The Power Stocke engines DO NOT typically need glow plug replacement, be SURE you test them before wasting your hard-earned $$$ on parts that still work as new.
The most common causes for hard starting relating the the GP system are: Bad (worn out) Glow Plug relay, and bad connections to the GPs at the "UVC" (Under Valve Cover) harness.
With the Wait To Start light ON, check the battery voltage at BOTH large terminals of the GPR (Glow Plug Relay), they MUST be the same measured to BATTERY GROUND, or at least within 1/2 volt of each other. Again, with the WTS light on.
If they are not the same voltage, then make sure the GPR is actually working. With the WTS light on, there should be battery voltage on one of the small terminals of the GPR, and ground on the other. If this is true, the GPR is bad. Replace and re-test.
You can check the glow plugs right at the 4 plugs on the valvecover gasket, when you pull the plug the 2 glow plugs are the outer most pins in the connector( one on each end) on the gasket side. Use a Ohm meter Black on ground and red on the outer most pin to check it. The ohm's need to be less then 2.0 homs. for it to be good.
On a 99 and up the glowplugs are the 2 outer pins on each end of the plug.
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