piopio

EJ20G High RPM Misfire, Where do I Start?

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I'm looking for advice regarding chasing down an intermittent high rpm misfire. The car is a GF8 impreza with an ej20G engine. Under high load high and over 5500RPM I am suffering a loss of power and torque and what feels like a misfire. Under light load I don't think I feel a misfire. So far I have replaced all plugs and tried gapping to 0.6mm. I have also tried a different set of coils with no luck. Otherwise the car drives perfect at low RPM other than sometimes taking a long time to start when hot. The engine is high km with 250,000km. Is it worth doing a compression check?

 

I am at the point where I am not sure what to replace next. I'm thinking crank and cam sensors. Or doing the 2002 WRX coil pack upgrade. After that I feel a I might have to start looking a general engine health closely? 

Edited by piopio

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10 hours ago, piopio said:

I'm looking for advice regarding chasing down an intermittent high rpm misfire. The car is a GF8 impreza with an ej20G engine. Under high load high and over 5500RPM I am suffering a loss of power and torque and what feels like a misfire. Under light load I don't think I feel a misfire. So far I have replaced all plugs and tried gapping to 0.6mm. I have also tried a different set of coils with no luck. Otherwise the car drives perfect at low RPM other than sometimes taking a long time to start when hot. The engine is high km with 250,000km. Is it worth doing a compression check?

 

I am at the point where I am not sure what to replace next. I'm thinking crank and cam sensors. Or doing the 2002 WRX coil pack upgrade. After that I feel a I might have to start looking a general engine health closely? 

couple of quick questions.. 

Does it do this when the motor is cold?
Does it get worse as the engine warms up?

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On 19/10/2020 at 8:46 AM, madmike said:

couple of quick questions.. 

Does it do this when the motor is cold?
Does it get worse as the engine warms up?

Seems to be slightly better on long drives. Otherwise its consistent.

22 hours ago, Loren said:

0.6 mm might be too big... also try new leads.

I gapped all plugs to 0.45mm and little to no difference, I will be trying new copper plugs instead of platinum. Coil on pack. 

 

 

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42 minutes ago, piopio said:

Seems to be slightly better on long drives. Otherwise its consistent.

I gapped all plugs to 0.45mm and little to no difference, I will be trying new copper plugs instead of platinum. Coil on pack. 

 

 

I'm wondering if maybe you are having the same issue i was having with my old motor..
Ihad a reall nasty miss and loss of power at 5500 RPM umder load that i could not figure out.. Tried everything.

I ended up chucking it on Keith Stewarts dyno and he diagnosed it as valve float.. This was an original unknown kms engine that apart from the miss at higher RPM's and load would perform really well...
Yours has quite hig Km's so its a possibility i guess...

Only way to really diagnose may be to chuck it on a dyno and get a full health check. 

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Does that motor have AVCS? Assuming no but stranger things have happened.

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Could be a number of things. Is the car jerking and back firing at all? Is it a hesitation or a miss? Does the idle run rough sometimes, or dip up and down? What does the boost gauge say? Is it still building boost up top? The not wanting to start when hot thing, it's common for that to be the ecu temp sensor which is on the under manifold cross over pipe. Cheap to replace, kinda a pain to access on some models. If that sensor was faulty it could be reading a false high temp and trying to save the engine? I don't really know how well these old ecu's worked so may not be that intuitive. IME on that model it's usually coil related, cracks in them etc. Check coils for any cracks under the rubber boots too. As a quick fix you can fill the cracks but the v7 coils is a good mod. And check the wiring isn't questionable too!

 

Intake - these models are prone to cracked and damaged vacuum hoses, some may not be sealing properly anymore. A leaking intake pipe, or gasket might be losing boost pressure as it builds higher. You can pressure test the system after the airflow meter. Also BOV leaks are common. 

 

Coils - This model crack coils often, check wiring to them as well. 

 

AFM - These are prone to failure on setups that run pod filters, in my experience they generally make the car run rough rather than miss so wouldn't go grabbing one just yet

 

Detonation - Is the car running proper fuel? E.g 98 or 100 octane fuel. Is the intercooler doing it's job properly? Are the fins bent to s*** and stopping cooling or is the bonnet vent for it still on the vehicle and pushing air on the cooler as it should.

 

Fuel related - A tired old fuel pump not keeping up later in the rev range as pressure drops, or an injector not flowing correctly, or even a pressure regulator not working or not recieving vacuum. Also the wiring for these pumps is known to fail and give issues, a common mod is a rewire of the pump and use the ecu to turn on a relay. 

 

Good luck 😁

 

 

 

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Update!!! I’ve managed to track down the issues! Well multiple issues.

 

The fix in the end was resetting the ECU after ensuring that the car was running the correct boost. It was hitting 14 psi which I suspect was causing knock, this was due to some race lab EEPROM. Then letting the car learn again on the stock map. 

 

you can really feel the more aggressive timing after the ECU learn and reset process. It now pulls harder throughout  the rev range and doesn’t fall flat at high RPM. 

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