Strange backfire under steady boost

My 2005 LGT is at the shop right now, and they're trying to figure this one out.

Under steady or heavy acceleration, moderate boost I get a pop, pop with power loss, then it works. Often doesn't happen again until next time I drive, but has been getting worse.

Sometimes flashes the CEL for 10 sec, then goes out, sometimes gives a cylinder 4 misfire code w/flashing Cruise light.

 

Have tried swapping injectors and coil packs, problem stays on cylinder 4.

 

Compression test ok, smoke test for boost leak ok, fuel/air sensor was destroyed on cylinder 4 (probably from backfire), replaced, swapped up-pipe w/catless, still backfired so swapped back.

 

 

Discovered someone put in the wrong plugs, changed.

Still backfires occasionally.

 

They don't think it's a burnt exhaust valve because compression was ok.

The mechanic mentioned that it's backfiring through the intake.

 

I've scoured the forums on many sites, any other ideas?

Edited April 2, 2020 by BlueGT05

Could be your tune, could be stuck open injector.

 

I'd first try new injectors

We've swapped injectors around, always cylinder 4.

They're going to try a leak-down test...

did you double check the #4 coil pack connector on the vehicle side? They get old and brittle and dont like make solid connection sometimes under heavy demand/load/gforce.

 

You can zip tie it with some good old dielectric grease just to test but make sure the pins are not bent as well etc.

 

What plus are in there now?

 

If its back firing through the intake thats not a great sign, did the problem just start? a lot of times that can indicate a cyl is getting incorrect timing and firing when an intake valve is partially open or 180 out of order.

 

Dave

It started recently and has gotten worse over time.

Runs fine at idle, only cuts out and backfires under what seems like moderate acceleration with boost. Pop, pop pop, and then it's good.

I'll have him check the wiring to #4 coil.

Well, wiring is ok, leak down test is good, MAF is good, O2 sensors are good.

They can't tell data-wise what's happening except it's hitting a lean condition on cylinder 4, more often when cold.

Have they done a smoke test to see if you have a crack in a vacuum hose ?

Yes, smoke test was good too.

They're going to check the cams and timing again - and some other sensor.

Edited April 8, 2020 by BlueGT05

What shop do you have this at? A Subaru dealer? A true Subaru Performance shop? Or local town shop that service mini vans, sedans, and trucks? Just asking cause I see many people take there cars to places I wold never take these cars to Only it it is your last choice or you have no option.

It's my usual shop and they do a lot of Subarus, Hondas, Toyotas, and other cars. Turns out the previous owner of this car also brought it here, so they know most of its history and installed the new radiator, new turbo, removed the banjo bolt screens some time ago.

What shop do you have this at? A Subaru dealer? A true Subaru Performance shop? Or local town shop that service mini vans, sedans, and trucks? Just asking cause I see many people take there cars to places I wold never take these cars to Only it it is your last choice or you have no option.

 

What a silly comment to make. Who here has ever had good luck at a dealership? Just because the name says Subaru over the bay doors, does not mean they know what they're doing or even care enough to do the job properly. I dont even want the dealer doing my recall work but I dont have any other choice.

Turns out it was a lot of little things: adjusted valves, verified timing, removed remaining banjo filters, cleaned VVT solenoid, smoke tested a second time (while hot) and found slight leak at both fuel rails, raised rails and cleaned injector pots, applied lube to seals and torqued rails to spec, cleaned intake system and combustion chamber. An air leak at the rails was causing everything to be off with the a/f mixture.

 

And now no backfire! Whew!

Fuel/Air sensor, bank 2:

congrats to the tech and the shop that did the job

 

eliminated the guessing diagnose

Who here has ever had good luck at a dealership?

 

Granite Subaru in Hudson, NH

 

Even with the changes they have gone through over the years, their Parts and Service departments have never strayed from anything but exceptional.

indeed