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  #1  
Old 05-14-2010, 09:20 PM
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How important is it to let the glow plug light go off before you start?

Normally, I do this religiously, however reading a post last night where someone was advised to try starting a cold engine without letting the glow plugs heat up on a car that they were looking to buy, I decided to try it for myself. My car (84 300SD) always starts up immediately, but this morning, starting it as soon as I turned the key, it cranked for 3 to 4 seconds before it started. I'm not concerned at all about this, I'm just wondering how important it is to actually use the glow plugs. I've seen several mechanics start the car without the light going off (warm engine, though). Does not letting the glow plug light go out have any negative effect on the engine?

Also, why don't newer (and larger) diesels have glow plug lights? I used to drive transit buses while in college. They all had Detroit Diesel 6v92t engines, and none of them had the "wait to start" feature or a glow plug light...
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  #2  
Old 05-14-2010, 09:26 PM
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newer diesels have faster cranking, better ring packs,and a MUCH higher fuel pressure injection spray. also, the fuel spurts directly into the combustion chamber in the piston itself, not a secondary combustion chamber like older diesels have.
on the older vehicles with glow plugs, the plug is heating the tiny prechamber and it's spray bulb so the fuel hits a hot surface when it is injected.
on a hot motor, it's not a big deal, but on a cold motor it's a good idea to let the glows run for 15 or more seconds, to keep cold fuel from soaking the rings and wearing the motor. the test is for seeing if the motor has sufficient compression to start without the glow plugs. if not, it's not gonna last as long as a stronger motor will.
it's just a buying tool, not a recommended way to start your motor.
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  #3  
Old 05-14-2010, 09:43 PM
aaa aaa is offline
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Quote:
Originally Posted by chrisbus View Post
cranked for 3 to 4 seconds before it started.
There's your reason to use the light. Starters do wear out, why use them more than you have to?
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  #4  
Old 05-14-2010, 09:52 PM
1983 240D auto
 
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Ha! I've just realized my relay probably doesn't work at all. I didn't think the light was even supposed to turn off... So it should actually go out before I start the car? I have an 83 240D that could probably start most times without the glow plugs. Are you saying I could be doing damage by not waiting long enough?

edit: wanted to add that my starter just begins to crank when my engine fires up.


Now off to see where the relay might be...
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  #5  
Old 05-14-2010, 10:07 PM
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the relay on yours should be on the fender, driver's side. follow the wires from the glow plugs to a box. that'll be your relay.
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  #6  
Old 05-14-2010, 10:09 PM
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The old Detroit diesels didn't have glow plugs, that's why they had no glow plug lights and why they typically had ether injection systems for cold weather starts.
My CAT 3208 has no glow plugs and no ether injection system. The engine is in my Wanderlodge and is designed to provide a complete impervious smokescreen in cold weather so no one can identify the vehicle.
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  #7  
Old 05-14-2010, 11:53 PM
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Thanks vstech! As always, your replies are very helpful!

Quote:
The engine is in my Wanderlodge and is designed to provide a complete impervious smokescreen in cold weather so no one can identify the vehicle.
Love it! ;-)
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  #8  
Old 05-15-2010, 12:43 PM
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But every once in a while, I start my Diesel engine without glowing, especially when the car sat without running for lets say .... a week, just to prime the upper engine with oil before it starts running....
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  #9  
Old 05-15-2010, 01:34 PM
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A warm engine does not require any glow so waiting isn't required. On the W140 the glow light goes out before the relay discontinues supplying power to the GP's so it seems to matter little.

Recently while fighting with my glow controller I removed the case and set the unit so I could watch it when I turned on the key and noticed that even after the dash glow light went out the glow controller was still activated and it didn't disengage for another 25 seconds or so.

When it's warm I seldom wait for the light to go off.

steve a
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  #10  
Old 05-17-2010, 12:15 AM
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My engine likes the glowplugs all the time...warm or cold, as long as I glow about 3-5 seconds (or longer) it starts perfect...otherwise its a few seconds of spinning then it coughs to life.....but the engine in my brother's 300D will start nearly instantly if it is warm without a glow. Very weird. It has about 65k less miles on it and less blowby, so the compression is probably higher....but mine has been the same for the past 90k....
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  #11  
Old 05-17-2010, 02:40 AM
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Quote:
Originally Posted by chrisbus View Post
Normally, I do this religiously, however reading a post last night where someone was advised to try starting a cold engine without letting the glow plugs heat up on a car that they were looking to buy, I decided to try it for myself. My car (84 300SD) always starts up immediately, but this morning, starting it as soon as I turned the key, it cranked for 3 to 4 seconds before it started. I'm not concerned at all about this, I'm just wondering how important it is to actually use the glow plugs. I've seen several mechanics start the car without the light going off (warm engine, though). Does not letting the glow plug light go out have any negative effect on the engine?

Also, why don't newer (and larger) diesels have glow plug lights? I used to drive transit buses while in college. They all had Detroit Diesel 6v92t engines, and none of them had the "wait to start" feature or a glow plug light...
I've also done this religiously (but for almost 27 years). Could that be a factor in the fact that I'm still using the original glow plugs and never had a relay problem? Also, minimal blowby!
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1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html
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