It’s nice to get an easy one once in a while amid all of the random failures that we usually have to fight through. But even an easy one, or should I say easier one demands both attention to detail and discipline to make sure that you haven’t missed anything significant in the early stages of the diagnostic routine.

Another shop brought us a 2001 Chevrolet Impala with a 3.8l engine, they reported that they had replaced the ignition switch but the car still wouldn’t start. Well, yea that car uses the pass lock anti-theft system, so when the ignition switch and lock cylinder are replaced, the system doesn’t recognize the voltage signal from the new pass lock sensor and the BCM reacts as if there is a theft being attempted. Attaching a scan tool and accessing the BCM codes there was a B2960 found for the passlock sensor signal valid but incorrect. That’s an easy fix and the system can be retrained with a re-flash routine in about ten minutes, or it can also be done with three ten minute key-crank-on cycles.

The rest of the story on this car was it was abandoned and impounded so it hadn’t run in a couple of years, which is why they had to replace the lock cylinder and key. Once the pass lock system was retrained the car was now able to crank and it fired but it stalled right away. With practice a technician can hear the way an engine tries to start and how it dies and actually recognize if a problem is specific to one or more cylinders, he/she can often tell of the car is too rich or too lean and if it lost spark or fuel to all of the cylinders. This one sounded rich, and it appeared to not be losing spark. Rich often allows for one or two cylinders to fire while cranking but it’s random, and opening the throttle normally allows more air in than the engine is getting fuel and the engine then clears up and starts running. The problem was this one wouldn’t clear out, in fact it would seem like it briefly cleared out and then got worse and would even kick back against the starter. At this point it seemed that I might just be discovering why someone walked away from this. Since the tech II was hooked up it made sense to check the essential scan data. Both the intake air temperature sensor signal and the coolant sensor signals reflected the ambient temperature of eighty degrees, the map sensor showed ninety-seven kilopascals which is close enough to normal ambient pressure, the cam and crank rpm signals matched when the engine was trying to start but the MAF sensor was jumping to thirty-five, then fifty- eight and up to seventy-six grams of air per second when it would stall and the engine just barely to one thousand rpm.  So that was clearly wrong, the sensor was way over reporting, but even then the engine just didn’t react to the throttle the way it should be expected to. Fortunately with the MAP sensor in the system, the MAF can be disconnected and then see how the system reacts. By disconnecting the MAF the engine now would start and idle, but it would not rev up when opening the throttle, in fact it sounded very labored. Looking at the MAP signal it was reporting about eighty kilopascals. That meant there was very little vacuum being generated, opening the throttle even the slightest amount had the MAP reading showing the same ninety-seven  kilopascals that was seen before cranking the engine.

One other important note was the fuel trims were taking away about twenty five percent of the fuel pulse. That meant that the computer was at least trying to compensate for the system being too rich. When you have a speed density system, which is what this became with the MAF disconnected the computer looks at the high manifold pressure as an engine that is working real hard to accelerate, so it calculates a long pulse width. But in this the airflow to match really wasn’t there so that ends up being too much fuel and the trims have to react. At this point there really was only two possibilities for the condition, either the exhaust was restricted or the camshaft in the engine is out of time with the crankshaft. Fortunately one simple test can provide the answer to which one it is and that’s do a running compression test with a pressure transducer and measure the valve opening events.

The first capture is cylinder number three running with only cursors measuring the entire cycle. That allows the time elapsed between the compression peaks to be measured, and then divided by four to give each piston stroke.

The second capture shows cursors set for the exhaust stroke, the pressure in the cylinder should go below atmospheric pressure before the piston gets to the bottom of what would normally be the power stroke of the engine, then the exhaust valve open at just about forty degrees before bottom dead center and since the cylinder is in a vacuum, the pressure rises because the exhaust is at, or slightly above atmospheric pressure. This second capture proves that the camshaft is in time.

 

Now all we need to do is speed the engine up. Notice how you no longer see the compression peaks, they are clear off the top of the screen, at the same time the exhaust stroke plateau has risen as well. The horizontal cursor allows that pressure peak to be measured and its over forty psi. That’s way too much back pressure and explains why the car won’t run above an idle.

 

At this point the discovered repairs need to be performed before any more diagnostics could be performed. There may easily be other issues that require attention, but sometimes you don’t have any choice because problems like these two will make proving anything else almost impossible IMO.