I have a 75 300D. Cold weather has brought rough starting so I started thinking glow plugs. I noticed the bus from 5th GP to 4th and from 3rd to 2nd are not heating and are covered with grease and dirt while the remaining get quite hot quickly and are clean. Are there bad plugs? Can I replace the bad plugs or should I go with 5 new ones which were last done March 04? Thanks for the help

Your GPs get their power from the firewall wire that is hooked to your #5 plug. (At least it is this way on a '77) So #5 and #4 does have power in fact if there is one plug that does not work then none of the plugs work. If you haven't replaced the plugs in the last two years then I would replace them and I also recomend getting the pencil type plugs from DieselGiant. I change my '77 to them and I am very happy with them. They cost more but it is worth it. Car never started smoother even with new plugs last year.

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1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C

My advice is also to get ride of those old "loop" style GPs and get the new style, you wont regret it. Much better system

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"