Measure the installed combination. Do not assume a lash-cap thickness is correct.
The HPFP must maintain preload at minimum fuel-lobe lift without exhausting its available travel at maximum fuel-lobe lift. Changing the camshaft, HPFP, pump follower, lash cap, valley cover or gasket can alter this geometry.
Minimum follower height → verify HPFP preload.
Maximum follower height → verify remaining HPFP travel.
Before starting
Use the HPFP, follower, lash cap and gasket intended for final assembly. Remove the HPFP outer spring for the measurement procedure.
Verify that both HPFP mounting bolts thread freely through the required range by hand. Bolt or thread resistance can interfere with the maximum-lift check.
An EN-51092-style gauge makes it easier to monitor follower position and locate minimum and maximum fuel-lobe lift. It does not replace the preload and travel checks below.
1 · Find minimum fuel-lobe lift
Rotate the engine by hand while monitoring the HPFP follower. Stop when the follower reaches its lowest position. Rotate slightly in both directions to verify the minimum. This is the fuel-lobe base-circle position.
2 · Verify preload
- Install the lash cap, if used.
- Install the HPFP gasket.
- Position the HPFP squarely over the follower without the mounting bolts.
- Press the pump toward the valley cover by hand.
The pump should require compression before its flange reaches the installed position.
The pump must be compressed to reach the mounting surface.
The pump reaches the mounting surface without compressing the pump mechanism. Correct the geometry before final assembly.
Do not stop after passing this check. Maximum-lift travel must also be verified.
3 · Find maximum fuel-lobe lift
Remove the HPFP and rotate the engine until the follower reaches its highest position. Rotate slightly in both directions to verify the maximum.
4 · Verify maximum-lift travel
Remove the HPFP gasket for this check.
- Position the HPFP squarely over the follower.
- Start both mounting bolts by hand.
- Alternate between the bolts, keeping the pump flange parallel to the valley cover.
- Use finger pressure only.
- Stop if the pump reaches a firm internal stop before the flange reaches the mounting surface.
Do not use a wrench or ratchet to pull the HPFP against the valley cover during this check. The purpose is to determine whether the pump reaches the mounting surface before its available internal travel is exhausted.
5 · Evaluate the result
If the pump reaches the valley-cover mounting surface by hand with the gasket removed, the maximum-lift travel check passes.
If the pump stops above the mounting surface, do not tighten it farther. Measure the remaining flange-to-cover gap with feeler gauges, then measure the actual gasket being installed.
Remaining installed travel = gasket thickness − no-gasket stop gap
Example: A .010 in. gasket and .006 in. no-gasket stop gap leaves approximately .004 in. of remaining travel.
If the stop gap exceeds gasket thickness, the installed geometry would attempt to drive the pump beyond the measured stop and fails the check.
6 · If lash-cap thickness changes
Repeat both checks. A thinner cap increases available maximum-lift travel but reduces minimum-lift preload. A thicker cap increases preload but reduces available maximum-lift travel.
Minimum follower height: preload present.
Maximum follower height: available pump travel remains.
Changing lash-cap thickness invalidates both previous measurements.
7 · Final assembly
- Verify the selected lash cap is fully seated, if used.
- Reinstall the HPFP outer spring.
- Install the gasket.
- Position the HPFP squarely over the follower.
- Complete final HPFP installation using the specified fastener procedure.
Technical reference: XDI GM Gen V HPFP installation procedure and GM EN-51092-style fuel-pump checking method.