Last summer, check engine light came on due to glow plug fault. I tested glow plugs with multimeter and found one bad. Car started fine with mild rough idle all throughout the winter. This morning I tested the plugs again and it looks like five out of the six are bad. Which is fine...they're all original. What I'm trying to figure out is how the car made it through winter with, likely, so many plugs out.

This will give me a good excuse to use the HHT emulator on Xentry to see if I can figure out exactly what's going on.

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

There's no need for a scan tool. It's simply going to say "Glow plug circuit voltage low/0 volts/open" or something like that.

The glow plug is open, simple as that. Just like a burnt out light bulb. Change plug, problem gone.

The glow plug can no longer provide continuity. Luckily your glow plug circuit isn't shorted, otherwise stuff starts smoking

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

That cars must be insanely hard to start with just ONE glow plug!

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

My mistake....was having continuity issues with the multimeter....just one plug out, #5.

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I had a similar run in as well, I like test lights much better because it's easier to get a positive response.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.