Help: Subaru WRX 2015+ knocking after engine rebuilt

Hello everyone,

I have a very weird problem with my car, I have a stock 2019 WRX and the engine failed around 50k kms due to cyl4 bearing damage. So Subaru replaced the engine as the car was in warranty. Around 110k kms, the engine failed again due the cyl4 bearing damage again because of my driving fault. I accidently shifted to 2nd gear from 6th gear. Anyhow, my mechanic rebuild the engine with Pmax cote bearings and rest of the engine was stock. As I started driving, i kept on feeling hesitation at random intervals. Me and my mechanic kept on digging in finding the root cause, O2 sensor was replaced, new spark plugs installed, I also bought a accesport to monitor the knocking and found that engine was recording knock very frequently and hence retarding the ignition timing. It was so bad that the DAM dropped to 0.121 and feedback knock reaches -12 degrees quite often.

We checked the knock sensors for any issues but could find anything and decided to install thicker head gaskets as the engine was rebuild 2 times. Now even after having thicker head gaskets to compensate the machining of the blocks and heads twice, the DAM still drops due to knocking. If I use 94 octane, then the knocking is very minimal but i want to use 91 as this car is my daily. I also tried to get a etune from a tuner, but he was also not able to figure out the issue and ended up retarding the ignition timing. Now may car runs ok with some knock sometimes but the is not even making stock power, feels like driving a toyota yaris.

I could find any information on internet regarding this issue and someone helping me to get this issue will be highly appreciated. I am really stuck and clueless here.

Hi. I assume you double checked timing belt marks and that is not the issue. Using higher octane fuel makes it less prone to knock- that's the indication that knock isn't false. Knowing that the engine is freshly rebuilt my suspicion is on pistons since you said the head gasket was replaced for thicker one. A friend of mine once got his engine built when his engine builder just bought and dropped in a set of pistons out of box without double checking them first. Ended up with increasing the CR almost 3 points... When I asked the guy why he didn't check the CR he said he actually did it but only visually and the pistons looked identical to him...

Nothing else comes to my mind unless it's one of those mistery cases then no logical causes can be found...

If you check spark plugs, does one cylinder look different than another?

I wonder if you have a iffy injector or during the rebuild it got muck in it.

I'm assuming you've checked your AFR's and that is not leaning out, but if one cylinders injector was being slightly underfueled that cylinder may knock, the ECU wouldn't know as it measures the AFR's of all cylinders together and may be compensating by overfueling the other 3 cylinders.

Long shot but sounds like you have a wacky issue

thanks for replying guys, i did use the same pistons and well didnt check for CR. @salco1987, your explanations makes lot of logic but how can i check for it? Sorry noob here.

pull the spark plugs out, and compare what the end of the spark plug looks like.

If one cylinder is running different that the others in terms of fueling, it will look noticeably different.

For example the 3 cylinders running slightly richer might be a bit darker, with a little more carbon build up on the tips

Where as the cylinder with the possible iffy injector would be running lean, and that spark plug likely would be cleaner than the other 3

Reading spark plugs is far from accurate, but typically if the engine is running happy, all 4 should look similar. If they don't, something is amuck, and at least then you might know which cylinder to focus attention on.

If all of them end up looking the same, then whatever the issue is, is likely no specific to the just one cylinder.

By the way, you never mentioned if your fuel mixture is still on target... Have you checked fuel pressure, fuel pump, fuel injectors?

The ECU has to be replaced after rebuilding that engine. Without doing so the AVCS system doesn't function properly which can cause all manner of issues.

Was it replaced, immobilizer reprogrammed?

That's something very out of the box about replacing the ecu?😮 Well ecu is still the same and immobilizer not reprogrammed

salco1987, sorry still haven't got time to check the plugs yet.

georg1970, afr readings is same when car knocking and when it is not, mechanic checked fuel system too, but couldn't find anything.

Moreover, i am trying to make changes in the tune slowly and by increasing the boost pressure a bit, retarding the timing a bit and made the afr richer have almost diminished the feedback knock if i do 3rd or 4th gear pulls. Feedback knock goes to -7 slowly only when i increase the load in 5th or 6th gear. 😶‍🌫️

The 15-21 north american WRX ECU performs a one time cam learning procedure when new. It cannot be re-initiated by any means, including with a dealer tool. The dealer and anyone else rebuilding the engine has to replace the ECU, reprogram the immobilizer, so the one time VVT learn can take place.