my legacy gt misfires when under boost, please help

HI guys,

 

I have a 2009 subaru legacy GT, the engine was swapped with an 2006 EJ20Y JDM legacy Gt. Now my problem is that when my car is running at low rpm it is perfect, but when it goes into boost at high rpm it misfires and the engine temperature goes way up although the fans come to help, but can u help me diagnose what my problem is? I changed the spark plugs, coils have been checked. Injectors also checked and cleaned, fuel pump is ok. One of my mechanic friends says it could be the headgasket. Any suggestions? Thanks guys.

Pressure test the cooling system. That way you won't be guessing if you have a headgasket leak.

The thing is that i don't have the kit, the country where i live there is not much subaru here, only a few and unfortunately the mechanics are not very familiar with this car, i don't have the pressure test kit nor does my mechanic, i know that when the car goes into boost after that the engine goes to knock and water temperature rises very much.

Do you have any smoke out the tailpipe? Any mixing of oil and coolant together in either the radiator or on the oil cap/on the dipstick? What kind of boost levels are you running?

 

Have you checked if the water pump is working OK? It's strange that you're misfiring with an overheat condition. That's typically either a valvetrain or head gasket issue.

No smoke, no sign of mixing oil and water, 14 psi boost, the thing is that the radiator is for the non turbo car but is working ok, water pump is working ok, when i press the accelator pedal a little and the car starts to accelarate and the rpm rises at a slow rate even to near redline it is ok and there is no misfire but when the car goes into boost when i just press the accelerator pedal quickly and a bit with force it misfires at 3800 to 4200 rpm.

Need to check the MAF, sometimes the front o2 sensor shorted can cause this. Not saying there aren't larger issues, but trying to get the internet to diagnose your car is insane.

Check the small stuff first, if you can't check the MAF you should take it to a shop. GL

 

P.S. are you running the correct ECU/tune?

14 psi is high for stock

Edited September 22, 2020 by Marvick08GT

The misfire is gone now the last run i did, but when the engine gets hot it goes into knock, this happens when the boost is high, and after that even at idle the coolant temperature is high and there is knock, i am also changing the spark plugs with the original type, i have another guess, since my car runs with an standalone ECU i believe there might be an issue with the map, where can i get the map diagram or spec?

Need to check the MAF, sometimes the front o2 sensor shorted can cause this. Not saying there aren't larger issues, but trying to get the internet to diagnose your car is insane.

Check the small stuff first, if you can't check the MAF you should take it to a shop. GL

 

P.S. are you running the correct ECU/tune?

14 psi is high for stock

 

Isn't stock 13.5 psi? Depending on what he is using for measurement and the conditions, that might be spot on.

Your car is a wildcard if it’s got a standalone. When you say high coolant temps, how high are they? It’s going to be virtually impossible to help diagnose that thing without plugging into it and looking at how it’s tuned and checking out some live data to see if something is off or a sensor not reading right that would put the tune in a parameter it shouldn’t be.

The misfire is gone now the last run i did, but when the engine gets hot it goes into knock, this happens when the boost is high, and after that even at idle the coolant temperature is high and there is knock, i am also changing the spark plugs with the original type, i have another guess, since my car runs with an standalone ECU i believe there might be an issue with the map, where can i get the map diagram or spec?

 

You need a custom tune. The MAP table from a stock ECU may not work in stand alone. It may have different size table, the other tables that affect the fueling are very likely going to be different. It might you started for a base but it will need tuning.

I'm trying to wrap my head around having the wherewithal to install and tune a standalone but not have a cooling system pressure tester. Best you can hope is that you deduce the problem before you blow a hole in a piston. There's no real way around proper diagnostics.

What is the gap on the spark plugs ?

 

0.028" is what they should be...? That's what mine are gapped at, in my 2.5L