1980 300SD - Starter disengages sometimes

January was cold in Vermont this year and it took me some time to get through enough of the cold starting checklist to get my 1980 300SD to start. There are still a few more things for me to get to, but I now
have a working 'cold start' routine.
Unfortunately, I also have a new problem. In the beginning, I would sometimes need to crank for 60 to 90 seconds to build enough engine heat to either start or drain the battery. I would only get through two or three of these prolonged cranking sessions and I tried to allow several minutes for the starter to cool.
Now, the starter engages and cranks and usually the engines starts within 2 seconds. However, if I haven't preglowed enough, I need to crank for longer. Sometimes the starter will stay engaged for 10 or 20 seconds until the engine starts. Sometimes the starter will disengage after 3 or 4 seconds. It sounds as though the starter pinion is slipping out the ring gear and the starter keeps turning but the engine does not. There isn't any metal grinding noises, just normal cranking or starter free spinning.
So far I have been able to get started each morning within 2 or 3 tries, but I am worried that if the weather turns cold (below -15C or 5F) that I may have to walk. Can anyone comment on what might be going on and what I ought to check?
Thanks, Ed
--
P.S. As a note to the cold start FAQ maintainer, Walmart sells a
portable gadget they call a 'Jump Starter 500 Amp'. It's about $70
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my 240D was starting poorly. At westfield state college i would have to go out to the parking lot, and run it for a little while every 3 or so hours on really cold nights, or it wouldnt start. my girlfriend at the time thought i was nuts. Bridegwater Motor Works discovered i had a fouled injector and 2 of 4 glow plugs were dead. They swapped those out and it ran like a top in the winter. Get new glow plugs and make sure the current going to them is good - you can check with a Haynes manual and a multimeter.
To clean out the injectors, get it out on the highway and get it going at a good click with lots of power service in it. RT 89 is perfect for this. As far as winter oil, Amsoil series 3000 has been a dream in my TDI. good luck with the turbo diesel.
On my 240D one glow would get it through all but the coldest nights. I think i had a big ass die hard battery on it.
Ed wrote:

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Sounds like the starter is bad and the solution is to replace it.
If you're going to stay in Vermont, I'd get an engine heater and stop going through hell. It's also better for the engine, as you've likely found out with the starter going. There are various types available, block heater that goes in a freeze plug, coolant heater that goes in lower radiator hose, etc. All of these are for overnight heating.
I have the same car and take trips up to Killington to go snowboarding. Since I stay at hotels, I can't be sure I could use an overnight heater, so I want something that can heat it up quickly. I came up with what I think would be a good solution for me.
There are small tank type heaters available from JC Whitney that connect in the heater coolant circuit. I scoped it out and looks like I could install one in the hose from the back top of the engine to the a/c servo. I checked the servo and in the off/home position with the engine not running, the water flows through this hose, through the servo and back to the lower front of the engine. The heater is 1500 watts which should get the job done quickly.
The other part of the plan is to use the existing aux water pump that is in the heater loop to circulate the coolant. The pump has a connector about 6 inchs away that is easily accessable and pulls apart. A cheap small 12 volt power supply could be used to power the pump during the heating process, the pump only draws .7 amps.
I think this system would warm it to make starting easy in about an hour or so. In the morning all you'd have to do is run an extension cord, plug in the heater and the power supply, pull the pump connector and hook it to the supply. The only tricky part is to find a good spot for the heater, I think there are a few possibilities.
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