1992 300D Block Heater
 

Bought a '92 300D in Sept. with 98,000 miles on it. Love it! Guy we bought it from stored it in winters and never hooked up the block heater. It is now January in the upper mid-west and of course cold. Car is garage kept and so far we haven't had any issues with it starting even w/o block heater. Questions: Is it imparative to connect heater block? Are we possibly causing any engine damage by not plugging it in at night? Car has been running awesome and we make sure it's good and warmed up before we drive. Any other diesel engine winter care tips would be appreciated.

If you are having no problems starting it, then I woulnd't even worry too much. You won't hurt your engine unless it's really struggling to fire up over and over again, plus it's garaged which will help. If you are feeling inclined you could hook up the heater and have it ready to go just in case you find yourself needing it for any reason, that's why I have it installed on my diesels even though I never have used them yet.

Owners manual states plug in when it's below 15 Deg F. If its garage kept it usually doesn't get that cold unless its an out building.

Both of mine (same car as yours) start EASILY down to 0F with less than one full glow cycle....I assume I could go lower, just hasn't gotten that cold yet. For some other testimonials see http://www.peachparts.com/shopforum/showthread.php?t=292575.

If you find it's having trouble at a particularly temp, try two full glow cycles. Pay no attention to the idiot light....you will hear the relay click after 30 seconds or so.

Use of a high quality 5w40 synthetic will help your cause.

Obviously, there's some very cold temp at which you would NEED block heater....but, in your car, I suspect it's pretty low.

Incidentally, no real need to let it "warm up"....just start the engine and drive slowly.

Welcome to the forum!
the answer is already covered, but I thought I'd agree. the car will have no damage from starting without the heater. with the heater, you will have heat in the car sooner. that's about it. there is no reason to wait at idle befory you drive away, just start the car, give it at least 30 seconds before you put it in gear, and drive sensibly without flooring before the motor temp breaks 40C drive and enjoy!
be sure that you give it adequate glow time before you try to start, but if you didn't give it enough time, be sure and hold the starter in until it starts. it's more damaging to the starter hitting multiple starts of a cold motor, than holding it in for 30 or so seconds to ensure the car starts properly.

Maybe due to the 15W-40 motor oil they expect to be in the car? Doesn't the 15W mean use to 15 deg F in winter.

no.
15W is the viscosity of the oil at 32F and the 40 is the viscosity of the oil at 250F or something like that. it's a flow number that lubricants are rated at.

I have pretty much the same car you do. I have used the block heater this year, even though I don't need it in my area, so that heater starts blowing heat within seconds of starting the engine.

'92 300 D Heater Block
 

Thanks all for your invaluable input. Haven't really had anybody to ask these questions. BTW. We also put snow tires on this vehicle and it makes a world of difference, barely any slipping or sliding. This is important as we get a good 60-70 inches of snow thru the winter season here. A myth surrounds these older MB diesels that they don't perform well in the winter. We have found it to be to the contrary.