- - - - ------=COMPRESSION TEST 101=--------- - - - - -

[c.d.iesel]

ALUMINUM HEADS? loosen the spark plugs when cold,

and snug up to warm up the motor

remove all the plugs

battery must be fully charged and starter motor

spinning like mad

disconnect your ignition PRIMARY - even more

important if you have a HugeBox-o-Transistor mega-spark system

hold the throttle fully open during cranking

valve clearance should have been adjusted first- COLD -

only crank each tested cylinder 4 revolutions

record each of the 4 pulse readings of each of the 4 revolutions

the 1st pulse of the needle should be more than 1/2 of the 4th pulse

for example, if 185 is the target:

30-60-90-120= there's a leak all the time and uniform in size like

a burnt valve, or severly burned out head gasket

90-100-110-120 = that cylinder is sealing up to a point

but begins to leak - rings, leaking head gasket....

120-145-165-185 = is normal

adding a little oil to a weak cylinder and re testing will only show

some sealing by the rings - but indicate a direction to look

...next proceed to the Cylinder Leak Test

[Zenon]

disconnect your ignition secondary - even more

important if you have a HugeBox-o-Transistor mega-spark system

This could be bad. If you just disconnect the secondary (the coil's spark wire) then the coil will still be developing the spark voltage and in fact, because there is nothing connected, it will climb way higher than normal. This still has to go somewhere and could arc externally or potentially cause a fault in the coils internal insulation. It could also cause very high back EMF on the primary that could kill a pertronix or other transistor trigger.

You could ground the wire from the center of the coil so the spark gets dumped in a controlled fashion, but it is far safer all-around and probably easier to disconnect the low-voltage, primary side of the coil so that it never develops the high voltage in the first place.

Either disconnect the points wire at the dizzy or otherwise remove all connections from one side of the coil primary terminals - it does not matter so long as the circuit is broken. With an MSD or other CDI setup, I would diconnect the power to the box itself since it will develop 300V or more at the coil primary and I am not sure what will happen to the capacitor's charge if the coil is disconnected. Better not to have any random high voltages around!

Otherwise, great how-to C.D. The tip to break the plugs loose and then just snug-up before you warm up the engine could be a head saver.

regards,

Zenon

[steve k.]

It is now published to the FAQ.

Thanks a lot, C.D.

steve k.

[c.d.iesel]

my error - PRIMARY side of ignition disconnect

also for all you meg-a-squerters with powered

fuel pumps and injection systems - de-power yer pumps

so the possibility of FIRE is eliminated!!

[Zenon]

my error - PRIMARY side of ignition disconnect

also for all you meg-a-squerters with powered

fuel pumps and injection systems - de-power yer pumps

so the possibility of FIRE is eliminated!!

I figured you probably meant primary but that's what can be good about Wikis and forums like these - others will hopefully jump in to correct a possibly dangerous typo. Still there may be confusion in other's minds about the primary/secondary terminology and anyway it is nice to know why to do things a certain way, so I elaborated. No disrespect intended!

Regarding EFI cars - good point. A tii is the same or worse since the cold start injector flows a lot of fuel during cranking and if fuel reaches the injection pump at all it always comes out in a very ignitable, 400 PSI atomized mist! I guess you can disconnect the fuel pump while it's idling and let it die to make sure there is no more fuel supply. That's what I do with any EFI car - I pull the FP fuse or relay and let it idle until it dies - then I know there is almost no pressure left in the fuel system.

But then again, mechanical-pumped carbed cars are always fuelling, too, and would be harder to shut off the fuel supply, though holding the throttle open probably reduces intake vacuum and velocity so much at cranking speeds that there is not much fuel delivery. If there's an idle cut solenoid, you can always unplug that for good measure, I guess.

And don't smoke while doing any of this, either! ;-)

regards,

Zenon

[ColinK]

For an EFI car, shouldn't the AFM or MAF (if equipped) be disconnected?

I know on my e28/m30 disconnecting the AFM per the Bentley manual changed the CR about 20 PSI. I registered 170ish on all six.

[Zenon]

For an EFI car, shouldn't the AFM or MAF (if equipped) be disconnected?

I know on my e28/m30 disconnecting the AFM per the Bentley manual changed the CR about 20 PSI. I registered 170ish on all six.

Good point. I guess a flap-type AFM or a CIS fuel regulator thingy is restrictive enough to affect the reading but a hot film/wire type mass air flow meter would not be a problem.

BTW, the MS EFI system uses no AFM of any kind - it uses manifold absolute pressure and RPM as the primary inputs.

regards,

Zenon