Hey guys I've been searching for about two hours now but I cant really find the correct search parameters to find the answers I was seeking...

Basically I was looking to see what exactly defines "compression"... like for example, (real life example ) if a compression test came out as 0 compression, does that mean that something in the cylinder is not working at all (air, fuel, spark) and so the cylinder gets NO compression? What typically causes a 0 compression (dead cylinder I'm guessing) cylinder?

What would I go about checking first? I know that there is spark because I did the spark test (plug 1/4 inch from block and start car ... this scared my friend hehehehehe). So that would leave air and fuel.... (this is for a friend's Fuel Injected car btw, and there are two cylinders that have 0 compression, 1 and 3) I am guessing that since I am getting Fuel to all the other cylinders that that can knock out the idea of fuel? I have a carb'd setup so I'm not sure about that.

Are there 4 fuel injectors? each one fuels the two closet cylinders (1 and 3, then 5 and 7, 2 and 4, 6 and 8) ? I am guessing not, I am pretty clueless on FI's....

So what could be wrong with the air then if it's getting fuel? I am assuming that even with a massive vacuum leak, you could run the car easy enough to atleast idle the car?

I am hoping that it is not this way, but could it be that something on the crankshaft is not holding up to the 1 and 3 cylinder and so the piston does not move up or down?

I'm sorry for the length of this post, I am trying to learn, as well as help a buddy. Thanks in advanced guys :hail: :hail: :hail:


EDIT: spelling/grammer

 

Think of the engine as a big air pump. There are four steps to the "pumping" process, and all are neccecary.

Intake: Piston moves downwards in the cylinder as the intake valve opens, sucking in the fresh mixture of air and fuel.

Compression: The piston moves upwards as the intake valve closes, "squeezing" the air/fuel mixture.

Power: As the piston reaches the top of the bore again, the sparkplug fires and ignites the mixture, causing a controlled burn. The piston is thrown downward in the bore, forcing the crankshaft to rotate.

Exhaust: The cylinder then moves upwards a final time, but this time the exhaust valve opens. The inert gas is pushed out, and then the process starts all over again.

By not having a piston seal in the piston, none of these processes can efficiently take place.

There could be a few reasons for this. The piston rings could be worn out. The rings are what seals the piston to the cyl. bore. If they are worn, some of your mixture escapes to the crankcase, and the rest of the unignited mixture is vented out the exhaust. A burnt or bent valve, or a valve that isn't seating properly could also cause no compression. You could also have a leaking head gasket or a cracked or out of round cyl. wall.

There are 8 fuel injectors on the port fuel injection, and even 9 on some of the earlier TPI motors. One for each cyl, and then an extra cold start injector. If you have TBI, then you have 2 large injectors in the throttlebody.

Hope that helps you some

 

Something keeping a piston from moving up and down would be a catastrophic failure and there'd be more obvious signs of a problem than a dead cylinder, like a broken, bleeding block. Low compression could be caused by leaking head gaskets, burned valves, broken rings, worn cylinder walls, or any number of things. What kind of car is this on?

 

For 0 compression, the first thing that jumps to my mind is a stuck valve. Pull the valve cover and watch the valve action of both the intake and exhaust.

 

many thanks you guys, now that I have a good idea of what I'm looking at I think we'll start out small with things that should probably be done anyways like a head gasket, but we'll pull the valve covers off tommorow and make sure all the valves are good and no lifters are sunk or anything. Thanks again you guys, I am now smarter BTW, I totally didnt even think about the 4 stroke cycle haha.