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Pressure Transducer Waveforms
Posted to Technical Tips Forum on 10/7/2009 41 Replies

1996 Chevy S-10 2.2 Liter engine. Customer says he requires a new engine. He said he heard a knocking noise and eventually it got to the point it will not start. He asks me to either install a used Engine or get this engine running so he can trade it in.

So I have the vehicle towed in and get in the vehicle and crank it over.Its turning over real quick and sounds like there is no compression.(The kind of sound you hear when your cranking over an engine with a timing belt failure)A quick note* on the scanner a code showed up for knock sensor. I get out of the vehicle and I can smell the fresh scent of raw fuel in the air,a visual showed no signs of rods hanging out the side of the block or pan. I decide its time to check some basics with the scope and a pressure transducer, I remove the spark plug on cylinder #1 and I can see the gap is about twice the size of what it should be.

I install a 500 PSI snap-on pressure transducer into cylinder #1 and I hook up a secondary lead from the scope and test leads to cam and crank sensors. I crank the vehicle over and this is the waveforms patterns I get.[1996 Chevrolet S10, Engine/Propulsion waveform] We can see from that pattern we have secondary ignition firing, a cam and crank signal (out of sync compared to [1996 Chevrolet Cavalier, CKP Sensor, CMP Sensor Waveform] ) and a pressure transducer waveform that has an unrecognizable signature pattern. With the fact its obviously out of time, I then decide to remove the valve cover and loosen off each rocker and examine the valves with my borescope (no valves appear to be bent from what I could see with the borescope)I decide I should do a leak down test anyways to verify no valves were bent.(This test indicted a minimal leak down rate on all cylinder) "So good no valves are bent" I then decide its time to remove the timing chain cover and this is what I see [1996 Chevrolet S10, Engine/Propulsion photo]. Wow approx 9 teeth out. I install a new timing chain and tensioner/oil change and button the front cover back up. I then take another capture of this vehicle cranking over [1996 Chevrolet S10, Engine/Propulsion waveform] Well that looks better its now in sync.Now lets see if this thing starts [1996 Chevrolet S10, Engine/Propulsion waveform]Hey its running. Now since I havent buttoned up the valve cover yet I decide to take some running compression transducer waveforms with different scenario's, I loosen off different rocker combinations and capture the corresponding waveforms [1996 Chevrolet S10, Engine/Propulsion waveform] Some interesting patterns and pressure's displayed. I then check each cylinder separately and note the captures [1996 Chevrolet S10, Engine/Propulsion waveform] and a closer look at cylinder #1 [1996 Chevrolet S10, Engine/Propulsion waveform] I finish putting it all back together,installed new spark plugs cleared the code and started it up running on all 4 cylinders, It ran very nicely [1996 Chevrolet S10, Ignition scandata] And the Happy customer was very surprised he didnt need an engine.

A little note, when using the 500 PSI snap-on Pressure transducers they will give you a fairly good representation of the running/cranking compression waveform pattern itself, However the pressure Scale will be off to some duration because of the inability to maintain the proper scaling. When examining the running pressure waveforms with a good sealing engine generally you should note where the exhaust pattern peaks and this should theoretically cross over the 0 PSI line to confirm the exhaust is actually escaping the engine as shown here [2002 Ford Focus, ECM/Inputs/Outputs waveform]

Hopefully this helps somefolks out.

Kevin from Manitoba

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