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2016 Legacy Surging

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Ok, new to the forum, having a problem I can’t find a cure to. About three weeks ago, my wife’s Legacy started surging. It started with a P0420 code on the Cat. Thought maybe O2 sensor so started there and replaced both. Problem persisted and same code popped. Went to replace the cat, and it had been punched out (branded/salvage title). New cat didn’t fit, so I went ahead and threw some non-foulers on the downstream sensor to fool it into thinking the cat was there and working. No more codes, but still surging. Thought maybe a fuel delivery issue so I changed the pump. Didn’t do the filter though but that’s probably gonna be next. No dice with the fuel pump but the problem persisted. The freeze data showed a throttle position problem. Put a new throttle body in but still surging. In park if I revv It up to around 4K it’ll stutter/surge. Under load it pretty much does it from the get-go and I can’t get it above 30-35mph and it stutters violently. I’m kinda at my wit’s end with this car. Any ideas??
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Dear god stop throwing parts at it! Expensive parts too.

Surging sounds like an air leak. Car is sucking in air from somewhere it shouldnt. I would start there searching the engine rubber hoses, intake, maybe a smoke test before ever touching the Cat. Check fuel pressure too. The money you already spent you could have got a descent real time scan reader to monitor the sensors and engine to tell you whats happening.


There is no way your Cat was bad at such an early stage in its life. The P0420 code is singularly responsible for the dumping of many good Cats every year, to the tune of millions of $$$, all makes of cars. That code should be banned lol :)

Edited by iyalla
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I’ll look into that, thanks! It’s weird. If I’m in park, I can Revv it up til about 3700-3800 rpms before it begins to surge. If I’m in drive, it begins almost immediately under load. In manual mode, I can accelerate and shift ok but no power. You think an air leak could really cause that? And as for the code, I agree! I’m going where my limited scan abilities have told me. Fortunately I can get a refund on all parts but the fuel pump since it was already dunked so I guess I can just leave it as a reserve if the other one dumps on me.
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