I bought an 850 turbo and the guy said that he had troubles getting it to start, but when he banged on the fuel pump it was fine. Once we got it running it ran great and I took it home. I then replaced the fuel pump and it still wouldn't start so i replaced the ignition coil, and it started up! but then it was being all jumpy and even died in flight. I then replaced the fuel filter thinking that maybe not getting enough fuel was the cause, but it still didn't fix the issue of the idle being really rough. I had my dad mess around with the engine while it was running and the idle went back to being smooth after he pressed on the distributor cap, so I looked into it and the cap and rotor needed to be replaced badly. once i got those in there, it ran like a champ! But then i get out there this morning to get to school, and it wont start again....I've been told that it could be a cam sensor, but that is $160 this broke college student doesn't have, any ideas on what could be the problem and how to fix it?
edit: my dad just looked at the engine and said that there might be a vacuum leak, could that be the cause of the problem?

 

Ok,, here is a very quick and easy way to tell what is causing the problem. An engine needs 4 things to run. Fuel, air, spark, and proper timing. Get a can of spray starting fluid. Take the hose off the air filter box and spray a short two second burst of starting fluid into the hose toward the engine. Try starting it. If it starts and runs for a couple seconds you have a fuel issue. If not, it is either spark or timing. Caution, do this outside and only use the starting fluid a couple times as it is very flammable.

 

Going by what you said, you already have done most of a stage 0, so I would finish the ignition system with new spark plugs and plug wires. If the cap and rotor were that bad, chances are the plugs are just as bad too.

Also noteworthy are vacuum lines. The age of these cars are significant, so a good vac leak can really mess up the idle or totally stall your car, so yes check the vacuum lines. Both of these are cheap to do and do wonders for these cars. I am pretty certain it is one or the other.

 

The cam sensor is very very easy to test. There would be no reason for anybody to replace one without testing it.

There are 3 wires going to the cam sensor, and there will be 12 volts across the outside wires. The middle wire reports back 5 volts half the time and zero volts half the time. If you take it off the engine, you can actually stick a metal object in the gap and trigger it yourself.

It should also be setting codes if it has sensor problems.

Good reading: Volvo 850 Maintenance And Service Documents