Performing A Compression Test
A handy little test that you can perform at home to gauge the health of your pistons and bores.
Why Do A Compression Test? The simple test can indicate if you are having problems with oil seals, damaged bores,
incorrectly seated valves, worn rings or a leaking head gasket.
How To Perform A Compression Test Firstly you need a compression test gauge which retail for about £15 upwards.
Make sure it has the M12 fitting so it fits the engine.
You now nead to prepare the engine for testing.
1 - Remove the top 3 spark plugs, this allows the gauge to screw into on of the holes so
neither of the other 2 cylinders create compression which could change the readings.
2 - Remove the HT leads from the lower spark plugs, the upper spark
plugs should already be unplugged before you removed them.
3 - Unplug the connectors to the 3 injectors at the top rear section of the engine.
An easier option to this is to either pull the fuse out for the fuel pump or disconnect the fuel line
from the fuel rail or the fuel filter. If you disconnect the fuel pipe, make sure you stick it in a
bottle to collect the fuel that will be pumped out as you perform the compression test.
4 - Screw the gauge into the spark plug hole of the first cylinder.
5 - Start the car. The smart has an auto start that turns the car over
for about 4 seconds, if the engine doesn't start, it automatically stops.
6 - Read the pressure reading from the gauge and write it down.
7 - Unscrew the gauge and perform the above steps with the other 2 cylinders.
I Have 3 Readings, What Now?
Compare the readings, depending on what your gauge displays you will either have PSI or BAR.
Petrol 450 (600cc and 700cc)
Ideally the figures need to be as close to 12 BAR or 180 PSI as possible.
The smart engine tolerance is +/-25% so can be anywhere between
9 BAR or 135 PSI and 15 BAR or 225 PSI
But ideally you will want to see all the pressures being very similar.
Exact figures and tolerances to be added soon.
Other engine models to be added soon.
Don't attempt this unless you have removed all 3 top spark plugs and unplugged the connections to the injectors. The 600cc and 700cc engine has 3 more spark plugs at the bottom of the engine which you leave in for this test. If the injectors aren't unplugged, the petrol will be ignited by the lower plugs and come out the top spark plug holes as a big firey mass. The 1000cc fortwo 451 only has the 3 spark plugs but you should still disconnect the injectors to stop the cylinders from filling with fuel.
Hot Or Cold?
There are pros and cons to both but ideally you should do the compression test on a warmed up
engine. This obviously only applies if the engine actually runs. As engine components heat up
they will expand meaning you get a better seal and a more accurate compression reading.
However, working on a cold engine won't burnt your hands and fingers.
A cold compression test will usually result in slightly lower figures.
If your cold compression figures are very low or very high I'd recommend a hot test to clarify.
Many professionals will do a cold and a hot test to compare figures.
Wet Or Dry?
A dry compression test is what is explained above. If any of the cylinder compression readings
are low you can try a wet test. With a wet test you pour a tablespoon of oil into the affected
cylinder and perform the compression test once again. If the resulting compression readings are
OK then you probably have a fault with the piston rings. If the reads are still low you probably
have a fault with a valve. It will either be scored, burnt, damaged or not seating correctly.
The Compression Is Way Down, What Does It Mean?
As stated before, it could be oil scraper ring, damaged bores, incorrectly seated valves,
worn rings or a leaking head gasket. There is a way to narrow down the search.
If compression is low in one or more cylinders, you can isolate the problem to the valves or
rings by squirting a little 30w motor oil into the cylinder through the spark plug hole and
repeating the compression test. The oil temporarily seals the rings.
If the compression readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings would tell you the cylinder has a badly seated valve.