vacuum strength

RazorRacer · 08-02-2002, 03:33 PM

I am new to working on cars, but have not found this subject covered here, yet.

At idle, putting a finger over the tip of one of the vacuum hoses, how much of a vacuum should I feel?

At 6000 rpm, repeating the same, how much of a vacuum should I feel?

I am not feeling a vacuum under either of the conditions above. What creates the vacuum and what should I look for to see if it's working? How might the car act if there was no vacuum - ever?

This is for a 78SC. Thanks in advance.

dean · 08-02-2002, 04:41 PM

Vacume is created by the piston moving down the cyl. If the throttle is closed you get higher vacume than with the throttle open. There are vacume ports above and below the throttle plate. So at idle in the ports above the throttle you won't feel any vacume

RazorRacer · 08-03-2002, 07:30 AM

That's good to know. I was thinking there was a vacuum pump or something that could fail. I suppose if the engine is running, then you have a vacuum.

So now the only thing to check for is vacuum leaks.

Thanks.

Wayne 962 · Wayne 962's Garage

Vacuum leaks decrease the amount of vacuum that you can feel. Also, if you have a wider hose (larger inner diameter) you will feel a lot more force on your finger.

Best way to measure is with a simple vacuum gauge. They are available most everywhere for only a few dollars. I think that we sell a few in our tools section...

-Wayne

john70t · 08-03-2002, 08:49 PM

You should definitly get a vaccuum gauge. They are the quickest way to judge the condition of an engine.

Theres three different kinds of vacuum: throttle, ported, and mainifold.

Throttle is before the butterfly and (I think) should be 10-15 inches Hg at idle( the amount mercury rises due to vacuum in a calibrated tube) and 18-22 at WOT(wide open throttle).

Ported is taken right next to the throttle. 0 at idle (becuse air is going through the bypass) and 1-22 at WOT. The EGR is run off this, but must also go through a wax pellet thermal switch so there is no hesitation on warmup. Vaccum is also stored in vaccuum amplifiers (don't ask me about the 79 accord which looked like a bowl of spagetti under the hood).
You should check a vacuum routing diagram if possible to make sure there isn't something holding back the vac.

Manifold is after the throttle body and the most important. It should be 18-22 at idle, 0 at snap open throttle, and jump up briefly to 40ish on snap closed. The engine is an air pump and you can check how well it works by taking a reading from the brake booster hose (or another if more conveininent)

In short:
A low reading(16in) could mean late ignition timing, a stretched cam-chain or a worn engine.
A wandering needle could be vacuum leaks or fuel flow problems
A rapidly bouncing needle could mean a bad/sticking/misadjusted valve.
A slowly dropping needle (from throttle vac) could mean clogged exhaust.

RazorRacer · 08-04-2002, 07:42 AM

This is sooo cool. I'm 40 years old and just learning about working on cars. Just got Bosch's fuel injection book, too.

Thanks for the help on vacuum stregnth. I'll get the gauge, too.