Vacuum leak problem

[mjr45]

I found a massive vacuum leak where the throttle body connects to the intake manifold. Took it off and replaced the gasket and still no joy. As much as I hate the idea, I'm contemplating trying some RTV or Copper gasket spray on the thing. Both throttle body and intake are flat as far as I can tell. I need some help on figuring this out. Thanks.:angry

[Diseazd]

What carbs are you running? SU's or side drafts?

[mjr45]

No carbs, efi. All the vacuum lines are new, including all the HVAC mess, I'm only getting 11.5 to 12 on the vacuum gauge.

[grantf]

well have you tried fastwomans yogurt cup test? Vacuum leaks can be hard to trace. The main culprit on my car ended up being the intake/exhaust gasket but still I have lower vacuum than desired. BTW is your avatar your car? I like the paint scheme.

[Zed Head]

How do you know the leak is at the interface between the throttle body and manifold? Have you checked the BCDD? It has a gasketed mounting surface also.

[FastWoman]

There's a common and understandable misconception that the only cause of low vacuum would be a vacuum leak. In fact if an engine is running too rich, a vacuum leak would INCREASE engine vacuum, because the engine would run and pump more efficiently with the extra needed air. What you need to do is to verify that you have a tight intake (e.g. my patented "yogurt cup" test ), and if that checks out, then address fuel/air ratio issues. When your engine is happy with the fuel/air it is getting, it will run at a "normal" vacuum, irrespective of how it's getting the air.

[mjr45]

grantf, yep thats my car and no I haven't tried fastwomans test, simply because I can hear and feel the leak at the throttle body. The other way I know is by squirting a small dab of water at the spot and you see it get sucked in. I put 2 gaskets on it yesterday and it helped a little but still running 11.5 to 12 on the vacuum.

[mjr45]

Fastwoman, I am going to try your yogurt test this AM and you just gave me some needed insight, this motor is running way way rich, the plugs are jet black and covered in soot. I thought it was because of the vacuum leak that it was running rich. Thanks

[mjr45]

OK, tried Fastwoman's yogurt cup test and even used a very nasty cigar to see where leak was and it is from the butterfly inside the TB, leaks out the sides where the TPS and the throttle spring are located, this happened on 2 TB's. I can't tell if there is any type of seal in these locations.

[Captain Obvious]

There are no seals on the throttle shaft.

However... That's quite the leak to cause that dramatic of a drop in manifold vacuum. Is your throttle shaft all floppy and loose in the throttle body? I would be surprised if you didn't have something else going as well.

[mjr45]

Nope the throttle shaft is not loose. I thought I would try to adjust the AFM to lean out the fuel and when I did the engine idled at about 4K and would not decrease with any idle screw adjustment so I put the AFM back to the starting point and no change, this was with a different TB, so tomorrow will change back to the old TB and see what happens, but I suspect that something went wrong with the AFM, I followed the directions and never let the spring loose in the AFM. Right now I'm so pissed I may just junk the whole EFI and switch to carbs!

[Captain Obvious]

Nope the throttle shaft is not loose.

If that's the case then I think you've got other leaks elsewhere. You'd be hard pressed to suck that much air past a well fitting throttle shaft.