What indicates that a valve lifter has a leakdown problem?

The presence of noise from lifters primarily at highway speeds suggests a leakdown problem. When a valve lifter exhibits a leakdown issue, it may lose hydraulic pressure due to wear or internal damage. This loss of pressure leads to inadequate lifter operation, often resulting in increased noise, particularly under higher engine speeds or loads, such as those experienced on the highway.

At lower RPM, quiet lifters might suggest they are properly filled with hydraulic oil, whereas noise at higher speeds indicates that as the engine demands more performance, the lifters are unable to maintain adequate pressure. This can happen because when the engine is at a lower speed, the hydraulic lifters can compensate better for small leaks, but as the speed increases, the demand for oil pressure increases, and the leaks become problematic.

In contrast, noiseless lifters during startup might suggest they are functioning normally, while even clearance all around typically indicates proper performance and function of the lifter. Therefore, the specific manifestation of noise only at highway speeds strongly points to a leakdown issue in the lifters.