Misfiring VW AWM Engine
This was a fun one. A 2002 VW Passat with 1.8T AWM engine. The car came in with misfire / most noticable at idle. Smoothed out ok when the throttle was opened up. We tested all the basics: checked the diagnostic error codes, smoke-tested the intake for vacuum leaks, data-streamed the mass-airflow sensor signal, etc, etc. The basics of the engine all appeared good but the engine ran pretty poorly at idle and it kept throwing error codes. Interestingly one of the error codes it threw more consistently was a camshaft-crankshaft correlation error (sorry, I can’t remember the specific code number). Naturally we checked the valve timing by checking the timing belt – everything lined up.
So we decided, since we had a spare (known good) camshaft sensor on hand that we’d try to swap it and see if the sensor was producing a poor signal. That didn’t help. We inspected the crankshaft sensor too – it appeared to have been replaced once but there was nothing about it that made us think it needed replacement.
The engine had a leaky valve cover gasket and the oil was contaminating the spark plug boots so we decided, as a smart next step, to sell the customer a new valve cover gasket with the bonus of allowing us to inspect the camshafts, etc while the valve cover was off. Here’s what we found…
EXHAUST CAMSHAFT TIMING
Check the red-circle in the picture – the exhaust camshaft timing was dead-on.
Notice also the cam lobes, etc all look decent.
Now – check out the Intake Camshaft:
WOW – look that – the intake timing is off by one tooth!
Even closer inspect revealed this:
Notice that chunk of material in the red circle? It’s part of the timing chain’s tensioner’s lower guide. Evidently the tensioner had a problem with the lower and the intake camshaft jumped one tooth. So why didn’t we see this when we checked the timing belt? Well, the timing belt aligns the exhaust camshaft with the crank – not the intake camshaft. This was the timing tensioner can vary the intake valve timing by increasing and decreasing the tensioner on the chain between the two camshafts. We replaced the the tensioner assemby and the timing chain, reset the valve timing, and retested the engine: fixed!