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The importance of checking engine mechanical conditions during drivability diagnostics

Saturday, September 1, 2018 - 07:00
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This diagnosis begins like many others have in the past. The local guy from the used car lot drags in a "great deal" he picked up at the auction. You know the vehicles — clean, southern, low mileage, and (oh yeah!) it doesn't run! This time it happened to be a 2008 Jeep Liberty 3.7 brought in on the trailer. It would crank and attempt to start, smelt heavily of gas when it did run, accompanied with a heck of a lot of mechanical noise from under the hood. Prior to dropping it off the throttle body was replaced as well as the intake manifold gaskets and spark plugs. However, now that it is at our shop it is time to see how much of a "great deal" this vehicle really was.

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Figure 1

Now that the vehicle is in the bay it is time to get to work. Where to begin? I initially scanned the vehicle for codes to see if there was anything that might help give some direction. There were approximately 12 codes stored in the ECM, but at first glance none of them seemed to be of any value for the current situation. (Figure 1) It was obvious when I pulled the vehicle in there was a hard misfire on one or more cylinders, along with a fair amount of popping and mechanical engine noise, so I made an attempt to use scan data to see what cylinder(s) were having a hard time but at this point I was not able to get the vehicle to run long enough to gather any useful data. Seeing that gathering scan data was not going to be an option it was time to move on.

Figure 2

The next step in my diagnostic routine was to set up my Pico scope and run a relative compression test on the engine to gather a "general health check" of the engine. With the heavy amount of mechanical noise, you could hear when the engine did run, I figure this would be the best next step. Often times the relative compression test can give us an insight to the cylinders’ ability to seal and show us how they are in comparison to each other. I am sure if you have used this approach you are familiar with the classic waveform and the benefits it can hold. After disabling the fuel pump and unplugging the ignition coils, I clamped my high amp clamp around the battery cable and set up a trigger on the #1 ignition coil (Figure 2) Now we are ready to crank the engine!

Figure 3

This is where we gather our first bit of usable data and direction. Have a look at the waveform we gathered and tell me what you see. (Figure 3) The blue trace is our starter current and the red trace is our #1 ignition coil trigger. Quickly we can see that the ignition timing seems to be about right occurring just before TDC. Ok we can put that in our memory bank. Let's focus on the starter current now. At first glance, on the starter current waveform, we might think there are two "good" cylinders (the higher humps) and perhaps three other cylinders that are not contributing as much (the lower humps.)

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But before we draw any types of conclusions based on this let's have a close look at the amperage scale (Figure 4). We can see that the amperage draw of the higher humps is near 300 amps! Experience will kick in at this point and tell us that "normal" starter current draw is around 150 to 200 amps on these engines. After observing this the big question becomes, how can the starter draw too much current? Can a cylinder have too much compression?

Figure 4
Figure 5

After some pondering and knowing that these 3.7's along with the 4.7 liters are notorious for the cam followers (rocker arms) falling off, can we make a hypothesis based on this data as to what might be going on in this engine? After all we are trying to gather as much data as we can before we open it up. Knowing the firing order 1,6,5,4,3,2 and knowing the companion cylinders 1 and 4, 6 and 3 and 5 and 2, could this help us (Figure 5)?

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