Vacuum gauge reading low possibly? Help

[user removed]

mwebb has it right.

An automatic in gear will have a slightly lower reading than in neutral.

The flutter does not exist in my echo with the source in the booster line.

Most boosters have the check valve in the booster where the hose connects.

If you connected it after the check valve the vacuum would not drop after you shut off the engine.

Typically a fluctuation in the needle indicates a weak cylinder, not producing the same power as the other cylinders.

If your idle is rough then that would be another indicator of a mechanical problem.

Most problems would give you a MIL malfunction indicator light, unless they were very minor, below the threshold of the point where the mil limap would come on.

There are many different reasons for a fluctuating vacuum guage, if it is because something is not right with the engine.

weak valve spring
valve not sealing properly
low compression on one or more cylinders
head gasket
intake manifold gasket
plug misfiring
bad ignition signal
injector not spraying properly

thats just some of the problems that could give you a fluctuation.

the purge line for the fuel tank will give you a fluctuation that would probably change somewhat depending on the fuel level in the tank, since the evaporative emissions control system maintains a slight vacuum in the tank.

full vacuum would collapse the tank so its a regulated vacuum that is very small compared to the highest manifold vacuum you would see.

regards
Mech

[racprops]

I am a old machinic as well and I would aggree if it was not the fact that a bouching pointer is not all that unlikely with todays motors.

So watch and see it the pointer bouches on a cold motor...egr does not normally cut in until the motor is warmed up.

Second I would shut down the egr to test the gauge before I tore into the motor.

Last is of course a dry and wet compression test.

My 2 cents worth.

After all I have tons of power and get a nice clean 29 MPG stock with a bouching pointer...

And have done so for the past couple of months.

Rich

[mwebb]

Originally Posted by Chalupa102

MWebb; in case there is an issue with incorrect cam timing, like if there's timing advance or retard, how would that effect fuel economy?

it depends on more than a few things ,
imho
no one should ever tamper with cam timing .
but , there are always exceptions ... to every rule .

your 2005 corolla has variable cam timing , you should not frak with it.

in my experience whenever i encounter a cam timing error ,
performance is way off , the people are not concerned with fuel economy as much as they are concerned with poor driveability and the
CHECK MONEY light glowing orange in their instruments cluster .



having said that
i am tampering with cam timing on a geo metro ,
so far ,
my test results are not what i was hoping for - the screen cap above looks neat , but it is flawed ,amplitude values are low , the connector to the pressure transducer was leaking , however ,
it does show EVO occurs at 126 degrees after TDC and EVC occurs at 366 degrees after TDC .
with a stock OEM cam and stock OEM timing values .
that is already very advanced for a stock EVO value
typical values are closer to 150 degrees after TDC.

i have installed a 3tech economy cam and a +10 degree cam wheel , now that i have a new connector for the pressure transducer ...
i just need a spare hour or so to connect up ...

[mwebb]

since most people are not familiar with running compression waveforms ...

there is three stacked up the bottom one is "good" relative to the upper two , it shows 720 degrees of engine rotation and what pressure or vacuum is present in the combustion chamber at what degree of engine rotation

720 degrees is two complete turns of the crankshaft

there is no spark , there is a pressure transducer in the spark plug thread connected to a digital storage oscilloscope .

below zero is vacuum , above zero is pressure , in the combustion chamber that fluke pv350 pressure transducer gets zero set to local ambient barometric pressure prior to capturing a waveform which is zero on the screen s

[Chalupa102]

^ Some pretty interesting stuff, mwebb.

Thanks for all the input guys.

Last night, i found the original line that came with the gauge and replaced it with the other line that i had on it. The needle still shakes, but has decreased and doesn't do it as often now.