TOPIC: MAP sensor relocation using PCV manifold vacuum

Here's a mod I did today. Ron sent an email to me re: how he did a MAP sensor re-location mod on a Honda. This got me interested, so I went to ebay to look at MAP sensors for my '02 Outback. A key thing i saw, was that the MAP sensor tip is a cylinder shape. Ron told of how he blocked the MAP intake man. port on the Honda w/ an inverted vacuum cap. That would work OK, but I did it my way.

IF you had not read what Ron sent out, the central idea is to have the MAP sensor see a lower pressure (higher vacuum) condition than stock. Ron pointed out that the PCV vacuum port is typically put so it sees as high vacuum as possible. I had done the PCV re-route this summer, so the PCV vac. port was capped. Meanwhile on my Outback 2.5 L engine the stock location for the MAP is at the front end of the intake plenum, on top.

This MAP mod is simple. I went to my local Pink 'n'Pull junkyard, and got another MAP sensor from an identical engine. I pulled the stock MAP sensor, replacing it w/ the junkyard one to reliably seal the hole in the intake man. Then I pulled the PCV manifold vac. port cap, and ran a 3/8" gas hose from that to a 3/8" to 5/8" hose connector fitting. The stock MAP sensor working end- the part going into the intake manifold, cylinder shaped--this went snugly into a short piece of 5/8" hose. The open end of the plastic hose connector is capped w/ a blue silicone cap. The parts store didn't have a 3/8" to 5/8" straight connector, just the tee I had to use. I finished off with 2 screw clamps over the 5/8" hose to guarantee no vac. leaks.

I started 'er up, just to make sure-it started and ran fine.

To me, at least, this is really a combo of the PCV re-route, WITH the MAP sensor re-location! I had wondered about the "MAP Sensor re-location mod" for years, Not anymore! Ron told me today that most MAP sensors are cylinder-shaped. That fact greatly simplifies doing this.

THANK YOU Ron!!!

PS- note, that, in the 4th and last pic below, you will see Two MAP sensors. The OEM one is hooked into that Tee and kinda sideways, loose off the intake man. The second MAP, is in the original stock MAP's location, plugging the hole- it's the one from the junkyard. The silicone O ring on these MAP's seal tightly.
Tracy G

you could probably get a plate that would cover/seal that hole, Tracy...I've seen them for similar ports, like when people talk of deleting their EGR system on some other forums. Your solution is Good thinking - don't think I'm criticizing!

It would be easy for me to do this mod as well, since I left the stock orifice PCV valve in place in the manifold right behind the TB when I did the re-route: I'd just need to remove the vacuum cap that's there now, then run a hose from it to the MAP sensor and plug the hole that the MAP came out of. (I also have a backup plan if that gives funky results or throws codes...) Maybe I'll jump on this right now while I'm waiting for today's work to come down the line...
photos and mileage updates to follow if it's a success, and/or a description of workarounds as necessary...stay tuned!!

Hi again, Greg. Well, for me, doing the MAP mod came down to simple mechanics. Once I realized that the MAP sensor working end was cylinder shaped, I figured it all out quickly. Then I spoke to Ron who told me most MAP's do have a cylinder shaped end going into the manifold. This solved a little mystery that had been in the backround for me since I became a G-Man.

Other G-Men back in the day spoke of re-locating a MAP sensor, but seemed to keep their methods hidden. I had visualized physically moving the MAP sensor to another unexplained location. I foresaw having to make a new mounting hole, setup, plus extending wiring harness, etc. But, using various hoses,, clamps, and plastic or metal hose tee's or adaptors, like Heater hose adaptors, it's easy to do.

And, a trip to a boneyard, and scoring a MAP sensor from another Subaru was to me the way to plug the original MAP sensor hole in the manifold. Turned out- that someone had pulled the intake manifold from an engine identical to mine, and it was on the ground, loose. so I also got a look at the manifold's underside, the port and injector layout, etc.

Another useful tip learned in Gadgetman Land!

One again a big Thank you to Ron, who gave the needed info!

Tracy G

In the relocation process, it is VASTLY important that you realize two things.

First, that you MUST relocate to a lower pressure point. While rare, some engines are already configured in such a way that the sensor is located in a low pressure point. Not ALWAYS is the PCV port the best place to route it to, and by making this change you may lose a little. If this happens, then simply try attaching it to another point. The LOWEST pressure will be either at the throttle body in the area opposite The Groove (or where The Groove would be placed) OR as close to the intake valve as possible.

