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Another pressure transducer case study
Posted to Technical Theory Forum on 10/4/2014 20 Replies

Hello All. I was called to a general repair shop to help in diagnosing this vehicle. This vehicle had a very rough idle and hard to keep it running at idle. There was a code p0300 and po172 for running rich . Looking at the data and trying to keep the vehicle running with foot on throttle , the fuel trims were negative approx -18 LT and -10 ST. The map sensor was showing 2.5 volts with part throttle indicating this vehicle was exhibiting low engine vacuum and supplying to much fuel ,thus the negative trims. I did not see a vacuum leak and there wasnt an egr to speak of. I did a cranking relative compression test with pressure transducer in cylinder 1.(fuel disabled) [2003 Chevrolet Cavalier LS, Waveform] You can see number 1 cylinder is lower than the others. Here is another waveform zoomed in showing vacuum transducer pulses in red and exhaust pulses in green. [2003 Chevrolet Cavalier LS, Waveform] You can see everything is labelled . EVO,EVC,IVO,IVC. When the intake valve opens for cylinder 1 , u can see the pressure drop nicely in the compression waveform. However, running compression, its a different story.

The running compression shown here [2003 Chevrolet Cavalier LS, Waveform] for cylinder 1 , shows the running compression high at almost 160 psi. Jumped timing chain or slack in chain? Zoomed in running compression shows more interesting tidbits. [2003 Chevrolet Cavalier LS, Waveform] Exhaust valve opens at about 15 BBDC which is kind of late. I find some interesting details here .Notice the intake waveform flat spots labeled.I know the timing chain has jumped because of high running compresson on cyl1and low running vacuum. This intake waveform ( the shape of the flat bottoms) ,could this be slack in the timing chain? From the waveform , it seems like the intake valve closes and there is a flat spot (delay) then another intake pull? I wonder if this technique can be used to spot chain slack? Also note when number 1 intake valve opens, there is a negative pressure drop initially in the cylinder but after a short time with intake valve still opened according to vacuum wave, a positive pressure is increasing.Is this compression from another cylinder ? Myself , i find the in cylinder pressure transducer and vacuum and ex transducers tell more of the story but , of course ,it takes more time, especially if the plugs are buried. Thanks for reading.

Mark from Ontario

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car Vehicle Data

2003 Chevrolet Cavalier LS 2.2L

Engine2.2 L
Trans4-speed Automatic (Electronic)