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Old 04-30-2013, 06:20 PM   #1
jhenry1983
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Member#: 269361
Join Date: Jan 2011
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Location: Los Angeles, CA
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2011 sti
03wrx Rallycar

Default losing maf voltage when boost comes on

Was wondering if anyone has seen this problem.

Version 8 motor with 03 usdm harness added in avcs ( was checked and its correct) JDM ecu

This problem happened after taking the setup from my 95L and putting it into my 2003 wrx rally build

Mods are ;
3 port ebcs
Walbro 255lp fuel pump
34mm restrictor w/ prodrive turbo inlet
Aem short ram

We had the car on a dynopak with the dyno holding the rpm max at 3k. So all that can change is throttle position and boost.

As soon as it goes from -1 to 0 boost the maf voltage drops from 2.54 to 2.4 and continues downward 2.35 2.2 then after about 6 cells jumps to 2.84

While this is happens and the maf voltage drops, the car leans out pulling 40-50% fuel . At idle its super rich and it leans out on boost.

What we've done to attempt to remedy it;
1. Switched and replace MAF sensor
2. Replaced 02 sensor
3. Replace fuel pressure regulator and confirmed it was working correctly
4. Checked for vac leaks - nothing
5. Smoke test - nothing
6. Vac test - nothing
7. New spark plugs - nothing
8. All grounds were sanded for full contact - nothing
9. Checked mapping and its fine

While the maf isn't plugged in it runs and idles great, just a bit rich

If someone has seen the MAF voltage drop like that and then spike back up in a pull please help me out, were in a time crunch and every reputable Subaru shop is stumped.
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Old 04-30-2013, 11:37 PM   #2
pmugabi
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Not seen this but probably it's a change in the map that some one didn't document and that you're not looking at. Have you tried reverting to a stock map (maybe with just an injector rescale if needed) to see if that fixes the problem? At the low loads at which this happens, a stock map should still be safe enough to complete the test.

Does a voltmeter at the MAF show the same voltage during the problem?

Maybe try swapping the ECU as well.
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Old 05-01-2013, 01:25 AM   #3
jebjkey
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2006 Evolution MR

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The voltage the MAF sensor puts out has nothing to do with the map. The map only controls what the ECU does with the signal.

Is the intake (before the MAF sensor) stock? Some aftermarket intakes have turbulence can cause bad MAF readings.

Where is the restrictor in the intake path? Try swapping intake pieces (before the MAF sensor) with a stock setup.
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Old 05-01-2013, 11:59 AM   #4
jhenry1983
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03wrx Rallycar

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I'll try reverting to a stock setup to see if the filter is the problem. Ive already tried a spare ecu, and the problem persists.

The restrictor is right on the cold side of the turbo machined to fit and used an O ring so no leaaks also used 2 lock screws so it can't move
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Old 05-01-2013, 07:17 PM   #5
jhenry1983
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Tried stock air box with no luck , same problem .
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Old 05-02-2013, 10:42 AM   #6
jebjkey
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2006 Evolution MR

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This is very interesting.

Just to make sure I am on the correct page, your intake flows like this [correct?]:
stock airbox [or AEM SRI] w/ stock MAF sensor -- stock after-MAF hose -- Prodrive turbo inlet -- 34mm restrictor -- turbo cold side -- intercooler -- intake manifold.

Just to put everything in one sequence: MAF v is normal at idle and goes up normally as throttle is opened. Except when MAF v hits 2.54, instead of continuing up as it did previously, it starts lowering until it reaches 2.2 and then it does a step jump to 2.84 where it continues on as it should. And it does this with two different MAF sensors, a stock airbox, and a AEM SRI.

When is the last time the MAF sensor operated correctly? Before the 34mm restrictor was installed?

Can you look at the signal with a O-scope or something? Maybe the signal from the MAF sensor has some electrical noise that is disrupting the normal signal at some harmonic frequency based on engine RPM [or something] and it just happens to show up at those MAF v values.

