They'll get you every time! For some reason it's been "vacuum leak season" for me and my rides over the last few months. Here's been my expereince recently with 2 different vehicles- an 85 LG-4 computer controlled carbureted 305, and my 91 GMC Jimmy daily driver with 4.3L TBI V6..... The 'Maro.... My brother having bought this turd off a guy in VA now feels obligated to get it running right. It's his daily driver. It didn't idle right and wouold occasionally throw a Check Engine light at idle (O2 reading too lean). He did plugs and wires, cap, rotor, etc. just on general principle. The stuff he was taking off looked fine and there was no improvement with the new parts. Engine still hammering with a dead miss at idle and the idle speed is too high- 1000 RPMs even with the idle screw adjusted all the way closed. Compression check said all was fine, mechanically. I suggested replacing ALL the vacuum lines in case of a vacuum leak (how prophetic!). Did so, little better but still not right. "MUST BE THE CARB" he said. Oh boy, here we go! He drives up and we do a rebuild on the carb. I am finding NOTHING wrong with it during the rebuild. Put it back on, no improvement. Maybe the EGR valve isn't getting fully closed at idle. Replace EGR valve, no improvement. We then crack into the anti-tamper plate that prevents access to the IAB valve to adjsut the idle mixture. 2 full turns richer and the check engine light FINALLY goes out! But... it's still hammering pretty good at idle with the miss. Now I'm getting pissed. I just KNOW it's not valvetrain problems or an ignition problem. In the back of my brain there are "VACUUM LEAK" alarm bells going off like a 5-alarm fire. But we already checked for that, DIDN'T WE?? I shoot around the engine with carb cleaner looking for vacuum leaks. Can't find any. But MAYBE they are not EXTERNAL VACUUM LEAKS. I mean, this engine must have more than 100 ft. of vacuum lines on it hooked up to *** knows what kinda emissions devices all over the body fo the car, right? The leak could be SEVERAL FEET away from the motor itself. I go around the engine pinching off vacuum lines with a pair of pliers. I finally get to a big 7/32" line that comes off the manifold behind the carb that goes to a vacuum reservoir above the brake booster. This reservoir, we discover supplies vacuum to the HVAC doors behind the dash. Engine immediately idles down about 300 RPM and gets smooth as baby's butt. BINGO, BABY!! The reservoir had apparently suffered catastrophic internal failure and was an "open door" to the outside air! As soon as we fixed that ALL of the carb settings went back to their original position and the car runs like a champ again, hot or cold! It's like it jsut rolled off the showroom floor. Oh yeah, almost forgot, we can now select the air to come out of the dash vents or the defrost vents where before it would only come out of the heater vents regardless of what you selected on the HVAC controls. The FI V6 Jimmy... Bought it back in June as my daily driver and it runs OK, but seems a little "flat" off idle and the engine knocks like a disco on cheap gas, despite the fact I had done a complete ignition tune-up on it when I bought it AND HAD REPLACED ALL THE DRY-ROTTED VACUUM LINES. Threw the Diacom on it a few weeks ago and find the O2 voltages are way too low (original 165K mile O2 sensor) and the engine is running with most of the "block learn" values down around 90-100 (really leaning out the mix). OF COURSE!, I think. Run lean, more chance for detonation and a lean flat-spot off idle. I replace the original O2 sensor with a new one. Slight improvement but not what I'm hoping for. And the spark knock has gotten WORSE!! ARGH!! I decide to check the timing (should have done this first). I jumper the ALDL terminals to kill all ECM-supplied advance and fire it up. Timing mark is sitting on 9* BTDC. Spec is 0*. Yep, that'll do it! I back the timing down to about 1* BTDC. Spark knock is gone, finally, but now the thing IS A TOTAL PIG unless you lay into the throttle quite a ways. It'll surge forward for a second, then fall on it's nose, then slowly build up speed, then BAM! it'll take off like it's got a rocket strapped to it's butt. It's practually undrivable in traffic. And the idle is for crap- worse than ever. I'm starting to doubt the wisdom of ever touching this damed thing. I shoulda lived with it like it was. But I'm now on a mission. I check the EGR system- try runnign around with EGR valve unplugged. Maybe a little better, but not much. I replace the EGR bypass valve filter although there is nothing wrong with the old one- no improvement. I make sure the spark advance from the ECM is working by revving the motor at idle- it's fine. I check fuel pressure, also fine. I throw on a new fuel filter jsut for good measure- no difference. Those alarm bells are going nuts in the back of my brain again- "YOU'RE GETTING A LEAN STUMBLE" they keep saying (yelling). How is that possible?? I repalced all the vacuum lines when I bought it. Where could the extra air be coming from?? The alarm bells still won't go away. I stand and watch the engine idle for at least 10 minutes. I'm in a trance. I let my mind go blank (yeah, I'm not kidding- I get Zen with this stuff when I get stuck on a problem). I next find myself holding a can of carb cleaner over the engine spraying around here and there. I eventually shoot a bit on the front of the TB, down by it's base where the vacuum lines go into it. The engine revs WAY up every time I shoot it. BINGO. BABY! The PCV line (a factory molded line- which is why I didn't replace it with the rest of them) was ripped 3/4 of the way around! Underneath, naturally, where you couldn't see the rip. Replaced it with some 3/8" hose I had laying around and suddenly IT'S A NEW TRUCK! It NEVER drove this well before. Idles smooth, pulls evenly, no spark knock. And the PCV valve isn't buzzing at idle like it has been since I bought the truck. I'm smiling. NEVER EVER UNDERESTIMATE THE DEVASTATION A LITTLE VACUUM LEAK CAN CAUSE! Now I'm wondering if my 94 Formula's slight off-idle hesitation could be.......................... nah! COULDN'T be that!

