i had my car plugged in last night and i went out and started it this morning; it started right up but after about 5 min of riving the glowplug light came back on and stayed on for like a min then went off. then after about 3 mins it came on again and i looked at the volt gauge i have installed and i could tell that the glowplugs were def being lit up and when the light went off the voltage went back to normal. im going to let it sit outside for a little bit and go try it out again but does anyone have any idea why this would happen?

thanks- J

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1985 mercedes benz 300D turbo diesel
1998 VW GTI VR6
1996 VW GTI 2.0
1999 saab 9-3 turbo
1987 300sdl (sold)