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Back To Basics

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Back To Basics

Spark

   There are a lot of silly little things here that can go wrong here. Most of the rough running and no start complaints can be traced to an ignition system basic. The first is the distributor cap and distributor rotor. It is surprising how many people don't even think of the distributor cap and distributor rotor as a cause of a problem. Distributor caps have to work under very severe conditions. Heat and high voltage take its toll on the distributor cap. They start to crack at the towers or inside the distributor cap itself. Distributor rotors burn out their tips or the center electrode burns out the center of the distributor rotor and the spark grounds to the distributor shaft.

   I know a mechanic with 25 years of experience replace a timing chain and gears on a on a Chevy 350 that, he found out later, had a cracked distributor cap. And the kicker was another long time mechanic agreed with his diagnosis. An unneeded repair due to not checking the basics.

   Spark plug wires are another place that can cause a variety of driveability problems. They work hard also. They have to carry very high voltage in conditions that are less than ideal. When they get old they can start to crack. If there is a ground close enough, the spark will take the shortcut and ignore the spark plug. Remember, electricity is lazy and will always take the short and easy path. And it's a lot of hard work the jump across that spark plug gap.

   An easy and simple way to check spark plug wires for cracks is to use a spray bottle to spray the wires with water. If there are cracks in the wires you will see and hear the sparks jumping to ground. Doing this at night makes seeing the spark much easier.

   Spark plugs can go bad internally as well. Inside most spark plug wires is a carbon core. After a while this core can crack and small gaps develop in the core. This makes the resistance in the spark plug wire increase to the point where the spark won't travel to the spark plug, or get to the spark plug greatly weakened.

   Another thing is spark plug wire routing. Some cars, Chryslers especially, are sensitive to spark plug wire routing. If they are too close to each other, they will develop cross fire and lead to all kinds of driveability problems. So when you replace spark plug wires, make sure you route them in the same way and that any protective shields or wraps are put back on.

   If it's been a while the best thing to do is replace the spark plug wires so there is no question of their quality.

   Now we go to the spark plugs. Here we have to check for the proper spark plug type and gap. Also "reading" the spark plugs will tell a story of the general engine condition. The brand of spark plugs you use can make a big difference. I am a great believer in using the OEM spark plugs in an engine. I would use Motorcraft in Ford products, AC Delco in GM products and so forth. Japanese cars do not, for some reason, like American spark plugs so I use only NGK brand spark plugs in them and Bosch spark plugs in German cars. So if you have any doubts about the spark plugs, replace them.

Basic Engine

   Check the ignition timing. It may be computer controlled and no provision for adjustment is made, but how do you know the computer has it set right? It only takes a couple of minutes to check and if it's off, you found a problem. If the ignition timing is off, and provision for adjustment is provided, then adjust the ignition timing to the correct specification.

   Check the engine vacuum with a vacuum gauge. At idling speed, an engine at sea level should show a steady vacuum reading between 14" and 22" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 23" or more.

   If you have a cylinder misfire, check the compression in that cylinder. If it's good then you know the engine itself is sound and the problem lies elsewhere. If you have adjustable valves, make sure they are adjusted properly before doing anything. A tight valve will cause a misfire or backfire.

   Copyright © 2000 - 2003 Vincent T. Ciulla All Rights Reserved

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