Throttle Body Vacuum/Boost Leak

[ion] C2 · 09-02-2018, 03:25 PM

So the throttle plate shaft itself is leaking air when I boost leak test. You know how you open the throttle plate by manually moving the part the cable is connected to? If I wiggle that my air leak changes significantly. It's apparently leaking air between the shaft and the main body of the housing.

From some Googling it sounds like there's a bushing that is worn that causes this. Anyone know a place to send it to that can do a good job on a rebuild?

I'm still working on having a bigger custom manifold made with a used 75mm throttle body that hopefully doesn't have this problem, but I'd like to fix this in the meantime.

[ion] C2 · 09-04-2018, 03:33 PM

I picked up a spare stock throttle body from a parts yard and it seemed like the shaft was pretty solid. Installed it on the car and pressurized and this one leaks too. Maybe they all do that? Maybe I just have two high mileage ones.

From searching around other car forums it seems many run into this issue and fix it by replacing the o-rings and oiling/greasing them up at install. Not sure if our throttle bodies have o-rings but I'm sure I can tear apart the other one I have now and see what I can do. Edit: Took apart the throttle body, there are no seals aside from the bearings. Turns out a fuel injector o-ring is the perfect size for a space inside there, so I put a viton o-ring in and re-assembled. It might seal, but I haven't installed and boost leak tested it yet because it turns out me fixing my other hose leak seems to have fixed my high RPM coastdown/revving up issue. The seepage in the throttle shaft itself doesn't seem to cause any big issues. Might swap it out for shits and giggles just to see if my o-ring job helped.

[ion] C2 · 09-05-2018, 04:48 PM

Intake Manifold
The intake manifold has two pinhole like leaks at weld areas. Easy to fix, but will have to remove it all and coordinate with my welder.

Throttle Body
I installed my old throttle body that I put a thick o-ring in - when I pressure tested like that it was much slower to leakdown back to zero from 20 psi. Notice a big difference while driving too. It doesn't rev up when I disengage the clutch and it doesn't hang at high RPMs or rev up as I coast down to a stopsign etc. So the throttle body leaking is definitely a huge source of the overall vacuum leaks. Major driveability boost.

It seems throttle body shaft leakage is a pretty common issue across all platforms. The DSM guys have a company that makes special high pressure seals. The stock LS1 seal is designed to hold vacuum, not boost, just based on the way it's shaped. So when I do make my fancy bigger manifold, I'll have to look at the throttle body I'm using and make sure I can seal that. Or make a custom throttle body that has specialized shaft seal areas. One guy made a custom seal for the stock LS1 one, I'll likely take it all apart and spec it out and see what's available.