Basic No-Start Guide: Part 3 – Engine Compression

We’ve reached part 3 of 3 in our basic no-start guide.  An engine needs three basic things to start, that’s spark, fuel, and compression.  Without all three of these, your car simply will not run.  Part three addresses compression, the last thing you will check if you have verified the engine has strong spark on all cylinders, and fuel is being supplied.  It is also the way to tell if your engine is in good shape, or needs a rebuild.

If a cylinder has a compression leak, it will have lower engine compression than it should. If the compression is leaking, your engine power and efficiency will suffer. If something is broken, like a broken valve or blown piston/piston ring, the engine will simply stop.  The most common cause of LOW compression is leaking at one of the piston rings or valves. If a seal is damaged or the cylinder is excessively worn, problems could result from lack of pressure, including low power output (especially on hills), cylinder misfires, or starting problems. The most common repairs include replacement of the exhaust valves, head gasket, and piston rings.

Engine Compression Testing

Universal Engine Compression Test Tool RentalA compression test is used to assess the mechanical health of the engine in terms of the compression that can be held within the combustion chamber. It measures the compression that results from the pressure created by each piston as it travels from bottom dead center (BDC) to top dead center (TDC) with the valves closed. A compression test can be done at home in about 15 minutes using a compression tester. eEuroparts carries a universal engine compression tester that will test compression up to 300 PSI with adapters included for almost any type of internal combustion engines. Here’s how you use it:

  1. Check the oil level and warm up the engine to operating temperature (if running).
  2. Disable ignition by unplugging the coil or coil pack and removing the fuses for the fuel pump and fuel injection
  3. Mark your spark plug wires for which cylinder they correspond to and remove the spark plugs and wires. Examine your spark plugs. All should look the same. A damaged plug, broken electrode, discoloration, or oil on a plug are obvious problems
  4. Use a removed spark plug to determine which adapter should be used for your engine
  5. With the appropriate adapter, screw the engine compression tester hose into cylinder 1 by hand
  6. Have a helper depress the throttle and turn the engine over for about 5 revolutions (it will turn over but will not catch)
  7. While the engine is turning, watch the tester gauge – it should bounce until it gets to its highest compression and will hold
  8. Release gauge pressure, and repeat with the other cylinders
  9. When done, replace the spark plugs and wires, and enable your ignition

How To Use A Compression Test Tool

“Healthy” engine compression levels vary with different engines, but typically compression should read over 100 PSI per cylinder with no more than about 10 percent variation between the highest and lowest readings.

A low compression reading in only one cylinder often indicates a bad exhaust valve in the cylinder. Low compression found in adjacent cylinders can be a sign of a blown head gasket. If all the cylinders show low compression levels, it often indicates worn out piston rings or excessive wear to the cylinders, which would require a full engine rebuild.

It can be hard to tell in this case whether the valves or the rings are the cause of low compression. One easy test that can help narrow it down is to squirt a small amount of oil into the cylinder through an empty spark plug hole to temporarily seal the rings before repeating the compression test. If it still comes up low, it is more likely to be a problem with the valves while compression readings read higher, the problem was more likely to have been caused by the rings or cylinder.

A compression reading that is significantly higher than the rest often indicates high buildup of carbon on the valves.  ZERO compression indicates you may have a bent a valve due to slipped timing, or other catastrophic failure completely letting all of the air out.  The next step here is to remove the head and examine these components first hand.  If you have low compression, continue on to the next step, leakdown testing.


Engine Leakdown Testing

The next step if you have unhealthy compression readings is to have your mechanic perform a cylinder leakdown test. For a leakdown test, the engine is set at TDC and the cylinder in question is filled with pressure. The tester measures the volume of air needed to maintain recommended pressure within the cylinder. Here are the steps:

  1. Disconnect the battery of the car, set the parking brake, and remove the spark plugs and wires.
  2. Insert a long screwdriver or extension into the spark plug hole to gauge the depth of the piston
  3. Turn the engine by hand using a socket on the crankshaft, and stop when the screwdriver stops rising or falling and the piston is at its lowest point (BDC)
  4. Using a leakdown tester (different from an engine compression tester), screw the tester into the sparkplug hole and zero the gauge
  5. Connect compressed air and open the regulator on the tester until the system is pressurized to the recommended amount
  6. Record the leakage percentage
  7. If leakage is more than 8%, remove the oil dipstick, radiator cap, oil filler cap, and open the throttle body and listen for air at each one. Also listen at the exhaust tailpipe.
  8. Cylinders 1 and 4 will be at BDC at the same time, while 2 and 3 will be at TDC. So reset the piston height to test the opposite two cylinders

How To Do A Leakdown Test

A leak from the oil filler cap, dipstick tube, or PCV valve may indicate worn piston rings. Air coming from the exhaust may mean a burned out exhaust valve. Air coming from the intake, carburetor, or throttle body may indicate a burned out intake valve. If two adjacent cylinders have similar low leakdown readings or you can head air escaping from another cylinder, or see it bubbling up through the radiator or coolant reservoir, it is most likely caused by a failed head gasket or a cracked cylinder head.

Saab 900 Compression Test

2 thoughts on “Basic No-Start Guide: Part 3 – Engine Compression
  1. Gerard Laprairie

    Can you confirm if a SAAB V6 Engine will fit into a SAAB 900S Convertible 1995 model which currently has the 4 cylinder 2.3L non-turbo and what about the Transmission etc. is it possible and has it ever been done before.

    • Adam Goral

      Anything is possible with enough, why not LS swap it? Just kidding. The NG900 actually did come with a V6 engine called the B258, but we can’t recommend installing it. The B234 4cyl engine in it is a great, strong, efficient motor with plentiful parts. Some consider it one of, if not the best engine Saab ever made. Why not consider adding a turbo instead to unleash the full potential? It would be much easier and the results will most likely be better.

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