90 cavalier won't start

i have owned the a 1990 cavalier z24 for a little over a month and it ran fine up until a couple days or so ago. everytime i drove the car somewhere the check engine light would come on once the car got up to temperature. i only spent $400 on the car so i didn't want to put any money into it and the car ran fine so i didn't do anything about it. i drove the car home and that was the last time it ran. nothing was different except the check engine light never came one. i thought it was kinda odd but didn't pay it much mind until the next morning i went to start it and it won't run. it rolls over fine but won't fire. where should i start looking first? it's a manual car with a 3.1 engine.

Get a scan tool and pull the trouble codes.

i'm not sure what happened but i just went and tried to start it and it started right up. i drove it a couple times after that and the check engine light never came on. i was able to check the codes with a tech 1 and it said it was an egr problem. i don't remember what the code # was. what would cause it to run one time but then just not. i was driving today and i had the rmps up close to 5 and then the check engine light came on and is now on every time the car get up to temp. so would being at a higher rpm make the egr valve stick and then maybe it finally got stuck all the way open and wouldn't start and after sitting for a while finally came unstuck? is the egr valve bad and should i buy a new one, if so where could i find a used or good priced one b/c all i've found are ones for $150-$200.

Look for a bad vacuum hose to the EGR first. The computer only checks when the EGR is hot. Don't buy a used one. Check around for price but buy one that is a name brand or you might still have problems.

i replaced the egr valve and cleared the code and it ran fine for a week. today i went out and it wouldn't start. could it be a crank or cam sensor? what can i do to figure this thing out?

When it will not start see if you have spark ? Also check your fuel pressure.

i checked for spark between the coil packs and the spark plug wires and got nothing. but since it will sometimes run and other not, wouldn't that mean it's a wiring problem and not the coil pack?

Yes either a wiring problem or a sensor not telling the computer to fire. Try and pull your trouble codes again.

the crank sensor was bad. it had a crack the whole way around in the plastic. thanks for all the help.

When it will not start see if you have spark ? Also check your fuel pressure.

I was under the impression that 89-90 up 3.1s used electronic EGR valves-- my 90 Lumina did....