7.2 Oil Pump Inspection, Overhaul, & Relief Valve Service
Key Takeaways
- Oil pumps are positive displacement pumps (gear, gerotor, or variable displacement vane) driven by the crankshaft, camshaft, or intermediate shaft.
- Critical oil pump clearance measurements include gear end play (side clearance), outer gear tip-to-housing clearance, and gear tooth mesh backlash.
- The oil pressure relief valve regulates maximum pressure by bypassing excess flow back to the oil pan sump or pump inlet when hydraulic pressure overcomes spring tension.
- A stuck-open relief valve causes low oil pressure across all speeds, whereas a stuck-closed valve creates dangerously high pressure (>100 PSI) that can burst oil filters.
- Replacement or overhauled oil pumps must always be primed with clean engine oil or assembly lube before initial startup to establish suction lock.
Oil Pump Inspection, Overhaul, & Relief Valve Service
The oil pump is the heart of the engine lubrication system. Driven mechanically by the crankshaft, camshaft, or intermediate distributor shaft, the oil pump is a positive displacement pump designed to deliver a specific volume of oil for every revolution of the engine. Because it is a positive displacement design, all oil entering the pump inlet must be discharged through the outlet unless bypassed by the pressure relief valve. Technicians mastering the ASE A1 examination must understand oil pump types, precision clearance measurement procedures using feeler gauges and micrometers, relief valve overhaul steps, and critical pump priming techniques.
Types of Oil Pumps and Operating Principles
Automotive engines utilize four primary oil pump designs:
- External Spur Gear Pump: Contains two meshing spur gears enclosed inside a cast iron or aluminum housing. The drive gear is powered by an shaft, while the driven gear idles on a stub shaft. As the gear teeth unmesh on the inlet side, a partial vacuum is created, drawing oil from the pickup tube. Oil is carried around the outer perimeter of the housing in the spaces between the gear teeth and housing wall, then squeezed out the discharge port as the teeth mesh on the outlet side.
- Internal/External Gear (Rotor or Gerotor) Pump: Consists of an inner drive rotor with external lobes and an outer driven rotor with internal lobes located inside the pump housing. The inner rotor has one fewer lobe than the outer rotor (e.g., 4 inner lobes, 5 outer lobes). As the rotors spin, the expanding cavities between lobes draw oil in from the inlet port, and contracting cavities force oil out the discharge port. Gerotor pumps offer smooth flow and compact sizing.
- Crankshaft-Driven Direct Gerotor Pump: Mounted directly on the front cover of the engine block, the inner rotor of this pump is splined directly to the front snout of the engine crankshaft. This eliminates intermediate drive shafts and belts, operating at 1:1 engine speed.
- Variable Displacement Vane Oil Pumps: Modern engines equipped with variable valve timing and fuel efficiency controls often use electronically or hydraulically controlled variable displacement vane pumps. An eccentric slide moves relative to a rotating vane rotor, adjusting pump chamber volume to match engine oil demand, reducing engine parasitic drag.
Oil Pump Drive Mechanisms
- Distributor / Camshaft Gear Drive: Older pushrod engines drive the oil pump using a gear on the camshaft that turns a distributor drive shaft connected via a hexagonal drive rod.
- Direct Crankshaft Snout Drive: Modern overhead cam (OHC) engines mount the pump directly on the crankshaft snout.
- Chain or Gear Drive: Some engines use a dedicated chain or gear set driven off the front of the crankshaft to operate a lower pan-mounted oil pump.
Oil Pump Disassembly and Precision Clearance Measurements
When overhauling an oil pump or evaluating a pump during an engine rebuild, clean all components thoroughly in solvent and inspect the pump housing and cover plate for scoring, gouges, or wear ridges. Precision clearance measurements must be performed using feeler gauges, precision steel straightedges, and micrometers.
1. Gear-to-Housing End Play (Side Clearance)
End play measures the gap between the flat face of the pump gears/rotors and the inner surface of the oil pump cover plate. Excess side clearance permits pressurized oil to slip back over the top of the gears into the low-pressure inlet side, dramatically reducing pump efficiency at hot idle.
- Measurement Procedure: Place a precision steel straightedge flat across the oil pump housing mounting face over the gears. Insert a feeler gauge blade between the bottom of the straightedge and the top face of the oil pump gears.
