Used Car Condition - Is a Compression Test a good proxy of engine health? (and what's a leak down test?)

Question

I'm looking to buy a used car and aside from the advice here (which is mostly functional) I want to know if the oil has been changed "enough" to prevent

1. gunk build up that could starve parts from being lubricated
2. premature engine wear

I've heard of a compression test that measures how effectively each cylinder will hold pressure.

Q1: Is that enough of a proxy to ensure there isn't gunk build up (i.e. if there isn't excessive wear on the rings then they've probably changed the oil frequently enough to avoid blockage due to old oil build up.

Q2: if there is gunk/oil build up, will changing the oil a couple of times (i.e. every 1500 miles for 3 times) provide enough detergent to clean out some mild gunk build up.

The challenge is I'm religious with changing the oil and had some cars go very very far but I've bought them new and babied them. I'd like to buy a couple of used cars and very much want to avoid buying something that's been abused.

Anything insightful or actionable is much appreciated.

Thank you

Answer 1

A compression test will only give you a general idea of engine health

• It WILL NOT help you to understand if there is gunk buildup in the engine

• It WILL NOT tell you if you have a specific issue with your valves or rings

• It WILL give you an idea of whether you have good compression or not.

A compression tester will give you an idea of whether you cylinder is maintaining the necessary pressure or compression in order to operate properly. If you compression reading are below the manufacturer specifications then you will need to further troubleshoot using a leak down tester to discover if it is one of the following that is your issue.

The problems you can discover through a leak down test are the following.

• Bad head gasket

• Leaking exhaust valve

• Leaking intake valve

• Bad rings

Here is a link to a good article regarding how to use a leak down tester.

• What is a leak down tester and what can I do with it?

Answer 2

It is good to check first if the coolant in the radiator has oil droplets inside, and if the oil (most of the times using the dipstick) has water inside.

Then with a compression test you will be able to tell if the piston rigs are still ok, if the valves are sealing the chamber properly. However in order to understand how good or bad the motor is, you will need to have the manufacturer compression specs of the vehicle.

Since you removed the spark plugs to do the test, you can check them too, along with their leads. From the spark plugs you can tell how rich or lean the mixture is, or if the motor burns oil. Also inspect the leads for damage, especially on their ends that they meet the spark plugs and on the other end that they meet the coil pack.

Finally one way you might (depending on the car, 80% of the time that is doable) be able to tell if the motor has done often oil changes is with your engine off, open the oil filler cap, using a torch try to see the camshaft below. If the lobes are clean (no blackish sticky oil on them) then the car had often oil changes.

All the above they will give you a quite accurate image of the condition of the engine.

Answer 3

A 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.

A 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).

A 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 leak down.

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.