Engine and/or exhaust flutter at 2000-2500 RPM

[howardc64]

2000 V70XC SE AWD 167k miles. 2.4 LPT engine. Purchased it recently to fix up to help someone. Stumped on 1 remaining problem.

Engine revs prefectly smooth at all RPMs in park. However, has a either slight miss or exhaust flutter (hard to distinguish) at 2000-2500 RPM and more prominent under load (like accelerating up a hill from stand still). All accessories off (no AC, ECC off) Here is what I've checked so far. Engine perfectly smooth below and above this RPM under load.

• VIDA DiCE reports no codes. There is an intermittent ECM 4801 code. Rear O2 was changed 20k miles ago. No record of front O2
• VIDA's ETM test shows max 2+ degrees of difference between TPS, well within Volvo's spec of 13+ degrees max tolerance. ETM is yellow labeled so been replaced at least once.
• Upper torque mount is new, lower transmission torque mount has no lateral play indicative of broken small bushing. Motor mount on driver+passenger side looks good (no collapse or crack). All rubber bushings (no poly)
• Rehabbed the PCV system. Bypassed the factory boost circuit with larger 5/8 ID hose (tapped it above the oil trap for intake manifold banjo nipple of course) Updated Banjo bolt to newer design with check valve. Blew air through all channels and components in the PCV circuit and engine block and all are clear. No oil leak anywhere
• Got an extra coil and rotated it through all 5 cylinders. This rules out any marginal coils. Replaced plugs and no change to above symptom
• Got an extra fuel injector and rotated it through all 5 cylinders. This rules out any marginal injectors.
• Replaced TCV (had one in hand) and all hoses to silicon. Tapped the turbo CBV connection on intake manifold to a pressure gauge. Good boost, can get 6+ PSI. Tested turbo CBV and opens under vacuum.
• Replaced all EVAP hoses to silicon. Replaced one blown check valve (closest to the intake)
• Replaced passenger axle, replaced front CV joint on propeller shaft, rotated back tires to front
• Front tires are tight when off the ground. No play vertically or horizontally. All ball joints in the steering linkages are nice the tight.
• No obvious exhaust leak cracks or soot around exhaust manifold, turbo, flex down pipe
• Unplugged the exhaust CVVT solenoid, makes no difference. Haven't check the CVVT HUB for angular play
• Serpentine belt tensioner roller has a little angular play. Not enough to make noise. Only noticed during inspection while belt was off
• Compression test shows about 190 PSI across all 5 cylinders. Didn't do leak down (no tool)
• Car seems to be down about 3+ MPG (17) from the 2001 V70XC I owned previously (20) driving on same road+patterns (some hills, suburb, no highway). Maybe related to this flutter or need a front O2.
• New synthetic engine oil, PS fluid, drain+filled ATF, flush brake fluid, 20k mile old angle gear so new AG fluid in there (no leak)
• No leak anywhere in the engine (PCV, turbo oil lines etc...)

I tend to think while the ECM 4801 code might point to a failing cat and/or front O2, engine revs smooth and RPM climbs quick above 2500 RPM so unlikely to have a plugged cat.  Anyhow, seems like I've checked air, fuel, spark, turbo, compression, and mounts. What else is there to suspect?

UPDATE:

Had a second set of ears listen to this. There is no power loss so we thinks its not a miss. Its exhaust noise. We did find a small 3/4" leak on the output of the cat where it attaches to the pipe going to the mufflers. I'll start by plugging this leak. Pic attached.

 

[gdog]

Sorry I don't know too much about the '99+ MY cars but from what I do know (ex-tech) and have read, the first things I would do is monitor the front O2 output and the fuel pressure in real time during the failure mode.

maybe you could post a video?  Hard to understand what you're trying to describe...

[howardc64]

Thanks for the note. The flutter is a bit subtle sound wise and can be felt through the accelerator. I'll see if it comes across on video.

Good idea on live O2+fuel pressure monitoring. Looked at O2 on my VIDA before, just wasn't sure what it was suppose to be. I'll do this test next. Thanks

[loseR99]

If it is that much more apparent under load I'd suspect the front O2 to be on its way out.  Especially given the intermittent catalyst efficiency code.   

[howardc64]

If it is that much more apparent under load I'd suspect the front O2 to be on its way out.  Especially given the intermittent catalyst efficiency code.   

Yeah, given 170k miles and perhaps never changed and seem lower mpg. I ordered a Bosch O2 sensor to replace it anyway. Will report back when that gets installed.

[loseR99]

I had this happen on a 99 R.  I had gone through everything, new silicone lines throughout,  silicone turbo hoses and radiator hoses,  new pcv system,  tcv, maf, etm, plugs, coils, you name it!
 

Hit the road to my parents house in FL and 2nd day i was there,  had a no start.  Ended up being the O2.  The thing just failed after the road trip down,  luckily found a NAPA that had the oem bosch part in stock,  had to haul ALL GLORY TO THE HYPNOTOAD to Tampa from Orlando to pick it up for a Sunday drive home.  After the new O2 went in it ran like a beast.  Smooth as glass and my mpg's went up about 3-4mpg. 

[howardc64]

One thing that made me suspect not the front O2 causing the flutter sound/vibration (I know its hard to describe, kind of like a non firing cylinder but much less pronounced) is that it does this when cold. My understanding is ECU runs in open loop and front O2 readings are ignored when its cold.

But anyway, good to change at 170k and see if I can recover some MPG regardless.