Hello everyone! I have a '12 MCS N18. It is stage 2 from Mario @ RPM. I have always had some boost fluctuations when I step on it. (Any gear, and even before I had it tuned) I have a video of my boost guage so you can see what I mean. You can hear the engine also increasing/decreasing in power and you can also feel it. I've been running it like this for awhile but I've been putting it to the side as its not a huge deal unless I am pretty deep in the throttle.


With the research I have done on NAM and google, I have come to the conclusion that either the wastegate needs replaced or the wastegate hoses need replaced. Can anyone confirm that this is correct or point me in the right direction? Thanks everyone! Cheers.

 

Ya check to see if any of the soft tubing vacuum lines are soft, if so replace them. If that still doesnt fix it, then check and see if the wastegate is closing all the way with a hand held vacuum pump. It shouldn't take anything more than 15inHg to fully seat it. Can also hook your boost gauge up to the vacuum pump on the car to see if its pulling over 25inHg to check and see if that is bad as well.

 

I think a tear in the diverter valve diaphragm could potentially cause that. The original ones did not seem to have a very long life, there's a new revision that's supposedly more reliable.

 

A torn diverter valve would cause a boost deviation code CEL. He's doesnt have a low boost issue, its just going from 20psi to 15psi, or what ever max boost then falls to Xpsi then back up again rapidly.

 

Also try letting off the throttle completely, like above 3.5k rpms then floor the car and see what the boost does. Then try it again but in boost, so 3.5k rpms then slowly get into boost like 5 psi then floor the car and see how it reacts. When I had issues with boost fluctuation it was a little more stable when flooring it after getting off the throttle in higher rpms.

 

NeonLed, I just read through your threads from a year ago when you were having similar issues. I noticed someone chimed in and said they had issues with the K&N intake, which is actually what I am running as well. That could be a possible cause, because that intake went on VERY shortly after I got the car so I'm not sure if my problems only started after that point. The K&N intake also does move the MAF sensor slightly so maybe something isnt right with it. I will put the stock air box and filter back on and report back if that makes any difference..

 

Mine does the same thing, and mostly when I gun it and get hard on the boost in lower gears - not so much in 5th or 6th gear and I floor the throttle. Not sure what the reason is, but I suspect its a limitation of the control software gains in modulating the waste gate.

I am on a Manic stage 2 with a Forge diverter valve and AEM intake.

 

I am also not opposed to adjusting the wastegate if anyone thinks that might help...It also might help if I give my total mod list.

RPM stage 2
Forge intercooler
Miltek catless DP
2nd cat delete
K&N air intake


NeonLed, It does not help if I am higher in the RPMS and then slowly build boost, it actually is a little worse. I suspect its worse because it builds boost a lot quicker and then cuts it off quicker and then starts oscillating from there at the same rate. On a cold start, my boost guage is in the vacuum state obviously, but it rapidly bounces from maybe 5 to 15 very rapidly, maybe 5 bounces a second. Is this normal, or could this help us diagnose the other boost problems? I will get a video of it on my next cold start and post it

 

Just checking in to see if you ever got this sorted out...similar mods...same issue.

Thanks!

 

Hey Takrdown, I am still dealing with this issue for the most part. I have been dealing with it since the start of the thread and have tried a numerous number of things. 1) New diverter valve (Updated design). 2) Boost control solenoid 3) Removing DP and inspecting wastegate door. 4) Wastegate arm adjustment.

I have only recently started to make progress on the issue. After shortening the wastegate rod a good bit (10 turns or so), it really helped my issue. I wound say with confidence that it did, but I only did this about a week ago, so far so good. Other times, the issue would be fine / better, but shortening the wastegate arm did the trick, so far. Holds 22 psi nice and smooth, as it should (Stage 2).

So, the best advice I can give is to not waste any money on the new diverter valve or a new boost control solenoid, this did not help one bit so save your $$$. Glad to help

 

So my drivetrain mod list is as follows:
-DDM Race Intake
-ebay Wagner Replica Intercooler
-Milltek Catless Downpipe
-No cats to a fJCW exhaust
-RPM Stage 2

So what I have replaced so far with subtle improvements, but no overall fix:
-Turbo replacement with an OE Borg Warner JCW turbo from FCP Euro which included a new DV
-New Cold side charge pipe into the manifold as the one the last guy did was suspect at best
-New boost control solenoid
-New valve cover/PVC system with all new gaskets (this made the most difference, but didn't fix the problem)

I'm picking up a new turbo vacuum line (OEM) from FCP Euro tomorrow (it's nice only being 45 min from them)

At this point I'm kinda lost and think a lot of it is the tune. Hoping the new vacuum lines will do the trick, but I'm not hopeful. I think the next step is to get the intake valves carbon cleaned, but every source I've talked to said these N18s just don't have the issue the N14s do. I know I dealt with a ton of carbon buildup issues with my N54 135, but the N18 was later engineering than that.

