Can anyone tell me what is going on with my car, 2003 MCS? I have a 15% pulley and a cat back exhaust. Just put it on the dyno and I am down 10hp and the car is only putting out 7psi of boost. Is there a valve or something that could be causing this boost issue? No check engine lights are on.

Thanks

 

It is most likely the supercharger by-pass valve is not fully closing. To test it, take a couple of zip ties and tie it shut. If your boost goes up dramatically, then that is the issue.

 

bad bypass valve and/or bad belt (old and stretched or too long). Oh, third possibility is that the tensioner is shot. It's possible but happens much less.

Matt

 

Check for leaks. Intercooler boots, by-pass valve bad, slipping belt. There's a lot of things you can check.

 

Thanks I had a feeling it was something like that. I will take it apart and check the valve. The belt is only a year old and it was the smaller napa belt.

 

especially if you're taking the car to red line. I've seen new belts slip. Also, if you have a plot of your dyno run, if it's a slipping belt, you usually see a dip in the HP curve as the revs climb (doesn't slip till you start hitting some boost). If the BP valve isn't closing, the whole curve will be lower.

Anyway, you should find it with a little work...

Let us all know what you find.

Matt

 

Same belt 5 years later, shame on me, especially given banging off redline is practically a daily occurrence.

 

The Dyno graph goes up nicely but then just evens out and stays that way till the end. Not the nice increasing power curve. I will check the belt also.

 

Does the bpv suck or blow?

 

She's gone from suck to blow!!!

 

and on the Internet I can never tell, the bypass valve allows air to go from the high pressure side of the SC to the low pressure side.

Just a levelling off of the HP curve would suggest a leak. From the high pressure side to the low pressure side (BPV) or to the outside air. Belt slippage tends to be a ragged drop or dip in the HP curve.

Now you know where to look, let us know what you find!

Matt

 

Ah I see.
Thanks!

 

Well got a couple minutes today and pulled the bypass valve off and sure enough it was f'd up. Pulled it apart and found that the spring had actually broken inside of it. So that sure made a huge difference in the performance!!! Thanks for all the input. It sure made getting to the bottom of this issue a lot faster!

 

There is or was a lightly used Detroit Tuned BPV for sale in the marketplace....

 

is about $50... The used DT was $100, and a new DT is what, $125? Anyway, you can wire tie it shut till you get the replacement, if you haven't already fixed it.

Matt

 

I ordered it first and had it delivered before we ever took it apart. it is easier to return something than to have a car apart and nothing to drive.

 

I'm new to the forums but have the same problem with my Mini.

Dr. O,

When you say zip-tie the valve shut are you saying to zip-tie the valve as if it was always in the position while the car is off? I only ask because I'm not sure if these valves are normally open or normally closed with vacuum...