I'm pretty sure my harmonic balancer is on its way out, but would like some tips for confirming the diagnosis/checking other possibilities before I order parts.

Car has gone into limp mode twice at full throttle/high revs (about 5000) on the freeway. Both times, it has run normally after a restart. INPA shows a fault code 5768 Electronic Throttle Control Monitor Level 2/3 Mass Air Flow Calculation. I know a few things can cause this, but I suspect the harmonic balancer is on its way out, since the car has also recently developed an intermittent rhythmic chirping sound from the left side of the engine, cycling up and down with engine speed (common at startup, usually goes away after a bit, comes back sometimes when shifting to reverse or cornering left or right). Seems like a dying crank pulley would explain both issues, but what else can/should I check before ordering a replacement?

Car has also thrown an error 312 (O2 sensor circuit high voltage, sensor 2) - is there any reason to think that might be related? That error has only occurred once, unlike the 5768 code.

 

If there is rubber residue around the pulley or if you are able to push the pulley in so that it contacts the engine, the chances are good that pulley has started to separate.

The chirp cold also be an idler pulley bearing.

Going to move you to the Stock Problems/Issues area.

 

Very common to see this with a failing crank pulley so time to upgrade to the ATI damper
https://www.waymotorworks.com/super-...ulley-r53.html

the 02 code isn't likely related, but I would fix the damper then diag from there.

 

X3 Yep! Crank pulley!

 

Think I might have found the problem....took the belt off and couldn't find any give, wiggle, wobble or whatnot in the crank pulley. Put the belt back on, and found a surprising amount of slack remaining until I gave the tensioner a little assist with a pry bar. Seems like this would fit the symptom? A little slip in the belt under hard acceleration, supercharger doesn't spin up properly, and we go into limp mode?

 

Do you have the history of the belt and tensioner? Could it be wrong belt after SC reduction pulley installed? Or just failing tensioner, and if so how does it fail?

Would like to know what is/are the cause of belt slip.

 

Tensioner is either original or has about 70k miles on it (engine was replaced under warranty after catastrophic coolant loss - I assume they kept the original tensioner). Belt has been on for about two years, and looks just fine. Everything is stock.

As to how the tensioner could be failing, I assume either the spring is getting weak or something's up with the damper. I might be able to tell which it is once it's off the car. But either way, the tensioner is reluctant to return to its proper position after the spring has been compressed, so I figure it's not putting enough tension on the belt, which is then slipping under high-load conditions (i.e. high revs and open throttle).

 

Thanks for the specifics. You should get a 15% pulley while you are at it. You will also need a shorter belt and a set of colder plugs.

 

Thanks to everybody who contributed. I replaced the belt tensioner, the stationary pulley, and the passenger-side motor mount (because, although it hadn't collapsed, it did dump a nice squirt of oily fluid onto my hand when I removed it), and can now run wide-open-throttle all the way to redline without anything untoward happening. And the chirp-chirp-chirp is gone. VICTORY!

 

Good to know you got the problem corrected. A whine is much better than a chirp.