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Old 10-05-2012, 12:23 AM   #28
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Originally Posted by Special J View Post
If this was electrical, you should see some kind of change on the MAFv. Instead, you've got a solid MAFv, a small change on the MAF, and a big change on the correction. I'm inclined to think mechanical, like turbulence or resonance, and possibly the tune reacting too soon, too much, or both. Just my .02.
MAFv does change, you just have to look very closely to see it, because the MAF scaling is extremely sensitive at low voltages. There's a blip around the 400 mark on the x axis that stands out if you look closely. If that trace was rescaled by a factor of 10 I bet we'd see it a lot more clearly.

And I'm betting that this is electrical.

It that would also explain a problem that I had a long time ago. I was running an in-the-fender CAI that I had assembled from 3" silicone tubing, elbows, joiners, etc, and a 6" long aluminum MAF housing that had a tab that I bolted to to chassis deep in the fender well. It worked great for months...

And then one day my idle totally sucked. I was like I had a vacuum leak, but I couldn't find one. To troubleshoot, I hung the air filter and the MAF housing off of the turbo inlet like a hot-ram intake, and that worked fine. Then I tried adding the various elbows and straight bits in different combinations to find the one that was leaking, and I got a totally reasonable idle every time. Satisfied that all of the parts were OK, I bolted the MAF housing to the chassis deep in the fender-well, and re-assembled the parts in the CAI configuration, and the idle sucked again. I went back to short-ram, got a good idle, re-assembled the CAI with the filter sticking up in the air, got a good idle. Re-assembled the CAI with the filter in the fender well and the MAF housing bolted to the chassis, and I it idled like I had a leak again.

The significance of the stuff in italics never occurred to me at the time, and if it was not for this thread, I probably never would have figured it out. At the time, I couldn't figure it out, which really pissed me off. I ordered a KSTech intake and ran my parts in short-ram configuration until it arrived.

So yeah. If you want to run MAF I think you need to either insulate the MAF housing from everything around it, or ground it. I lean toward insulating it, because I have no idea whether there will be side-effects from grounding it. Subaru didn't plan for that, so there might be current flowing between the housing and the chassis. (In fact, there has to be some, else grounding wouldn't matter.) You might eventually cook something sensitive inside the ECU.

Have you tried measuring the voltage between your MAF housing and your chassis? I'm pretty sure you'll find that it's non-zero.
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