BLUF - Engine isolated from drivetrain is creating a sine wave that emulates a wub-wub-wub noise.



Background -

I woke up to this noise one morning out of the blue. The night before it wasn't there, nothing had been changed within the previous month mechanically (or electronically). At first I thought it was an exhaust leak from hell - until I changed every single gasket from the Exhaust manifold back to the test pipe/exhaust connection. No dice.

This sound is separate from the drive-train as a whole. In neutral, it makes the noise. Running up to WOT in gear, it makes the noise. Coasting down the freeway and holding the engine speed steady in neutral, makes the noise.

Here are some video's for your viewing pleasure -

Idling, Parked.

Placing load on engine while in gear (Most prevalent IMO a T+10s). The raspy sound is from the dump tube, which was installed one week after this noise appeared.


I have plugged my laptop into the car, and using Evoscan nothing appears out of the ordinary. The car still pulls like a freight-train, no knock, it is just this sound. It is driving me half insane. I've cleaned out the MAF Sensor, and checked for Vacuum leaks (none found using carb cleaner and spraying the lines off the IM, and the connections at the IM). My engine mounts are 75A Innovative's installed 10 weeks prior.


So the puzzle is, what is causing this sound, what is the fix for it, and where do I paypal the correct answer cash.

 

Today if time permits I am going to remove the accessory belt and get the engine running long enough (10-15 seconds) to identify if the vibration is caused by one of the accessory pulleys failing. If that does not work, I will switch the crank pulley with a known good unit (one off an engine that doesn't make its owner want to light it on fire) and repeat the test.

Is there anyway that the balance shaft could have mysteriously jumped its timing putting it exactly 360 off? That will be the last item I can check due to the amount of material that has to be removed to check it.

 

Question... When you installed your dump tube, did you use a divided gasket? If you're not sure, pull it and check to see if the gasket divides your normal exhaust flow from your wastegate side.

 

Yes, I used the MAP Divided gasket and have adjusted the preload of my wastegate. At idle, minimal leakage - can't feel or hear anything - is escaping the dump tube.

I removed the accessory belt and no change to the vibration. Also checked to make sure nothing was wedged between the engine/transmission/transfer case that could transfer the vibration. Mechanical timing was verified and all pieces are lined up as they should be at TDC. Also changed the PCV valve because why not at this point

Still need to switch out the crank pulley tonight, and I want to start disconnecting motor mounts 1 at a time to see if isolation can be achieved.

 

Played around with different Maps for the ECU tonight, nothing killed the vibration or even dampened it. Reduced the torque on the passenger and drivers side mounts, no dice. Sprayed out the inside of the throttle body w/carb+map cleaners, fm couch.

 

kinda weird. i couldnt see the parked video. i feel like my car does this time to time but with poly mounts and no balance shafts, i hardly notice body noises anymore. you have a oscilloscope? use it to probe abound the motor, anyplace it could mount to the body.

 

Hope it shows up for you this time.

No oscilloscope handy, I'm looking at biting the bullet and making the three hour drive to EvoDynamics to gain some insight on this

 

Quick update - it was the balance shafts. The tension pulley went bad and allowed to belt to loosen. This led to the belt stripping its teeth down to nubs.

 

thats good to hear, like i said i have no balance shafts and poly mounts. it may not be as smooth as stock but iv gotten so used to it, i love it.

 

Next engine is having the balance shafts removed, they seem to be just another point of failure.