Some basic troubleshooting concepts

I see it all the time on FaceBook and other forums for mechanical advice: people, usually dudes, that cannot think straight. Troubleshooting is logical experimentation, like science, only generally a little faster and looser, so the job gets done in a reasonable amount of time. If you can't think straight for one reason or another, you can't troubleshoot. Reasons for not thinking straight vary, but they are usually to do with making assumptions that just are not correct. 

I have to admit, there's also a lot of ignorance and plain stupidity. But technique is not going to help the latter, and most ignorant people don't know what they don't know, so you can't help that either.

So lets make a list of some techniques to avoid making incorrect assumptions while troubleshooting mechanical repairs.

First, up, read the manual. There are times when you can't get one, that's for sure, with older machines or custom and homebuilt ones that never had a manual in the first place. But 99% of the time there is a manual, and you can generally find it for free online. 

Use Google. Search by the name of manufacturer, the model number, or name and year and any other identifier. For example, my home emergency generator is a "Generac Guardian 0043892 7kW manual". Always add the terms "manual" and ".pdf" after the identifiers. That tends to throw the actual manuals further up the search results and weeds out all the commercial sites. These sites will sell you a manual. But if you look hard enough, some kind person will have scanned it and uploaded it somewhere.

Here's the generator search results: Generac Guardian 0043892 7kW .pdf

You can see that the first page is the manufacturer's support page, with links to the correct manuals.

Now let's try my boat motor, a 1979 Mercruiser 120. The first several listings turn out to be commercial sites, or the more popular newer Mercruiser engines.

But bingo! Half way down the second page of results we find https://web.statler.wvu.edu/~mathews/boat/pdf/manual/Chapter%20Six.PDF

I rarely strike out finding the right manual.

Why do we read the manual? Mostly to make sure we understand how the machine works. Is that a centrifugal advance mechanism or does it just look like one? Is that oil temperature switch open/off or open/on? Does it have a timing chain or belt? Not every detail is relevant, but the big details are rarely irrelevant.

Focus on the troubleshooting table. These are rarely complete, but they will usually list all the most common snags. What isn't covered in the table can be figured out if you know how the machine works.

Second up, troubleshooting requires active testing. Once you know how the machine works, there is nothing you can learn by sitting and staring. You have to start running tests. Consider the symptoms first. Is that backfire through the muffler or the carb? Is there a knock or a ping? Develop a mental list of possible issues. 

The most common example of such a list is the old "fuel, fire, compression" sequence mnemonic for gasoline engine troubleshooting. To run. the sequence, you must test each in turn. 

Which brings me to my third up item:

Only test or change one thing at a time. Don't go tearing the machine down willy nilly in hopes of striking it lucky and finding that one clear symptom that tells you what the problem is. What you're likely to find in that case, especially if the machine is getting worn out in general, is a bunch of unclear symptoms, any one of which could be the real snag, and you'll have to sort out which one by reassembling and testing again and again tediously. Try to keep the machine intact until you have some good clear reason to strip, and when you do strip, have some destination in mind that will help solve your problem. 

So, for instance, if you have a noise like a rod knock, but it could be a ping in your classic VW engine, don't strip it down to the rods until you've tested for pinging. The test for pinging, or preignition, is to retard the timing experimentally a degree or so at a time to see if it goes away. 

Fourthly, replicate. Scientists and engineers know that to know something for sure you have to keep testing until you've eliminated the possibility of a chance occurrence. So test it again, and again. retard the timing? The pinging goes away? OK. Advance it and see if it comes back. Then retard again. A few iterations and the likelihood of other hidden causes begins to diminish to an irrelevance. 

Fifth and last of all, sleep on it. Or rest. If you get tired, your brain will slow and eventually stop. The answer is more likely to come to you in the middle of the night after a few hours sleep than if you strip it down and reassemble it for the nth time without finding a damn thing. Or sit back. Look at something else. Drink a a cup of coffee. Take a walk. Eventually something, a new fact, a new idea, a new understanding, will make itself apparent and you can go back to the machine.