compression testing

[gsx600]

Hi there, anyone know if it's possible to compression test the engine out of the car ? I have a fly wheel and starter on the engine, but when apply power to the starter motor directly, only the motor turns with out engaging the flywheel.

Probably something stupid like pre engaged starter that needs another power source to energise the motor ?

Engine a late Rover V8, and wanted to test just in case problems before swapping

Ta

[deltaf]

Ok, disconnect the MAIN positive power lead from the starter motor and use a little flylead to test the other connections on the solenoid (round cylinder fixed to the top/side/bottom or the motor).
What you should see if you look at the starter solenoid is one or two 6mm spade terminals sticking out.
Touch the flylead to the spade and you should see the pinion on the starter fly out into engagement (pre engaged).
If that works, then reconnect the main power lead (remove the flylead first) and then touch the flylead to the spade.
The motor solenoid should operate and the motor should turn.
If it dosent, either the solenoids gone for a burton, or the spade terminal isnt the right one..(can be up to 2 of em).
HTH...ps, watch yer fingers.

[moomin]

Errr if you are testing for compression, then surely you want to turn it over by hand anyway

moomin

[deltaf]

Wont ever build enough speed to check the compression level tho if you do it by hand.......please feel free to try it tho

[moomin]

maybe my understanding of compression testing is wrong then... feel free to enlighten me

moomin

[Paul V]

moomin said:

maybe my understanding of compression testing is wrong then... feel free to enlighten me moomin

Bung a compression tester in the spark plug hole, spin the engine on the starter to get the reading

[nighthawk]

I hope you have that thing securely fastened to something.

Engines tend to move round a bit when cranked over out of the car, you'll not be wanting to use your foot as a chock i can assure you.

[moomin]

PaulV said:

Bung a compression tester in the spark plug hole, spin the engine on the starter to get the reading

that's what I thought, but assumed it was done cranking over by hand rather than on the starter. you could argue you'd get leakdown, but then that is worth knowing too...

moomin

[deltaf]

Theres a dedicated tester for doing leakdown which is more useful than a compression check of the usual type.

[moomin]

sorry to keep going on... the leakdown test you surely do by hand? cos you need to hold it at TDC for the cylinder under test surely?

moomin

[deltaf]

Correct. Leakdown tester plugs into the spark plug hole, engine is turned to tdc for the cylinder on test.
Pressure from a compressor is applied (car in gear say 5th to stop it doing a run-awa ) and read from the gauges the percentage of leakage.
Spoonful of oil in the plug hole will confirm/deny piston ring or valves leaks.
HTH

[moomin]

thanks Deltaf
I'm still a bit boggled as to why you can't do compression via the same method - you would only need to turn the crank half a revolution to compress a cylinder which is easily do-able by hand.

okok, I'll stop going on about it now! I should have gone round to help my friend do one on one of his three (count em) dead VHPDs last year... talk about a cheesy engine...

moom

[Mr2Mike]

moomin said:

thanks Deltaf I'm still a bit boggled as to why you can't do compression via the same method - you would only need to turn the crank half a revolution to compress a cylinder which is easily do-able by hand.

Turning the engine by hand would be so slow that the small amount of leakage that even a perfectly good engine has would cause all the compression to leak away before you could ever get a reading. Because of the extra volume that the compression tester itself introduces, you need several revolutions of the engine to get a true reading anyway.