Compression Impressions

Sick engine or a good engine? How do you tell? Does it run good? Smoke on deceleration? Blow oil smoke on acceleration? Leak like a sieve? Drip all over the driveway? Yeah.. it does all that? Then you might want to either run like the devil is after you or embrace the block like a long lost buddy.

Either way you need to consider doing a compression test before you even go further. This is going to tell you just how good or bad an engine is. For an Alfa Romeo engine there is one sure way to know and a right way to do it as there is a wrong way to do a compression check.

First off you need a good quality compression tester. My choice is the kind that screws into the spark plug hole. You can’t always hold a rubber plug style compression tester in the spark plug hole while trying to figure out how to turn the engine, especially if the key is in the ignition and that is the only way to make the engine go around. A remote starter button solves some of that issue. Most automotive parts stores have compression gauges that will do the work you need to find out the compression.

Here are the steps to doing a quality compression test to test:

  • Remove the spark plug wires from the spark plugs. Number them with masking tape to make sure they go back on the one you took it from. Sometimes people have put in the oil pump wierd and the cap is off from where it should be. Number 1 is usually the front plug hole closest to the radiator on the distributor. Some times it is 180 deg. out and it could be in the back hole closest to the starter. Lay them down away from anything that can catch fire or cause a fire. Usually I lay them on top of the radiator or remove the whole cap as one assembly. If you leave the cap on, pull the main wire from the coil so it doesn’t spark or remove one of the leads like the one that goes to the distributor.
  • Make sure the engine is up to operating temperature BEFORE you start the test. If you have just driven the car, wait a while for it to cool down some. Cold engines do not always give good results.
  • Get some compressed air and blow out the dirt and crud that accumulated around the base of the spark plug. Put a towel over the top so you don’t blow crud all over your nice car or in your face. You do not want any of that to fall down in the cylinder and screw up a good reading.
  • Turn the spark plugs a 1/4 turn and tighten. Turn the engine over a few times to blow out the loosened carbon build up that might have fallen on the valve face or piston.
  • Remove the spark plugs noting color (should be a light grey/brown/white). Anything black is oil or heavy carbon build up from oil or leaking guides. Keep them in order unless you plan on putting new ones in. NOTE OF CAUTION!  Removing spark plugs from an engine at operating temperature can cause the aluminum threads to strip out.   This can ruin the cylinder head.   Do not remove any spark plug that continues to offer resistance after turning it more than one half a turn.   If this happens allow the engine to fully cool down.   If it is still difficult to remove the spark plug it is best to remove the cylinder head and will cost $$$$$$. If you take out the plug and it is stripped you will have to have a Helicoil kit installed in that hole. This means aluminum chips down the cylinder, drilling and installing the Helicoils.
  • Install the compression gauge until it seats.
  • Open the throttle all the way open and keep it open for the next step.
  • Turn the engine over 7-10 times until you get a maximum reading.
  • Write it down #1 ____ #2_____ #3______ #4______
  • There should not be a difference of more than 10-15% between the cylinders. If there is you should rerun the compression test to see if you get the same results.
  • There are some variables that affect the readings obtained from compression testing. They are cranking speed, altitude, temperature, worn camshaft lobes, and high performance long duration profile camshafts. The cranking speed needs to be maintained the same for each cylinder. This may mean jumping your battery to maintain the speed. The effect of the two camshafts above is the same, one of lower readings. Compression data is usually based on 14.7 atmospheric pressure and 59F at sea level. There are factors to compensate for the different altitudes and the corresponding temperature differences.
  • Normal should be about 160-180 lbs per cylinder. Depending upon the compression ratio of the pistons you can have as high as 200 lbs per cylinder.
  • Anything below 150 would indicate some wear of the rings or valve lash is out of tolerance. Should you see 125 or less on a cylinder you have a potential problem.
  • You say you got a very low reading on one cylinder but all the rest are good? Take some compressed air and inject it in to the cylinder while the engine is turned over. This will blow off carbon that might be stuck on the face of the valve. Rerun and see what results you get. Still the same? Not good.
  • Turn the engine over to where the piston is at top dead center (TDC). How do you know it is TDC? When looking down in the spark plug hole you see no valves open and the top of the piston is as high as it can go. Put in some moderate (50 psi) air into the cylinder with the low compression and see if you can hear air coming out of the intake ports or out the exhaust. This is a burnt valve if you hear escaping air.
  • Low compression of 150 or less…put a table spoon of motor oil down the spark plug hole and rerun the compression test. The oil seals the top ring. The compression goes up, you have worn rings. If the compression stays the same you have bad valves.
  • If two adjacent cylinders have low compression it is often caused by a head gasket that is leaking between these two cylinders.
  • If you are fortunate you have a leak down tester. I don’t have one and haven’t used one but they are used in the aviation industry to measure the differential of pressures. You can read more about it here – Leak Down Tester
  • If you notice smoke on deceleration, you are pulling oil down the intake guides past the seal. Time to consider a valve job.
  • Blowing oil smoke and increased oil consumption? This with low compression across all cylinders means your rings are worn past their life. Sometimes the ring land (the space where the ring sits) gets to wide from all the up and down motion, creating a space where the ring can float and bind, causing the rings to break.
  • Sometimes out of adjustment valves can cause a wide variance in compression readings. Hopefully when you run the test your valves are within a reasonable tolerance to factory specs. This isn’t usually a big deal because if the valves are sealing tight, then there is no leakage of air unless it is down past the rings.
  • Reinstall the spark plugs with either the old ones cleaned up, or new ones. I usually rotate the plugs in reverse order when re installing. Put just a bit of anti-seize on the threads and tighten until the plugs seal. Some torque to 25 ft/lbs. If you have weak threads I wouldn’t go that high. Then reconnect everything and you should be good to go.

These are just some of the basics of running a compression test on an Alfa engine. Just remember to open the throttle WIDE open when running the test and 7-10 revolutions.

Any questions? Be sure to post them below and I will attempt to answer them.


About alfadoc

I have been involved in Alfa Romeo cars since 1970 when I bought my 1600 Veloce spider. It was in 1972 that I started the Veloce Register for 1600 cars. It was in 1998 that I added the Sprint Veloce, 750 Spider Veloce and Sprint Speciale registers. Currently I own 2 Alfas: a 1994 164 Quadrifoglio.and a 1991 Spider "Automatic"
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