I recently had my spark plugs changed and at the same time had the shop do a compression test. I was originally going to do a leakdown test but the tech said it wasn't necessary unless something odd showed up in the compression test - is this reasonable logic? Everything was fine in the compression test and the car is running as strong as usual so the leakdown test wasn't performed.

The reason I did this was I bought my car CPOd and didn't have a thorough inspection done ahead of purchase (Figured CPO meant a solid seal of approval). After reading some mixed reviews of the CPO program, I figured I'd have the car re-inspected when it was in for service - hence the compression test. I still have >1 yr of CPO left so I figured if anything was off I could get it addressed.

 

I have always done the leak down it's more work but it is more accurate. You are most likely fine with the compression test.

 

Always do a leakdown before a compression test whenever possible...

 

I would do both - leakdown and compression.

There are differing opinions on what readings are good vs bad. For leakdown, I like to see less than 5%, and preferably less than 3% or 4% each. In my experience the measurement is not that precise though so it is kind of a judgment call. Sometimes can vary on technique used.

On the compression test, I usually like to see all cylinders within 10% of each other and perhaps determine if there is inconsistency with the leakdown result.

 

What is the effort required for a leakdown test? How many hours of a techs time should it take?

Given that I've now already changed my sparkplugs, I'm wondering if it's worth the effort to go back and do the leakdown? That said, there is something to be said for peace of mind and that's really about the last thing that I can do to make sure my car is at 100%. Rather than wait for the last month before my CPO runs out, I'd rather be pro-active in making sure there are no issues that should be covered under warranty.