Simpson MSH3125-S Gas Pressure Washer

Simpson MSH3125-S MegaShot 3100 PSI 2.5 GPM Honda GC190 Engine Gas Pressure Washer

Simpson MSH3125-S Gas Pressure Washer Review

The Simpson MSH3125-S Gas Pressure Washer is not perfect. But it does provide value for money. And this residential machine is powerful enough to handle most medium- to heavy-duty cleaning tasks.

My pressure washer arrived from Amazon in a slightly damaged box. I was apprehensive to open the box, but the contents were not damaged. There were no oil stains, and Simpson included all the necessary components. I specifically listed each of these points because buyers have mainly complained about these issues. 

If you're purchasing this pressure washer from Amazon, try to schedule delivery for a day on which you will be available to inspect the contents. Alternatively, check the items in the box before leaving your Home Depot or Lowe's store.

You can assemble the unit quickly. I needed 19min but could have shaved off 5min - if I had read the instructions. I have explained the assembly and start-up process in the FAQ. My description uses images from the Simpson electronic manual so you should be able to follow the method quickly. The FAQ also has tips on getting your Simpson Pressure Washer started up quickly and safely.

Next, you will need to check the sturdiness of the frame. Tighten any loose bolts - there are direct benefits to not injuring yourself or damaging your property. Follow the rest of the instructions in the FAQ until you're ready to start the machine.

Starting the machine can be tricky. 

The pressure washer is in a closed system - there's pressurized water on the pump suction, but no discharge pressure relief. If you pull the starter cord, but the engine fails to start, then you will quickly compress the inlet water. Here's a simpler, safer way to start the Simpson MSH3125-S Gas Pressure Washer...

Find an assistant. Ask your assistant to fully depress the wand trigger. Then, start the engine. It should start at the first attempt. I've tried the method described in the manual, but that procedure is less reliable. Pressing the trigger effectively opens the water flow to atmosphere, relieving the back pressure on the pump. 

A pressure washer is like a sophisticated pencil. You don't care how it works, but if it breaks you want someone there to fix it.

Unfortunately, this is where Simpson fails to deliver.

The Simpson customer service department does not have many positive reviews. They are reluctant to solve warranty claims, especially those related to third-party vendors such as Amazon. Luckily, the manufacturer's poor service is offset by Amazon's excellent customer service. The lesson here is to choose a seller that will counterbalance Simpson's lousy customer support.

Technical Specifications & Warranty Information

Technical SpecificationsSpecifications





Power Source


Water Pressure

3100 PSI (maximum)

Flow Rate

2.5 GPM (maximum)


34.75" x 21" x 23.25" (L x W x H)


65 lb

Engine Manufacturer

Honda GC190

Fuel Tank Size

61 fl. oz

High Pressure Hose Length

25 ft. (7.6m)

Warranty Information


1 year

Honda Engine

2 years


90 days

Frequently Asked Questions

Q: How do I assemble and start the pressure washer?

