Replacement Valve for 551-33-1 | Sierra 18-99528

Sierra Replacement Valve | 18-99528 | CDI 551-33-1 Gear Lube Pump | Check Valve

The Sierra #18-99528 is the replacement check valve for the CDI 551-33-1 gear lube pump. The check valve in a gear lube pump is the one-way valve that allows gear lube to flow from the bottle into the pump and out through the nozzle under pump pressure, while preventing gear lube from siphoning back after the pump stroke. A failed or worn check valve causes the pump to lose prime, deliver inconsistent flow, or fail to hold pressure between strokes — making lower unit gear lube fill frustrating and inaccurate. Replacing the valve restores full pump performance without replacing the entire pump assembly.

Part Details:

• Part Number: 18-99528 (Sierra)
• Manufacturer: Sierra
• Type: Replacement Check Valve — Gear Lube Pump
• Fits: CDI 551-33-1 gear lube pump
• Replaces: CDI 551-33-1 valve
• Condition: New

Additional Information

How to Diagnose a Failed Gear Lube Pump Check Valve:

The check valve is typically the first component to wear or fail in a gear lube pump. Signs that the valve needs replacement:

• Pump loses prime between uses — you have to re-prime the pump each time you pick it up, even mid-service; a functioning check valve holds fluid in the pump body between strokes
• Inconsistent or weak flow at the nozzle — the pump strokes but delivers less fluid than normal per stroke, or flow drops off after the first few pumps
• Fluid drains back into the bottle — after pumping, fluid siphons back rather than staying in the hose; the check valve is not seating
• Pump requires excessive strokes to deliver a quart — a pump that used to fill a lower unit in 10–15 strokes now requires 30+ strokes

For a complete gear lube pump service setup, also see:

• Sierra Gear Lube Pump Hose Assembly (18-99526) — replacement hose for the CDI 551-33 pump kit; also replaces CDI 551-33HB
• Suzuki OEM Gear Oil Pump (990C0-99002) — OEM Suzuki bottle-mount pump for Ecstar quart bottles

Frequently Asked Questions

1. How do I confirm this valve fits my CDI gear lube pump?
The Sierra 18-99528 is the replacement valve for the CDI 551-33-1 pump. Check the model number on your CDI gear lube pump body — if it is the 551-33-1, this is the correct replacement valve. Contact us at MacombMarineParts.com if you need to confirm fitment for a different CDI pump model.

2. Can I replace just the valve or do I need a new pump?
In most cases, replacing just the check valve is sufficient to restore full pump performance. If the pump body, hose, or nozzle are also damaged or degraded, those components should be replaced at the same time. Replacing the valve alone is significantly more economical than a complete pump replacement when the pump body is otherwise in good condition.

3. Is this valve difficult to replace?
Check valve replacement on a CDI 551-33-1 gear lube pump is a straightforward service task — the valve is accessible by disassembling the pump body. Refer to the CDI 551-33-1 pump's service instructions for the correct disassembly procedure and valve installation orientation. Installing the valve backwards will prevent the pump from functioning.