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Compression waveforms, Part 13
Posted to Technical Discussion Forum on 11/7/2013 41 Replies

A 2000 VW Jetta is towed in for a stall, no start. Customer says it stalled suddenly, a few days ago, but restarted immediately. This time it stalled and would not start.

I scanned it and found some codes, most interesting among them was a 16725 (P0341) CAMSHAFT POSITION SENSOR IMPLAUSIBLE SIGNAL. I grabbed a Modis ULTRA and scoped the crankshaft and camshaft position sensor signals. Seeing both signals in tact gave me a pretty good idea of what the problem was. A quick search here on iATN turned up Michael Webb's contribution, which confirmed my suspicions. I didn't calculate the number of degrees the engine was out of time, but it certainly seemed like quite a bit.

I have no experience with broken/jumped timing belt issues on these engines, but they are apparently of an interference design. The engine cranking cadence was even, did not utterly lack compression, and was free of any horrible sounds. I thought I would use my pressure transducer and my Firstlook sensor and see what I could discern about the state of the engine.

I started here:

Cylinder #1 cranking compression with exhaust pulse waveform

I see a symmetrical tower and even exhaust pulses. I was looking for evidence of an asymmetrical tower, and/or any uneven exhaust pulses. I then swapped the Firstlook to the intake:

Cylinder #1 cranking compression with intake pulse waveform

There I was looking for any disturbance in the intake pattern, something like Matt Batulis's contribution.

I felt pretty confident that resetting the timing belt would get this engine running again, and that it would run well. My thinking was that I saw no evidence of compression leakage in cylinder #1 while using the pressure transducer, and both the intake and exhaust pulses were even. The intake pulses, in general it seems, are especially susceptible to almost any mechanical issue. Cranking, the exhaust pulses only seem to show pretty severe issues.

I contacted the customer and explained why the engine wouldn't start, what condition had caused this, and what the risks were in moving forward. The customer decided to speak with his wife, and do his own research. When he called back he was prepared to have the vehicle donated to charity. He felt the risk was too great to even attempt to re-time the engine. It was at this point that I went into detail on the pressure transducer and pulse waveform testing I did -- and how, while I couldn't guarantee the engine would be fine, I was reasonably confident that it would be. I was prepared to offer to re-time the engine for no charge if it proved not to work out, if he would agree to have us do the repairs if it did. I didn't need to, though, as he approved moving forward.

I lined up the crankshaft, and found out exactly how far off the camshaft was. The white mark is the timing mark, the green mark I made to clarify where the rear cover pointer was aimed. A closer look at one of my cranking compression captures in comparison to Kerry Marion's contribution shows the difference when viewed with a scope. The belt was tight and in tact, but I did notice the tensioner. I reset the belt position, using the existing tensioner -- which seemed to work well enough for testing purposes.

The engine fired up right away and ran very smoothly.

I took two more captures before I called to give the customer the good news:

Cranking compression capture

Running compression capture

Like I said, I've never run into an out of time 2.0 AEG engine before... and all I had to go on was that it was supposed to be interference. I felt approaching things as I did gave me enough information to feel confident in recommending the customer spend money by having me reset the timing. In the end I did give all the information I had to the customer, let them evaluate their tolerance for risk, and make their own decision.

Shannon from California

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car Vehicle Data

2000 Volkswagen Jetta GLS 2.0L

Engine2.0 L
Trans4-speed Automatic (Electronic)