2005 OBXT sounds like a diesel when gently blipping from idle

[serx7]

Last week I had a failed clutch replaced with a Competition Clutch Stage 2 Steelback Brass Plus (recommended by the tuner/shop I went to). Clutch seems good, engagement is quicker than stock and a little harder to modulate but it reminds me of an upgraded clutch I had in my 1st ever car back in the early 90's).

 

Right after the clutch installation work, I noticed 2 things:

 

1)

The starter motor cranking sound is now MUCH more subdued than it was before the clutch replacement. I thought I had fluid in my ears the 1st time I started the car after the clutch job, but yup, the starter sound is way quieter now. I guess no complaints there.

 

2)

If I gently blip the throttle from idle (eg: blipping before engaging the clutch for a normal take off from a stoplight), the motor makes a noticeable diesel-like sound for a split second as the revs jump slightly and then sounds normal after that. I don't recall hearing this sound before the clutch replacement, or at least don't recall hearing it loudly enough to get my attention. The sound does not resemble what I'd expect to hear if there was an exhaust leak of some sort (it's more deep metallic in tone than 'farty' like an exh leak). I haven't tested this from cold idle (thicker oil) but will probably test that tomorrow morning.

 

The shop did mention that the 6MT (Spec B swap) was missing a support pin of some sort, which they replaced using an identical part from a spare tranny they had in the shop. Not sure what the actual part was called but IIRC they said it was 1 of 2 such pins, which help to take the load off the other trans mount bolts.

 

I don't -think- the sound indicates bearing/oiling issues (famous last words). I've had rod bearings go in 2 prior cars (the 2nd one was in my 2005 Saabaru 9-2X Aero aka WRX Wagon) and in both of those cases blipping from idle was fine but the tell-tale metallic clattering sounds would occur under light load right around 3000 rpm.

 

W/ the OBXT, under my regular operating conditions (cruising, boost, engine braking, various rpm b/n 2k and 6k+), the engine sounds the same as it has all along. If I hold say ~1000 rpm and blip from there as opposed to from idle, the diesel-like sound is either not heard at all, or very barely audible. Warm/hot oil pressure is ~20 psi at idle (775 rpm target), ~60 psi at 2000 rpm and sits b/n 78-80 psi from 2500 to 6400 rpm, which is what it's been like since I bought the car 3+ yrs ago. Oil is fresh Rotella T6 5W40 w/ a new OEM filter. I changed the oil this past wknd.

 

Below is a video link whose audio contains the diesel blip sound. In the clip, I gently blip from warm/hot idle, let the rpm re-approach idle, blip again, lather rinse repeat a few times.

 

https://www.youtube.com/watch?v=XxkosDd382c

 

Any feedback would be appreciated.

[doobaruGT]

Hmm...

 

Just running this through in my head and I came up with a few thoughts and questions.

 

 

Questions

1. Is this from a warm start? You mentioned you would check from a cold start so I figured that's what you meant.

2. Do you know if they disconnected your battery?

3. Do you have fueling "upgrades"?

 

Here's what I'm thinking preliminarily:

 

I just had my trans out of the car a few weeks ago and of course, to the trans off, you have to take the exhaust off. I know you addressed the exhaust leak, but can you spot any sooty deposits around the flanges? This is pretty easy to ruin without proper torquing sequence. Still, your video sounds pretty clear and I'm with you, it doesn't consistently make the sound that you would expect with increased exhaust.

 

I asked about the battery because your car may be trying to relearn fueling. I've reset the ECU a few times where it had a lot of trouble relearning. it was to the point where it would bog and even stall 3-4 days after the reflash. This has only happened once or twice. You haven't mentioned anything like that so I'm guessing it's not at that point.

 

My third thought has to do with your fueling upgrades (if any). Our cars do have little stumble to them especially at low load. The sound I heard in your video reminds me most of what it sounds like when your car is struggling to maintain a proper AFR target, similar to when the ECU has been reset and it wants to stall out.

 

I'm thinking that after your exhaust leak inspection, you could check AFR's (if you can) and see what it looks like with that little throttle blip. Does it go rich? Does it lean out? That might give you some indicators about where to go next. Could be a simple as a tuning issue or new plugs.

 

All in all if you're like me, and it sounds like we're not too different, you're just noticing things because you're more highly aware of your car since someone else just tore her apart to "improve" a part. I drive myself crazy sometimes picking up on things that were there the whole time GL, my friend! Don't do too crazy.