Question 1

my pressure washer makes a really loud knocking or banging sound from the pump when it's running.

Accepted Answer

DIAGNOSIS A loud mechanical knock from the pump area is serious. It's often a connecting rod inside the pump that's come loose or is hitting the housing, or a severely worn crankshaft bearing where the pump connects to the engine. FIX Stop using it immediately. This isn't a fix-it-in-five-minutes issue. You need to isolate the sound. First, remove the pump belt cover (if equipped) and see if the sound follows the pump speed. If it's definitively from the pump, the unit likely needs a pump replacement. These axial pump designs aren't really meant for user-level internal repairs on the crankcase. Continuing to run it will turn a pump problem into an engine problem if a rod breaks. IF THAT DIDN'T WORK Before condemning the pump, double-check that all mounting bolts are tight and that nothing external (like a loose hose or guard) is vibrating and hitting the pump housing. Also, ensure the pump and engine pulleys (if belt-driven) are aligned and tight. A loose pulley can make a horrific banging sound. HEADS UP This kind of sound is different from the lighter "clicking" of a normal working pump. Don't ignore it. The forces inside a pressure washer pump are immense, and a catastrophic failure can send metal parts flying. If it's a loud knock, plan on a new pump.

Question 2

engine starts and runs okay but there's no pressure at the gun, just a garden hose stream. simpson megashot.

Accepted Answer

DIAGNOSIS The pump is turning but not pressurizing. This usually means the water is bypassing the pump internals completely. The first place to check is the thermal relief valve, a safety feature on the pump that opens if the pump overheats. FIX Locate the thermal relief valve. On the MSH3125 pump, it's typically a small, hex-headed brass fitting on the side or top of the pump head, often with a spring inside. If the pump has overheated (from running without water flow), this valve can stick open. Try tapping it gently with a wrench to see if it re-seats. If that doesn't work, you may need to remove it, clean any debris, and ensure the spring moves freely. Sometimes simply removing and reinstalling it resets it. IF THAT DIDN'T WORK If the relief valve is fine, the issue could be severely worn inlet or outlet valves inside the pump head. These are little check valves that can get pitted or have their seals worn out, letting water flow backwards. This requires a pump head rebuild kit to replace all the valve seals and springs. HEADS UP A stuck thermal relief valve is a symptom, not just a cause. It means the pump got dangerously hot, likely from dry firing or a restricted water inlet. Always address the root cause (check inlet screen, never run without water) or you'll just burn out the new pump.

Question 3

the unloader valve on my ms3125 is leaking water really bad from the sides when i'm using it.

Accepted Answer

DIAGNOSIS The unloader valve assembly has internal seals that direct high-pressure water. A leak from the body means those seals are worn or damaged, or the valve body itself has a crack (less common). FIX You need to rebuild or replace the unloader valve. First, relieve all pressure and disconnect the spark plug. The unloader is the brass block where the high-pressure hose connects to the pump. It's held on by several bolts. Remove it. You can often find a rebuild kit (like a "gun/unloader seal kit") that contains the specific O-rings and seals. Disassemble the valve carefully, noting the order of the springs, plungers, and seals. Replace all seals with the new ones from the kit, lubricating them with a tiny bit of silicone grease before reassembly. Tighten the body bolts evenly. IF THAT DIDN'T WORK If it still leaks from the same spot after a rebuild, the metal valve body may have become scored or cracked from freeze damage or grit. In that case, you need to replace the entire unloader valve assembly. HEADS UP When you reassemble, be very careful not to overtighten the bolts on the brass body. They only need to be snug. Over-tightening can warp or crack the housing. Also, always run the machine for a minute with the trigger gun pointed in a safe direction after any repair to check for leaks before you start working.

Question 4

my megashot won't start at all, not even a sputter. i checked for spark and it's good, gas is fresh.

Accepted Answer

DIAGNOSIS With good spark and gas, the engine should at least cough. The fact it doesn't points to a lack of fuel actually making it into the cylinder. On these, the most likely cause is a clogged pilot jet or passage in the carburetor from old fuel residue. FIX You'll need to clean the carburetor. Remove the air filter cover and the filter. Then, disconnect the fuel line (pinch the spring clamp). Remove the two bolts holding the carburetor to the engine. There's often a gasket between them. Once off, remove the float bowl (single bolt on the bottom). You'll see a small, main jet and an even smaller pilot jet (sometimes called the "idle jet"). Use a can of carburetor cleaner and a fine strand from a wire brush or a dedicated jet cleaning tool to clear the tiny hole in the pilot jet. Spray cleaner through all the small passages in the carb body. Reassemble everything. IF THAT DIDN'T WORK If it still won't start, check that the choke plate is actually closing fully when you move the lever. If the linkage is bent or disconnected, the engine isn't getting the rich fuel mixture it needs to start cold. HEADS UP When reinstalling the carburetor, make sure the small spring that connects the governor linkage is hooked back up correctly. If it's not, the engine will either not run or will immediately rev to dangerous RPMs when it starts.