Simpson MSH3125-S Gas Pressure Washer
  • Place the handle into the frame. Depress the snap buttons, and slide the handle assembly onto the frame until the buttons snap into place.
  • Torque all bolts, and check the sturdiness of the frame. Loose bolts may lead to oil leaks, catastrophic` destruction and personal injury. Those are all bad outcomes!
Simpson MSH3125-S Gas Pressure Washer
Simpson MSH3125-S Gas Pressure Washer
  • Place the nozzle holder onto the panel (located on the handle assembly) and push each nozzle holder into place. Remove the colored quick connect nozzles from the plastic bag and insert them into the correct grommet on the nozzle holder.
  • You should secure the nozzle holder with an adhesive. The vibrating machine may lead to lost or damaged nozzles.
Simpson MSH3125-S Gas Pressure Washer
  • Attach the wand to the gun, and tighten securely.
Simpson MSH3125-S Gas Pressure Washer
  • Pull the quick-connect coupler back and insert the nozzle. Then release the quick-connect coupler and twist the nozzle to secure it in the coupler.
Simpson MSH3125-S Gas Pressure Washer
  • Attach high-pressure hose to gun. Tighten securely.
Simpson MSH3125-S Gas Pressure Washer
  • Connect the high-pressure hose and the garden hose.
Simpson MSH3125-S Gas Pressure Washer
  • Fully open the COLD inlet water.
  • DO NOT use hot water. 
Simpson MSH3125-S Gas Pressure Washer
  • Squeeze and hold the trigger until the water flow is steady. Then release the trigger. 
  • Switch on the machine. Pull the starter cord to start the engine.
  • DO NOT start the motor until you're ready to begin washing.
  • REMEMBER: Add engine oil, gas and a fuel stabilizer. Read our FAQ for additional information.
  • If you're having problems with starting (or restarting) the motor, try this tip. Ask someone to depress the wand trigger when you are ready to start the machine. This method will reduce the back pressure on the pump during start-up. Pull the starter cord while the trigger is depressed.
  • If the engine does not start after a couple of pulls, squeeze the trigger of the gun to decrease pump pressure. Pull the starter cable slowly until resistance is felt. Then pull the cord rapidly to avoid kickback, protecting against hand or arm injury.
  • Check the choke position. If the engine is cold, close the choke. Open the choke if you are restarting a warm engine.
  • Bypass mode allows water to re-circulate inside the pump when you do not depress the trigger. This feature enables you to release the trigger gun and move around without having to switch the motor off in between washing actions. You are, however, not allowed to operate the unit for more than two minutes without pressing the trigger - this action may overheat and damage the pump. Do not let the pressure washer run for more than two minutes in bypass mode. Switch off the engine. Then relieve the pressure in the wand during these long breaks.

Q: What home improvement applications can the machine be used for?

  1. Restoring and cleaning a wood deck
  2. Cleaning a fence
  3. Removing barnacles from  a boat hull
  4. Washing your car or motorhome
  5. Sidewalk
  6. Driveway
  7. Restoring a pool
  8. Removing salt or corrosion off a vehicle’s underbody
  9. Cleaning gutters
  10. Removing old paint
  11. Cleaning animal pens
  12. Cleaning farm equipment
  13. Washing concrete
  14. Restoring patio or lawn furniture

Q: Can you leave the machine running without discharging  water?

You should not leave the pump running (even for a few minutes) without depressing the trigger. There is pressure relief valve to allow water to escape before damaging the pump. Releasing the trigger will not automatically stop the engine. Therefore, stop the motor during washing breaks. Do not start the engine until you’re all set to begin washing. If you stop discharging water for more than a minute, release water into the air to relieve the pressure. Else, shut down the engine to prevent overheating the pump.

Q: Does the washer contain a detergent tank?

No. There is no integrated detergent tank. You can connect the clear hose to your detergent container. The venturi connection will siphon and mix the cleaning solution with water.

Q: Is there a low oil alert?

The engine shuts off automatically when a low oil alert is triggered.

Q: Can I adjust the pressure to wash my car?

Yes. Refer to the list of nozzle colors to find the appropriate one for washing your car. Pressure is proportional to force and inversely proportional to area. 

Q: What are the water requirements?

Cold tap water is recommended. The preferred inlet pressure range is 20 – 100 psi.

Q: Can I use bleach as a cleaning solution?

Do not use bleach. Use only soaps and chemicals designed for pressure washer use.

Q: I purchased the pressure washer from Amazon. Can a local warranty shop repair the machine? 

Amazon is not an authorized dealer. Local warranty shops will only cover the warranty for units sold by authorized dealers.

Nozzle Colors - An overview


Spray Pattern


Recommended Surfaces


0 degrees

for spot cleaning of hard, unpainted surfaces or hard-to-reach areas

unpainted metal or concrete - do not use on wood


15 degrees

for intense cleaning of hard unpainted surfaces

grills, driveways, concrete or brick walkways, unpainted brick or stucco


25 degrees

for standard cleaning nozzle for most applications

yard tools, sidewalks, lawn furniture, unpainted siding, stucco, gutters and eaves, concrete, brick surfaces


40 degrees

for cleaning of painted or delicate surfaces

auto/RV, marine, wood, painted brick and stucco, vinyl, painted siding


low pressure

for applying cleaning solutions

low-pressure spray is safe on all surfaces. Always verify compatibility of cleaning solution before use

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