The gap is necessary because dishwashers create a lot of heat and humidity. It can damage the counters to be exposed to these elements too closely. Most manufacturers recommend a gap of 1/2-3/4 of an inch.
Generally, most appliance manufacturers recommend a minimum of 27 inches or more between the edge of the dishwasher door when ajar and the facing cabinets.
You will need to purchase a pre-finished cabinet frame filler. They typically come 3" wide x 36" long. The dishwasher must be removed, the filler installed and attached to the cabinet and the the dishwasher installed mounting to the filler.
Many dishwashers have 1/2" - 3/4" gap or clearance to the counter top. This is not unusual. Many manufacturers require a certain amount of "clearance" to the top of the counter top/sides of the cabinets due to heat and humidity that comes of the machine. That's why I suggested a 1/2" plywood and nothing thicker.
Just get it in a spray can at the hardware store and spray into the gap. It will then expand to fill the hole. Once it is dry you can clean it up a bit to be more presentable with a hacksaw blade. This method will only work for small-sh gaps though.
Dishwashers require leveling when installing them under the counter. If the dishwasher is out of level, water sits at the low point. Eventually, the standing water causes odors, and attracts mold and mildew.
The tilt back will provide adequate water for a good spray. Water is best deeper in the back than at the point of the float. You cannot find this in very many install manuals as well as troubleshoot advice online, but it is a truth that should be used.
A dishwasher that leans in one direction can cause water to leak. If it leans forward, you might notice water leaking from underneath the door. A minor leak might go unnoticed, but the dishwasher might start leaking so much that the water pools on the floor near the appliance.
Corner locations need at least 2 inches of space between the side of the dishwasher door (when open) and the wall or cabinet. Check for any obstructions that would cause the dishwasher motor to touch the floor of the opening, such as carpeting or loose, uneven tile.
Standard Dishwashers
The vast majority of undercounter dishwashers measure approximately 24 inches in width, 24 inches in depth and 35 inches in height with adjustable feet. This means that the rough opening in your cabinets must be 24- by 24- by 35-inch.
The appliance's water inlet valve may not be closing properly. The water inlet valve, which is often located at the bottom of the appliance, allows water to flow into the dishwasher tub throughout the cycle. If it doesn't open and close properly, your appliance may overfill and the excess water will likely leak out.
Leaky gasket: A faulty door gasket is one of the most common causes of dishwasher leaks. Water on the kitchen floor in front of the dishwasher could indicate leaking door gasket.
The air gap is a chrome plated cylinder located on the back of the sink where the faucet is located. Not all areas have plumbing codes that require an air gap, so you may not have one installed with your dishwasher drain. The air gap prevents water from a clogged drain from backing up into the dishwasher.
For softer clogs, pop the cap off of the air gap, and place a cardboard tube from a roll of paper towels over the hole. Blow through the top with a good amount of force, and any material should come loose. If this doesn't work, though, you can use either a long bottle brush or a wet/dry vacuum to remedy the issue.
A foul smell or dirty water squirting out of the air gap vent holes indicates a clog inside the air gap. The fix is a simple cleaning with household disinfectant.
If your dishwasher doesn't have an air gap, its drain hose may be looped up and attached to the bottom of the countertop near the sink. Although this is not code-compliant in most places, it serves the same purpose as the air gap—to prevent drain water from siphoning back into the dishwasher.
One of the most common reasons for water at the bottom of your dishwasher is because food particles or other debris have clogged the drainage system. Check the dishwasher drain and clean the drain basket around the lower sprayer arm, unclogging debris with a wire hanger.
Air gap fittings have a decorative heading that extends above the counter, usually next to your faucet or soap dispenser. This heading houses the vertical air gap. Air gaps are notched with holes that will release water into your sink if the drain becomes clogged or the tubing is obstructed.
Most of the undercounter dishwashers measure approximately 24 inches in width, 24 inches in depth, and 35 inches in height with a variety of feet. The opening in your cabinets must be at least 24 by 24 inches. It's possible that the dishwasher is somewhere between 23 1/2 by 23 1/2 and 34 inches.
You may need to remove the kick panel to get a grip on it. If that's the problem, try screwing the adjustable front feet down a bit to snug things back up. If that's not the case, check for damage to the door near where the hinges attach. There may be deformation of the sheet metal.
Dishwashers that aren't level can lead to unwanted vibrations and noise. Fortunately, all dishwashers are equipped with two adjustable legs on the front of the machine. Raising the leg on the left will raise the left side of the machine, while raising the right leg will lift the right side during leveling.
Each time the unit tilts forward, it gradually moves out of its “normal” position, and over time it can shift to the point where the front of the unit is no longer flush with the surrounding cabinets. The most common reason why dishwashers tilt forward when opened is because of improper installation.