Tech Tuesday : Basic Engine Troubleshooting

Last time I went over the function and parts of a basic old-timey engine. This time it’s about how to find out what is wrong when it’s not running right. Hit the jump and even you too can fix your uncle’s 1969 pedo van down by the river. I can’t do anything about the wall to wall shag carpet though.

A basic engine needs three things to run correctly; Air, fuel, and spark. If it’s missing one of these or there is a disproportional amount of any of them, it will run rough, or not at all. So, I’m going to run through how to figure out what is wrong with that old wreck in your front yard.

To guide you along with your quest for engine nirvana, I suggest you get a manual for your car. The typical Haynes manual cost around $15 for cars in print. It’s the best $15 you will spend on your car if you plan to wrench on it yourself. It will contain step-by-step procedures, labeled pictures of the offending parts, fluid quantities, and tolerances it takes to keep your car running well.

The most common ailment is something in the ignition system. Whether the timing is off or a spark plug is broken, there is a common method to find out what is wrong with your engine. You have to approach it scientifically (for great science!), one part at a time. I usually start with the plug. If the engine has a misfire I go get a piece of chalk and put a mark on the exhaust manifold/header right where the exhaust ports are. You then fire up the engine. The marks will burn off once enough heat is built up, except for the missing cylinder.

Now you have pinpointed where the malfunction is, so it’s time to diagnose what is amiss. The first thing I like to do is unplug the bad cylinder’s wire and put a dummy plug in the wire. Touch it to the engine and have a friend turn the car over. Try not to touch the car, and wear rubber sole shoes. Trust me, you don’t want to know what a 20K volt milliamp charge feels like (but it’s fun to do to your friends). If you see a spark, that means the wire is good and chances are the plug is bad. I change my spark plugs about every two years. I’ll also keep the old set of plugs to use as dummies. If there is no spark, take a wire from a good cylinder and plug it into the bad spot and check for spark. If it’s still not there, the distributor cap is bad.

If you have no spark anywhere it’s best to put a new distributor cap and rotor in anyways. They are a cheap part and it’s good insurance to look further up the line. If it’s still not working, then it’s probably the coil. Now would also be a good time to make sure that the battery is charged, but if the starter didn’t work you would have figured that out already. Also check to make sure that all the wires are snapped on tight. Next, you’ll want to check the points in the distributor to see if they are gapped correctly and not burned out. Or do yourself a favor and get a Pertronics pointsless system. Its expensive, but adjusting points sucks. Are you too hardcore for the Pertronics? Then check your car manual for the clearance. If you still have no spark then take off the distributor cap and see if it is spinning when the engine is being turned over. I have seen distributors break or strip gears before. It’s uncommon, but has happened. If that’s the case, then you have a larger problem. Now you’ve got to get out the shards or pieces of gear before they block or destroy the oil pump. Also, check how loose the gear is on the distributor. Just wiggle the rotor back and forth. A little is good. A lot of slop is bad.

So you have spark and still no go. Next up is the fuel system. The question here is whether or not the fuel is getting into the cylinder. Try cranking the engine for an extended amount of time, then pull out a spark plug. It should be wet and reek of gas. If it isn’t, replace the fuel filter. They are cheap, and are good insurance. Unplug the coil wire and crank the car. You should have fuel moving through the filter. If it doesn’t, then it’s time for a new fuel pump. If you’re getting fuel through the filter, then get ready to remove the carburetor. Once you have the carburetor off, remove the bowl. The bowl is a reservoir where the gas is stored until it’s sucked into the engine. If the bowl is full then maybe the carburetor is bad or blocked. Some performance carburetors have a sight glass on the bowl making this step much easier.

Now it goes without saying that if you are working on a car that has been sitting for some time that fresh ignition parts and gas is required. These parts tend fail with no use. Plus, any gas that’s been sitting for more than 6 months is suspect. If you plan to store a car with gas in it, there are a couple of gas preservers that work fantastic, and save you the hassle of cleaning the carburetor from the gummy deposits and replacing dried out seals. Cleaning a carburetor is best left to a professional, but if you are handy, interested, or flat out broke, there are some good manuals out there to help you go step by step through a rebuild. Just make sure you have the right manual for your carburetor. Various carburetor companies build carburetors that work differently than others.

Before you go and rebuild the carburetor, check for the air portion of the equation. Make sure that nothing has set up residence in your intake. And check the compression of the engine. Most stores carry a cheap compression tester. It’s the best way to check the state of your engine in the combustion chamber. Check your manual for acceptable compression pressures. If you have low compression, it’s time to think about saving up for a rebuild. Low compression usually means bad/broken rings or bent or burnt valves. No compression means it’s not going anywhere soon. Holes in pistons and broken valves are full rebuild time, and full rebuilds are far from the topic of this story.