98 Silverado will not start!!

I have q 1998 Chevy Silverado extended cab C-1500 pickup with a 4.3L engine, auto/overdrive, A/C. After the truck sites for 4 or 5 days sometime not every time it will not start. It just turns over when I try to start and when I let go of the key it acts like it tries to start. Sometime it will backfire some unburnt gas back through the throttle body. Somke will somrtime come out of the tail pipe. Sometime while cranking the engine will turn over like the timing is way off. It turns real hard and will almost stop. I think is might be unburnt gas in the cylinder head. After a while and a jump it will start. It runs ragged for a few seconds and then smooths out. It will start and seems to run fine after it starts the first time. The has only happend after it sits for a long time. I live in sunny Siouth Carolina and it has not been very cold anytime this has happend. Took it to local parts stoew and they plugged in their computer and no codes. Any suggestion on where to start

I have q 1998 Chevy Silverado extended cab C-1500 pickup with a 4.3L engine, auto/overdrive, A/C. After the truck sites for 4 or 5 days sometime not every time it will not start. It just turns over when I try to start and when I let go of the key it acts like it tries to start. Sometime it will backfire some unburnt gas back through the throttle body. Somke will somrtime come out of the tail pipe. Sometime while cranking the engine will turn over like the timing is way off. It turns real hard and will almost stop. I think is might be unburnt gas in the cylinder head. After a while and a jump it will start. It runs ragged for a few seconds and then smooths out. It will start and seems to run fine after it starts the first time. The has only happend after it sits for a long time. I live in sunny Siouth Carolina and it has not been very cold anytime this has happend. Took it to local parts stoew and they plugged in their computer and no codes. Any suggestion on where to start

Welcome on your first post.
Mileage and tune up history?
Hard to start cold.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.

You could have excessive starter motor draw.

Bought several years ago had 108000 miles changed plugs, rotor, cap,wires, fuel filter, and had trans. serviced. Now has 133000 miles. Don't have pressure gauge will have to get one. She started fine this morning temp last night around 50 have started it fine with the temp in the low 30s. Only happens after it sits for about a week.

Check your battery connections for the heck of it?

Check your battery connections for the heck of it?
New positive bolt and spacer, cleaned and put noncorrosive grease. Had battery checked at auto store and it checked good, alt. checked good but machine was broken and could not check starter. I will go back and have the starter checked. :banghead:

also check the distributor cap.
Mine wouldnt start every so often (http://www.automotiveforums.com/vbulletin/showthread.php?p=4578815#post4578815). Would 'hiccup' a little fire like it was thiiiis close to starting and do the other things like you described. I also live in SC and it started right up that cold week or so we had... It would start when it was 20 in the morning but wouldnt the next morning when it was 60 (the weathers screwy down here :screwy: , im a yankee), but it also didnt start a week or so later when it was about 35-40. It takes absolutely the smallest bit of moisture to affect spark in the cap.. which could wreak havoc on the spark going to the plugs. If its in the tightness of fitment you can use large zipties or a small bungee to make it tight to see if it makes a difference..

I have q 1998 Chevy Silverado extended cab C-1500 pickup with a 4.3L engine, auto/overdrive, A/C. After the truck sites for 4 or 5 days sometime not every time it will not start. It just turns over when I try to start and when I let go of the key it acts like it tries to start. Sometime it will backfire some unburnt gas back through the throttle body. Somke will somrtime come out of the tail pipe. Sometime while cranking the engine will turn over like the timing is way off. It turns real hard and will almost stop. I think is might be unburnt gas in the cylinder head. After a while and a jump it will start. It runs ragged for a few seconds and then smooths out. It will start and seems to run fine after it starts the first time. The has only happend after it sits for a long time. I live in sunny Siouth Carolina and it has not been very cold anytime this has happend. Took it to local parts stoew and they plugged in their computer and no codes. Any suggestion on where to startI bought 98 siveraldo with 190,000 miles, it had same problem starting, turned out to be distibutor cap letting moisture in. The bolts holding cap on were not tight. Further investigation I found the bolt holes stripped on distributor. Distibutor made of some carbon material, talked to technican, was told this problem is common. Bolts can be easily over tigtened. New distributor, cap, rotor button, plugs took care of problem. Good luck..