If your starter easily cranks a cold engine, but “drags” or cranks very slowly when hot, there may be a “heat soak” problem. Before you condemn the starter as bad, you should perform a system diagnosis of the battery, cables and starter. For a good overview of these tests, visit www.bbbind.com – click on “Technical Information”, select “Training Videos”, and view “How to troubleshoot a starter problem”.
Assuming that the diagnosis results are within acceptable ranges and the starter cranks fine when cold, it is quite possibly a “heat soak” condition. What exactly is starter “Heat Soak”? Without getting into the physics of thermal conduction or Newton’s law of convective cooling, it is simply a matter of the starter absorbing and retaining heat from under-hood sources such as the exhaust manifold. This absorbed heat adds resistance to the electrical conductors inside the starter. The additional resistance results in the starter requiring more amperage than it normally does when cool. Once under-hood heat warms the battery, cables, and starter, a “tipping point” is reached concerning the overall amount of resistance a starter can accept (and still function
The TOTAL effect of all this unwanted resistance is a dramatically reduced current flow, which in turn means the starter is limited in current, and therefore either turns very slowly or not at all. The starter solenoid is equally vulnerable to this condition, and thus may not activate the starter at all. Another overlooked contributor to “heat soak” is corroded battery cables. At some point, cleaning the battery terminals and connectors may no longer help if the corrosion has already spread throughout the length of the cables. The cable’s outer insulation “hides” the corrosion.
Heat soak can usually be solved by installing a heat shield on the starter; this will reflect the heat away from starter. In fact, many vehicles are originally equipped from the factory with a heat shield. Unfortunately, they are often discarded when the starter is replaced. You can likely find a used heat shield at a salvage yard, or source a new one from your local parts store. Other fixes can involve upgrading the battery cables with a heavier gauge cable or installing a higher capacity battery. Anything that will keep the starter cooler or allow more amperage to the starter should help considerably.