Car is a Civic VTi 99. After taking back from garage following radiator replacement I found that it sounded quieter, less crisp, less peppy and vibrant than before.

I thought the engine may have been damaged from overheating, however car has since been investigated, ignition checked, compression tested and dynoed and results show a healthy engine producing close to factory BHP. According to the dyno tester the Dyno graph suggest no symptoms like running rich or lean, exhaust backpressure etc.

This makes me wonder what the problem might be and now I'm thinking maybe while the previous garage was test driving it they hit the exhaust as the car is very lowered and it has damaged the internals of the exhaust somehow leading to the sound changes? Could this happen and if so, how would the internals have been damaged to cause such changes and how can I diagnose this?

Bearing in mind engine, combustion and delivery seem to be fine, could there be anything other than the exhaust which might be causing said issue?

  • There is more to engine sound than just the exhaust. I'm not familiar with Civics, but did they need to remove (part of) the engine air intake to replace the radiator? If you had a leak in the air intake prior to re-installing the car would have produced more sound, although it isn't very healthy for the engine to bypass the air filter. – MadMarky Dec 14 '18 at 8:43
  • @MadMarky No, they didnt need to remove the air filter. – James Wilson Dec 14 '18 at 18:51

The difference in sound you're hearing is almost certainly due to the leaks in the manifold.

Assuming the car is on a stock exhaust (or close to it) then the majority of the sound you hear while driving comes from the engine bay - the "fluttering" sound produced by exhaust gases escaping from a leak in the manifold would certainly cause the aural symptoms you describe.

Damage later on in the exhaust system or to the internals (such as to the silencer or catalytic converter) would be more likely to make it sound louder than the other way around.

  • there is currently a small leak 2mm in diameter, would that be enough to cause the sound reduction as described? – James Wilson Jan 5 '19 at 20:40
  • @jameswilson hard to be sure without seeing/hearing it but that certainly sounds plausible – motosubatsu Jan 5 '19 at 22:32

If the battery was disconnected, the keep-alive memory in the ECM might have reverted to default. That memory uses your driving style to make corrections to computer engine controls. Take the car out for a good hard run of strong acceleration, shift points, etc. Make the ECM learn what you need from it.

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