I'll let you know if the vacuum lines do anything and I'm all ears for other ideas.

 

I would think the new turbo would have the wastegate adjusted from the factory as it is brand spanking new....hmm...I need to take my mighty vac and check the wastegate.

 

Here’s my update: my car no longer has this boost surge

What fixed it? A new tune from @Lou@Prototype-R Turns out the Manic calibration just wasn’t tuned right. It only took 2 rounds of data logs and calibration adjustments for Lou to make my boost curve rock solid.

For those of you still dealing with boost surging, get in touch with Lou to discuss your options.

 

Just went for another drive, shorting the wastegate rod did the trick so far. I will paste the link to a video that I took of the boost gauge during a 3rd gear pull. Holds boost great as it should. This seems to have done the trick. I would consider getting re-tuned from Lou, but after spending $949 on stage 2 RPM, another tune would probably not be worth it, unless there is some performance advantages

 

So I replaced the turbo vacuum lines (which was a bit of a PITA when you have big hands btw). And it didn’t fix a thing.

I’m a bit frustrated with this tune and the support that comes with it. I’ve been told on multiple occasions that something must be wrong with the car and having a tune only magnifies the problem. I don’t buy it anymore.

While I agree that canned tunes are not as good as custom tunes, there shouldn’t be this much of an issue with this many cars. I had a JB4 tune on my 135…zero problems. I have a BPM tune on my supercharged E90 M3 with zero problems. Is this platform that problematic?

I may have to throw in the towel and contact someone about a custom tune and swallow the $800 this tune cost me, not counting of course the close to $1500 I spent in parts fixing a car that didn’t need fixing.

 

'12 N18 powered MCS here, also with boost oscillation on accel in the mid-RPM range.

After inspecting the WG flapper, adjusting (a few times) and replacing the WG actuator, I'm trying to avoid spending more time/effort/money doing things unless I'm certain they'll work. There's a lot of posts here and elsewhere about messing with various aspects of the wastegate without much luck.

I stumbled across this thread:
https://www.northamericanmotoring.co...eleration.html

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Makes sense to me, so I'm thinking of giving this a shot.
Issue feels like some input in the PID boost control is messed up, causing the output to go underdamped and oscillate. If it was just the wastegate or a line, I feel like it wouldn't be an oscillation, just failure to hit boost target?

 

Just a heads up I had the oscillation issue with the tune and I had just put in a brand new OEM turbo. A bad wastegate want the cause.

 

Well, I scored a spare turbo with a good WG at the junkyard a few weeks back, so just a matter of swapping the turbo or housing if it is indeed bad.

I received a new Hella throttle pedal from Turner Motorsports. Will be at least 6 weeks before I get around to installing it, but I'll post back here with results.

Edit:
Well, here I am, at least 6 weeks later as promised...

I realized this behaviour happens for me even while cruise control is active and the throttle pedal is not used, so I don't think this is my issue.

 

Long time ago I know but did you solve the problems?

 

No, I have had zero luck.

I've adjusted the WG rod 3-4 times to every random recommendation I've found on the internet ('151.5 mm' or whatever that picture shows, which is definitely wrong. Also tried to set it so it has just fully seated by: 15inHg, 7inHg, currently 12inHg based on Lou's old Tigger thread when he tested his stock one. Might mess around with it more, but it doesn't seem to make a huge difference and there's zero official direction in the TIS from BMW/Mini.)
I snapped the N18 rod due to a seized adjuster nut, and eventually swapped a N14 WG actuator.
Fixed the N18 actuator and reinstalled (this is when I tried the current 12inHg setting)
Borescoped the WG flap and found nil play or stickiness
Removed BOV spacer and installed the updated part number DV
Tested the vacuum pump, and vacuum tested the lines/reservoir and haven't found any leak.
Recorded WG actuator vacuum under driving conditions (low tech, tee + gauge + video) and just gut feeling makes me think it's oscillating really slowly, so maybe it's the pressure converter valve slowly dying/not reacting quickly to inputs.

I think the issue is fairly mild right now, and I only drive this car once every few weeks so I haven't been in a rush to go further.

I want to smoke test/boost leak test the car, and if I find nothing, I might take a quick stab at 6~7inHg again now that I swapped back to the correct WG actuator, even if it's definitely a fool's errand. If that doesn't work, I'll throw a new pressure converter solenoid at it.

2025-09-06 edit:
Issue was still happening.
Sometime 2024, I set the WG seat point to 7inHg per Lou/Tigger's old thread, no change.
Purchased new Pierburg pressure converter solenoid, did not install yet.
OFH oil leak became obvious.This made me remove the downpipe for access, and I found the WG flap does have play, so this is my new train of thought, and seems quite likely at this stage.