compression test is a proxy for "there's a problem" but you'll probably need to go and do a
leak down test to identify specifically what's the problem.
As stated above these tools are helpful in identifying problems with the core of the motor - how effectively will the pistons, rings, valves, etc function or is there excessive wear or damage. There are lots of additional reasons the motor may not work (carburetor, electrical, etc) but these 2 tests assess the state of the inner mechanicals of the motor.
compression test will tell you how much pressure each cylinder will hold when operating. This is good for comparing against
- manufacturer design specs (what should it be)
- differences between cylinders (i.e. 2 of the 6 can't hold pressure - that's a problem).
Leak Down Test can be used to find the source of a leak by identifying where air that is pushed into the cylinder is coming out. Each cylinder is tested by pushing air into the spark plug hole when the cylinder is Top Dead Center (TDC) so all the valves are fully closed
- Is air squeezing past the rings and coming out the PCV valve? Well then the rings are bad
- Is air coming out the exhaust? There's something wrong with the seal of the exhaust valve(s).
- Is air coming out the intake? There's something wrong with the seal of the intake valve(s).
leak down test https://mobiloil.com/en/article/car-maintenance/car-maintenance-archive/how-to-do-a-leakdown-test
It seems that a
compression test is your first check and you may or may not proceed to a
From a car buying perspective, if you're looking for a good used daily driver I'm inclined to look for a car with even readings across all cylinders via a compression test. I recently spoke to a Mini Dealership that will perform a car inspection with a compression test for $250. Not a bad investment if you want to rule out buying a $10k used car only to find out its got major motor issues. Sounds like rebuilding a motor can cost $3-8k.
From a car owner perspective, I might use the leak down test in order to figure out which specific set of cylinders need help but personally if I've got to go that far I'm thinking I'd either ditch the car or rebuild the motor.