kevsgt

Car has been doing this sporadically for a few years. There's no CEL. At first I thought it was related to CAI, but now I think not. With each thump there is a barely noticeable instantaneous increase in RPMs.

https://www.dropbox.com/s/tmp4rbq85a...%20PM.mov?dl=0

Derf00

I don't have audio on my work PC but if it's affecting the RPM's and there's no CEL i would guess the CMCV's (butterfly's) that are built into the intake manifold. They are designed to improve emissions at low RPM and are closed at idle. If they are getting stuck closed as you slowly increase the RPM, the thumping is most likely when they snap open. This would affect your RPM's for second as the engine compensates for the extra air being let in when that happens.

http://www.mustangv8.com/fr/moteur-g...71070096;image

They are controlled by a motor at the back of the intake manifold.

kevsgt

Any ideas how to verify that as the cause? Is the motor replaceable?

Derf00

The motor is replaceable but I’m not sure on how to troubleshoot it. You could disable it temporarily by disconnecting its power cable at the back of the intake manifold on the passenger side. It’s crammed between the intake manifold and the firewall. Once disconnected, see if the sound still exists. If it’s gone, there’s the problem.

If that is the issue, you might just need to clean the plates like you do a throttle body.

08'MustangDude

Unplug the power/harness as mentioned, and if the noise stops, that's it.
Spray in some Seafoam at 2000 RPM, or other cleaning agent.

kevsgt

Ok I researched what the CMCV's do, but it sounds like they don't start to open until higher RPM's. The video I posted was car just sitting at idle. So I don't see how they would be the cause, unless they are somehow not sealing closed at idle and letting air leak thru?

As far as unplugging the motor, do you see a problem with unplugging after the car has been started? That way I can wait until I hear the noise (it doesn't always happen) and then unplug it and see if it stops immediately. But I'm unsure if that would cause any problems, or just throw a temporary CEL.

I thought I saw a post about the SCT tuner having capabilty to lock the butterflies open, but I'll have to look into this.

08'MustangDude

Yes, you can pull the plug while the motor is idling.

Even if the CEL comes on, it will eventually go out after you hook them back up.

A $19.98 Walmart bought scan tool should be able to clear the codes.

During a bad rain/snow storm, my active shutters stuck on my other car, and that
cheap scan tool cleared the code.

kevsgt

I unplugged it. No change. Still not sure what's causing the thwump sound. It's louder with my ear up to the air filter.

Derf00

Dirty throttle body plate or, if you are an '05, you have a carbon filter inline between the air filter housing and the throttle body. Remove it if it's there and try cleaning the TB with TB cleaner.

kevsgt

Car has been doing this more often again, perhaps related to the colder weather. Mainly happens after starting the car and then settles down. Any other ideas how to troubleshoot it?