Great stories. Ones to remember. Thanks for Sharing.

Man you've got that right. Vacuum leaks are the worst.
Followed closely by Vibrations, and Noises. They sucks.

I've got to agree here. I hate vacuum leaks with a passion. However, I've got the opposite problem. Every car I own always runs rich. The '84 had too much fuel pressure (blew the needle off the seat) and had the canister purge hooked up *** backwards. The Olds is also running rich at cruise, but I'm not tracking it down till I get the carb working right (my instincts are screaming "canister purge valve" here too). The '79 vette runs rich on occasion (at least it smells like it), had a problem with the choke not working for crap (replaced it with an electric one, WOW, it's amazing how much power it has when the secondaries actually open!). Even the 'Hawk has started running rich on me, my fuel mileage is crap lately (16/21 vs. the usual 19/26)--I'm suspecting crappy factory plugs here.

I'm not complaining though. Rich problems are easier to track down than lean ones. There's only so many places extra fuel can get into the exhaust.

<-------runs outside and checks PCV valve, canister purge, vac resevoir, brake booster vac and all other related hoses.

If you think that's bad wait until I post about my expereinces with screwed up EGR systems! Double UGH!

I know THAT feeling. I don't much care for EGR myself, thank *** I don't need it on the 84. I'm not looking forward to having to deal with it on the 91...

Man, Damon.....now you have me thinking about a stumbling-at-idle problem I'm having on my Thunderchicken......guess I'll have to go "Zen" on that for awhile longer, THANKS TO YOU!!
(jk)

My Code 44 O2 sensor lean i tracked back to my Fuel injectors, they were clogged all to hell. 1/8 tank of high octane fuel + 2 full obttles of that fuel injector cleaner did the job.

I'm bringing this back from the dead, because there was a little tidbit in here that I never saw before in all my research and asking questions here on the board in search of (leaky) vacuum lines:
Pinching them with pliers to check if they are leaking later down the line.

Aces!
I've been using WD-40 to search for a leak.... doesn't seem to work worth crap.... even spraying it down the primary venturi doesn't do anything...
I'll have to pick up some carb cleaner.

Wow, this IS old! 3 years old!

