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Old 10-12-2004, 01:42 PM
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Don Moyer Don Moyer is offline
 
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Troubleshooting Exercise

Because so many of the problems we experience within the ignition system (I'm estimating approximately 90%) end up being caused somewhere within the primary circuit, I'm prone to forget about the secondary ignition system in troubleshooting exercises. This point was driven home for me a few days ago, when I had an opportunity to respond to a recent rebuild customer's request for assistance. I thought you might be interested in what I discovered and join me in taking "a closer look" at the troubleshooting process in general.

TROUBLESHOOTING EXCERCISE:

Preliminary Observations:

When I arrived at the boat, the engine was only running on two cylinders (#3 and #4). Cylinder # 1 had zero compression due to a stuck exhaust valve, and #2 cylinder was not firing due to a weak and/or intermittent spark.

The plug from #1 cylinder was covered with carbon, and it also had water droplets hanging from its electrodes, which I concluded to be the result of condensation. The plug from cylinder #2 was dry, and from its condition, I would have predicted that it could have been functioning normally.

The crankcase oil had a "milky" color which was indicative of a small amount of water present. I concluded that the evidence of water in the oil was consistent with what would be expected from condensation developing within the combustion chamber of the first cylinder, and working past the rings.

NOTE: Due to the presence of detergent in modern motor oils, even a tablespoon of water in the crankcase can produce a grayish emulsified look to the oil.

Investigative Findings:

Being unable to free the exhaust valve in the first cylinder by working through the plug hole, I removed the head. With the head removed, I was able to free the valve by sequentially tapping it down, and turning the engine over on the starter to force it back up.

While the head was still removed, I noticed a thin rusty colored residue that was spread evenly throughout the second combustion chamber in the head. This discoloration was the only clear physical evidence I could find that was indicative of a pre-existing anomaly that seemed to predate even the stuck valve in the first cylinder. By contrast, the carbon and condensation in the first cylinder seemed somehow "fresher" than the discoloration in the second.

After a liberal application of Marvel Mystery Oil to all valves and cylinders, I reinstalled the head, and replaced the entire secondary ignition system, including the rotor, cap, wires and plugs. The engine then sprang to life, with good power on all 4 cylinders.

Primary Cause, and Sequence of Events:

1) The primary cause of the failure was a breakdown within the secondary ignition system (I'm guessing the wiring harness) which caused intermittent firing of the plug in the second cylinder.

NOTE: We had been using factory OEM wiring harnesses as of the time we had rebuilt this gentlemen's engine, and we had, over the years, experienced several problems with these wire sets, which prompted us to look for another source. We have currently settled on plug wires produced by another supplier which are of significantly better quality.

2) Very limited use of the engine since rebuilding (approximately 5 hours in almost a year), coupled with the cool operating temperature resulting from the poor ignition within in the second cylinder, eventually caused the exhaust valve to stick open in the first cylinder.

Corrective Action:

Replacement of all secondary components of the ignition system (including an improved plug wire set), and encouraging the owner to avoid continued short periods of engine operation. In general, when an engine is started, it's best to continue using it at a normal cruising power setting until it's fully warmed up before shutting it down (45 minutes to an hour, minimum).

Follow-up Questions:

1) Why did the exhaust valve in the first cylinder stick open, and not one of the valves in #3 or #4?

A: Cylinders #3 and #4 had each other to keep themselves warm, at least warmer than the first cylinder, which was all by itself on the other side of the anemic #2.

2) Why didn't the exhaust valve in the second cylinder stick, instead of the valve in #1?

A: I'm not sure. But, given the very minimal use on the engine (hardly ever getting up to a normal operating temperature), the stage was set for a valve to stick, even without anything else going wrong. It could be that the valve in the first cylinder was on the verge of sticking in any case, but it certainly became more likely, given the demise of cylinder #2.

3) Why couldn't the stuck valve have been the primary cause?

A: Two reasons: First is a matter of cause and effect. It seems to me much more plausible that the condition in cylinder #2 could lead to (or at least accelerate) the condition in cylinder #1, than to conclude that the stuck valve in cylinder #1 could somehow lead to the ignition problem in cylinder #2.

Secondly, there's the concept of inevitability. Had we somehow diagnosed the defect in the secondary ignition system before the valve in the first cylinder failed and were able to get the engine up to a more normal operating temperature and power range, it's very likely that the valve in the first cylinder never would have stuck in the open position.

4) Why all the fuss over failure sequence, primary cause, etc.?

A: So we know what to fix. While this failure sequence is relatively straightforward, in other cases, it may not be so clear. Understanding the primary cause of a problem with as much specificity as possible makes it possible to spend maintenance dollars wisely. It makes much more sense to spend a couple hundred dollars on a primary cause (that we're sure of) than on something that is merely an effect of the real (primary) cause.
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