just normal exhaust flow. The flapper only opens one way, which is with the flow of exhaust. A weak gate will blow the flapper open sooner.

[FlyboyS4]

What response from the wastegate are you using (initial arm movement, movement a particular amount) as the indication that you've set the preload to 14 psi?

The curve being below 14 psi doesn't seem unusual to me, you set the preload to 14 without any pressure acting on the wg door, with pressure acting on it from the exhaust a slightly lower curve seems right to me.

The drop in boost pressure with the wg preload has me thinking the wg door is opening with increasing engine speed. As others have said, it seems likely the wastegate isn't functioning adequately.

[Dr Awesome]

I checked them when I put them in 500 miles ago. Hot sides and flapper arms are genuine BW and they looked good when installed. Only way to check that would be to remove DPs correct and physically check, right?


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Disclosure: I have not done this but am diagnosing a similar wastegate issue.

I think removing the downpipes and visually inspecting is the only way, but I have yet to do so myself. I want to replace my wastegate actuator because I think that is my culprit. In my case, symptoms are similar to yours (low ceiling and slow spool) but my boost leak test found a leak in the diaphragm of my driverside wastegate actuator. If you can get the wastegate actuator arm off of the flapper, you could try to move the bushing in and out of the turbo to check for play. In fact, I bet you could also tell just by trying to wiggling the arm side to side (a good one shouldn't move), but I think a visual by removing the downpipes would leave all of the guesswork out of it.

It’s not backpressure, it’s just normal exhaust flow. The flapper only opens one way, which is with the flow of exhaust. A weak gate will blow the flapper open sooner.

Hi CEL,
I'm trying to learn as well and was wondering if you could help me understand. Does this mean that his wastegate flapper/bushing (the component in the turbo hotside) is weak, or his wastegate actuator that pushes the arm is too weak?