Onan Oil Pump Rebuild Guide

Many owners of Onan generators, particularly those found in RVs and other recreational vehicles, often ponder the specifics of their engine's lubrication system. A common question that arises is: "Does my Onan have an oil filter?" The answer to this question is not a simple yes or no, as it depends on the specific model and year of your Onan generator. However, the information provided below focuses on a crucial aspect of the lubrication system: the oil pump itself and its potential for repair and maintenance.

Understanding Your Onan's Oil Pump

The oil pump is the heart of your engine's lubrication system. Its primary function is to circulate oil throughout the engine, ensuring that vital components are adequately lubricated, reducing friction, and dissipating heat. A properly functioning oil pump is essential for the longevity and performance of your Onan generator.

While many modern engines feature a spin-on oil filter to remove contaminants from the oil, older or simpler designs, or specific components within a larger system, might not. The provided text details the process of rebuilding an oil pump, suggesting that while an external filter might not be present on all Onan models, the internal oil pump is a component that can and sometimes needs to be maintained.

When to Consider an Oil Pump Rebuild

Several indicators might suggest that your Onan's oil pump requires attention:

• Low Oil Pressure: This is the most significant symptom. If your generator's oil pressure gauge reads lower than normal, or if an oil pressure warning light illuminates, it could indicate a problem with the oil pump.
• Unusual Noises: A worn or failing oil pump can sometimes produce whining or grinding noises, especially when the engine is under load.
• Engine Overheating: Inadequate lubrication due to a faulty oil pump can lead to increased friction and heat, potentially causing the engine to overheat.
• Visible Oil Leaks: While not always directly related to the pump's internal function, leaks from the pump housing or associated seals can be a sign that the unit needs maintenance.

Rebuilding Your Onan Oil Pump: A Step-by-Step Approach

The following guide, derived from the provided text, outlines the process of rebuilding an oil pump, focusing on common wear points and ensuring proper reassembly. This process assumes you have the necessary parts, including a gasket kit (e.g., #120-0161), which typically includes gaskets of varying thicknesses (.005", .007", & .009"), and potentially a new Woodruff key.

1. Initial Inspection and Disassembly

Before diving into the rebuild, it's crucial to inspect the existing components. The text highlights the importance of checking the gear faces for wear patterns. If the gear faces appear to be in good condition, the primary focus of the rebuild is often replacing the Woodruff key, which is a common wear item.

Gear Play Check:

To assess the condition of the smaller oil pump gear and its shaft, perform the following check:

1. With the large drive gear held in place, gently try to turn the small oil pump gear back and forth.
2. Minimal play indicates that the Woodruff key is likely in good condition.
3. Significant play or slop suggests wear, and you'll likely see this when the gear is removed.

Shaft Play Check:

1. After removing the small gear (be careful not to lose the C-clip!), check the drive gear shaft for play within the pump housing.
2. A small amount of play is acceptable.
3. Excessive play, accompanied by obvious wear on the shaft, indicates that the shaft or housing may be worn beyond repair.

2. Cleaning and Preparation

Proper cleaning of mating surfaces is paramount for a successful rebuild. The text strongly advises against using metal scrapers or screwdrivers to remove old gasket material, as these can easily cause gouges and scratches.

Recommended Cleaning Method:

• Use very fine Scotch-Brite pads along with a suitable solvent.
• Clean both the pump cover and the housing mating surfaces thoroughly.
• Specialised spray gasket removers can be helpful if the gasket is particularly stubborn.

Scratched or damaged mating surfaces can lead to leaks, negating the effort of the rebuild.

3. Reassembly - Key Considerations

The reassembly process requires careful attention to detail to ensure the pump operates smoothly and efficiently.

Installing the Woodruff Key:

• Lubricate the large shim washer before placing it back onto the drive gear shaft.
• If installing a new Woodruff key, it might not slide easily into the shaft slot. Avoid using metal tools to force it.
• A small wooden dowel tapped gently with a hammer is the best method to seat the key correctly.

Securing the Small Gear:

• Place the small pump gear back onto the shaft.
• Ensure the C-clip is installed with the “rough” edge facing away from the gear.
• Crucially, triple-check that the C-clip is fully seated in its groove on the shaft. It may appear seated with a click, but often requires a secondary, firm push to be truly secure.

4. Gasket Selection and Cover Alignment

This is a critical step that determines the pump's operational smoothness. The goal is to use the thinnest gasket that allows the gears to turn freely without binding.

Gasket Selection Process:

1. Start with the thickest gasket (often white).
2. Place the pump cover onto the housing and finger-tighten the bolts.
3. While turning the drive gear, gently move the cover around its mounting holes. The objective is to find a position where the drive gear turns without any resistance or binding.
4. Once a smooth-turning position is found with the thickest gasket, try the next thinner gasket (e.g., yellowish). Repeat the process of aligning the cover and checking for free rotation.
5. If the pump still turns freely, try the thinnest gasket (e.g., blue).

Important Note: During this alignment process, only finger-tighten the cover bolts. You need enough tension to hold the cover in place, but the ability to adjust its position.

5. Tightening the Cover Bolts

Once you've identified the thinnest gasket that allows smooth operation, the cover can be properly tightened.

1. Carefully tighten the cover bolts in an alternating pattern (side to side, back and forth).
2. Continuously check that the pump still turns smoothly as you tighten.
3. Aim for a snug fit. A final torque of approximately 10 foot-pounds is recommended, but “good and tight” without excessive force is acceptable.
4. If the pump begins to bind as you tighten the bolts, you likely need to revert to the next thicker gasket.

6. Priming and Testing the Pump

With the pump reassembled, it needs to be primed and tested to ensure it's functioning correctly.

1. Pour some oil into the pump's inlet/pickup area.
2. Turn the drive gear. It should feel smooth, with a slight resistance as the oil is drawn in and compressed.
3. Add more oil and continue turning the gear counter-clockwise (when facing the gear). Oil should begin to flow out of the feed hole.
4. Final Test: Screw on the pickup tube, submerge the pickup end in fresh oil, and turn the drive gear until oil flows consistently from the feed hole. Cover the feed hole with your finger; you should feel pressure building.

FAQ: Onan Oil Pump Maintenance

Q1: Does every Onan generator have an external oil filter?

A1: Not all Onan models are equipped with an external spin-on oil filter. The presence of a filter depends on the specific generator model and its design. However, all engines rely on an internal oil pump for lubrication.

Q2: What is the most common part to fail in an Onan oil pump?

A2: The Woodruff key, which secures the drive gears to their shafts, is a common wear item. Other potential issues include wear on the gear teeth or the pump housing itself.

Q3: Can I use regular RTV sealant instead of a gasket?

A3: It is strongly recommended to use the correct gasket kit, especially the precision-thickness gaskets provided for cover alignment. RTV sealant may not provide the necessary clearance control for the gears to turn freely, potentially leading to binding or insufficient lubrication.

Q4: What happens if I use too thick a gasket?

A4: Using a gasket that is too thick will increase the clearance between the gears and the pump housing, reducing the pump's efficiency and its ability to generate adequate oil pressure. In severe cases, it can prevent the gears from turning at all.

Q5: What torque should I use for the cover bolts?

A5: While specific torque values can vary by model, a general guideline is around 10 foot-pounds. The critical factor is to tighten them evenly and snug them sufficiently to prevent leaks without causing the pump to bind.

By understanding the components of your Onan's oil system and following these maintenance and rebuilding steps, you can help ensure reliable performance and extend the life of your generator.