Just a question ...

I have read on the technical book for the diesel W123s that when the engine is already hot (above 70C or 160F or something like that) the preheat light on the dashboard should not be lighted and you can start the engine directly, without preheating.

But on my 240D, the preheat light is always on, even when the engine temperature is about 90C (or 195F). And if I remember well, I believe that when my 240D was youger, the preheat light didn't light above 70C.

So, how can I fix that ? Do I have a sensor problem anywhere?

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in France : 240D 1981 380 000 km
240D 1984 252 000 km
300D 1978 325 000 km
in the US : Ford Tempo 1993 70K

gnolo,

I think the duration of the light being on changes with engine and ambient temperature, but it should always go on when you turn the key. I also think there is a minimum time the glow system will cycle the plugs on for, and the manual note is to tell you if you are in a hurry, and the car is at the temperature it normally operates then go ahead and start it. I do not think there is a way to make the light not come on at all if the car is warmed up. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

And by the way, I have notice that when the engine is cold, it start better when I wait a few seconds after the extinction of the preheat light. My glow plug have been changed last year and they have today about 30 000 km.

If i don't wait and start the engine just after the extinction of the preheat light, the engine run jerky during about 15s.

I know that the glow plug are still operating during a while after the extinction of the preheat light on the dashboard, but why need I to preheat my engine more than it is required ?

__________________
in France : 240D 1981 380 000 km
240D 1984 252 000 km
300D 1978 325 000 km
in the US : Ford Tempo 1993 70K

gnolo,

That sounds like you have a single or maybe two glow plugs that are not performing correctly, or are on their way out. If they are burned out the system usually causes the glow plug indicator lamp on the dashboard to light up oddly. If they are getting poor electrical connection to the system, they will just not get hot enough in the allotted time. I do not think the timer circuit is "smart" enough to notice whether or not the electrical connections are good or bad as long as they are not open or short. I would check the connections, clean them and put them back carefully, then check the performance again. If it is still bad, maybe you have some bad plugs, in which case, take them out and check them with an Ohmmeter. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)