engine vaccum

[kandied91z]

can someone explain engine vaccum to me and why it's important with our tpi motors?

simply put i'm trying to find out what's wrong with my motor in the starting problems i've been having, i've been told it's not getting enough vacuum which i believe is controlled by the cam so i'm trying to take a step back and get alternate views on the working mechanics of the motor.

in other words my head hurts from looking up things to much and want to see what you guys consider it to be.

:D

[Corey]

Not sure where to start on this so I'll just start in the middle.

A good indicator of low vacuum is that the brake booster doesn't work well (brake pedal is hard to push, like when the car is off). You can also get a vacuum guage at any auto parts store for about $20 - $30.

Vacuum is at is source controlled by the design of the cam but many other things can affect it. Just for the sake of argument let's assume that you have a stock cam on a near stock motor that is supposed to pull a high vacuum. Leaks in the intake manifold will lower the vacuum at the throttle body (or carb) causing the engine to run lean. That also usually makes the engine idle like crap. You can try sprying carb cleaner around the gaskets on the intake to see if you notice any difference in the way the car runs. If so then you have a vacuum leak at that point and the gasket needs to be replaced.

Another thing that will cause a low vacuum is if the cam lobes are very worn and the valves aren't lifting as high as they should. You would need to take the valve covers off and rotate the engine manually and measure the valve lift to diagnose that.

Having said all that, and assuming the low vacuum hasn't been confirmed, look at a couple of other things...

Check the compression, if it is very low, like less than about 120 psi, the engine will get hard to start, not to mention will have little power.

Check the fuel pressure, be sure it is at least 30 psi or so, normally it should be around 45-50.

Check for a weak spark, take a spark plug wire off, lay it over a good piece of metal on the car, and have a friend crank it. You should see a bright blue spark in a regular pattern.

I'm kinda tired of typing now but maybe some of that will help.

Remember, it always comes down to air, fuel, or spark. Once that part is known the rest is easy.

[kandied91z]

thanks alot for taking the time....i'm just trying to figure out what's wrong with my car one step at a time.

:)