hidden vacuum leaks

[dlb]

i want to touch on this because it's easy to find obvious vac leaks by simply pulling the air filter housing off and looking for cracked, broken, or disconnected vac lines, or spraying carb cleaner in the suspected area while the car is idling and noting where you are spraying if the revs rise. there are, however, less obvious leaks that can occur. i'll list a few that come to mind for me but everyone else should chime in as well, as a cohesive list of these things should be a valuable resource to anyone dealing with weird engine behaviour.

- vacuum advance diaphragms on the distributor. if the vac lines are hooked up to the ports on the bottom of the distributor but the diaphragms in there are ruptured, the vac lines will just suck air. to test the vac advance diaphragms, hook up a clean hose to each port and give it a good suck. you should hear a click or pop as the diaphragm flexes, and then it should hold vacuum indefinitely. if you're not a germaphobe, stick your tongue in the end of the hose for 30 seconds or so. when you finally release vacuum from the line, you should hear another click or pop as the diaphragm returns to its resting position. if you can just suck wind, you need to block the line that runs to that particular port until you can replace the vac cans. note that there are two separate diaphragms in the cans so be sure to check both.

- charcoal canister. the charcoal can operates by mixing gas fumes from the tank with fresh air sucked in from the bottom port and then dumping those in the carb. unfortunately, as the charcoal in the can breaks down with age, it offers less resistance to the fresh air it pulls in through the bottom port. this ends up causing a lean mixture of fresh air/gas fumes, and can act as a vacuum leak if lean enough. the best way to diagnose it is to simply disconnect the charcoal can purge line from the carb and block it; if whatever symptoms you're experiencing disappear, you've found the culprit. to fix it, the best solution would be to find a newer charcoal can from a newer vehicle at a wrecker. another route is to add a valve into the purge line that goes to the carb. this will allow you to meter the amount of fresh air/gas fume mixture that the carb can suck. it's not a great solution because the mixture is still too lean but at least you can keep the total mixture being sucked low enough that it will not have the vacuum leak effect.

this is the valve i used to fix the vac leak problem on my charcoal can. cost $1 at the local pet store and fits in the vac lines perfectly. it's an air valve for a fish tank air pump.

http://ca-en.hagen.com/Aquatic/Aeration ... ries/A1172

those are two of the more insidious hidden vac leaks i've encountered so far. i'll add more as i think of them and encourage everyone else to do so as well.

[GLet71]

Thank you David, this is way easier than scrolling through the 8000 page PDF FSM! Keep 'em coming so I can get Leonard running proper~

[dlb]

thought of more today.

- EGR diaphragm. the flying saucer-looking part of the EGR houses a diaphragm just like those found in the vacuum advance canisters. if the diaphragm is ruptured, the vacuum that should suck, flex the diaphragm, and thus open the EGR valve, will instead suck air. you can check this by sucking on the hose connected to the port on the face of the flying saucer. same principle as checking the vacuum advance: if you can suck air, the diaphragm is done broke. plug that port and the other end of the line until until you replace the EGR.

- AAP diaphragm. this one isn't so much of a vacuum leak as it is a fuel leak. it's been covered on these forums often enough but i like cohesive lists. so on the front of the stock carb is the AAP port. when the engine is cold, vacuum is applied to this port on acceleration to enrich the mixture. if the diaphragm is broken though, the vacuum will just suck gas into the line. don't suck on this line unless you like the taste of gas! just unplug it from the AAP port. if any gas dribbles out, the diaphragm is ruptured. again, plug both ends until you can replace the AAP diaphragm.

- hell, check all of the diaphragms! same principles apply to all of them--choke opener, choke breaker, throttle positioner...

[deejay1272]

Nice post, DLB. I was just logging on tonight to see if I could get lucky and find an easy solution to my vacuum leak problem that I'm experiencing right now. Since I've deleted most of what you mention above, I'm going to have to take it down to basics and spray carb cleaner around until I find a spot that makes my RPMs blip. The only problem is that my Weber air intake is sucking in the fumes pretty much no matter where I spray the stuff. I'm considering making a temporary snorkle out of a wrapping paper tube or something, but that seems kind of tricky.

Have you ever rigged up a way for the air intake to suck in "clean" air from outside the engine bay?

[dlb]

i haven't needed to make any sort of snorkel to test for vac leaks. if you use the little red plastic tube that comes with the carb cleaner or WD-40 or whatever it shouldn't be an issue.

have you checked that the nuts mounting the carb are all snug? i've had them come loose and cause a vac leak. also, check the intake manifold nuts are snug too.

[deejay1272]

I knew I should have taken care not to lose that little red tube thingy. I'll track that down and check the intake manifold nuts (I've already checked the carb nuts and they're fine).

Thanks for the input...

[4wdchico]

Yes, it is good to remember that not all vac circuits have vac applied when the engine is at idle. Even then the vac states change between cold & hot. So a vac leak may be missed if the tester relies solely on the carb spray test or raw propane test, as the pro's do, while the engine is idling to find vac leaks. The type of testing that you are promoting is called functional testing in the auto diagnostic trade and it is a very important concept.

The FSM is actually pretty good in showing how to test all of the vac circuits and their components for function. Yes, it takes a most of a day the first time you check the full vac system, but you will come away with a much better understanding of how the system works. I guess that I would go so far as to say that w/o testing the whole system, subsystem by subsystem, one will never fully grasp how the system works as a whole.