Vacuum leak?

[peeeot]

Recently, I found out that the vacuum reservoir for the cruise control and climate control systems on my 1991 Cutlass Supreme was disconnected, and the operation of the car improved pretty dramatically after I fixed it. Since then, I've been wondering whether I've been missing any other vacuum leaks, since I thought the way the car drove beforehand was normal, and now it is obvious to me that it was not.

 

In order to figure out whether I had any vac. leakage, I thought taking an intake manifold vacuum reading would be a good place to start. I hooked my vacuum gauge up to the vacuum modulator port at the throttle body (3.1 v6). Every late-model car I've checked has had a very steady intake manifold vacuum reading of around 21 inches of mercury, but mine was reading only 18 inches at idle speed.

 

Is this normal? If not, how do I find the leak?

[1990lumina]

say carb cleaner all over the vacuum hoses...if the idle changed then you've found a leak

[slick]

^^ That method will also work to check bad intake gaskets as well.

[patgizz]

i use starting fluid, carb cleaner doesn't evaporate fast enough for my liking(fire hazard). nor does carb cleaner give as difinitive a change as starting fluid.

[peeeot]

I will try using fluid to check for leaks, but first of all, does anyone know whether 18 inches is abnormal? I'm assuming it is, but I don't know for sure.

 

Also, if it IS low, what other causes besides vacuum leaks should I check? The factory shop manual is so confusing to use that it isn't helping me much with this. Can't even find an intake manifold vacuum spec!

[95oldsVan]

Using the vac gauge is a great idea and place to start.18 is a bit low,but isnt abnormal either from what ive seen.However,some detective work is in order if it should be closer to 21.

 

If you have the pcv valve on the back of the engine,check the hose there too.They crack,hardened and brittle.Check map sensor hoses/vac lines as well if equipped.

 

Also,do this test...unhook your vac line to the brake booster,take it off and cap off the area tightly where it hooks up to the engine.If you cant do that,plug hose with a fat screw laying around the home somewhere.See if the vac readings change at all.If they get higher,the problem lies closer or in the brake booster.

 

There can be a small leak coming from the grommets/hose from the vac line to the booster plug.Have your brakes felt a little harder to push down sometimes?

A leaking brake booster,even a small leak,can cause vac problems.

 

Spraying carb or brake kleen is a great way to find vacuum leaks.If you cant hear any hissing,spraying areas where vac lines and gaskets are,will notice a change in idle if there is a leak.If there is a leak,and the spray stuff gets sucked inside,It will also stink up the exhaust fumes so if youre behind the car when it happens,youll get watery eyes and irriated nose,so be careful around the fumes.

It will make the exhaust smell weird for a minute.

 

Also,theres another way to look for leaks,but isnt very reliable...with the car off,find someone who smokes or can smoke on a cigar for ya.Hook up a hose somewhere to a vacuum port and have them blow smoke inside the hose and look around the engine area for any smoke that may be leaking out somewhere.

 

You may need to adjust the lighting around the car,or with a flashlight,at certain angles,see if theres any noticeable smoke leakage out of places that shouldnt be leaking smoke.if it comes out the throttle body or intake air snorkel,thats probably normal.I usually remove the air box and hose and tape off the TB opening and cover it with a plastic bag and rubber band.Do not smoke near the battery either and keep the lighted cigs or cigars away from the engine!

 

Good Luck!