Bleeding The MAP Sensor


Here is Gus Mahon's procedure (picture by me) on how to install a MAP sensor bleed to raise your overboost shutdown point.  The disadvantage to this design is that it slows down the response of the MAP sensor because of the restrictor.


Bleeding the MAP sensor line is only safe if you do it carefully, while monitering your fuel richness on an [A/F] gauge.  The more air you bleed out of the line, the higher the cutout point rises, and the leaner your fuel curve gets.  If you've got at least .9 volt on your voltmeter (while attatched to your oxygen sensor), or at least 9 lights on your Cyberdyne gauge, or a low, safe temp on your EGT gauge, then you're safe.

If you only want to raise the cutout to 18 or 20 lbs, there's an easy way .  Put a restrictor in the MAP sensor line (black rubber tubing) about 6" from the MAP sensor.  Make the restrictive hole about .027" to .033" in diameter.  NAPA and some stores sell plastic vacuum line connectors with a small brass pinhole in the middle.  Chrysler calls it an "orifice"; that's how it's listed on your white underhood vacuum diagram.  See if you can get one in the correct size range, or buy a smaller one, and drill it larger.  You can get creative, and melt the end of a plastic connector shut with a hot knife blade, then make a pinhole in it with a hot stick-pin or sewing needle.

After you install the restrictor in-line with the MAP, cut the line in between the restrictor and the MAP sensor, and install a "tee".  One nipple of the tee will be empty.  Put a one-way air valve (plastic check-valve) on this tee, so that air can blow out of the line, but cannot be sucked in.  Put a small needle valve, or aquarium valve, on after the one-way valve.  It will blow out into the atmosphere.

Crack open this valve a ***TINY*** amount, so that you can barely blow through it, and you have just raised your cutout point!  Make sure you're not leaning out too much as you find your new cutout point.  It might be 16 or 17 if you barely opened it.  Open it a ***tiny*** bit more.  Now your cutout might be 18 or 19, etc.  Leave it as low as you can, so that you will not lean out excessively.

IF YOU OPEN THIS VALVE TOO MUCH on the first try, you will be inviting DISASTER!!  If you are patient and slow, making sure you're not leaning out, you will be happy.

Note from Russ:  you will notice that the MAP sensor signal is only modified for boost, not for vacuum.  This is accomplished by the check valve, which turns the bleed off when the system is in vacuum.  That way,  the engine still runs normally when not in boost.
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This page is maintained by Russell W. Knize and was last updated 09/17/98. Comments? Questions? Email

Copyright © 1996-2003 Russ W. Knize