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Old 04-18-2009, 05:21 PM   #1
Scooby Specialist
Member#: 59182
Join Date: Apr 2004
Location: N. Idaho
2002 WRX GG>>
2016 Focus RS

Default Boost Drops to 0 psi @ 5K RPM in 2nd and 3rd (4EAT) - Fixed - ?

Short answer is that after I pulled the turbo and re-did my intake - modified to a 45-degree airbox and related hose replacements - it now works OK - no Boost Drop! I put the turbo BACK IN and still ran fine, so it was NOT the turbo, but not sure was causing it!

I’ve posted this on the LegacyCentral BBS, too.

’90 Subaru Legacy, w/a ’93 EJ22T swapped in.

When WOT in 2nd or 3rd gear (auto) @ approx. 5K RPM boost suddenly drops and the car stops accelerating - but continues to run fine. The boost drops to 0 and stays there as long as I keep the pedal to the floor...I've continued to run the car this way for @ least 5 seconds. The car decelerates but continues to run - not lurching/sputtering/etc….Just a smooth decrease in speed and rpm. Only after I let off the throttle and reapply it does boost come back normally...until I hit 5K rpm and it drops again.

Here's a list:

- Running 17 psi w/the new Revtronix chip that overrides the stock ECU”s parameters, such as max. boost (8.7psi), fuel cut (@ 13psi), and allows me to use a different MAF and bigger fuel injectors. I’m helping do some final testing on the chip.

- Does w/the stock ECU and an FCD/MBC set to 16psi, too - so it's not the chip.

- It started in the last month, as temps have risen, during testing of the chip… I put many miles/hours on the chip before the issue appeared. I don’t believe it’s the chip but the chip may be pushing the car’s stock components to a ‘new’ level, esp. w/the warmer temps.

- It doesn't sputter or miss, and no CEL.

- If I let off the gas and then floor it again, it repeats the process – accelerates to 5K and boost drops to 0.

- If I left off the gas, but only go to ½-3/4 throttle it'll continue to accelerate and boost stays solid @ about 15psi, and pulls to 6K before shifting to 4th.

- It’s heat related, as it doesn’t do it on the 1st full throttle run when the car has just reached ‘operating temp’, or when it’s less than 40 deg. F.

I’ve replaced the following parts w/other used parts- nothing’s helped:

- Bypass Valve (BPV)

- Hooked up a manual boost controller (MBC) and boost control solenoid (BCS)-both a stock and aftermarket.

- Wastegate actuator.

- Tested and adjusted the TPS.

- Checked for leaks many times.

Updated Summary:

When WOT in 3rd gear (auto) @ approx. 5K RPM boost suddenly drops to 0 and the car stops accelerating - but continues to run fine. It doesn't sputter or miss, and no CEL. If I let off the gas and then floor it again, it repeats the process. If I left off the gas, but only go to 3/4 throttle it'll continue to accelerate and boost stays solid, and pulls to 6K before shifting to the next gear.


- It's temperature related…..When the car is just warmed up, the 1st full throttle run works OK. It starts to occur on the 2nd or 3rd WOT runs….usually occurs in 3rd gear 1st, but once car is fully warmed up, occurs in 2nd and 3rd gears. It is very consistent and repeatable.

- If I only use 1/2-3/4 throttle I can accelerate past 5k rpm and get to 6k rpm and it shifts normally.

- If I let off on the throttle after boost drops, let the car slow a bit and then hit it, boost comes back until I hit 5K again.

- It started this spring when temps got above 40-ish F.

- It occurs w/the Revtronix chip and 480cc injectors/JECS MAF @ 17psi.

- It occurs w/the stock ECU and 440s/Hitachi MAF/FCD/MBC @ 15+ psi.

- It occurs w/an MBC set to 15+ psi with or w/out the chip.

- 14 PSI seems to be the max I can run w/out the boost drop occurring.

- AFR readings:

o With Stage 2 chip/480cc's/JECS MAF I'm getting from 10.95-11.11 AFR in 2nd gear full throttle. There is one 'dip' where it drops to 9.94 when the boost drop hits. It then goes up to 12.2-12.6 @ full throttle when the boost drops to 0.

o W/the stock ECU/MAF/440cc injectors I was able to see in 2nd gear under full load that I'm running about 10 AFR 'til I hit the boost drop @ 5K rpm, then it goes up to 12. But is never above 14 when I'm on the throttle. 3rd gear looks essentially the same. Car idles @ 14 AFR.

What I’ve replaced:

- 4 different BPV's – 3 stock; 1 Bosch

- 3 different BCS – 2 stock and 1 GM

- 2 different ECUs

- 2 different TCUs


- 2 TPS –the whole throttle body

- Coil pack – diamond and stock

- New plugs – tried gap @ .025 and .030 (smaller gap suggested/required by Revtronix for chip)

- Ignitor

- Wastegate actuator – both were the newer WRX ones. Tested @ 10psi.

What I’ve tested: (all tests done @ Off or Idle – not while issue occurs)

- TPS – some values out of range. Still occurs w/separate TPS harness.


- Press. Exch. Sensor

- BCS (?)

If there's a leak or other issue w/my setup does it sound like it's before or after the turbo?

My Setup:
- '93 EJ22T swapped into a '90 Legacy.
- '06 WRX TD04
- '06 WRX TMIC
- Revtronix Stage 2 Chip (plugs into stock ECU)
- 480cc injectors (chip corrects for this)
- JECS MAF (chip corrects for this)
- Stock NGK plugs .030 gap
- Intake modified to remove resonator and snorkus (in fender well) - uses stock air box.
- Turbo 4EAT tranny and Turbo TCU
- 3" Turbo-back exhaust
- Bellmouth downpipe
- STi fuel pump
- Lightweight (not UD) pulley

Thanks for any input.
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Last edited by wtdash; 07-31-2009 at 04:33 PM. Reason: Update
wtdash is offline   Reply With Quote
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