- Typical Specification: 0.0015 to 0.004 inch (0.038 to 0.102 mm). If clearance exceeds specification, replace the oil pump assembly or lap the housing cover plate if permitted by the manufacturer.
2. Outer Gear-to-Housing Radial Clearance (Tip Clearance)
Tip clearance measures the gap between the outer tips of the gear teeth or outer gerotor rotor and the internal wall of the pump housing body.
- Measurement Procedure: Insert a feeler gauge blade between the outer tooth tip of the gear/rotor and the pump housing wall while pushing the gear toward the opposite side.
- Typical Specification: 0.004 to 0.008 inch (0.102 to 0.203 mm). Excessive clearance allows oil to leak around the gear perimeter during pumping.
3. Gear Tooth Mesh Clearance (Backlash)
Backlash measures the play between the meshing teeth of the drive and driven gears in a spur gear pump, or inner-to-outer rotor tip gap in a gerotor pump.
- Measurement Procedure: For spur gear pumps, insert a feeler gauge between meshing gear teeth. For gerotor pumps, measure the gap between the inner rotor lobe tip and outer rotor lobe tip at the point of minimum contact.
- Typical Specification: 0.003 to 0.007 inch (0.076 to 0.178 mm).
4. Drive Shaft-to-Housing Bushing Clearance
Inspect the oil pump drive shaft and housing bushing for play. Measure shaft outer diameter using an outside micrometer and housing bore with a telescoping hole gauge. Compare clearance to specification (typically <0.003 inch / 0.076 mm).
Pressure Relief Valve Operation & Inspection
Because positive displacement oil pumps output an increasing volume of oil as engine RPM increases, system pressure would rise to destructive levels (over 150 PSI) without regulation. The oil pressure relief valve controls maximum lubrication system pressure.
Components & Relief Valve Operation
The relief valve assembly is built into the oil pump housing or main gallery and consists of:
- A precision machined steel piston, plunger, or hardened steel ball.
- A calibrated coil tension spring.
- A threaded retaining plug, cap, or roll pin.
Oil pressure from the pump outlet acts directly against the face of the relief valve piston. When oil pressure force exceeds the calibrated spring tension (typically at 40–60 PSI), the piston compresses the spring, unseating the valve. This opens a bypass port, bleeding excess oil flow directly back into the oil pan sump or back to the suction inlet of the pump.
Relief Valve Inspection & Servicing
- Remove Plug & Spring: Unthread the retaining plug and remove the coil spring and relief plunger.
- Inspect Plunger & Bore: Inspect the relief valve plunger for scoring, scratches, varnish build-up, or burrs. Inspect the housing bore for galling. The plunger must slide smoothly through its bore under its own weight without binding or sticking.
- Measure Spring Free Length & Tension: Measure the uncompressed free length of the relief spring using a vernier caliper. Test spring tension using a valve spring tester if specs are provided. If the spring is distorted, corroded, or shorter than factory minimum free length specifications, replace the spring/pump. A weak spring results in premature pressure bypass and low peak oil pressure.
- Clean Passages: Thoroughly flush varnish and metal debris from the relief valve bore using solvent and compressed air.
Oil Pickup Screen & Suction Tube Diagnostics
The oil pickup screen and suction tube deliver oil from the pan sump to the pump inlet. Defects on the suction side severely impact oil pump performance:
- Pickup Screen Clogging: Carbon deposits, sludged oil, silicone RTV sealant beads, or shredded nylon timing gear teeth collect on the pickup screen. A clogged screen restricts oil flow, creating a vacuum on the pump inlet that leads to pump cavitation (imploding air bubbles), noise, and severe oil starvation.
- Pickup Tube Air Leaks: The pickup tube connects to the oil pump inlet using a bolt-on flange with a rubber O-ring seal or gasket. A hardened, flattened, cracked, or missing O-ring seal allows the pump to draw air instead of oil. This aerates the oil, causing low and fluctuating pressure readings and noisy hydraulic lifters.
- Pickup Screen-to-Pan Clearance: The bottom of the oil pickup screen must sit 1/4 to 3/8 inch (6 to 10 mm) above the floor of the oil pan. If an engine oil pan is struck by road debris and dented upward, the pan floor can press directly against the pickup screen opening, sealing off oil flow and causing total engine oil starvation. Always inspect oil pan bottom flatness during overhaul.
