Physics of cold starts - PeachParts Mercedes-Benz Forum

Chad300tdt · #1 12-14-2008, 07:58 PM

Quote:

Originally Posted by nickofoxford

Not at all, I used to think the same thing until I learned how the glow system works. The Glow plug light is only a recommendation to how long you should glow before you start. In reality the glow plugs are still being powered after the light goes out for a certain amount of time, my 240D for example is around 30 seconds. So cycling is only extra wear on your ignition switch, we all learn this trick sometime

. I really want to hook up an extra indicator light to tell me when my glow plugs really shut off. The relay is a good way to tell, if your driveway is as quiet as a mouse in the morning.

What he was talking about is letting the glow plug relay click off at around the 30 seconds you refer to not waiting for the ready to start light to go off. (at least that's what I interpreted) A second complete glow cycle DOES benefit a start in very cold temps. I do it on both my w123's and it makes for better starts.

You don't need a light to indicate when the relay clicks off, just listen for it. It's a fairly loud click at least on the newer glow relay systems.

nickofoxford · #2 12-14-2008, 08:37 PM

Quote:

Originally Posted by Chad300tdt

What he was talking about is letting the glow plug relay click off at around the 30 seconds you refer to not waiting for the ready to start light to go off. (at least that's what I interpreted) A second complete glow cycle DOES benefit a start in very cold temps. I do it on both my w123's and it makes for better starts.

You don't need a light to indicate when the relay clicks off, just listen for it. It's a fairly loud click at least on the newer glow relay systems.

OHH gotcha, I always manage to look like an idiot on here somehow.. it definitely will help as you said, Is there a way to just modify the set glow cycle or does that just require a whole new relay? Where I live the street noise in the mornings makes it hard to listen for the relay, so a light would help out. It's more of a hit or miss situation, depending if my neighbor takes his 240D by my house at the right time.

Chad300tdt · #3 12-14-2008, 09:51 PM

Quote:

Originally Posted by nickofoxford

OHH gotcha, I always manage to look like an idiot on here somehow.. it definitely will help as you said, Is there a way to just modify the set glow cycle or does that just require a whole new relay? Where I live the street noise in the mornings makes it hard to listen for the relay, so a light would help out. It's more of a hit or miss situation, depending if my neighbor takes his 240D by my house at the right time.

There is a modified relay available from Bosch. I think it's called an afterglow relay and it requires the use of plugs that can handle the extra glow time.

There is also a modification where you disconnect the violet/purple wire which I believe disables the safety shutoff timer so you get extra long glow cycles that continue after the car is started. I haven't researched this much, so I don't know if this is safe or how the plugs actually end their cycle once the mod is implemented.

I just replaced all my plugs with Monark brand plugs, which I've recently heard glow hotter than other brands but have a slightly shorter life (they're also made in Germany). I don't think I need an afterglow system and I'm happy with glowing twice when necessary. Usually 1 full glow cycle suffices, and in warmer weather the "ready to start" condition indicated by the light is enough.

I imagine an extra indicator light wouldn't be too difficult to add. I wonder if splicing one into the plug circuit so would cause any problems? I could only imagine maybe a very slight drain on the power sent to the plugs.

pizzachef · #4 12-14-2008, 10:22 PM

Hey, as all that unburnt fuel builds up in the combustion chamber, doesn't that keep increasing the compression ratio?

My 240D smokes a LOT when its starts and the engine is cold. Its almost embarrassing. I thinks its unburnt fuel, not oil, but I could be wrong...its a light gray smoke and it decreases substantially after running for a few minutes. But even when its warm, it smokes a little at idle, but runs very smooth. By the smooth running, I'm assuming the injectors are spraying nicely, but that's me assuming.

Could my IP be injecting more fuel than it needs to? And why would it smoke so much when its cold? An incomplete burn? I hope this still fits in the physics of cold starting...

katja · #5 12-14-2008, 10:36 PM

I've never known a diesel to NOT smoke, run loud & some what rough on a very cold start - except for maybe the very latest engines. Look at all the YouTube videos of cold starts and whether it's an Detroit, Cummins, Mercedes, VW, or Izuzu they all do that. Cold fuel & cold cylinders mean the fuel isn't combusting completely. Hey, the smoke is part of the fun on having a diesel!

So, even though I have a garage and an outlet to plug in the block heater, I left my 240D outside in sub freezing weather all day and started it tonight at somewhere between 18-20 degrees. Glowed for about a minute and it fired up in TWO seconds but of course smoked and ran rough for a few seconds (but it does that even in warmer temperatures). What impressed me is how good it started considering I have quite a bit of blowby, old injectors, the old-style loop glow plugs of unknown age, and around 50% biodiesel in the tank. I think the Mobil 1 synthetic 5w-40 really makes a difference, and of course my battery is good & valves are adjusted.

It sure is interesting to see how much variability there is between cars with the same engines (OM616 & OM617). Some really won't start at freezing, others will start right up below 0F. Of course if you have a chance to plug in by all means do it though, if nothing else you get heat quicker.

Hatterasguy · #6 12-14-2008, 10:58 PM

Nothing smokes on start up like a 2 stroke Detroit! On a cold, cold morning when we would start up the 8v71 TI's in the Rybovich, OMFG we would fog the marina out!

