Well hello all. This is my first post here on nam. I've been floating around for a while reading and getting to know more about these cars. I came from hybridz.org and the world of Datsun Z cars.
Anyways now I have an 03 MCS. I have approx. 92k on it, CAI, cat back exhaust. I'm not sure whether I have an underdrive pulley or not cause I read between 6-10psi on my boost gauge but my sc pulley says Alta on it.

Anyways I think I have a boost leak. When I hold half gas my boost will hit about 5 or 6psi then slowly leak down and the car will start to stutter a little. I had noticed this stuttering before the boost gause installed, but now I have evidence. Does anyone else have this problem.
Then also the Alta pulley? If this is some sort of underdrive pulley shouldnt I read more like 10-15psi?

 

Ok I just measured my sc pulley and its approx. 57.15mm in diameter.

 

Alta doesn't make an OEM sized pulley to the best of my knowledge. You should be getting around 15 to 16psi of boost. Are you getting good vacuum? You let off the gas, does the boost needle close immediately or does it slowly go down? If so, you probably need a Detroit Tuned Bypass Valve with the stronger spring. My 2005 MCS had a very weak bpv when I acquired it with only 37,000 miles on it. I now have close to 100,000 miles on it, and I still have excellent vacuum.

Not sure if the stuttering is related to a boost leak however. Do you notice this when you accelerate off of boost? Does it still stutter??

 

Yeah pretty much if i'm not all the way in the throttle then it does the stutter. Its worse the closer I am to full throttle.
Yes, when I let off the gas there is immediately no boost.
As for vacuum i'll have to check that in the morning. What should I be looking for as far as vacuum numbers when I hook my meter onto it? Or can I tell from my boost gauge? cause that reads approx. -21psi at idle.

 

Z Cars, eh? Small world! My first (and current auto-cross car) is a '71 240Z. Little bit simpler to work on than these MINIs, huh?

Anyhoo, my '05 with a 16% pulley reads around 15psi at full throttle. I believe stock was around 10psi, so you should be making more boost than that at more than half throttle. As OctaneGuy pointed out, a common place for a leak is the by-pass valve... I saw a post somewhere that suggested wiring it shut and re-checking boost levels to see if it's the cause. I had actually been suspecting mine, since I notice my boost seems to taper off at mid-throttle... However, I don't get the shudder. Perhaps it's something like an over-rich situation, since the computer thinks you're getting more boost? I'm not sure if the engine management monitors boost level or not...

I'm still kind of new to this car myself, but I've read up on boost leaks a bit since I'd been suspicious of mine - Hopefully it gets you a start at least.

 

Yeah man Z cars are sometimes way simpler. I had a 70, 76, 77, 83, and 84.

Anyways it seems like it must be this bypass valve. So where do I go about getting a detroit tuned BPV as octaneguy stated? And I know dumb question, but where about is this guy located?

 

Ok after doing a little more reseach I have found that the stock size of the pulley is 2.56in and mine is roughly 2.18in. So could the bpv really make me lose that much boost? Or could it be something else?

 

If your not hearing ANY vacuum leaks. The best thing to do is remove the stock bypass valve and check to see if it is closing all the way. Then check the operation of it, you can physically just try opening it and seeing if the spring in the actuator is broken. This has happened alot on the earlier 02-03. If you need one you can get a stock one or a DT valve. Here is a link to the valve http://www.waymotorworks.com/product...cat=270&page=2

 

Is the belt good?

 

I'm having the exact same problem. My DT BPV arrived, so as soon as I get back from vacation I'll be installing. Will be posting if that was infact the problem.

 

I have no idea if it matters to you or not, but be aware that the DT BPV is known to reduce mileage since the boots comes on earlier.

 

Well to throw out some data points on this...my '05 RMW tuned MCS has a 17% Helix Pulley, JCW 380cc injectors, bored out cylinders and larger forged pistons, DT BPV, RMW Header, Invidia Exhaust & Alta Intake. I have a 100 mile daily commute. I still get about 350miles to the tank of mostly freeway miles at over 75mph. I could probably get more if I wasn't always taking advantage of the 200 pounds of torque at every stop light, lol.

Richard

 

Yeah its not a big concern of mine.

Well I ordered up a DT BPV and new intercooler couplers. Hopefully that'll cure it. I'll post my results when I have everything installed.

I learn more and more about these cars by the day. Anything else I might look at?

 

FWIW, my MCS lost about 2-3 mpg when I installed a DTBV. I liked the on/off throttle response, but my wife didn't care for it as a passenger or driver. Mileage improved by the same margin when I went back to the stock valve.

 

Ok so I got the DT BPV which did help a lot with throttle response. I got new intercooler couplers and i've gone through most common places for vacuum/boost leaks and i'm still getting at the max 11psi.
I'm scratching my head over this one. My next thing i'm going to do is the belt. If the belt was bad though wouldnt I hear quite a bit of noise coming from it?

 

Yes, I'm aware of it - thanks for mentioning though. Coming from driving my Cayenne Turbo and other 5litre+ cars, the Mini as it is now is still very light on fuel

 

I also haven't quite solved this one yet. The new DTV is in and I'm also not seeing any increase in boost. I'm going to reinstall the entire tube section of the boost gauge. I'm suspecting that is where the fault is.

 

Yeah i've already checked all that when I had mine all apart and it was fine. I suspect its my belt. I just thought it would be more audible though.
I guess if it was a slight slip at higher RPM's I wouldnt hear it that much. (Someone confirm that please..)

 

What boost gauge are you using? And I assume you know for sure it's not faulty? Do you have access to a Scan Gauge II which can also read boost?

 

Okay I have a lame question...I read an article on a twin charged mini that got rid of the Bypass Valve and instated a Blow Off Valve Can I install an adjustable Blow off Valve instead...That way I can control my boost when i drive...I have a 17% reduction pulley with 420cc injectors just hate that bypass valve