Trouble starting in 17*f weather

[Biotour]

ive been having trouble with my 81 300sd lately, but only when cold out. its never seen the snow before so ive never had any trouble getting it started before, but the first trip to cold weather i killed the battery trying to start it. i gave it two glows before starting, waiting until the relay clicked off both times. i checked the glow plugs at the big connector behind the driver headlight. they were all around .8 ohms. i brought the battery inside and charged it, then just cranked maybe 3 or 4 times and while i was cranking, my friend unscrewed the oil fill cap on the valve cover, and right when he unscrewed it the engine started. this was about a week and a half ago, i thought it was a coincidence until tonight when the same exact thing happened.

it decided to snow yesterday, so today its freezing out and i try starting the car, figuring it would work fine because its been nicer out the past couple weeks.. killed the battery AGAIN! this time i pulled the glow plugs and replaced a couple.. noticed the first one had a loose connection to the wire harness. (maybe thats why my car's been stumbling a little bit for the first few minutes after it starts up??) i ended up getting the size 49 battery because i realized the one i had in there was the wrong size. it just died too quickly after trying to crank and glow only a couple times. so i put the new one in and cranked for a good 5-10 seconds, after doing two glow cycles. tried again. and again. then same thing- my friend unscrewed the oil cap thinking it would work again and it did.

just a side note- ive done a valve adjustment within the past 100 miles and i ran diesel purge through the engine, about 2000 miles ago. when it ran through it came out clear and didnt seem to have any affect on the running of the engine. the shut off valve at the back of the ip was replaced about a month ago, in case you were thinking somehow it was keeping the engine from starting. and im pretty sure it isnt air in the lines, i pumped the primer pump and could hear the overflow coming back out the ip, i just wanted to make sure the fuel wasnt frozen soild!

i don't think i have excessive blowby; the oil cap will just rattle on the valve cover when its unscrewed, it doesnt fly off.

anyone have any insight as to why unscrewing the cap helped the engine to start? i know there must be something else wrong, because today (before the battery died) i tried unscrewing the cap before cranking it, thinking, "just in case i can't get it started, this is what worked last time". maybe my glow plugs are all bad?? maybe its the excessive cranking that heats up the engine enough to get it started? and somehow pressure is building up in the valve cover, keeping it from starting? i dont know..