Alright, so I'm hoping maybe someone had a similar issue, because I'm at wits end. It's a 2002 (b6) a4 1.8tq with 150k on the clock:

I had a check engine light for implausible signal at the cps, and it was giving me a hard start (3-4 attempts at cranking usually). So the symptoms and the light pointed to needing a new cps, which I bought. Installed it.... same issue, same code. Thought the new cps might be faulty and grabbed a known good one... same issue, same code. Left it for a bit, and now got back into it again... now I have the cps correlation code as well as implausible signal. Checked the timing on both cams and they're fine. The engine felt like the timing was really retarded, so when that checked out I did a compression test. 100 psi across all 4 cylinders... and there's almost no vacuum in the manifold. In fact, my brake booster has stopped working.

So... what causes low compression on all cylinders equally if the timing isn't off? And, why the cps correlation code? Any ideas appreciated.

 

Is your head gasket OK? Maybe the engine just had it...

 

Well, no smells or smoke, and the liquids aren't mixing... so the gasket appears to be fine. My only thought is that maybe the cam chain tension key sheared off the back... I'll check on that and report back.

 

I haven't really heard of anything involving timing that is not cheap.

 

100 psi seems low. Did you have all 4 spark plugs out when you did the test? Did you crank for at least 4 compression strokes?

Do a bleed down test. Not sure how much work that is on a DOC engine but that should tell you where the compression is going; bottom end, or valves. Or if it is holding.

I would say your rings are shot. JMO

 

You need to have someone do a leak down test. This will tell you if it is leaking past the rings or the valves. It is a very simple test and wont take long to do.

 

when doing the compression test hold the accelerator pedal down. This ensures that the maximum amount of air is entering the engine. Otherwise your readings will be skewed. Try it again and post what you get.

 

Not needed if you remove all the spark plugs before doing the testing which is the correct way to do a compression test, plus the DBW throttle body does not open fully during cranking so pushing the pedal all the way down during the testing does nothing.