Thursday, Apr 25, 2013 at 19:23
You won't be disappointed with your 12HT. Great engine.
Now to answer your questions...
- A compression
test can be done a few ways. The common way is to take all injectors out and screw in a pressure guage into one cylinder at a time. You turn the key to rotate the engine and the compression will be read on the guage as cylinder pressure.
Now compare the readings. Around 10% variation isn't too bad. Less is best.
The next step is to squirt some engine oil into each cylinder. This temporarily seals the piston rings while you crank the engine over again for another reading.
On your lowest reading cylinder, if the pressure reading increases, it means you were losing pressure past your piston rings, which if a big difference, means an engine rebuild. If the pressure doesnt change then it means you are losing pressure past your valves, which means a cylinder head overhaul.
Another way of checking compression is to do a leak-down
test.
This is simply removing each injector again, and screwing in a fitting that is connected to an air compressor with a pressure guage.
You pump in, say, 200psi, and rotate the crankshaft via the harmonic balancer until all valves are closed. Each cylinder should hold pressure. If they don't, simply put your ear to the exhaust to see if it is leaking past the exhaust valve, put your ear near the air filter to see if it's leaking past the inlet valve, and remove your oil filler cap and listen there for rushing air which means your piston rings are not sealing.
For head gasket leaks, you need to warm the engine up so the thermostat is open and you are getting coolant flowing through your radiator. With your radiator cap off, look at the coolant flowing- if there are hundreds of timny bubbles in it, then your head gasket is blown between your cylinder and water jacket- so cylinder pressure is blowing into your cooling system.
If your head gasket is blown between your cylinder and oil gallery, then you can
test that by removing youir oil filler cap and starting the engine. Excessive fumey air being blown out is an indication of this.
As for sleeves in
the block... every aluminium block engine I know of has cast iron sleeves installed. Tour 12HT is a cast iron block so i'm pretty sure it shouldnt have sleeves. The only way I know of is to remove the cylinder head and see if it has been sleeved. Mosrt rebuilt engines are bored 30thou oversize, and up to 60thou oversize bores with larger pistonms to suit. You can get marginally more power due to larger capacity, but your engine can run warmer due to less cast iron as a heat sink before it reaches the water passages.
Personally, I wouldnt even consider or be worried about cylinder sleeves. If they are fitted and you have good compression then all is good. If they arent fitted and you have good compression, then all is good also.
If you listen to the engine running, leave all accessories connected first. Look for white smoke on start-up- less is best- and it should idle nice and low- around 600rpm.
If the injector pump needs a rebuild (BIG money), then the common way to keep the engine ideling
well is to increase the idle revs. So if its idling high, you could have a worn-out injector pump that is being masked up by higher revs.
I hope this helps!
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