Vacuum Line Leak Test?

Any tips for checking for Vacuum leaks?
i know sound would be a first indicator,
just was wondering, no problems im having with driveability
just want to find small issues before they grow!
Thnx

Use a vacuum gauge. They are cheap, and available at any parts store. If you have a SC Riv, install a Vacuum/boost gauge. The 3800 engine should pull a solid 20" of vacuum fully warmed up in Park.

Spray with brake cleaner.

96riv wrote:

Spray with brake cleaner.

Not any brake cleaner will work.

Starting fluid will work sprayed a little along each line and at connectors.

I have hears of using a propane torch, no flame, lightly hissing out gas with tip moved along lines and connections. When you move tip by a var leak idle will change speed. But i've never done that so you should see if another person posts to confirm this who's actually done it.

which line do you use for the test? ive done it and i get no vac on account of the throttle body suckin it out.what am i doin wrong?

robotennis61 wrote:

which line do you use for the test? ive done it and i get no vac on account of the throttle body suckin it out.what am i doin wrong?

The throttle body?

Start with the var tree on the supercharger.

i must have a fat leak somewhere. whenever i do the test all i hear is the sucking thru the throttle body. i swear to God i am retarded when it comes to mechanics

robotennis61 wrote:

i must have a fat leak somewhere. whenever i do the test all i hear is the sucking thru the throttle body. i swear to God i am retarded when it comes to mechanics

Dont beat yourself up
Its just one thing that u might be doing a little wrong
We all have stupid moments

albertj wrote:

robotennis61 wrote:

which line do you use for the test? ive done it and i get no vac on account of the throttle body suckin it out.what am i doin wrong?

The throttle body?

Start with the var tree on the supercharger.

i did.all i get is a loss of suction at the throttle body.i hear hissing thru there.any ideas?shouldnt i be getting total vacum?

So suddenly my vacuum dropped from 9-10psi to 3psi. I read in psi no in/Hg...

I have no idea why.. No vacuum leaks from what I can tell. Everything looks tight, I cant hear anything with stethoscope, and spraying aerosol around lines doesnt affect idle.

Anyone have any ideas on what I should look for? Car is driving fine and boost #s seem fine too.

Ive had and tracked down a couple vacuum leaks before and each time my fuel trims were skewed.

This is not the case now. Fuel trims appear fine.


EDIT:: Map sensor readings also appear to be correct on all my scans.

If youre reading a separate vacuum/boost gauge, then Id say it's gone screwy or the line got pinched somehwere. I had a cheap-o gauge do something similar years ago.

charlieRobinson wrote:

So suddenly my vacuum dropped from 9-10psi to 3psi. I read in psi no in/Hg...

I have no idea why.. No vacuum leaks from what I can tell. Everything looks tight, I cant hear anything with stethoscope, and spraying aerosol around lines doesnt affect idle.

Anyone have any ideas on what I should look for? Car is driving fine and boost #s seem fine too.

Ive had and tracked down a couple vacuum leaks before and each time my fuel trims were skewed.


This is not the case now. Fuel trims appear fine.


EDIT:: Map sensor readings also appear to be correct on all my scans.

Charlie, vacuum is measured in in/Hg, boost is measured in psi.(see gauge picture)  How are you measuring, or are you relying on the scan tool?  Vacuum gauges are cheap enough, buy one and hook it up.  Then spray around until you find the leak.  I have an analog Autometer boost gauge that measures vacuum and boost.

Yes, I understand that traditionally vacuum is measured by in/Hg. But on my scan tool I am using PSI for vacuum and boost. It's just converting it. Nevermind the units I am using to measure..
But if it helps...
What I have always seen

What I am seeing now


I already tried spraying and stethoscoping for vac leaks. I cannot detect anything. Also, fuel trims look fine. Not affected by any vac leak that I can see.

MAP sensor is looking fine too.

Is the idle settling like normal or has it been higher with the colder temps? Is throttle body returning to 0% TPS?

th3fr4nchi5e wrote:

Is the idle settling like normal or has it been higher with the colder temps? Is throttle body returning to 0% TPS?

Idle is normal and TPS returns to 0% when no throttle is applied.

It will be much easier connecting an inexpensive analog vacuum gauge and then spraying and watching the gauge. It will let you confirm the scan tool readings also.

LARRY70GS wrote:

It will be much easier connecting an inexpensive analog vacuum gauge and then spraying and watching the gauge.  It will let you confirm the scan tool readings also.

Thanks, I did this last night. Getting about ~6in/Hg. I usually see 20in/Hg

You mean 15-20in/Hg at idle right? If you're only getting 9-10in/Hg, there's a problem, let alone 6. Your cam isn't big enough to need an external vacuum source.

Abaddon wrote:

You mean 15-20in/Hg at idle right? If you're only getting 9-10in/Hg, there's a problem, let alone 6. Your cam isn't big enough to need an external vacuum source.

Right. I have no idea where to look for the issue. I've checked everything I know to check. Im out of ideas

Edited my post above... usually get 20in/Hg only seeing 6 right now..

Did you run the vacuum check from the Brake Booster hose? Right where it connects to the engine. Disconnect it, and hook the vacuum gauge up to the engine. That's the closest you're going to get to actual engine vacuum.

Abaddon wrote:

Did you run the vacuum check from the Brake Booster hose? Right where it connects to the engine. Disconnect it, and hook the vacuum gauge up to the engine. That's the closest you're going to get to actual engine vacuum.

I have not done this. So you want me to pull the hose off and connect to vac gauge, start the car, and see what that reads?

Yeppers. Forget all those baby hoses and lines. Go right for the source.

Speaking of that. If you really do have a drop in vacuum that high, you'd definitely get less brake booster assistance because of it.

Abaddon wrote:

Yeppers. Forget all those baby hoses and lines. Go right for the source.

Speaking of that. If you really do have a drop in vacuum that high, you'd definitely get less brake booster assistance from it.

Ok. I will check all this stuff and report back. Thank you

I'd keep the gauge hooked to that port. If you get a lower than "normal" reading, you can start disconnecting vacuum hoses and capping them off (engine off). Do sections at a time. See if you can find the general area where the leak is....if there is one.

EDIT: Another reason to use a manual Vac gauge....what if the sensor that reports vacuum is the one causing the problem?

Brake booster needs about 10" minimum to function. Do you have power assist? The booster itself can leak. Try pinching off the booster supply while watching the gauge.