Recently I noticed about every other morning, the glow plug light not staying on as long as usual. Starts but idles rough for a minute or two. Is it a bad relay? Thx

 

What year is your truck? After about 04.5, (or around that year), they started making it so the grid heater would only cycle when the air temperature inside the intake manifold only got below 55 - 60 degrees. But if it's that warm inside the manifold and it starts, but still idles rough, then it sounds like something is wrong. Relay sounds like it may be your issue. Someone else will chime in.

FYI, the 5.9 doesn't have glow plugs, it has a grid heater in the intake manifold which reaches about 500 degrees and pre-heats the air before it goes into the cylinder. Think of a toaster oven.

 

Its a 04.5 I just got it a couple months ago so I dont know much. It is getting warmer her in the am. So it will always start rough unless it is cold out? Thanks

 

When the grid heater is working and its cold outside, watch the Alternator. It will pulse everytime it turns itself on. Its really noticable at night cause you think you turn you lights out or something when it hits. If you don't see the alternator take a dive, you may have a problem there.

Otherwise it only comes on after the truck sits awhile and may only pulse 1 time or not at all.

 

I have tested my Grid heater's ability to affect how my truck starts. Now I understand why people delete them in warmer climates.

Even a 30* morning (Rare here in So. Cal.) I never wait for the light to go off. Truck starts the same reguardless.

Sure beats my previous MBZ 300 Turbo that would never start w/o the glow plugs. Sometimes cycled 3 or 4 x's and still a hard start.

These cummins start right now for the most part. Again fair climate.