lxcoupe

iTrader: (8)

Guys, I'm stumped on this one! I have an intermittent thunk/thump/thud sound coming from what sounds like the Fast intake. It doesn't do it every time, sometimes it will do it 10 starts in a row and sometimes it won't do it for 5 starts in a row. It's most audible on shutdown but you can kinda hear it on startup if you listen for it. It's a "thumping" sound almost like someone tapping their knuckles on a hollow plastic shape... like a Fast intake. Short background... h/c/i LS2 has around 7k miles on it, starts right up, runs like a scalded ape, no weird noises or behavior while running, good oil pressure, everything normal except this stupid sound. So I pull the Fast, disassemble, clean and reseal with silicone. Motor runs smoother than it ever has after resealing the Fast, but the sound is still there! I thought for sure that was it! If I remove the CAI the noise gets louder. The only thing I can think of is that the injectors need to be cleaned and are dripping after initial fuel pump prime and after engine shutdown, and motor is backfiring. This car does sit for extended periods of time since I have too many projects but I've been wanting to row gears lately and this sound really has me scratching my head. I'm going to send the injectors out tomorrow to be flowtested, but figured I would ask ya'll. I have the stock LS6 intake I'll throw on as well tomorrow night, even though I'm boggled as to how the intake could be the culprit since I built this motor 3 years ago and it's never skipped a beat.

slogo

Isn't that your throttle blade shutting?

lxcoupe

iTrader: (8)

No the throttle blade does not make a sound like that.

I believe, from looking at my own damn video, I might have resolved this. If you look closely the throttle body actually pops "up" during shutdown. I think my allen head t/b bolts were a few mm too long, I replaced them and put some nuts on for good measure. I'll report back if the issue reoccurs.

lxcoupe

iTrader: (8)

Ok guys I'm back to the drawing board with this one... pop sound on shut down still present. Any ideas?

slogo

So you confirmed that the throttle body blade does not make that noise, because it sounds like a throttle blade sounds when it bottoms out.

lxcoupe

iTrader: (8)

I haven't confirmed, I need to get another LS2 tb or swap on the stock LS6 intake with stock 78mm tb. Was hoping not to throw parts at it but I'm basically at that point. But good idea on the t/b...

slogo

Just remove your cold air intake and push the blade open and close with your fingers
you will hear it, no need to replace anything.

lxcoupe

iTrader: (8)

That was the very first thing I tried before I made a post. The blade doesn't make a any unusual noises when opened and allowed to snap back by hand. No binding or catching of the blade. I think if the tb was malfunctioning there would have to be a driveability symptom of some kind, whether a code, hanging idle, etc.

eb110americana

iTrader: (4)

There is something weird going on with your intake in the video, and it is not just the throttle body. The plastic part just downstream from the throttle is moving too. I cannot tell if the whole intake is moving with it. Is the upper intake manifold cracked at the base of the mouth? If not, is the manifold bolted down tightly? Grab the throttle body and try to move it up and down (car off).

If the intake isn't loose, check the engine and trans mounts.

slogo

If it will run, unplug the wire then kill it, see if noise stops.

lxcoupe

iTrader: (8)

Not sure what you mean by that? Anyways I threw on the LS6 intake and stock 78mm t/b and it definitely did something. Cold starts are snappier, more like a stock vehicle. Turn the key and fire sorta deal. I couldn't duplicate the noise after 20 starts/ shutdowns and the throttle response is infinitely better with the stock LS6 intake and stock LS6 t/b. I wish I could borrow a known good LS2 t/b so I could throw it on my Fast for testing but the car just runs waaaay better with the LS6, I'm tempted to leave it.

eb110americana

iTrader: (4)

I still think your FAST intake was loose. Did you try what I suggested?

lxcoupe

iTrader: (8)

I had the Fast off last night and micro analyzed it for cracks and there are none that the eye could see. I had all inner "halves" of the Fast on the work bench to reseal it last week and could not see anything either. I am using the button head bolts for the valley cover that come with the Fast and there are no witness marks on the bottom of the Fast where the button heads might be touching. There is some slight rubbing on the front base of the Fast with the top of the PCV tube tower on the valley cover, but with the Fast bolted down there is absolutely no loose movement from the front of the intake or the throttle body if I apply vertical or horizontal force to the front of the intake and t/b area.

My TEA 243's are milled .020" and I'm also using the thinner Fel Pro .041" head gaskets for 11.4:1 so I suppose it's possible that the Fast isn't seating properly but I doubt it. Now that I think about it I need to check clearance with the small head studs as I've heard of issues there.

I'll probably order another LS2 t/b after that.

eb110americana

iTrader: (4)

Here, I made a GIF for you from two screenshots of your video. Hopefully it will play correctly:

mOtOrHeAd MiKe

iTrader: (8)

LOL - It looks like it is nodding.

1989GTA

I wonder if that one bolt seen in the picture is tight. Maybe the bolt is bottomed out.

lxcoupe

iTrader: (8)

Ok guys I appreciate the input. A new AC Delco LS2 t/b has no affect, so the problem is with the Fast somewhere. Since I've resealed it, I have no other avenues to pursue with this Fast. LS6 will go back on! Might pick up a new Fast, not sure yet. Might go with a bigger motor . There is absolutely nothing visually, structurally, etc wrong with the Fast. Just one of those things!

SAPPER

iTrader: (10)

Its the TB a d there's nothing wrong with it. Being a DBW, its popping closed. My TBSS does the same thing and its all stock other than headers & CAI. Plus I've heard many vehicles do it when it has a CAI of some sort.
Some do it. Some don't. Mine does it and worries me none.

lxcoupe

iTrader: (8)

I've had more LSX vehicles than I can count and they've never done this... it's not normal.

lxcoupe

iTrader: (8)

Guys as a follow up to this post, I ended up trying a stock LS6 intake that I had on the shelf, along with stock 78mm t/b. That resolved the issue. I then reinstalled the Fast intake, along with a new GM LS2 t/b, and the problem came back. I then installed a new Fast 92 intake (and reinstalled the old ported LS2 throttle body) and the problem went away completely.