I just bought an 87 300d about 2 weeks ago and it has a strange problem. When you first turn the key to start (after waiting for the glow plug light to go out), it will not start no matter how long the engine turns over.

If you turn the key completely off and go to start again, it starts immediately with no problem. What could this be?

So this would lead me to believe it is some type of electrical issue that is keeping it from starting. But since it is a diesel, I would think that as long as you had the engine spinning and fuel provided it would start. Any ideas?

let the glow plugs run a full 45 seconds. not just the few seconds the light stays on.
if it now starts fine, all is ok. if not, you may have bad delivery valve orings, or some other fuel leak letting the fuel drain back, and when you crank it, you are getting the fuel flowing, then the second glow lets it fire.

Ok, it worked. I just left the key on for the 45 sec and it started no problem. It seems strange that I would have to wait so long. Is it the GP relay that's faulty or just glow plugs that aren't working properly? I got the car from my brother and he recently put all new Autolite glow plugs in.

it's not the relay or the glow plugs.
it's a combination of many things, that are lowering your compression.
the motor should start with no glow above say, 50°F if it can't it's due to compression loss. likely it's the valves worn in their seats. how many miles you got on that motor? ask Brian Carlton what a fresh head repair does for starting!
it could also be stuck rings, or worn cylinders, etc. bottom line is compression starts these cars. Glows bandaid the issue for longevity of the motor.

263k

Yeah, I definitely need the glow plugs to start even on a 70 degree day.

Can I do a leak-down test on a diesel?

I guess I would like to know if its the valves or the lower end.

Isn't there a test for blow-by using the oil fill cap? When I unscrew mine and leave it on the cover it just sits there. Is it supposed to float or wobble or fall off?

I just noticed you said autolight plugs.
odds are good that many of them are bad. autolight plugs do not like mb's for some reason.
they fail as soon as they are installed in the car.
pull them all, (I know, I know, huge pain on this motor) and install genuine bocsh plugs.
you will thank yourself with much better starts, and not have to think about the plugs for a while.
also, when the plugs are out, get in there with a ream tool. carbon builds up on the plugs, and can insulate them so that they do not heat the chamber at all.
oh, is your EGR functioning? how dirty is your intake? check out my thread about my new to me 87TD, I have a nice pic of my new EGR gasket... I recommend you get one like it.

When the car was still my brother's, I helped him take off the intake and clean it out. It was full of sludge.

I have no idea how the egr is working. As you can see this is all a learning experience. Vstech: I checked out your thread but couldn't find the pic of your gasket. Can you put it up or link me to it? Is there more I should know about the egr systems on these cars?

Just a little background info: i got the car for $400 and I love the way it drives so I am committed to getting this thing to run correctly. Right now i'm not even sure the turbo is boosting, it feels that slow. Just trying to fix one thing at a time.