1st Gen R53 Cooper S Most liked posts in thread: Low compression across the board. Verify timing? Bent valves?

Thanks Eric, I may give you a call tomorrow about that quick & dirty diagnosis. I'm actually going to do my own compression test tomorrow as well just to verify the shop's numbers.

I'm pretty damn sure. I rigged up a PVC cap to pressurize everything from my air inlet hose & have fixed every leak that I can hear or see by squirting soapy water. I've done this whole process about 5 times.

Nah, it's wasn't clattery, I can post a video on here later that I took of it that shows exactly how it sounded. I actually popped the timing chain tensioner with the timing cover off so I visually saw it go out & tension the chain. As for the lifter's caps, I have no idea. I don't even know what those are but I am suspecting the machine shop may have dropped the ball somewhere at this point. Care to elaborate on what I can look for if I pull the valve cover?

 

Okay, so yesterday I ran my own compression test & came up with:

Cyl 1- 150psi, 10 strokes
Cyl 2- 157psi, 10 strokes
Cyl 3- 140psi, 12 strokes
Cyl 4- 148psi, 10 strokes

So then, Cylinder 3 seems to be a bit of an issue but my numbers certainly aren't uncharacteristically low across the board as the shop had led me to believe. I used the same gauge on the same day to do a compression test on our other R53 that runs great & it was also in the 150's so I know my car's in the right range.

I also verified that my timing is, in fact, correct. After speaking with Eric @ Helix, I pulled my valve cover, rotated the crank 'till the two round holes in the cam gear were perfectly level with the top of the head, then used a long 1/4" extension to verify that all cylinders were sitting at the same height, and they were.

Finally, I ran a test similar to a leak down, where I pulled the valve stem out of the compression test hose & used it to feed compressed air into the combustion chamber of each cylinder while at TDC & listen for air leaks. On every cylinder, air was leaking very slightly out of the dipstick tube, meaning past the rings, but nothing through the valves.

So in a way, I'm back to square one, with the symptoms being:

-Very unenthusiastic starting

-When RPM drops to idle, it does not fall to a solid 800 RPM. Rather, it will usually fall under, possibly bog a bit, then eventually correct itself.

-Lazy throttle response

-Lumpy, resonant idle (all stock mounts)

My possible suspects are:

-Throttle body

-Partially clogged Cat

-Still a damn vacuum leak somewhere that I haven't found

Any suggestions at all would be greatly appreciated!!

 

That would be fantastic, do you know of any sure-fire way to test that I haven't tried?

-smoke test
-positive pressure applied through intake / spray soapy water everywhere / listen for hissing
-spray brake cleaner across every potential leaking seam & listen for changes in idle