Does it only do this when you use the dyno to hold RPM at 3000 RPM?
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Old 05-02-2013, 05:17 PM   #7
jhenry1983
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03wrx Rallycar

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Yes you have the flow pattern correct.

But it does this on dyno, street whatever. The main issue with the dropping maf voltage is when it goes into boost. The moment it goes 1psi of boost the voltage drops like I said and it then leans out.

So far we've addressed almost all possible issues and now we something.

We have been testing resistance in many areas mostly across connectors and the main ground and anything linked into the common ground with the maf

We found the rear harness (that plugs in near the fuel pump and fuel pump controller) when the fuel pump primes we lose all resistance to the MAF signal pin on the ecu. But when we unplug that white connector that allows the fuel pump to prime we have double the resistance to the same pin.

Now were back into figuring what's pulling voltage or grounding out back there, but it can't be much because its only fuel pump , transfer pump and controller. All emissions were deleted taking pins out of the connector, so its a start .

Going to replace the rear harness and start from there now
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Old 05-02-2013, 07:41 PM   #8
jhenry1983
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03wrx Rallycar

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Isolated the rear harness wiring and its not the issue. I'm stumped 100% and the rally tech is tomorrow
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Old 05-02-2013, 09:57 PM   #9
nmlittlebigman
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Is it possible that your turbo inlet is collapsing?
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Old 05-03-2013, 01:38 AM   #10
jebjkey
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2006 Evolution MR

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Quote:
Originally Posted by nmlittlebigman View Post
Is it possible that your turbo inlet is collapsing?
It would be very odd for the turbo inlet to collapse at such a low value unless it was damaged in some fashion. Since the turbo inlet is after the MAF sensor it would be impossible to explain why MAF v comes back to normal all the way to redline if the turbo inlet was collapsing. I have seen datalogs of a collapsing turbo inlet, they do not look anything like what the OP is experiencing.



jhenry1983:
Thinking about your fuel pump, does the MAF v problems coincide with the fuel pump duty cycle shift?

Also, when was the last time the MAF sensor was working normally and what have you changed since then?

Maybe a temporary "fix" would be to scale the MAF values at the problem voltage values so that you do not run lean while the problem is occurring. This is at the expense of running rich prior to the issue, but maybe a worthy compromise in the short term that will allow you meet our timeline.
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Old 05-05-2013, 08:31 PM   #11
BlazeRex
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Post-Maf, Pre-turbo leak. My $.02
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Old 05-09-2013, 07:46 PM   #12
jhenry1983
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2011 sti
03wrx Rallycar

Default

impossible for a leak there, turbo inlet has no fittings and no cuts / cracks to leak from. did a smoke leak test and found no leaks through the entire system.

I ran the high desert trails rally completely in the fail safe speed density and it ran a bit rich but overall good enough to what i needed.
unfortunately I didnt have the power for the long straits to place well in my class.

so now im back in the same boat attempting to figure out whats up . theres 2 roads I can take. 1) tap a IAT sensor in the intake manifold and tune for speed density to eliminate the maf sensor completely
and 2) Buy a new bulkhead wiring harness and put it in to eliminate all posibility of wiring issues.

Ive tested to much to know where to pick up,
all sensors have been replaced
fuel pump
fuel pump controller
grounds ( grinding grounds to bare chassis )
checking all wires for continuity to ecu , wire to wire, wires to grounds ect
checked tuning and MAP on ecu to ensure correct one is still there
swapped ecus numerous times.
eliminated TGV wires
smoke test
vacuum test
pressure test
compression test

if someone.has any idea why whats happening in the first post is happening please help im ready to pay someone who figures it out. already spent over 1-2k in diagnosing , parts, tests

even took it to a subaru tech whos been working at subaru for 30 years and even he was stumped.

help !
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Old 07-15-2013, 12:06 AM   #13
massiveal
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find a solution?
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