2000 Focus Zetec-rough idle, very rough under load.

Check plugs and plug wires or coils. Also do you have a check engine light on?

I'm not sure what you mean by "under load", maybe with a/c on, etc. Also is it really an engine misfire or a rough idle. If it really is a misfire, a scantool at your local parts store should be able to read a code for that. Sometimes a rough idle could be due to just a cracked or disconnected vacuum hose or dirty throttle body. You didn't say whether it runs ok at highway speeds. Sometimes a rough idle can be cleared up a lot with throttle body service or injector flush. Sorry, this is the best I can do with information provided.

misfire spark plugs coils wires did you wash engine water gets in under the intake to the plugs first if with a scanner check codes p0300 change plugs p0301 or 2 or 3 or 4 its one of the cylinders i would try just changing all the plugs first and start from there

Should be on or near the fuel injector supply rail.

If it is an ignition miss, it should turn the check engine light on and give you a code for which cylinder.
A fuel problem or mechanical problem would not always make the warning light come on. Once the engine warms up the O2 sensor gives the computer data to cut back on fuel.
You might want to check the computer for trouble codes to see if anything has been stored.

Ignition misfire, leaking intake gaskets, faulty fuel injector. Is the check engine light on and blinking with engine running?

spark plugs good

check that the EGR valve is not sticking or has a vacuum leak. This valve will not operate until the engine is warm.

check also the engine coolant temp sensor (ECT). This sensor tells the engine computer that the engine has warmed up, so if it is faulty, it could be sending wrong information to the computer. If the sensor is disconnected, wires to it broken, or faulty in full cold mode, it will tell the computer that the engine is very cold, like -40F. The computer will schedule fuel for cold conditions. This is usually way too much fuel for a warm engine and it will rich misfire, idle rough, and be difficult to start warm.

The ECT has two wires going to it and is dedicated to sending information to the engine computer. It has nothing to do with your dash gauge that shows coolant temp. That sender has only one wire and is dedicated to the coolant temp gauge.

Sounds like a fuel pressure problem. Get it checked.

Misfire is a common driveability problem that may or may not be easy to diagnose, depending on the cause. A misfiring cylinder in a four-cylinder engine is, pardon the pun, hard to miss. The loss of 25% of the engine's power output is the equivalent of a horse trying to run on three legs. The engine may shake so badly at idle that it causes vibrations that can be felt in the steering wheel and throughout the vehicle. The engine also may be hard to start and may even stall at idle, depending on the accessory load (air conditioning, headlights and electric rear defroster, for example).

When misfire occurs, performance suffers along with fuel economy, emissions and idle quality. And, when a misfiring vehicle is subjected to an emissions test, it will usually fail because of the unusually high levels of hydrocarbons (HC) in the exhaust.

What causes a cylinder to misfire? Basically, it's one of three things: loss of spark; the air/fuel mixture is too far out of balance to ignite; or loss of compression. Loss of spark includes anything that prevents coil voltage from jumping the electrode gap at the end of the spark plug. Causes include worn, fouled or damaged spark plugs, bad spark plug wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder.

"Lean misfire" can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. This can be caused by a dirty, clogged or inoperative fuel injector; air leaks; or low fuel pressure because of a weak pump, restricted filter or leaky pressure regulator. Low fuel pressure would affect all cylinders rather than an individual cylinder, as would most air leaks. A leaky EGR valve can also have the same effect as an air leak.

Loss of compression means the cylinder loses most of its air/fuel mixture before it can be ignited. The most likely causes here are a leaky (burned) exhaust valve or a blown head gasket. If two adjacent cylinders are misfiring, it's likely the head gasket between them has failed. Also, if an engine is overheating or losing coolant, it's likely the head gasket is the culprit.

Intermittent misfires are the worst kind to diagnose because the misfire comes and goes depending on engine load or operating conditions. They seem to occur for no apparent reason. The engine may only misfire and run rough when cold but then smooth out as it warms up. Or, it may start and idle fine but then misfire or hesitate when it comes under load. Also, it may run fine most of the time but suddenly misfire or cut out for no apparent reason. Intermittent misfires can be a real challenge to diagnose, so let's start with a steady misfire in one cylinder before moving on to intermittent misfires.

Hope this help.