Can anyone identify the symptoms of(possibly)a slipping belt?

I have an 04 R53 MCS (80K).
About a 3 weeks ago; I successfully preformed a 15% SC pulley mod, correct belt size and colder plugs.
The car ran great and the power boost is significantly noticeable. It's only been about 400 mi since then.
The Mini still runs/sounds smooth and the acceleration is awesome. I never really drive it hard.
Recently, however I've noticed while driving, from time to time, when I accelerate quickly but steadily (from 38k rpms) up to/between 45k - 52k rpms
I can feel a momentary surge = and then it boosts up again. Just for a split second. It is not sluggish or a misfire. It's like a quick mild off/on jerk. It is not constant.

It can best be described as if you're driving along with the the AC on and then turning it off and back on/off again.
You can feel the difference of how much the AC puts a load on the motor. I hardly (n)ever use my AC.
This is just meant to illustrate an example...but the symptom is exactly like that.
Looking under the hood while the engine's running; everything (on the belt) seems to be riding smooth.

Four months prior/Before the mod - I brought the Mini in to a local independent BMW/Mini garage to R&R a failed idler pulley, new belt and a "new" belt tensioner.
*Without accusation* - When I was preforming the SC mod; Although it was "clean", I wondered if the mechanic actually/honestly replaced the tensioner with a new one.

Notwithstanding,
Might it be something else perhaps?

Any insight, thoughts, speculation is welcome and greatly appreciated!

Addendum: The Mini runs awesome. Regardless: a new fuel filter will arrive tomorrow. I will (hopefully) be working on
replacing it this weekend. Just to rule it out. It's never been changed.

 

Traction control? Turn it off and see if that helps

 

I would suspect the Compressor Bypass Valve or maybe the (T)MAP sensors, but my best bet would be the valve

This thing

 

+1 on bypass valve being a very common reason.

 

You can test to see if it's the bypass valve by simply zip-tie'ing it closed and see if the problem still exists. (Some times the valve will flutter at certain boost levels)

But only do this as a test, do not leave it that way, it will throw the ECU off.

If it is the valve you can try a VGS mod to see if it fixes it, or purchase the Detroit Tuned Bypass Valve, they have been rebuilt and calibrated for this exact purpose.

 

I tried the VGS mod, but the power delivery gets on/off. I would not recommend it.

 

Hey guys thanks for the input.
Although, I don't believe it may have been the DSC, (I probably only used it 10 times in 17.5 years), I have read here that DSC seems to retard performance = acceleration. Thanks @Serafin . it was a helpful to know.
@nd-photo.nl - As always, Thanks for the photo! I haven't explored all the working parts under the hood (until things break/fail). I see in the photo, the bypass valve/brass nipple on the right is "capped off". Just
curious...Is there a reason for that?
@BlwnAway , How to Tests have always been a great help to us (water cooled) newbies. It's a great start. I'll give it a try!

On a side note: Since I tend to move a lot - I don't often have a lot of friends to call upon when I need another set of eyes/hands.
Since I'm used to cars that actually have a THROTTLE CABLE - I improvised technology; by making a mark on the belt with a grease pencil, set my phone/camera up in line with the belt and mirrored my phone to my laptop
while I sat behind the wheel revving the engine at different intervals/RPMS. The poor man's timing light trick for us "air coolers". Of course the car wasn't under load/driving....I experienced no symptoms... so achieved nothing more than entertainment value .
I'll try the diagnose by pass valve and keep us updated with any finding that might help others.

 

I had it prepared for the VGS (Vacuum Gain System) mod, but in the end I reverted back to normal. In a nutshell, tee off from the vacuum nipple on the intake manifold and connect it to the CBV actuator (that black thingy with the nipple).

The CBV is then operated by direct vacuum instead of "indirect" vacuum. Power delivery will be more direct, but it can get jerky

More info over VGS > https://www.northamericanmotoring.co...ystem-vgs.html

[edit]
Something else to check, the vacuum hose of the fuel pressure regulator tends to disconnect when the car gets older (rubber boot). Advice: replace it with a proper silicone hose.

