Doing engine compression test

hamhonda, Sep 9, 12:26am
Been a while since I did these so please let me know if I'm doing it correctly.
(usually one 4cyl petrol injected motors).
I'm getting the vehicle up to running temp first, then pulling out all spark plugs (and disconnecting the distributor) screwing the gauge into #1 cylinder and then cranking engine over for about 3 seconds - then repeat on each other cylinder.
Next though I think my gauge my be faulty, is a brand new one from B'n'T but occasionally after doing the test the needle will drop back down to zero before I press the release valve button.

toyboy3, Sep 9, 12:31am
Hold open the throttle let the cylinder get a gasp of air

mrfxit, Sep 9, 12:35am
All plugs out
Remove coil lead or coil power wire
Throttle wide open
4 compression strokes
OR>
Personally, I much prefer to use a remote cable for the starter motor
Bypass's the ign system that way.

hamhonda, Sep 9, 12:53am
Cheers, so just need to hold the throttle open.
Does it sound like the gauge we're using is faulty by not holding the pressure?

tintop, Sep 9, 1:00am
Yes - It should hold for a usable time until you press the relief.

ema1, Sep 9, 1:03am
Some gauges have a check/relief valve similar to a tyre valve in their stems check to see it it is correctly seated and firmly in place and or the valve seal is defective.
You may have to replace the valve thingy in the gauge possibly?
Its a mandatory requirement having throttle open during compression test as it allows a full head of air to be compressed in the cylinders being tested at any given time.
Important to disconnect ignition circuit as mentioned above before doing the test as well, disconnect coil 12v supply wire is best.

hamhonda, Sep 9, 1:06am
thanks for the help team, you've each earned a double ration a rum

ema1, Sep 9, 1:08am
Rum will have a similar kick to a belt from a live ignition circuit matey only thing some high energy systems could make the belt from the HT ignition circuitry. FATAL.!
So just be aware the precautions mentioned etc are adhered to . OK.!

elect70, Sep 9, 1:26am
I have a good quality comp tester comes with its own ply felt lined box but the relief valve is stuck open ,& is integral with the screw in boss . where could I find a rep[lacement . I tried ripco S c Bnt to no avail dont want to buy another 1 hardly gets any use .

melonhead1, Sep 9, 1:50am
Fell free to tell me I'm dumb but would you have to pull out the fuel pump fuse/depressurise rail on a modern car to stop fuel being squirted in? Or does this not matter?

intrade, Sep 9, 4:19am
the gague should move a secound nedle to the compression maximum and keep it there till you press zeroing button to drop nedle back for next test.= sounds like your tool is faulty .
better check your shrader valve is not loos and about to fall in to the engine?

Also mrfixit has said how to do you need disable all fuel and spark and hold open throttle so maximum air can get compressed. if the throttle is closed you draw a vaccume thats how the vaccume for brake booster is made and its why diesel need a vaccume pump because they dont have a throttle closed to draw a vaccume on the intake manifold .

intrade, Sep 9, 4:23am
yes it matters in a way , it wont affact compression inistially but can do when the boors have washed clean with petrol after a few times try , its why yes you do disable fuel and spark for comp test.

melonhead1, Sep 9, 7:48am
Cool bananas. Thanks.

marte, Sep 9, 11:39am
You should do a compression test like normal.
Then squirt in a few squirts of oil and do the test again.

This is so you know if its leaking thru the valves etc, or down thru the bore, past the rings.
The oil will seal up the rings a bit.
If the pressures low and it changes after the squirt, the rings are leaking.

ema1, Jul 14, 7:11pm
Can you unscrew the screw in boss to see from it's opposite end whether something is fouling the relief valve?
Might be a speck of carbon fouling it possibly, if you can unscrew it try a shot of compressed air to dislodge any possible fouling.
Failing that run some CRC or WD50 into the boss part AFTER removing it (if that's possible) that may clear what ever is fouling it?
Other than that I don't know where you'd get replacement parts for your gauge unit. Hope you have luck with it elect70 cheers man.