So I did the unrthinkable and bought a 2003 c4 cab in February without a service history. What can I say, the price was right. It had passed a trusted local PPI (somewhat limited in nature). They added a 3 year 3rd party power train warranty for a reasonable cost.

In the first few weeks, it started to throw a 3rd cylinder misfire code fairly routinely. Based on review of threads here, I replaced the plugs and coil packs. I took care of some easy consumables (no service history) and it has been running great since then. Runs smooth, pulls strong, and no codes. It doesn't blow any unusual smoke either.

Fast forward to this past week. I noticed a blister on one of my rear tires. The best deal on PS2s was at the Porsche dealership. On a whim, I asked if the car had been serviced there. Turns out, the PO used them for all service over the past decade or so. Had some good surprises like AOS, water pump, etc. last year, he started getting some misfires in cylinder 3. After changing the coil pack and plug, it persisted. They finally did a compression test (passed) and leak down test (87% cylinder 3!). At this point, he refused further investigation and presumably traded it in to another dealer.

Again, it runs great now and has so for the past several months. Would anyone pursue further diagnosis on this? I have convinced myself that a stuck valve has burned off whatever carbon may have been on it through aggressive driving.

 

No worries as long as you don't have any pending or active codes anymore. The DME is watching every single ignition for you so you can count on it.

If cyl #3 is leaking 87% now, your engine should run like crap

 

That's kind of what I was thinking. Thanks.

 

If you want to know for sure, test it yourself.

I have never met a technician as thorough or competent as I want them to be... Speaking as a former tech, and currently responsible for my company's warranty program.

 

FYI, you can usually "borrow" leak down testers from your local auto parts store.

 

Sadly the pinhead that did the leakdown test (it's actually a differential pressure test with orifice) didn't report where the leak was going. Grrrrrrr. This is not rocket surgery. Listen to the exhaust, intake, or oil fill cap when doing the test and the sound of the air escaping in this amount will be easy to determine. Exhaust valve, intake valve, rings.

 

Yep, not noting whether the leakage was past the rings (into crankcase) the exhaust valve, or intake valve was very amateur. Here we only use a leak down test to know WHERE the leakage is coming from, because when we perform one, we already know that we have an issue with compression.

That said, if the issue existed then, and not now, the engine probably had a lifter that was stuck, not allowing one of the valves to seal. We see this happen from time to time and it can clear it's self up. Thats about the only thing that would net an 87% loss of cylinder charge and not still be a problem (or even bigger problem) today.

 

Or the leak down was performed on TDC overlap?

 

Could have been, but it probably would have had 100% leak down in that case.

BUT they could have been this dumb. Very easily.

 

I'll call on Monday to see if anyone has more detail. This was a year ago, so if it is not on the computerized record, I doubt it exists. I think I'm hearing encouraging things from you guys though.

 

Grab up a cheap comp tester at Orielly's and do all six at normal temp. don't squirt anything in the jugs before taking the test, and hold the throttle all the way open when testing. This will get you a nice baseline which includes the adiabatic effect on the ring expansion. Mark the numbers down in your mx log, and if it comes up again, you have something to compare to. Use the same comp tester the next time, as they are notoriously inaccurate, but you are looking for a change, not an absolute value. i.e baseline is 166, 162, 170, 158, 167, 166, then if you come up with a test later, you can compare to pre-existing values. If it shows something different, proceed to the differential test(leakdown), and find out where the air is going.

 

Pull the fuel pump/ DME relay and turn the engine over while filing it on the starter... Post about 8-10 seconds of the engine just spinning over without trying to fire up.
I'll tell you if you have low compression or not.

 

Teek: I would do as Jake says. He has a "calibrated" ear! Pretty scary, but I guess years upon years of hearing then tearing apart plus understanding how everything works inside makes one hear things us regular guys miss!

 

Heck, I use a lab scope with an inductive pick up on the positive battery cable to perform a compression evaluation too.. It works much like my calibrated ear.

I perform a "compression test" on every car, every time I start it.. Comes from years of driving aircooled VWs more than anything else.

 

Will do. Busy week though.