If I owned that boat I would dump those old loud smokey lumps for a pair of Cummins.

kerry · #7 12-14-2008, 11:05 PM

Quote:

Originally Posted by Hatterasguy

Nothing smokes on start up like a 2 stroke Detroit! On a cold, cold morning when we would start up the 8v71 TI's in the Rybovich, OMFG we would fog the marina out!

If I owned that boat I would dump those old loud smokey lumps for a pair of Cummins.

My CAT 3208 would give those Detroits some serious competition.

pizzachef · #8 12-15-2008, 12:08 AM

Quote:

Originally Posted by katja

I've never known a diesel to NOT smoke, run loud & some what rough on a very cold start...

My wagon starts very well in the cold, very little smoke. And its just about nill at idle. On the opposite end of the spectrum, I get a plume with the 240 starts, anyone who gets in the cloud's way would have trouble breathing normally. So there's a big difference between the two and the 240 is not what I'd consider normal for an old diesel, but then, these are the only two I have experience with. If they all smoke a ton when they're cold, then I guess I just got lucky with the wagon.

tangofox007 · #9 12-14-2008, 10:58 PM

Quote:

Originally Posted by Chad300tdt

There is also a modification where you disconnect the violet/purple wire which I believe disables the safety shutoff timer so you get extra long glow cycles that continue after the car is started. I haven't researched this much, so I don't know if this is safe or how the plugs actually end their cycle once the mod is implemented.

Disconnecting the violet wire does not disable the timer, it does just the opposite-it allows it to continue to run. Ordinarily, the glow cycle is terminated (early) when the key is released from the start position, thereby cutting the cycle short as compared to what it would have been if there had been no start attempt.

kerry · #10 12-14-2008, 11:03 PM

Quote:

Originally Posted by tangofox007

Disconnecting the violet wire does not disable the timer, it does just the opposite-it allows it to continue to run. Ordinarily, the glow cycle is terminated (early) when the key is released from the start position, thereby cutting the cycle short as compared to what it would have been if there had been no start attempt.

There was a thread on this topic last year. I believe the consensus opinion was that the glow plugs cut out once the key is moved from the glow position to the start position.

tangofox007 · #11 12-15-2008, 10:47 AM

Quote:

Originally Posted by kerry

There was a thread on this topic last year. I believe the consensus opinion was that the glow plugs cut out once the key is moved from the glow position to the start position.

The consensus of opinion if frequently in error because one person gets it wrong and the crowd repeats "what he said." So instead of acting as a cheerleader for the uninformed masses, why not get out your multimeter (or look at a FSM) and determine for yourself what really happens?

kerry · #12 12-15-2008, 11:21 AM

Quote:

Originally Posted by tangofox007

The consensus of opinion if frequently in error because one person gets it wrong and the crowd repeats "what he said." So instead of acting as a cheerleader for the uninformed masses, why not get out your multimeter (or look at a FSM) and determine for yourself what really happens?

Yes, you're right.

For the record, here's an earlier thread on this issue which also references an earlier comment by Lance who has a push button glow relay and mentions the electrical draw issues of glowing and starting at the same time.

Glow plug timing on 123's

Chad300tdt · #13 12-15-2008, 11:10 AM

Quote:

Originally Posted by tangofox007

Disconnecting the violet wire does not disable the timer, it does just the opposite-it allows it to continue to run. Ordinarily, the glow cycle is terminated (early) when the key is released from the start position, thereby cutting the cycle short as compared to what it would have been if there had been no start attempt.

I understand that it allows the glow to continue after the ignition switch returns to position 2 (following a start from position 3). This is in the FSM and owner's manual which indicates to me that if you are starting the safety shutoff is ignored (or extended) and the glow continues as long as you're cranking to start and then shuts off when the key returns to position 2.

What I don't understand is, reports of the afterglow relay and violet mod giving up to 2-3 minutes of glow.

If the relay timer isn't disabled and you're only allowing the normal cycle length to continue after a start, how is the reported extended glowing possible? ... or was it misinformation?

tangofox007 · #14 12-15-2008, 11:25 AM

Quote:

Originally Posted by Chad300tdt

What I don't understand is, reports of the afterglow relay and violet mod giving up to 2-3 minutes of glow.

You won't get anywhere near 2-3 minutes with an original equipment relay. All you will get is whatever is remaining of the original glow cycle, which is around 40 seconds, depending on temperature.

The aftermarket Bosch relay will give a 3 minute afterglow, however.

http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1983@@1983&makeid=63@@MERCEDES%2DBENZ@@63&modelid=6232@@300TD&catid=241959@@Body+Electrical&subcatid=241967@@Fusebox+and+Relay&applicationid=W0133-1606169&mode=PA

Quote:

Originally Posted by Chad300tdt

If the relay timer isn't disabled and you're only allowing the normal cycle length to continue after a start, how is the reported extended glowing possible? ... or was it misinformation?

As designed, the glow cycle is terminated (regardless of time elapsed) when the key is released from the start position. Disconnecting the violet wire prevents the timer from ending the cycle prematurely, since it doesn't "know" that a start was attempted. So the cycle will run for its "normal" duration until it times out. So, the most that you could nominally expect is an additional 30 seconds, plus or minus.

Chad300tdt · #15 12-15-2008, 11:29 AM

Thanks.

I seems then that the cost of the Bosch afterglow relay would be worth it, compared with doing the "free" violet wire mod, for someone with a rough idle after starting.