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Help please. Boost cut and sometimes no boost!!! Stock 2007 LGT


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Hey guys. I juSt picked up a 2007 LGT and noticed after I got it that under ful load/ 3/4 load, getting into boost, it will just cut out. No boost. Then boost. No boost then boost. All very quickly while raising in Rpms. I can also hear what seems like some pressure being released to open atmosphere when the happens if i open the window. It drives ok without gettin into boost/throttle. Idles decent. Can tell there’s s misfire. And has a pending code for misfire #4 as well as 2 o2 codes. It has 120k miles on it. The engine was replaced by a dealership with a 90k motor when the car was st 95k. I cleaned the Maf, replaced intake filter, reset ecu, and It will boost totally fine the very first pull. Nice and strong. Then after a couple pulls it just boosts and falls on its face. Doing plugs tomorrow and compression test hopefully. Also put some injector cleaner in as well

 

Anyway, anyone else ever have this type of boost issue? On off on off on off through the RPMs? And also when it happens pretty bad. The check engine light will flash and put it back in S mode from S+. Any easy diagnostic tips? Any experience? Thanks guys

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Smoke test the boost control hoses

Inspect wastegate & actuator

Pull DP & check turbo shaft

Boost leak test

Swap coils, injectors, plugs to diagnose misfire

And as you mentioned, compression test

Could be the front O2 sensor, what codes do you have?

Post LV, or at least what are your fuel trims in the A/B/C ranges?

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Alrighty anything specific I’m looking for on the WG & actuator? And how do I do a boost leak test? (New to Subaru) I’m doing plugs tonight so I’ll swap coil pack over them so the injector after to determine which is the culprit. And would a bad o2 cause that? And the guy at o Reilly’s cleared my codes on accident -___- he was brand new so I’ll be waiting having to check them again tonight. What’s LV? And how can i determine a/f without a gauge? Thanks a lot!!!!!
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Check the vacuum hose to the wastegate actuator for leaks or cracks in the plastic T junctions.

 

I had this exact thing happen and the wastegate wasn't activating because of a vacuum leak - and the ECU was hitting boost-cut when it was spooling up.

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Really? Hmmmm so I sprayed brake clean all around the wastegate (cool turbo) and no raise in idle.. hmmm did it cause any cel codes? Didn’t get to comp test, o2’s or spark plugs today. Only replaced air filter, cleaned maf and sprayed brake clean and nothing so far
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The spray test is the "hicksta" method. Occasionally it gets lucky, but the only reliable way to test for vacuum leaks is a smoke test.

Furrhermore, an audible idle change is an extreme result. To use the spray test effectively, you have to watch the STFTs (fuel correction).

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Haha no worries. Just how my old buddy taught me how. So smoke test, how is this performed? Do i need a tool of some kind? Would an o2 sensor cause this sort of issue by chance? Replacing them tonight

 

You can buy a pressure tester, or you can build one. Home Depot plumbing department is your best friend. You will also need a pressure regulator and an air compressor.

 

I don't bother much with the smoke anymore. I used to have a cigar/dry ice chamber on the 1st on I built. Now I just listen for the leak. Once you get to know the vaccum routing, leaks start getting easier to find.

 

I will also throw this into the mix, the BOV. It's not common, but tI have had a bad BOV where the spring got weak and it would blow through under boost. However, it sounds like the WG as others have already said.

 

This diagram helps quite a bit.

fuel_intake_combined.thumb.jpg.912e062bc29998dea5433a97429ff3b0.jpg

CKE SSP product information and sales : chris.ckessp@gmail.com
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Haha no worries. Just how my old buddy taught me how. So smoke test, how is this performed? Do i need a tool of some kind? Would an o2 sensor cause this sort of issue by chance? Replacing them tonight
You can buy a paint can smoke maker off eBay for $80. You'll need a compressor and mineral oil with it. Or you can make your own smoke machine, dozens of examples on YouTube, etc.

A bad upper O2 sensor can cause a variety of problems.

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That is a handy diagram, never seen that one before. Where's it from?

 

It was my go-to until I got used to the layout. I actually 1st stole it off of this site a few years ago, then misplaced it. I like how is also shows the GS 3-port EBC since most folks who modify get a 3 port early on, beofre they find all of their intake leaks...So if the stock (2) port is on the car, there is just the single line to the boost/WG t-fitting.

 

I just did a google search.

 

"Subaru Turbo Vacuum Routing" and it pops up early in images.

 

https://www.google.com/search?q=subaru+turbo+vacuum+routing&source=lnms&tbm=isch&sa=X&ved=0ahUKEwju8OnPsYDaAhWkjlQKHS4DDr4Q_AUICigB&biw=1280&bih=669

CKE SSP product information and sales : chris.ckessp@gmail.com
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Alright so did comp test on number 4 first, 120/125 ish range.... can I rule out rings/valves?!

 

Thanks a lot for that diagram!!! Going to save that! So I’ll be doing rest of cylinders while i do plugs so I’ll update numbers soon. Also doing both o2’s as well. Going to swap coil packs with 3 and 4 and see if I can get the misfire on #3 instead of for, then doing same swap with injector after as well.

 

What should my 07 LGT run on the compression test?

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Finished comp test. Found out there’s 3 different style coil packs on here.. -___- all cylinders were 118-125. Swapped plugs. Couldn’t get o2 done tonight. Gotta work on the morning. But if still seems to be misfiring a bit. The #4 plug looked lean. Rest look old. Gapped them all ton .029. Resetting ecu currently. Tried to find that wg t and i didn’t see anything that looked bad. They seem a little weak maybe. Didn’t dig too far. Just without taking anything apart I seen what was visible. So I swapped coilpacks from #4 to #2 so we will see what the next check engine code says and rule out coilpack.
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