I really need help with this. Is it possible for the vent line in the fuel tank to be clogged causing so much pressure in the gas tank that it will not start? I'm leaning toward this being my gremlin. After I took off the fuel cap and released pressure it started right up...before it would just continuously crank. After the car starts the first time of the day it will keep starting until it sits for 2-3 hours or more...tonight it sat 2 1/2 hours and cranked for about 5 seconds then fired up...shut it down and cranked right back up. 87 GTA 5.7 TPI

Come to think of it I think it is sucking air into the tank not pushing the cap out...anyways...will either of these cause it not to start

Did you happen to try a different cap?

Not yet...I'm trying to make sure it's pressure related in the tank and not something intermittent under the hood. I plan on getting a vented gas cap if this is the reason the car won't start.

Which one?
If it is sucking in then more than likely your cap is not venting
( allowing air in as fuel is used up)
If it is blowing out then most likely the purge line to canister is blocked or the purge valve itself is not working resulting in a fumes ( pressure ) build up in the tank

I won't be able to check until the morning. Would either condition cause the car not to start?

Try driving with out the cap. IF it's good then a new vented cap is needed. IF NOT,then check per vetteoz.Good point vetteoz!!
Was the car worked on & now it's acting up?

The car's been worked on and acting up the last 3 weeks.. I fix something..runs great for a day..then the next day it's something new. That's the problem with letting it sit for so long I guess. It's been driven about 200 miles in the last 5 years...except I've put 600 on it in the last 3 weeks...so the weak links are starting to break. To check the purge line do I just pull the vacuum line off the charcoal canister and see if it's sucking?

That only checks the suction part of system
To check tank vent line ,disconnect at canister and try to blow back to tank. Not up to speed on 3rd Gens but Vettes have sections of rubber line that get blocked by rust flakes from the steel lines

There wasn't any pressure in the tank this morning because I took the cap off last night and reset it...this morning it started after cranking for a few seconds (it was 38 degrees this morning). I let it idle for about 10 min to warm up then shut it down and checked the tank...it appears that it is sucking air in.....I tried to feel for air coming out and smell gas fumes but I didn't....that's why I believe it's sucking air...I'll be driving it all day so I'll know for sure this afternoon. I'm still confused why the only time it wouldn't start is when it's cold. Unless it's because the fuel pressure in the lines are what's causing it to start when it's warm then when the pressure all releases it's not able to reprime.

I had the same problem, I changed the sending unit and the littel valve coming off the fuel line, seemed to fix it for me!!!!

Replace that little white or gray valve check thing that hangs under the gas tank. Its all by itself, its by where the rear brake line bracket is, and it only connects to the gas tank. Buy a new one from the GM dealership. Other thing it could be is something in the charcoal system, at least thats how the LS1 tank works.

Fuel injected setups should not have a vented cap, if I remember correctly. The other parts of the fuel/vent system are supposed to regulate that.

If the tank can't vent at all, it causes the electric fuel pump to work too hard, and that leads to pre-mature failure of the fuel pump. I had a new walbro fail in under 200 miles because it wasn't vented at all.

The cap isn't the stock cap so I"m getting a vented cap today.. I changed the pump a few weeks ago so I'm wondering if the line to the vent filter is pinched...with the other problems I've had it may just now be showing up. It ran great all day...about 6 starts...I'm letting it sit until tonight then trying it..thanks for all the help!

i reccomend trying the dealership to replace the vented check valve by the fuel tank's hose. Changing the cap is a short cut, not standard. not stock. the check valve is cheaper too. Check out Hayne's Manual at autozone,--->(page 4A-3)--> (diagram 5.12)--> (number 5 in the picture) replace that., not the check valve at the cansiter

A " non vented " cap by definition is one that will not allow fumes to be vented out to the atmosphere ( for emission purposes). A "non vented " cap however will allow air in to tank.
Can check cap operation by blowing / sucking on it; should only allow air to come into tank

The canister purge system removes the tank fumes to the engine by vac so in that case when the purge operates, the tank could be in vacumn if the tank is not vented through the cap ;there is no way for air to get in to replace fuel used up.

Started up this morning no problems but it was already 60 out...Tomoros I have a 400 mile drive so I'll have it figured out from that...hopefully.

I didn't have any problems today with the tank venting..I'm guessing something was clogging a line or something...hopefully everything keeps running good. Thanks for all teh help