Lower vacuum and boost after SBTB install.

[Sully008]

Hello all,

I recently installed the Accufab SBTB that I bought here and I'm noticing two things with the car that it didn't do with the stock throttle body.

1. Vacuum is lower. When I first installed the Accufab, it idled at ~15in. of vacuum. This week it now idles @ ~7in. At partial throttle (no boost), vacuum drops to ~10in. Stock TB was closer to 20in.

2. Boost at WOT was 10psi., after TB install it's ~7-8psi.

I immediately thought there was a leak somewhere so I re-tightened all the clamps on the intake and checked the vacuum hoses at the back of the blower to see if they were loose, but I didn't find anything out of the ordinary, and I don't hear any whistle or air leaks when I rev the engine.

I must be missing something. The only thing changed was the throttle body. The engine is stock, running a JLT and 3.2" pulley on the Trilogy. The car idles fine, drives and stops fine, but feels a little down on power because of the lower boost pressure. Could there be a leak in the TB itself? I've read that it could leak through the butterfly shaft. But wouldn't it have to be a large leak to loose ~2-3lbs. and 10in. vacuum? Any ideas?

TIA,

[MOTOWN]

You have a vacuum leak, spray the throttle body at the gasket with carb cleaner, if you hear the engine stumble theres your leak! most likely around the gasket.

They did have some issues with leaks around the butterfly shaft, but that was a few years ago.

[Sully008]

Thanks for the reply. I used permatex gasket maker to seal the TB to the inlet. It seems like there's no gap, but I'll check it this weekend. There could be a small leak there somewhere.

[ctrlraven]

Could always try and put the old TB back on and check again. If the problem is no longer present then you know it was the SBTB. If it is still present then it's another issue.

[Sully008]

True enough. I'll start by resealing the gasket and if that doesn't work, I'm putting the stock one back on because it certainly didn't do this with that one.

[MOTOWN]

Thanks for the reply. I used permatex gasket maker to seal the TB to the inlet. It seems like there's no gap, but I'll check it this weekend. There could be a small leak there somewhere.

You would be better off using the correct gasket for the throttle body.

[ctrlraven]

You would be better off using the correct gasket for the throttle body.
Grab yourself a cereal box, make yourself a quick gasket and try that.

[justbob]

I've never used gaskets, just sealant.


Self proclaimed Builder Of Badassery.

Buy it, Break it, Build it BETTER.
"Since 2004"

[Sully008]

I have the paper gasket that came with it, I figured that the Permatex would seal better. I drove the car home from work today and the gauge never really went below 5in. of vacuum until I was accelerating. Under WOT it didn't read over 5psi. I'm thinking the gauge has stopped working properly. The car still runs like a ***** ape. I've got to find/buy another one to test my theory.

[massacre]

You will see a slight drop in boost pressure with a SBTB, this is normal.

[RF Overlord]

I'm thinking the gauge has stopped working properly. The one that was supplied with my Trilogy kit was bad right out of the box. They sent me a new one and it's been fine ever since.

[Sully008]

You will see a slight drop in boost pressure with a SBTB, this is normal.

That's what I thought after searching the web, and I still think that's correct. It's the lower vacuum that has me confused, but I'll redo the gasket between the inlet and the TB in case there is a small leak.


The one that was supplied with my Trilogy kit was bad right out of the box. They sent me a new one and it's been fine ever since.

I just ordered a new one today and should be here Monday. The gauge barely responds to vacuum/boost any more, so I'm guessing it's toast.

[Vortech347]

I've never used gaskets, just sealant.


Self proclaimed Builder Of Badassery.

Buy it, Break it, Build it BETTER.
"Since 2004"

Ditto. I used to be a gasket nazi. Little bit of RTV black goes along way. No issues on the intake track of my Fox or 03' Cobra.

[RacerX]

With the Trilogy, the IAC is relocated. Did you block off the little slit in the sbtb so you're not getting air through there?

[Sully008]

With the Trilogy, the IAC is relocated. Did you block off the little slit in the sbtb so you're not getting air through there?

I didn't know there was a slit in there, so no, I didn't block this off. What would be the easiest way of doing this?

[RacerX]

Turn the gasket upside down.

[Sully008]

Thanks, I'll check it to see if there is a slit or not. I don't recall seeing one when installing but I've been wrong before.

[Sully008]

So, I just wanted to update this thread. No slit in the TB, reinstalled it using the paper filter that came with it, still unresponsive gauge. Got the new gauge put in, still nothing. :confused: Checked all the connections on the vacuum lines, and noticed the one @ the lower intake was loose.:down: Tightened that one up and the gauge now reads vacuum/boost again, ~15inhg for vacuum, didn't get a chance to get into boost yet, but it's real slow in indicating. I might have a kink or oil in the lines somewhere. I redid the one line going from the lower intake to the "T" for the gauge/FPRS, but I ran out of daylight last night to replace the one from the "T" to the gauge. I'm hoping new line will cure the slow reading. I'll have to do that this weekend.

On a positive note, I now have a 2nd boost gauge that I'll probably give to my brother. And the car still runs great, no poor idle, nothing. I might've been chasing the wrong problem in the beginning. :stupid:

In all honesty I haven't paid too much attention to the gauge until I installed this TB. I might've had this issue earlier and didn't notice it.

[Sully008]

Okay, so now I'm really confused. I worked on the car yesterday and tried to troubleshoot this gauge problem. Where I left off was I replaced the gauge with a new one, no change. Replaced the short section of nylon tube from the manifold to the "T", and got slow operation of the gauge. So here's what I did yesterday:

1. Put the new gauge directly on the short line off the manifold: Slow gauge movement.

2. Pulled the vacuum elbow fitting from the manifold, inspected/cleaned, reinstalled. Put new gauge on short line again: Slow gauge.

3. Just for fun, put the stock TB back on. Still slow gauge.

IIRC, the gauge should jump from vacuum to 0 with a stab of the throttle, correct? My gauge moves very slowly, if at all under part throttle. Only when I go WOT does the needle move with any authority.

I'm currently stumped as the only thing I changed was the TB. Does anyone have any ideas or suggestions on where I should look next? The car's still running well.

[justbob]

Check and make sure you do not have silicone blocking the hole for the adapter at the manifold. My dumb azz blocked it totally shut years back and that's all it was.


Self proclaimed Builder Of Badassery.

Buy it, Break it, Build it BETTER.
"Since 2004"

[Sully008]

I did blow the hole out with an air compressor, but I didn't actually check inside the hole to see if there was anything there. Guess I gotta take it out again. Ugh. There was a piece of teflon tape in the elbow which I thought was the culprit, but wasn't.


Check and make sure you do not have silicone blocking the hole for the adapter at the manifold. My dumb azz blocked it totally shut years back and that's all it was.


Self proclaimed Builder Of Badassery.

Buy it, Break it, Build it BETTER.
"Since 2004"