It's still true. A little set of needle-nose pliers used to pinch off vacuum lines is a very handy tool for quick-n-dirty vacuum leak tracing. Just don't pinch any of those hard thin plastic vacuum lines- they'll crumble to pieces.

Propane works well for finding leaks.

Am I correct in assuming vacuum leaks can ONLY originate from the intake manifold and carb? There are no other vacuum "sources" to worry about? I mean, intake gaskets, carb gasket(s), and any vacuum lines that lead to intake manifold or carb right?

Yeah, but not if the leak is at the end of a vacuum hose that's 5 feet away fron the carburetor.

Well yea, but if the line goes to the carb, and I pinch the line at the carb and it slows down... Then I know it's that line, and I follow it back to a crack/split right?

Yes, that's correct.

There can also be leaks between the carb and intake and between the intake and heads, but they can usually be diagnosed by more "traditional" methods like spraying carb cleaner at them and see if the idle picks up/smooths out.

The only places there can be vacuum leaks, at the end of the day, are things somehow connected to the intake. Only the intake generates vacuum, no other source anywhere on the engine can build vacuum.

everyone just be careful spraying that carb cleaner or whatever it is you're usin near the distributor or plugs .... unless you're tryin ta find out how ta set your engine on fire .... or worse!

I am reviving this post since i have a nagging vacuum leak that i'm trying to find.
Supplies Needed:
- Pliers
- Carb Cleaner
- Stethoscope

Ok heres the deal, when its idling it has a tiny lean stumble and theres a whining noise that "seems" to be coming from the a/c vents. At first i thought it was an AC whine coming through the radio because it was high pitched but it occurs when the radio is disconnected and the speakers are off. It whistles when its idling and fades away when i rev the engine, so it sounds definitely like an air leak. I disconnected and plugged the vacuum control line at the T junction where the brake booster gets its vacuum and it still does it. I listened all around the engine and it seems to be coming somewhere around the throttle body on my TPI. Now, i have yanked the throttle body a couple times and cleaned it out recently and cleaned the IAC valve with TB cleaner and replaced the TB gasket (mates the TB to the Plenum) and of course the IAC gasket . The bolts were kinda loose the other day so i torqued them, but its still making a hissing sound. LOUD. SSSSSSSSSSSS.... any ideas? I gotta try the carb cleaner, that seems like a good idea, once i put the new starter and power steering pump in......Thanks

wait. when i pinch the vaccume lines what should happen to the engines idle? Up/down/stay the same?

morbid, depends on the vac line.

Awesome post! My 88 GTA has been running lean as hell, I wish I had the Zen mind mastery that you have and looked into vacum leaks before.......now I know how to get this thing fixed!

im going to bring this back too, cuz i believe i have either 1, or many, vacuum leaks.

My maro idles high and drops in rpms every time it "rattles", sounds like it may be coming from the carb .. ?

And If i Just go buy some carb cleaner, and spray it at certain points, my idle should go up correct?

Wow, this this thread is back AGAIN! I wrote the original post in 2002- six days earlier it would have been in 2001 !

Well, it's still all true and it all still applies.

Yes, you can use carb cleaner to find vacuum leaks. Use the little straw they supply taped to the can so you can pin-point it rather than spraying all over the place. Use small sprays and don't get it near the throat of the carb or TB or it'll just get sucked into the engine and give you a false reading. Sometimes idle will go up, sometimes idle will go down, depending on whether your engine is running a little rich or a little lean to begin with. If there's a vacuum leak, you'll know it. What's supposed to happen is NOTHING. If you spray and the idle doesn't change you know there's no vacuum leak near where you sprayed.

And don't forget possible vacuum leaks far away from the engine- stuff way out at the end of vacuum hoses and such. Sometimes it helps to simply remove the hose (temporarily) and plug the fitting. That takes that whole line and everything connected to it out of the equation.

Last- if you have a a cc-Qjet on your motor a malfunctioning choke will often be the cause of a high/rough idle. If the choke doesn't open all the way the carb will hang up on the last step of the choke's fast idle cam (which is built into the choke linkage). Choke must get FULLY open to drop down onto the curb idle speed screw.