I have a 99 early series with 170,000miles on the motor. Recently I notice the truck seems lazy when pulling away and the boost reads 0 psi and there is an air leak noise from under the hood. Once it shows boost the leak noise stops until the truck evens out and starts to cruise, boost returns to zero and the noise comes back, As soon as you increase power the leak goes away, as long as there's load on the motor there's no noise but as soon as it's levels out here comes the noise. I have a biig boost valve onit and thought that was my issue but I removed it and went back to stock andit's still doing the same thing. My fuel mileage seems to have taken a crap too since this started, down to about 10MPG from 16.

Any guru's out there?

 

Check the boots below the spider, on both ends of the intercooler pipes, and the map sensor connections first.

 

check this out.

 

What is the testing procedure for the MAP sensor? My truck seems sluggish too until RPMs are built. I have replaced the boots with ones from RiffRaft
265,000 miles...

Jeff

 

You need to use a scantoll to see what the MAP shows as manifold pressure.

Regardless of having new boots you should do a boost leak check. There can be leaks that are not do to boot problems.

 

I should add that this seem like a valve issue as the noise turns on and off with any load on the motor, as long as she's making boost pressure she's perfect, it's only when you are coasting or there no load on the engine.

 

Tha last link in my signature has some general tests you can perform. Do a "boost leak detector" search on the forum, lots of help there as well.