- Engine: All 3500 Engines
Introduction
Do not perform any procedure in this Special Instruction until you read this information and you understand this information.
This Special Instruction describes the effects of hydraulic lock or piston to valve contact. To avoid further damage, replacement of parts is recommended.
Hydraulic Lock
When a liquid is trapped in an engine cylinder on the compression stroke, hydraulic lock can occur. The liquid cannot be compressed and the liquid does not have an outlet. The force of the piston on the liquid can result in damage to the engine.
Hydraulic lock has been known to cause the following damage:
- Cracked piston skirt on the two-piece piston
- Cracked cylinder liner
- Bent connecting rod
- A unit injector that is ejected from the cylinder head
- Injector hold down bolt is bent or broken
- Head gasket spacer plate is cracked or broken
Hydraulic lock can be identified by the following conditions:
- A coolant leak from the spacer plate under the cylinder head of the locked cylinder
- An injector that is ejected from the locked cylinder
The injector may be partially ejected or the injector may be fully ejected. In most cases of ejected injectors, the hold-down bolts are broken.
For electronic engines, the injector solenoid terminals will contact the valve cover. The contact of the terminals may generate a diagnostic for the injector. The terminals may also crack or break a hole in the valve cover. In some cases, both the spacer plate and the injector are affected. In those cases, the injector can be loosened in the bore. The injector solenoid may rotate and the solenoid can come in contact with the valve springs.
For high displacement engines with two-piece pistons, hydraulic lock can crack the aluminum piston skirt without cracking the spacer plate and/or without ejecting the injector from the cylinder head. It is possible to have a cracked piston skirt without any damage that is visible outside of the cylinder. The cracks in the piston skirt may not be visible.
When a hydraulic lock occurs, damage to other components in the cylinder pack can also occur. This damage may result in a catastrophic failure if corrections are not made.
Piston to Valve Contact
If you suspect piston to valve contact, it is important to take action as soon as possible to prevent additional damage.
Piston to valve contact can occur for a number of reasons. The following examples represent some of the most common causes of piston to valve contact.
Mechanical Wear:
As valve train components wear, slight changes can occur that affect the valve timing. Excessive lash can expose valve train components to shock loading, which may result in additional wear and even mechanical failure. To compensate for these slight changes due to mechanical wear, Caterpillar requires valve lash settings be measured and adjusted, refer to the Operations and Maintenance Manual.
Insufficient Lash:
If lash settings are too tight, valves can remain open longer, allowing the head of the valve to contact the piston at TDC.
Mechanical Failure:
Failure of any component within the valve train that alters when valves open, or how far the valve open can result in piston to valve contact. Examples include broken teeth in the gear train, slippage of press fit camshaft timing gears or damaged valve springs, camshaft lifters and rocker arm components.
Foreign Object Damage (FOD):
The introduction of debris in the combustion chamber or valve train or can interrupt valve timing. If significant enough, piston to valve contact can occur. The most common causes of FOD include other component failures in the intake or exhaust systems such as turbochargers, wastegates, and thermocouples.
Over Speed:
Excessive engine speeds can result in piston to valve contact if the valve springs cannot fully seat the valve prior to the piston reaching TDC.
Visually inspect the top of the piston. Inspection techniques vary, the typical method for in cylinder inspection is using a bore scope.
Parts to Replace
In the event of hydraulic lock or piston to valve contact, complete replacement of the cylinder pack for the locked cylinder is recommended in addition to replacement of the damaged parts. Do not reuse any components until the components have been checked for conformance with Reuse And Salvage Guidelines.
If corrective measures are not taken in the case of hydraulic lock or piston to valve contact and the cylinder pack is not replaced, further engine damage may occur.
References for Inspection
Reference the following publications in order to determine whether components of the cylinder pack can be reused:
- Reuse And Salvage Guidelines, SEBF8149, "Specifications for Connecting Rods Used in 3500 Family of Engines"
- Reuse And Salvage Guidelines, SEBF8059, "Procedures and Specifications for Pistons"
- Reuse And Salvage Guidelines, SEBF8063, "Visual Inspection of Connecting Rods"
- Reuse And Salvage Guidelines, SEBF8290, "Visual Inspection of Two-Piece Pistons"