Motor Age Garage: Cardinal Rules

Jan. 1, 2020
In my part of the world, business is cyclical, like the phases of the moon or the changing tides. Some days you are busy, and some days you're slow. And sometimes, you are slammed. This Mustang came in on one of "those" days: I've got 10 cars on my s

This is the story of a failure, not a success. Some rules should never be broken.

In my part of the world, business is cyclical, like the phases of the moon or the changing tides. Some days you are busy, and some days you're slow. And sometimes, you are slammed. This Mustang came in on one of "those" days: I've got 10 cars on my schedule, all waiting for me to diagnose a variety of complaints, and of course, they all want them back by the end of the day. To say I was busy would be an understatement.

THE FIRST MISTAKE

The complaint was simple enough. As is my usual practice, I first verified the rough idle and stumble by taking a test drive. When I got back to the shop, I hooked up my scan tool and found code P0301 (Cylinder Number One mis-fire) and P0316 (misfire detected on start up) stored in the Engine Control Module (ECM).

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 on the Number One wire. The wires looked like original equipment, and a closer inspection revealed signs of leakage in the others.

I knew that bad wires were a common cause of coil failures, so I hooked up my scope to Number One and saw a low firing line and spark line that looked an awful lot like what I am used to seeing when the coil fails.

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.

OK, I got this one nailed. I order a new coil and ignition wires and move on to the next car on my list.

SHORTCUTTING A DIAGNOSIS

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. I took it for another test drive, and the original codes were back.

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.

Relearning the Rules

VEHICLE: 2003 Ford Mustang DRIVETRAIN: 3.8 V-6 Automatic transmission MILEAGE: 65,261 miles COMPLAINT: Vehicle idles rough, stumbles. MIL on.

Yes, I was busy. Yes, I had pressure on me to get the cars looked at and diagnosed. Yes, I was in a hurry and didn't follow the rules.

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 Number One cylinder wasn't good.

If I see a low cylinder indication on this quick and dirty test, I follow up with a normal compression test. 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 was; however, the coil was a rushed diagnosis. The low firing line 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, and saw what I wanted to see based on an assumption.

NOW LET'S FIX IT

The next step I take when I have a low compression reading is 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 it's compression stroke, the tool pressurizes the cylinder and will give you a reading as shown in Figure 3. The left side reading is line pressure; in this case, it's 90 psi. The right side gauge reads the pressure in the cylinder: 70 psi. That's a 20 psi difference, or a little more than 20 percent of line pressure. Standard specification is no more than 10 percent difference.

With the shop air line connected, I removed the oil fill cap, radiator cap and air filter housing. That 20 percent of air pressure is going somewhere, and you can actually hear it escaping. That's the nice thing about this tool. It allows you to hear if the loss of compression is from the valves (air escaping from the throttle body or exhaust pipe), the rings (air escaping from the oil fill) or from the head gasket (air escaping from the radiator).

This one was a no-brainer. Air was rushing out of the throttle body with no evidence of air flowing through any of my other checkpoints. OK, now I've got it: The intake valve is leaking. I got authorization to remove the head, confident that this was the problem.

With the head removed, I verified the valve was leaking by pouring solvent into the intake port and looking to see if any leaked past the valve on the combustion chamber side. It began to pour out as soon as the solvent got to the valve face.

But because I had been burned on my first diagnosis, I needed to be extra thorough. I also checked the installed valve height to see if there might be a problem with bent valves or recessed valve seats and found no problems there. I inspected the push rods for damage, and the cam lobes for wear. While the head was off, I rotated all the cylinders to the bottom of their travel to look for damage to the cylinder walls. Everything looked good.

I got the head back a few days later and reinstalled it on the Mustang. I turned the key, and the miss was still there.

WHAT HAD I MISSED?

You've got to know that I am really upset by now. I felt I had done a thorough diagnosis, and I had definitely found a major flaw in the leaking intake valve. Thinking that maybe the machine shop had done something wrong, I checked cylinder leakage with my tester. This time, the results showed no leakage.

But what about compression? Again, I got a low reading on the Number One cylinder. What is going to make a tight cylinder low on compression? The only answer I could come up with is that the cylinder couldn't breathe. However, I had checked the valve train and had found no problem.

I pulled off the valve cover and rechecked the valve operation, measuring opening and closing heights of the valves on Number One and comparing them to Numbers Two and Three. I could not find the problem. There was only one answer left: It had to be in the piston. A broken ring, maybe?

Again, I removed the head on my way to the piston, and here I'll tease you with the photo in x. Do you see what I should have noticed the first time?
With the piston removed, the problem was obvious. Looking at Figure 6, do you see what should have caught my eye? The interesting thing about this failure is that the rod bent almost perfectly along its axis, effectively shortening its length. Other than that, there were no other symptoms — no noise, no vibration and no bearing damage.
Look closely at the stain on the cylinder wall where the ring travel ends near the top. The cylinder in the foreground is Number One; Number Two is behind it. Notice how the stain is thicker on Number One, showing that the piston wasn't reaching TDC. I should have caught this when I had the head off the first time. Would you have caught it?

What would bend the rod? Perhaps it was hydraulic lock from a leaking head gasket. When I first performed my visual inspection, the oil level was correct with no sign of intermix. Coolant levels in both the reservoir and radiator were correct as well, and there was no air escaping thru the radiator during the first leak-down test.

However, the customer forgot to tell us that he had just had the car serviced less than 60 miles earlier. He also neglected to tell us that he had had an overheating problem that was the result of low coolant. But the previous shop had told him they couldn't find any leaks. They topped off his coolant, and he hadn't given any more thought to it.

While the car was fixed, this is not a success story. My first broken rule was not performing a thorough diagnosis and verifying the fault before I recommended the coil pack and ignition wires. My second broken rule was not determining the cause of the intake valve failure.

Yes, I was under pressure, but that's no excuse. I'm a professional, and I take a lot of pride in my work. This is one I am ashamed of. So learn from the lesson I had to relearn: Be thorough in your work. It will save you grief in the long run, and help you maintain your reputation as a professional.

PETE MEIER is an ASE CMAT, a member of iATN, and a full-time tech with CarMax in Tampa, FL. He started doing oil changes and minor repairs more than 30 years ago and brings a variety of experience to bear. His current job involves all manufacturers' lines, and, as Pete says, "provides me constant opportunity to learn something new." Diagnosing electrical and driveability problems are his favorite challenges.

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