Vacuum diagnosis: Am I missing something?

[Twilight Fenrir]

Okay... So, I'm pretty sure I've got a vacuum leak on my '66. My fan runs constantly, except when I accelerate hard, the fan turns off, then as soon as I hit cruising speed, it turns back on... Okay...

Anyway, that's not the anomaly.

As I understand, the way this is supposed to work is, you start with all the vacuum lines disconnected, and ports capped except the one line you use to hook up the vacuum gauge. Then you uncap each port and hook up a line until the vacuum drops, then chase it up the tree doing the same thing...

Damndest thing is, when I uncap a port, before putting the line back on, my vacuum INCREASES, rather than decreases... Am I missing something here? O_o Or just don't fully comprehend how this is supposed to work?

[Graniteman]

Here's a real simple (and old time) way to tell if you have a vacuum leak; pull your air cleaner and get a good size rag/shop towel, have your motor warm and at idle, with the towel block off all air flow going into the carb. I mean smother it, if it speeds up before dying you have a vacuum leak!With no vacuum leaks your motor should of course dye quickly.
At this point you'll know if you do indeed have a leak(s) and you can begin to search for them in ways you likely know; of like spraying carb cleaner at potential leak areas, etc.

Hope this helps, good luck.

Mike
TOA # 70

[xgecko]

I have often wondered how long the rubber hoses that run darn near every mechanical device under the dash would last and I dreaded replacing them.

As Graniteman said using carb cleaner or even water in a spray bottle can help track down a leak. The idea is that when you hit a leak with either of the above you will hear a change in the idle.

However, your approach can work as well, but keep in mind you may still want to use some water to help track down the leak. If you can get the leak to suck in some water it will produce an audible change in the idle which may suffice to track it down. The gauge may also show a twitch, but the sound might be easier to manage while you are not easily able to see the gauge.

I am actually down to just the line to the trans and that is about it since my ventilation system is at present not very useful given how much heat builds up under my hood. I really need to figure out how to dump all that heat even once I install AC as I do not want the AC to be working against engine heat that infiltrates the passenger compartment.

Anyway, between what Graniteman and I offered you should be able to track it down. Good luck!

[69W34]

Sometimes the simplest approach may prove to be the easiest. Disconnect and cap/plug all vacuum lines at/from the motor, this includes any control lines connected to the thermal distributor switch (if so equipped), the transmission modulator and vacuum booster also cap the PCV line at the carb.
This then completely isolates the motor eliminating any parasitic loses, now take a vacuum reading and adjust carb if and as needed to smooth out the motor. Then you can begin to check the intake sealing at the heads.
Once satisfied the intake is not a problem begin reconnect the lines both independently / one at a time so that no more than one system is connected at a time, starting with the small stuff i.e. Heat /AC control - then distributor vacuum - transmission modulator - brake booster - then the PCV. If any you detect any stressing of the motor as in shaking / missing / higher RPM (a condition similar to have the PCV line open) as if it is sucking wind after a given line is fully reconnected its a good bet there's and issue some where in that system.
Finally reconnect all systems except those (if any) that seem to present or have issues.
One note: you can also have the vacuum gauge connected as you walk through this
check list, you will be able to see a slight drop and/or a flutter of the gauge if a leak is present in a given system. Also keep the PRMs at or near idle 650-750 as the motor is more susceptible to extraneous air incursion that will result from the a fore mentioned engine behavior.

[Schurkey]

Okay... So, I'm pretty sure I've got a vacuum leak on my '66. My fan runs constantly, except when I accelerate hard, the fan turns off, then as soon as I hit cruising speed, it turns back on... Okay...

Fan? You don't mean the engine fan, you're talking about the HVAC fan...right?
First Guess: Air leakage at the vacuum reservoir; probably a failed check-valve, but could be split vacuum hose, cracked canister, etc.

A talk with Greg "The Comfortron Guy" is in order.

Anyway, that's not the anomaly.

As I understand, the way this is supposed to work is, you start with all the vacuum lines disconnected, and ports capped except the one line you use to hook up the vacuum gauge. Then you uncap each port and hook up a line until the vacuum drops, then chase it up the tree doing the same thing...

Not my first choice for diagnosis.

Damndest thing is, when I uncap a port, before putting the line back on, my vacuum INCREASES, rather than decreases... Am I missing something here? O_o Or just don't fully comprehend how this is supposed to work?

Easy. Engine fuel mixture is too rich. Uncapping a vacuum port provided extra air. Idle speed increases, which improves vacuum in part because the centrifugal advance may be moving the timing.



You're going to do real vacuum-system work? You need a hand-held vacuum pump. My first vacuum pump was a plastic-bodied Mityvac, it lasted about a week before the pump section was contaminated with brake fluid, and it never worked again. My second was a Snap-On, took two hands to pump it. Very inconvenient, but it lasted forever and never had a failure. Disappeared after a garage fire. I replaced it with a Mityvac metal-bodied unit, which had no end of problems until I replaced the check-valve with a Holley accelerator-pump check valve. Now it works fine. The current-production Mityvac is a different unit than what I have, apparently made in China, and therefore not recommended.

Pumps I have zero experience with:
http://www.amazon.com/Actron-CP7830-Han ... acuum+pump

http://www.amazon.com/Alltrade-648744-P ... acuum+pump

Some vacuum systems have designed-in leakage (the headlight switch on my '66 has no seals at all--just machined metal-to-metal contact--but for testing individual components, you'll do well with a hand-held pump.

[Twilight Fenrir]

Thanks for the input, but... I'm not a fan of spraying stuff around my engine bay to look for a leak And there are waaay too many hoses and connections and components that aren't under the hood. I do have the Comfortron, and power trunk release, so I've got a few extra miles of tube than a base setup.

69W34: that's what I had in mind to do... What flummoxed me was the increase in vacuum/rpm's when I had an open port... that seemed contrary to how it should work to the best of my knowledge.

Schurkey: Yes, I mean my blower fan, not my radiator one.

Engine fuel mixture is too rich... Are we talking about the idle screws? I've suspected I've got a problem with my carburetor and its fuel delivery for quite some time, but I haven't quite sorted it out yet. I took it apart and rebuilt it, but didn't seem to help much.

Haha, the same thing happened with my plastic mightyvac... it will still pull a vacuum... but the gauge got brake fluid in it, and the whole thing just dissolved. I'll look into finding a better one...

Thanks for all the input everyone

[Graniteman]

...............................interesting guys, I've been using my current plastic MityVac for about 25 years with no problem................

[69W34]

" 69W34: that's what I had in mind to do... What flummoxed me was the increase in vacuum/rpm's when I had an open port... that seemed contrary to how it should work to the best of my knowledge."

Right ..though you may recall .... As an engine idles as an begins to run out of fuel it will speed up slightly before it dies So If memory serves ( though I could be wrong here on the numbers the best air /fuel mixture is 13/14 to 1.) As the air /fuel mixture becomes leaner (16/17-1 I really don't know exactly) the more efficient it is as burns off .... Now, and here's the .... BUT .... the engine can't sustain a lean burn as it is simply to hot as carburetors can't control mixtures precisely enough.
Enter computer controlled fuel injection and one of the biggest reasons we are seeing 200K plus miles out of today's motors, a clean burn provide for less cylinder wall wash down and oil contamination and less exhaust contaminates.

Okay off my stump now . . .