Charles C. Roberts, Jr., Ph.D., P.E.


A specific type of automotive engine damage occurs when water enters the engine and is compressed in one or more cylinders.  Figure 1 illustrates the damage sequence often called hydrostatic lock. The left drawing in Figure 1 shows water entering a typical automotive engine cylinder during the intake stroke. Water, unlike the fuel/air mixture, is incompressible and during the compression stroke, locks the piston in place as shown in the drawing at the right of Figure 1.  This condition tends to overload the connecting rod, causing a bending failure of the rod and severe engine damage. A common way for water to enter the engine is driving through water sufficiently deep to allow injecting into the air intake system of the vehicle, such as crossing flooded roads.




Figure 1


Figure 2

Figure 2 shows a failed connecting rod that sustained hydrostatic lock. After stalling in a flooded street, the vehicle operator started the engine, which suddenly failed. In this case, the connecting rod bending failure resulted in a fracture through the cross-section of the connecting rod. Figure 3 is a view of a connecting rod that sustained a hydrostatic lock bending failure without a fracture.

Figure 3



Figure 4

Figure 4 shows an engine that operated poorly after the owner drove through a flooded street.  The lower piston in the photo (red arrow) should be at the same height as the upper piston (green arrow), but is not. The lower piston is displaced downward relative to the upper piston, suggesting that the connecting rod is shorter or bent as a result of hydrostatic lock brought on by water ingestion into the engine.  

Figure 5

Other evidence of water ingestion into the engine may exist in the form of mud and water debris patterns. Figure 5 shows a water damaged air filter, which is often damp and crumbles when handled. Water marks on upholstery and carpeting in the occupant compartment are additional clues that corroborate the assessment that water may have entered the engine. Finally, an oil analysis2 may add further information regarding the condition of the engine. If wear metal analysis yields evidence of long term related engine damage, then it is possible that the engine failure was not related to hydrostatic lock, absent any other evidence as exemplified in Figures 2 through 5.  It should be noted that some engine failure modes such as connecting rod bearing wear-out2 can cause connecting rod failure, which is not related to hydrostatic lock.




  1. Roberts, Charles Jr., "Motor Vehicle Accidents Caused by Mechanical Failures," Claims Magazine, January 1988, p33f.
  2. Roberts, Charles Jr., "Engine Oil Analysis," Claims Magazine, February 1990, p58.
  3. Roberts, Charles Jr., "Truck Component Failure Analysis," Claims Magazine, August 1996, p64f.
  4. Roberts, Charles Jr., “Throttle Mechanism Failure Analysis,” Claims Magazine, May 2001.



© C. Roberts Consulting Engineers