Oil Pump Priming, Installation, & Replacement Protocols
Installing a replacement oil pump dry is a major technician error that can cause instant engine damage upon startup.
Oil Pump Priming Protocol
Positive displacement gear and gerotor pumps cannot pull air effectively when completely dry. A dry pump will fail to create a vacuum seal, spinning in dry air while the engine runs without oil pressure for several minutes.
- Priming Procedure: Before installing the pump cover or mounting the pump to the engine, pack the internal gear cavities completely with clean engine assembly lube, clean petroleum jelly (Vaseline), or fill the pump body with clean engine oil while rotating the drive shaft by hand. The petroleum jelly or lube seals internal gear clearances, creating an immediate liquid suction lock that pulls oil from the pan upon the first turn of the crankshaft.
Installation & Pre-Lubrication
- Replace Gaskets & O-Rings: Always install a new pickup tube O-ring seal and front crankshaft seal (on OHC pumps). Lightly coat O-rings with clean engine oil.
- Torque Fasteners: Align the pump housing to the engine block dowels and torque mounting bolts in a cross pattern to manufacturer specifications.
- Engine Pre-Lubrication (Pre-Oiling): Before starting a newly rebuilt or re-pumped engine, use an external engine pressure primer tank connected to an oil gallery port to pressurize the system with 30 PSI of clean oil, or use a drill motor to spin the oil pump drive shaft (on distributor-driven engines) until oil flows out all rocker arms and main pressure is verified on a gauge.
Precision Oil Pump Clearance Limits & Measurement Methods
| Clearance Test | Measurement Method | Typical Factory Specification | Consequence of Excessive Clearance |
|---|---|---|---|
| Gear End Play (Side Clearance) | Steel straightedge across housing face + feeler gauge to gear face | 0.0015"–0.004" (0.038–0.102 mm) | Internal oil bypass past gear faces; low hot idle pressure |
| Outer Gear Tip-to-Body | Feeler gauge between outer gear tip and housing wall | 0.004"–0.008" (0.102–0.203 mm) | Reduced pumping efficiency; loss of volume at elevated RPM |
| Gear Tooth Backlash / Mesh | Feeler gauge between meshing gear teeth or rotor lobes | 0.003"–0.007" (0.076–0.178 mm) | Gear noise, accelerated tooth wear, pressure drop |
| Pickup Screen-to-Pan Gap | Depth micrometer/clay measurement from screen to pan floor | 1/4"–3/8" (6.0–10.0 mm) | Restricted oil flow if pan dented; air pickup if screen too high |
Relief Valve & Pickup Tube Diagnostic Matrix
| Component Condition | Physical Finding | Pressure Result | Engine Symptom | Corrective Action |
|---|---|---|---|---|
| Relief Valve Stuck Open | Debris/varnish holding plunger off seat | Low pressure across all RPMs | Lifter noise, oil light ON at all speeds | Disassemble, clean bore, or replace pump assembly |
| Relief Valve Stuck Closed | Plunger seized in bore | Excessive pressure (>90 PSI) | Ballooned oil filter, blown filter gasket | Clean relief bore, verify free movement, replace pump |
| Relief Spring Weak/Shorter | Free length below minimum spec | Low maximum pressure (peaks early) | Pressure stops rising above 25–30 PSI | Replace relief spring or complete oil pump assembly |
| Pickup O-Ring Damaged | Hardened, cracked, or missing seal | Low/fluctuating pressure, aerated oil | Foamy oil on dipstick, lifter noise | Replace pickup tube O-ring seal |
| Dented Oil Pan | Bottom of pan pressed against screen | Severe oil starvation | Rapid oil pressure loss under acceleration | Straighten or replace oil pan and inspect screen |
A technician is overhauling an internal gear (gerotor) oil pump. To measure the gear end play (side clearance), which tools and procedure should be used?
During an engine rebuild inspection, a technician measures the uncompressed free length of an oil pump pressure relief valve spring and finds it is 1/8 inch shorter than factory specification. What is the consequence of reinstalling this spring?
Why MUST a replacement oil pump be primed with clean engine oil or assembly lubricant prior to initial engine startup?
A vehicle experiences severe oil pressure fluctuation and oil foaming on the dipstick after an oil pan replacement. Inspection reveals no oil leaks, and oil level is correct. What is the most likely cause?