Motor Age Garage: Cardinal Rules - This is the story of a failure, not a success. Some rules should never be broken. - Search Auto Parts | Automotive News
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?
Figure 5 Can you see what I should have seen?
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?
Figure 6 I'd say this is a problem.
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.
Figure 7 A
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.
Figure 7 Notice how the stain left by the ring travel on Photo A is thicker than on Photo B. Photo A was not reaching top
dead center (TDC) because of the bent rod.
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.
Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model.
Articles by Peter F. Meier
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