 

@nd-photo.nl Thanks for that explanation on the VGS.

I had to go run some errands across town so I didn't have a chance to dig into ziptieing the by pass valve as described by @BlwnAway .
I did a 30 mi rndtrp on the hwy - Oddly enough the aforementioned symptoms did not re-occur. Even @92mph - nothing!
I'm not convinced it was just a fluke or isolated symptom...I'll do another test drive and see.

As to the vacuum fuel pressure regulator hose (you so kindly pictured): Is that the small hose that runs under the (passenger side) of SC intercooler horn?
It connects to the slim black canister looking "fuel recirculation valve", then under the SC horn to a 'T' connected hose?
If so; After the pulley mod, I tripped the SES light resulting in a P1477 fault. I found this hose was disconnected and the rubbery elbow had literally decomposed into mush.
I changed it out with a heat resistant rubber hose/tube (direct = no elbow).

Is that what you have described/pictured?

 

OOOPS!
Wait a minute... It's THAT hose
I just googled "Mini Cooper S fuel pressure regulator hose" = Ugh! what a PITA to R&R that looks to be.....

 

Yes, that is the Fuel Pressure Regulator vacuum line.
what you've done to fix it is appropriate, and should be fine as long as it's a tight fit.
And yes, that hose being disconnected or cracked can cause real issues when it comes to leaning out the fuel mixture. (The T is probably there for a boost gauge, that's where most people tap in.)

Zip-tie'ing the valve closed basically takes the vacuum operation out of the picture, so if the zip-tie helped, your problem is almost assuredly the BPV.
And while the VGS mod will sometimes help, your better of skipping it and rebuilding or replacing the BPV.
But...
If you removed the zip-tie and the problem didn't come back, there's a chance that the BPV was just sticking or gummed up, and just not operating properly.
As mentioned before, the Detroit Tuned Bypass Valve, was created especially for this problem and comes with a stiffer spring and realigned butterfly, for best seal possible. The stiffer spring is there to eliminate flutter in the butterfly under low vacuum (higher RPM & Boost), which it sound like what your dealing with.

 

Don’t mean to go off topic lol but since we talking about vacuum issues.

maybe you can advice.

i do have the VGS mod. And i noticed my vacuum on idle is -4 that’s seems high. Got cam as well. Tune etc.

do i have a vacuum leak? i noticed my boost would top out around 14 on my ODB2 but only around 10 on my boost gauge. It’s connected through a vacuum T ( VGS line )

Thank You

 

Quite possibly, I would have said it's possibly the Cam, since they can reduce vacuum, but it your still seeing proper boost and vacuum through the OBD-II but not you're actual gauge, then it's of course pointing toward a leak somewhere in the gauge line itself.
I'd check all the associated lines and connections in the gauge chain, looking for cracks or loose connections.

 

And what about the -4 vacuum? Isn’t it suppose to be around -20?

 

Yes, should be, but again, the Cam could be having an effect on that.
(If a Cam effects your vacuum, it will be most noticeable at idle)

Check all your lines and go from there.

 

Ok - I woke up early this morning and took the Mini out for a quick spin (4mi ±). I took advantage of the absence of Sunday morning traffic and happily twisted the rpms/mph to where I previously (occasionally) felt the slight "jerk".
Still I could not recreate the symptom that had occurred the weeks prior (i.e.:the reason for my initial post). Curious; I started to dig in/take things apart so I can get a hand on the bypass valve.That's a fair amount of disassembly...Just about to twist the snorkel off -Sure enough,
I bumped the tray and spilled an intercooler cover bolt down in the engine bay. Never to be seen again. Dismayed - I stopped and went no further. Decided to run a quick OBD 2 scan just for the heck of it, which turned up clear - EXCEPT for a 0456 "Tank~venting system~very minor leak detected" fault code.
I've already replaced my cracked gas cap 3 weeks ago with Mini OEM. So I it's anyone's best guess as the result of throwing this code.
As @nd-photo.nl pointed out - It just might be a failing fuel pressure line/hose responsible for the code showing up on the OBD. However - back to the power "jerk"; I suspect @BlwnAway may have a poignant assessment regarding a "sticky" (failing) bypass valve.
The Mini seems to run fine for now. With your help at least I'll have an idea of what to start diagnosing/replacing if it starts acting up again....