Question 5

oil is leaking from the bottom of the pump on my simpson, looks like it's coming from a little plug. is that bad?

Accepted Answer

DIAGNOSIS That's the pump oil drain plug. If it's leaking, the seal (usually an O-ring or crush washer) has failed. The pump uses a special non-detergent SAE 30 oil, not engine oil, and if it all leaks out, the internal components will overheat and seize. FIX You need to replace the seal. First, catch the old oil in a pan by removing the drain plug and the oil fill plug (on the top of the pump). Once drained, inspect the drain plug. It likely has a small rubber O-ring or a fiber washer. Replace it with an identical new one—hardware stores often have assortments. Clean the threads, install the new seal, refill with the correct pump oil to the level indicated in the sight glass on the side, and reinstall the fill plug tightly. IF THAT DIDN'T WORK If it still leaks from the same spot after replacing the plug seal, the threads on the pump housing itself might be stripped. In that case, you can try wrapping the plug threads with Teflon tape designed for oil, or you may need a slightly oversized plug or a helical thread insert. HEADS UP Check the pump oil level via that sight glass every few uses. It should be in the center of the glass. Cloudy or milky oil means water has gotten into the pump oil chamber, which is a more serious issue requiring a full tear-down and seal replacement.

Question 6

pressure washer surges and pulses really bad, can't hold a steady stream. it's a megashot 3125.

Accepted Answer

DIAGNOSIS That pulsing is almost always caused by air getting into the water supply side of the pump. The pump is trying to pressurize a mix of air and water. FIX Start at the source. Make sure your garden hose is connected tightly to both the spigot and the washer's inlet. Run water through the hose with it disconnected from the washer to purge all air before connecting. Next, check the inlet water filter—it's a small screen inside the female garden hose connector on the washer. Unscrew it with pliers, clean any debris, and re-seat it with a little Teflon tape on the threads to ensure a good seal. Finally, with the machine off and trigger gun closed, turn on the water supply and squeeze the trigger for a few seconds to purge any remaining air from the pump and hose. IF THAT DIDN'T WORK If it still pulses, the O-ring or seal on the inlet side of the pump itself might be worn or damaged, allowing air to be sucked in even under pressure. This requires disassembling the pump head to inspect and replace the inlet seals. HEADS UP Never run the pump without water flowing through it, even for a few seconds. That's called "dry firing" and it destroys the internal seals and pistons almost instantly, which will also cause pulsation.

Question 7

just got my ms3125 and it runs for a minute then dies like it's out of gas, but the tank is full.

Accepted Answer

DIAGNOSIS Yeah, this one's common on new units. The fuel tank cap isn't venting properly, creating a vacuum that starves the engine of fuel after it burns through what's in the carburetor bowl. FIX First, try running it with the gas cap slightly loose. If it runs fine that way, you've confirmed it. The fix is usually simple: remove the gas cap and check the small vent hole in the center of the plastic cap. Poke a thin wire or a paperclip through it to clear any manufacturing debris or blockage. You should hear a slight hiss of air when you open the cap on a warm engine if it's venting correctly. IF THAT DIDN'T WORK If the vent is clear and it still dies, the problem might be a stuck or gummy float valve in the carburetor from old gas or factory preservative oil. You'll need to remove the carburetor bowl (single bolt on the bottom), clean the bowl and the small needle valve the float controls, and reassemble. HEADS UP Always use fresh, ethanol-free gas if you can find it. The ethanol in standard gas attracts moisture and turns to varnish surprisingly fast in small engines, causing these kinds of fuel flow problems.

Question 8

my simpson megashot starts fine but the pump just hums and won't build any pressure, what gives?

Accepted Answer

DIAGNOSIS That's the pump motor humming but the piston assembly inside isn't moving. On these, the most common culprit is that the pump was stored with water in it over winter and the piston is stuck or seized. FIX First, relieve all pressure and disconnect the spark plug wire. Remove the four bolts holding the pump to the engine. You'll need to separate the pump from the engine crankshaft. Once it's off, you can try to manually turn the pump's input shaft with a wrench. If it's stuck, you'll need to disassemble the pump head (the part with the water inlets/outlets) to free up the pistons. Often, tapping the pistons gently with a rubber mallet after applying some penetrating oil will break them loose. Clean out any scale or debris and reassemble with pump oil. IF THAT DIDN'T WORK If the pump shaft turns freely by hand but still hums when powered, the issue is likely the unloader valve (the brass block where the pressure hose connects) being stuck in the bypass position. Remove it, disassemble it carefully, clean any debris from the small spring and plunger inside, and reassemble. HEADS UP Never, ever run the engine for more than a few seconds with the pump humming like that. You'll burn out the motor coil in under a minute. If you can't free it manually, you're looking at a pump replacement.

Simpson MegaShot MSH3125 Gas Pressure Washer

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