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

Dec. 1, 2017
Every technician has been there — deep in the diagnosis, tunnel vision sets in, and often memorable mistakes are made. But it's only a failure of you don't learn from the experience!

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.

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.

When connected to the 3.8’s #1 hole, the left side gauge displayed the line pressure of 90 psi. The right side gauge reads the pressure in the cylinder, showing 70 psi. That's a 20 psi difference, or a little more than 20 percent of line pressure.  Not a lot, but standard specification is no more than 10 percent difference.

With the line still 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 you should have heard the expletives that followed!

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 #1 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 #1 and comparing them to #2 and #3. I could not find the problem. 

There was only one answer left. It had to be in the bottom end.

Again, I removed the head on my way to the piston, and here I'll tease you with the photo in Figure 1. Do you see what I should have noticed the first time? 

Figure 2 - I still don’t know how the #1 rod was bent, but it caused the piston to fall short of TDC and lowered the effective compression in that cylinder.

With the piston removed, the problem was obvious. Looking at Figure 2, 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 #1; #2 is behind it. Notice how the stain is thicker on #1, 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 and the failed rod went undetected, or ignored, during that repair. 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.

Learn from My Mistakes

Hindsight is always 20/20, they say. My first mistake was not performing my normal diagnostic routine and checking engine integrity from the start. My second mistake was not considering that a 20% leakage rate (a nominal amount) would result in a compression reading of only 60 psi. The two did not agree with one another, what the ECM would flag as a “rationality” error. Now, in my defense, who among you would have expected to find a bent rod or would have caught the visual signs that were present especially considering that there was no other evidence present? Even so, the visual cues were there when I had the head off the first time, and I missed it. When troubleshooting any kind of problem, remember what Spock told Captain Kirk – “We must fall back upon the old axiom that when all other contingencies fail, whatever remains, however improbable, must be the truth.”

About the Author

Pete Meier | Creative Director, Technical | Vehicle Repair Group

Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels. 

Connect with Pete on LinkedIn.

Sponsored Recommendations

Best Body Shop and the 360-Degree-Concept

Spanesi ‘360-Degree-Concept’ Enables Kansas Body Shop to Complete High-Quality Repairs

How Fender Bender Operator of the Year, Morrow Collision Center, Achieves Their Spot-On Measurements

Learn how Fender Bender Operator of the Year, Morrison Collision Center, equipped their new collision facility with “sleek and modern” equipment and tools from Spanesi Americas...

ADAS Applications: What They Are & What They Do

Learn how ADAS utilizes sensors such as radar, sonar, lidar and cameras to perceive the world around the vehicle, and either provide critical information to the driver or take...

Banking on Bigger Profits with a Heavy-Duty Truck Paint Booth

The addition of a heavy-duty paint booth for oversized trucks & vehicles can open the door to new or expanded service opportunities.