 

Fingers crossed that we fixed it

 

Well,...It looks like I'll be replacing the bypass.
Mid afternoon today, merging from one interstate to another - traffic was at a standstill. Major accident up ahead.
While sitting in gridlock for a few minutes, my GPS brought up an alternative route.
When traffic was clear; I hit the gas to merge back on to the previous hwy. Chirped the tires in 1st & 2nd (gotta love that wind-up.)
Then *Fuf* - RPMS dropped from 4.8k rpms to 3k rpms = NO Boostt!
I took it easy until my next exit. While driving the alternate route, the Mini started acting normal again.
*Bypass valve*

That's the good news.
The bad news is that; During the past 2 weeks I began developing some anomalistic swelling, bruising and welts in my hands and forearms
coupled with severe shortness of breath.
Bloodwork came back - They determined I have Lupus.

I hope the bypass valve can last long enough until I can actually hold a wrench again,(let alone make a fist).
It may be a couple of months. Time will tell.

Anyway - You guys were right!
I'm so happy I don't have to guess.

Thank you!!!

 

Sorry to hear of the medical condition and hope you get better soon.
Glad you figured out what was wrong and can fix it.

 

I'm sorry to hear that. Best wishes for a quick and full recovery.

 

just reading this. I got cam n big valve head and all too but I'm still running 13lb of vacuum and 16lb of boost even when I had stock sc and 17% and now on sprintex..

 

so sorry. My wife AND I suffer from Lupis , her woss than I BUT I Have advanced rheumatoid arthritis with it so there days simply moving is rough and pain med route is a nogo for me. Wish you luck in your battle, I had to learn to live with and around my condition and still am learning what my body cam and can't handle and on what days .
I have my old spare bpv I can send you so you can try that , but ziptie ur bpv shut and if the car runs great , if it does that's ur problem.

 

Man - I can't thank all of you enough who have helped me/us to sort out our Mini/mechanical issues.
As I may have mentioned before; I've never owned a car that I haven't built myself. The Mini is my first
water cooled, computer controlled vehicle that I've owned. I love learning about it almost as much as I love driving it.
Thank you for all your kind heart well wishes while I deal with yet another bout with this disease.
It certainly wasn't my intention to inject the forum with personal issues.. However your replies have certainly revealed what a tremendous group of people you guys are = An unfailing willingness to share - regardless.

@MiniManAdam - Thanks for the generous offer regarding your bypass valve. I actually went ahead and ordered an OEM from WMW...nonetheless, Thank you again. It was very cool of you to offer!
On the side:
Yeah, you know what we go through when these unprecedented flare-ups occur. After 15 years before my present diagnosis of Lupus - Major flare-ups are almost like clockwork = about every 2½ - 3 years. I also have RA and are always severe. Sometimes 3-6 months before recovery. Sucks when you can't even do a simple task like putting on your shoes because your extremities are so badly inflamed and bloated. That is - If you don't end up passing out from exhaustion before then.

Anyway..
When my hands no longer resemble something hanging in a NY deli;...I'll be back at it again

Thanks again, to all of you!!

 

best of wishes bud . Hope the bpv fixes ur issues.. make sure to check the vacuum hoses as much as you can while replacing it just encase one has come off or loose as its common for these.

 

Ok - Being the the compulsive /"can't sit still for a moment" person that I am; I went out and managed to wedge my fat, swollen hands to get to the bypass valve.
I was determined to try the ziptie test but quickly found that the actuating lever has a minor tendency to stick ever so slightly. There is a fair amount of spring resistance.
It does slightly stick for a split second but snaps shut tight like I think it should. My new WMW oem bpv should be here next week.