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Don't let tunnel vision get in the way of your diagnostic process

Friday, December 1, 2017 - 09:00
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In this month’s Tech Corner, I would like to share an experience I had when still full-time as a technician. It’s an experience I’m sure most of you have also enjoyed, or not, depending on your outlook on life. You know, the old “glass half full” kind of thing.

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The car in question is an older Ford Mustang with the 3.8 liter V-6, with a hard misfire on cylinder #1. Follow along and see how you would have tackled this one!

The First Mistake

The customer had brought the Mustang in for a complaint of a rough idle and stumble on acceleration. After a short test drive, it was easy enough to tell that there was a serious misfire going on. I hooked up my scan tool and found code P0301 (cylinder #1 misfire) and P0316 (misfire detected on startup) stored in the Engine Control Module (ECM). 

Figure 1 - The answer to the Ford misfire is in this picture. Do you see it?

This vehicle uses a Direct Ignition System (DIS) that fires two plugs simultaneously. Opening the hood, I could hear the distinctive "tick" of a spark jumping to ground outside the cylinder. Looking a little more closely, I could see the spark jumping to the valve cover from the #1 wire. The wires looked like original equipment, and a closer inspection revealed signs of leakage in the others. 

On this type of coil, one plug is "positive," and one is "negative." When the coil discharges, current first travels to ground thru the negative plug, then back to the coil through the positive plug. When the coil is stressed, the internal insulation can fail, reducing total coil output. In this low state, there is just not enough voltage left to jump the gap on the second plug, even though the first plug continues to run just fine. That's why it's possible to have a DIS coil with one dead plug. 

Thinking I had this one nailed, I ordered a replacement coil and ignition wires and moved on to the next car on my list. Time is money when you’re working flatrate! 

The Second Mistake

When the parts arrived later in the day, I pulled the Mustang back in to the bay. It is a simple installation and took no time at all. I cleared the codes and went to verify the repair. Have you guessed yet? The miss was still there, and the MIL light was back on. 

You would think that after all the time I've had in this business I would remember my personal rules regarding diagnostics - Never take a shortcut, especially on a misfire code. 

A misfire code can be set by any condition that doesn't allow for complete combustion in the cylinder. My normal procedure is to first do a relative compression test to ensure the engine is mechanically sound. Doing that test now indicated that the #1 cylinder had a problem.

If I see a low cylinder indication on this quick and dirty test, I follow up with a normal compression test. 60 psi was all I got on the misfiring cylinder. What I found had me muttering a few words under my breath. I was kicking myself for breaking the rules, and now I had a major engine fault to explain to my customer.

Was the original repair necessary? Replacing the ignition wires may have been; however, the coil was a rushed diagnosis. The low firing line I had seen on my scope was a result of low compression – not low spark energy. Remember, the firing line is typically affected by pressure, gaps in the system and the amount of hydrocarbons available for conduction. The scope was trying to tell me something. I just wasn't listening, instead choosing to see what I wanted to see based on an assumption. 

What’s The Fix?

The next step I took was to perform a cylinder leak-down test. This test uses a tool called a differential cylinder pressure tester and has two gauges on it. One indicates line pressure (supplied by shop air), and the other is the pressure being contained in the cylinder. When connected, and with the cylinder to be checked at TDC of its compression stroke, the tool pressurizes the cylinder and you compare the two pressure readings on the gauges.

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