I’ve been burning about a quart of oil every 400 miles so decided to do a leak down and compression test.

The compression test went well with 150, 145, 140, and 135. The battery seemed to be getting weaker so the lower readings may be higher. They connectivity reduced from 150 to 135.

Leak down didn’t go so well. I bought a Harbor Freight tester and it seemed to only test with about 25 psi even though I was putting 100psi into it. The first cylinder showed 10% leak.

My question is, Is 10% at 25 psi the same as 10% at 100psi? Shouldn’t I be testing at a higher psi?

I am returning the tester since I can’t hardly adjust it precise enough. What is a good tester to use and does anyone rent them? Autozone does not.

 

The leak down test will highlight ‘where’ you have a bad component, either rings or valves, or something else. It’s not so much the pressure, but where the air is coming out.

at least, that how I understand it....

 

That’s what I was trying to do, but I don’t think the tester was working right.

 

What is your year, model, and mileage? To me that is a considerable amount of leakage. I am just now gaining familiarity with these cars so bare with me on the questions. Do you have a turbo engine? I’m not sure if all “S” models are turbo charged... All the N12.14,13,16.... is still confusing to me.

Anyway, if you do, then I understand that they are direct injection engines too. If this is your case, you may have intake valves covered in residue preventing your valves to seat fully. I know this was the case with the initial BMW engines with DI. There are several cleaning methods but inspection is the first step after ensuring leakage. I would assume you could remove the plastic intake manifold and look into the valves.

 

First step is to get a good tester. The HF testers are junk. I got an OTC brand tester from Amazon for about $50. Much better. With what I’ve learned about these engines over this year from my rebuild (you can see the thread in the Rebuild category), my initial guess is build up on the valves. If the valves aren’t closing and sealing completely, you’ll get air leaking passed. If you want to get a visual confirmation, pull off the intake manifold and look down the ports with either a bore scope (you can pick up a good one that links Bluetooth to your iPhone/ipad from amazon for about $40-$50) or a mirror & flashlight. That will tell you immediately the bulk of the problem if there is severe build up.
Another possible cause is a leaky turbo allowing oil to cross over and get burned/used. Good way to check is to pull the bumper cover (surprisingly easy) and disconnect the intercooler, then stand it up on one end and see if oil drains out. I propped mine up in a bucket and let it sit for 20 mins. You’ll know pretty quick if there’s oil in it.

If you’d been good with regular oil changes (depending on the history of the car), then the piston rings are most likely not the cause and doing their job.