The second thing you must remember it that you are once again changing the fuel trims. As the ECU will ALWAYS want to use the baseline settings as burned into the ECU's bios, you MUST allow the ECU time to respond appropriately. So, if you see a loss initially, give it time to adapt, and during the adaptation process (as ALWAYS once a modification affecting the combustion rate occurs) you MUST USE THE POWER, for it is ONLY when the engine us under a load will it make its adjustments.

Stop using the power and you will lose any potential gains. Using the power regularly must become a habit. If you fail to change your driving habits then the computer will ALWAYS revert to its factory settings!

And that is (as we all well know) NOT conducive to the most efficient use of your fuel.

Have fun in Gadgetman Land, guys! Thank you ALL for sharing.

I can't tell you how many people are looking at your posts, but it's growing every day! So, your words here are adding to the lives of MANY who will never post, and never say "Thank you!" but you are making this world a better place for us all.

And you have my gratitude for all the work you are doing and HAVE done to solve this most important issue for all mankind.

Ron

Thanks for pointing these things out, Ron! If I could I would hang a flashing light on this post- it's that important!

This fact of using the Power, especially during the time right after Grooving and the other mods we do, seems a holy Grail sort of info point to me. Though it seems counter- intuitive on it's face, I will stress this fact to all from now on.

This kinda ties in nicely, to my idea from a while back, of calling our group of mods the Power System.

It's even a little like saying to someone-" If you want to Lose Weight, then you have to eat more donuts at first, and then once in a while later!"

Tracy G

I finally got around to implementing this, and while it's still early, initial results are looking good. In addition to the MAP enhancer modification I've mentioned, I'm cautiously optimistic that I'll get at least an incremental mileage gain.
All I had to do was take off the vacuum cap from the PCV vac source (intake manifold), extend the nipple with some hose, move the sensor and plug the hole the sensor came from.
I'll add pics once I can get some good ones. If you'd like to see just what I've done so far, check out my megathread for my ride (I call her Wendy) that documents each step I've taken along the way:

GadgetmanGroove Forum - 2007 Buick Rendezvous 3.5litre V6 - Groovy Forums

Right, Greg> This is a simple mod. In the past, I was thinking that it was necessary to physically move the MAP sensor, and re-mount it in another place. This to me, would mean making a new hole and mounting arrangement elsewhere in the intake man. then extending the wiring harness to the new spot. Seemed a hassle.

Instead, we pull the MAP, plug it's old hole, then bring the PCV's vacuum to the map, using hose's, adaptors, clamps, etc. About ten times easier. Then too- if it's seen best to fashion a hole in the manifold in a place, like close to the intake valve at the end of a runner. Then this is still less work than physically relocating the MAP. Bring the vacuum TO the MAP, not the other way 'round!

Tracy G

Yessir, that's it indeed...plus, with length (and possibly diameter) of the hose used, you might be able to tune the range of what the sensor sees.

On that point, Greg-- my gut tells me to keep the hose as small an I.D. as possible. That gives less volume of air to lower the "strength" of the vacuum "signal". Sound right?

Tracy G

It does sound right, but something in my gut tells me that the computer is looking for a waveform with rapid changes to average rather than a constant.
I may start looking for a hose to fit inside the hose I’m using to reduce the volume as you suggest to test the theory, but I won’t be able to get to that until the weekend. I’m tempted to match the ID of the hose to the exposed area of the sensor, or come as close as possible.
As usual, stay tuned for updates...

If you've come to this topic/thread from the megathread of what I've done to my daily driver (whom I affectionately call Wendy), It occurs to me that I'm due some photos/updating of the process to show where she's at under her bonnet.
Stay tuned - it's too dark outside at the moment to take photos!!

GregK wrote: Yessir, that's it indeed...plus, with length (and possibly diameter) of the hose used, you might be able to tune the range of what the sensor sees.

Greg (and everyone else), there are two things worthy of consideration with the MAP.

The MAP sits (ALWAYS) on the outside of the fluid (air) stream. This is also ALWAYS the HIGH pressure zone at any point in the system (save in the throttle body itself). Extending the sensor tip further to midway in the flow will provide lower pressures.

When you're considering hose size, this is good. But if (when) a reduction is required, MAKE THE REDUCTION AS CLOSE TO THE SENSOR AS PRACTICAL. The reduction in size also affects the flow of vac to the valve.

If you want the absolute BEST mileage possible, the littlest things can make the BIGGEST difference! (Like a 3/16" deep groove doubling fuel mileage)

Ron