The Three Things it Takes to Make an Engine Run

by Jim Deutsch

The internal combustion, reciprocating engine needs only three things to run.

Spark

Fuel

Compression

Yep, that’s it. Nothing more, nothing less.

Is there spark?

The spark needs to occur at somewhere near the correct time. This is a function of the spark plugs, the wires, the distributor, the timing, lots of things come into play here. We can assume that if you are getting a spark at the plug then you’re probably ok. This is unless the distributor is 180 degrees out. Not that above I used the term "at somewhere near the correct time".

Let’s see if you have spark. Remove one of the spark plugs and unhook the wires to the others. Put the spark plug in the cap and lay the plug body (so it’s grounded) on something like the exhaust manifold and have your assistant turn the engine over as though you were trying to start it. The reason we unhooked the other plug wires is that we don’t want the engine to start while you’re under there. A four cylinder engine will start on only 3 cylinders. You have spark at the plug? Good! No spark? We probably need to look at the distributor, coil, points, etc., then.

Is there fuel?

Are the plugs wet with fuel? If you can smell raw gasoline on them then you’re probably getting fuel to the cylinders. No fuel on the plugs? Hmmm. Is there fuel in the tank? Don’t laugh. It’s happened!

Are the float bowls for the carbs full? If not check the pump. Don’t assume anything. Look things over. Try squirting a little fuel into the throat of the carbs. Does the engine fire and then die when you do this? Perhaps we are not getting fuel to the cylinders. Look the carbs over. If the car has been sitting for a while then you’ve perhaps found a stuck needle in the seat and there is no fuel getting to one of the carbs.

Is there Compression?

Here’s where it can get expensive. Unless you’ve lost compression on several cylinders (2 out of 4 or so) you should be able to make the engine run. If more than one cylinder is low on compression then we’ve got a problem. If two cylinders out of four are low this could be a bad sign. It could also be a good sign. Two cylinders side by side being low on compression can be as simple as a bad head gasket. It can also be a bad sign as this problem can be caused by a cracked head. If all the cylinders are above 75 pounds then the engine has enough compression to run. If only one cylinder is low on compression then it could be a head gasket in this case too. Remove all the plugs. Unhook the coil wire from the coil. Have your assitant spin the engine with the starter for 6 revolutions with a compression gauge in the plug hole. Record the compression. Do the same for the other cylinders. Now, if you have good compression in all the cylinders, somewhere in the neighborhood of 125 pounds or better, we’re good to go.

If you have a low cylinder(s) we need to do one more test. With the cylinder that is low, squirt in some motor oil. Four squirts will do. Now check the compression again. If the compression RISES noticeably then you have bad rings. If the compression stays the same then you can bet it’s either a valve problem, a blown head gasket, cracked combustion chamber or something in that area.

Again, remember that these are relatively simple engines in our little cars. They don’t require a great deal of equipment to work on but they do require some tlc. Get back to basics when there is a problem. Don’t assume anything. Start at the beginning with the THREE things our engines need to run.