The Simpson AAA Triplex Plunger Pump Kit #90034 is a direct-replacement pump for gas-powered pressure washers with a horizontal-shaft engine, built to restore full 4400 PSI at 4.0 GPM performance when a factory pump wears out, leaks, or loses pressure. It's engineered around a triplex plunger design with dual oversized ball bearings that reduce internal friction, a stainless steel integrated unloader with an easy-start system, and PowerBoost technology that increases pressure at the nozzle for faster, more efficient cleaning. A built-in sight glass replaces the traditional dipstick, so you can check oil level and condition at a glance, and thermal relief protection helps prevent the pump from overheating during extended runs.
Specifications
- Pressure/Flow: 4400 PSI at 4.0 GPM
- Pump type: Horizontal-shaft triplex plunger pump
- Shaft: 1-inch diameter keyed shaft (fits engines such as the Honda GX390)
- Minimum engine requirement: 11 HP / 340cc gas engine
- Inlet/outlet: Garden hose inlet; quick-connect high-pressure hose outlet
- Included hardware: Siphon tube with filter, mounting bolts, shaft key
Built for these use cases
- Replacing a worn or leaking factory pump on a Simpson or comparable gas pressure washer instead of buying a whole new machine.
- Restoring lost pressure or flow on units that have started pulsating, surging, or running weak after years of use.
- Upgrading a lower-output pump to 4400 PSI / 4.0 GPM for tougher jobs like stripping paint, cleaning concrete, or heavy fleet washing.
- Commercial and rental fleet maintenance, where triplex pumps are preferred over axial pumps for longer duty cycles and rebuildable service life.
- Contractor and industrial cleaning setups pairing the pump with an 11HP+ horizontal-shaft gas engine for daily use.
Will the 90034 pump fit my pressure washer?
It's designed for gas pressure washers with a horizontal-shaft engine and a 1-inch diameter keyed shaft, such as the Honda GX390, and uses the SAE-standard mounting bolt pattern common to most gas units. Confirm your engine's shaft diameter and mounting hole spacing before ordering, since pump kits are not universal across every shaft size.
What horsepower engine do I need to run this pump?
AAA specifies a minimum 11 HP (340cc) gas engine to run the 90034 at its full rated output. On engines below that size, the pump's pressure should be turned down using the adjustable unloader knob to avoid overloading the engine.
How do I install the pump?
Apply a light coat of anti-seize to the engine shaft and shaft key, align the pump and engine keyways, and slide the pump fully onto the shaft before securing it with the included mounting bolts. Most installs take under 30 minutes with basic hand tools.
My pressure washer won't build pressure after I installed the new pump. What's wrong?
This is almost always trapped air rather than a defective pump. Disconnect the spray gun, turn on the water supply, and let water run through the hose and pump for one to two minutes until it flows steadily with no air bubbles, then reconnect the gun and start the engine. Also confirm the garden hose has full, unrestricted water flow, since a starved pump will not build pressure regardless of condition.
How do I know if I need a new pump instead of repairing the old one?
Watch for oil leaking from the pump body, milky or cloudy oil (a sign water has entered the oil chamber), sloppy or worn plungers, and a drop in pressure or a pulsing spray pattern. If a repair would cost more than roughly half the price of a new pump, replacing the whole pump kit is typically the more economical option.
Discount Cleaning Products is an authorized dealer of Simpson and AAA Pumps replacement parts, and our US-based customer service team can help confirm shaft size, engine compatibility, and installation questions before you order.
