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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