I don't know anything about cars, so these might sound like stupid questions, but bare with me please.
2001 C70 Convertible with about 180,000 miles on it. Im only the second owner so it hasn't been passed around a lot.
-In the past month I have had to put a lot of coolant in it twice, but I see no puddle and I see no leak, could it be a different problem?
-Car is starting to shake when I drive it all the time, but does not when I am stopped. Lug nuts are all tight, I just got done checking them and tires are all fine. I have heard it could be a few things and I was just wondering what everyone else's thoughts could be.
Im too broke at the moment to take it to a shop, and I would rather not die while driving it so I would like some tips on how I could check/fix these problems myself.

 

Disappearing coolant is likely a bad head gasket, are you seeing any faint white smoke out the tailpipe at any time? It could also be a very slow leak that is evaporating on the hot engine before you see it on the ground. A shop can pressure test the system and try to see where the leak is, but at your very high mileage a head gasket leak is most likely and unfortunately an expensive repair.

Shaking could be an out of balance tire, bad tire, or a bad tie rod, wheel bearing, or ball joint. This is a safety issue and you should take it in ASAP, have a mechanic get it up on the lift and look around. Again at your high mileage there are many possibilities. An out of balance tire would probably only shake at speeds above 45-55 MPH. Bad tie rod, wheel bearing, or ball joint may do it at all speeds. If it only shakes under braking then you could also have a severely warped brake rotor, a bad wheel bearing could actually shake less when braking b/c the brakes stabilize the bad bearing. Also a trashed CV joint could cause a shake too. Someone needs to poke around under the car on a lift and the problem should turn up.

 

Do you smell anything strange (usually sweet) when you turn the heater on? If so, it is probably leaking coolant from the heater core.

As for the shake, do you hear any noises when driving, or is it just shaking? If you jack the car up, can you shake the wheel and feel any play in it? Shake it from side to side and from top to bottom. Push it in and out. If it has play from side to side, probably a tie rod end. Top to bottom play is probably a wheel bearing. In and out play could be an inner tie rod end. If none of these are an issue, then you might need an alignment. Check the tires for uneven wear (bald on the inside of the tire, but tread on the outside, etc).

Here's a good video that covers some of those problems. This guy's channel has good information in general and I highly recommend watching it.


Good luck and let us know if you find anything.

 

Pressure Test - You can "borrow" one from a auto parts store. Those kits come with instructions or you can look up on google/this site.
Monitor your tail pipe - the exhaust color will be an indication. Check your floor mats. IF soaked heater core is busted (which is probably whats happening). Very easy fix.

Regarding tires, in addition to what Cactuslegs mentioned, you can also try rotating tires. That might help. Also, you can inspect each rim to see if the weights are missing.

As far as mileage, my '98 is only 50 miles short of 300,000 miles - I still cant get myself to sell it. So you have ways to go..

Post back what you find.

Enjoy the car