94 GMC Sierra not running right, need help
 

I cant figure out whats wrong with my truck, I have to pump gas to start it, sometimes it idles bad, it spudders while driving, hesitates and rpms drop when i punch the gas stalls out when stoped, seems to be worse the colder it is outside. Ive changed fuel filter, spark plugs and wires, distributer cap and roter. any ideas I dont know what to do next.

Check for a vacuum leak at the base of your throttle body by spraying carburetor cleaner around the base. If the RPM changes when you spray the cleaner there, then you've got yourself a vacuum leak!

Have you checked your fuel pressure?

someone checked my vacuum for me and told me that there was no leak.
no i havent checked the fuel pressure.
And i forgot to mention when I bought the truck it was sitting for about 2 years

Have you put new gas in it yet and run it for a while?
TB may be plugged up from sitting and need cleaning/rebuilding

Yeah you could very likely have bad gas in there ... gas can go bad after just one year. Plugged fuel filter perhaps too.

Ive had it for about 6 months now, changed the fuel filter 2x.
Could the fuel pump get cloged up from sitting?
there were times the truck would run great, then all of a sudden back to crap again.

Yeah definitely check fuel pressure, also is your check engine light on at all?

It must be a fuel supply issue... If it won't start unless you pump the throttle? And it runs good periodically.... This could be a sign the fuel pump is on it's way out, or the fuel pickup is beginning to plug with slug or rust.
There is possibly a fuel filter under your truck on the frame rail and a smaller one at the throttle body.
Another possibility is the two injectors are beginning to clog

ya, the check engine light comes on randomly

Scanning the codes wouldnt hurt

I had what seems to be the exact same issue with my '95 Sierra. Took me some time, but finally identified it to be a bad fuel pump. Typically when fuel pumps die, they will completely stop functioning, but these ones can still put out a bit of pressure. Hardly any, and not enough to maintain idle/run, but enough to get the truck started.

Go to an auto-parts store that will rent tools and get a fuel pressure test kit. These years should be around 14-16 psi if memory serves me right, but verify that before attempting.

I bet you you'll get a few psi if any at all. You'll have to drop the tank or remove the bed to access the pump, which sits inside the tank and removed through the top. I used a jack to hold it up while I undid the straps, lowered it, disconnected all the tubing/wiring, and slid the tank out to work on. I'm not sure the condition your truck is in, but I ultimately had to replace a lot of the rubber fuel tubing, including the sending unit, so it was a bit of a pricey replacement, maybe 300. Don't go cheap on a fuel pump, or you'll be doing this project again in a few years or sooner.

With my truck, the connecter between the pump and the sending unit shorted out and melted/charred. The pump might be good, but I didn't want to take any chances. How the whole thing didn't go up in flames is beyond me. I think what ultimately happened was the previous owner (to whom this problem happened to before he sold it to me), let the gas varnish, which put extra load on the pump. The pump drew too much current and it fried the electricals. Ran great once replaced, just had to improvise some fuel lines with pieces of tubing and plenty of screw clamps.

It's definitely a task you can take on yourself. An extra hand helps.

when i start my truck i have to pump the gas pedal to start it.
how do you check the fuel pressure?

My truck, when I had this problem, would start but quickly die unless you repeatedly pumped the gas pedal.

To check the pressure, call your local auto parts store. Many of the main carriers (e.g. autozone, O'Reily's, etc) offer "free" tool rental, meaning you purchase the kit and return it for a full refund. They will provide you a fuel pressure test kit.

The proper way to test the pressure is fro one of the schrader valve on the fuel line. The kit should explain this. Sometimes, though, these aren't present and you need to test it at the throttle body, under the air filter where the fuel lines enter the pressure assembly. The kit should have a TBI adapter that inserts between the incoming fuel line and the pressure assembly. A pressure gauge would then attach to the adapter. Run the truck and operate the gauge as per instruction.

Speaking from experience, attaching the TBI adapter is a real pain in the ass and I don't recommend it. If you don't have a schrader valve on a fuel line, I would just improvise an attachment directly to the fuel line right before the fuel filter. Use hose clamps or what have you. Compare the read pressure to spec.

On a 94 truck, it's TBI and wont have a schrader valves from the factory. You'll either have to unscrew the fuel filter and use an adapter there, or you can buy the actron tester that's designed to screw into the fuel like right by the throttle body. I would recommend the actron kit, because that way you don't have to mess with rusty fuel lines just to check pressure. When I did the filter on my old 89 pickup, I ended up breaking one of the lines, and it was only to install my $10 fuel filter .... so if I had one of these actron testers, I would have saved myself some grief.

It's your call, but testing fuel pressure can be more of a hassle than running to the autoparts store and grabbing a tester. You'll need one of two adapters to do it, or you'll need to pipe in some rubber hose somewhere and use a generic "T" adapter and some hose clamps. Really though, the actron tester is the best option for these trucks. I eventually made the leap and spent the cash and was glad I did. I bought a second adapter and left the two installed and they don't leak or anything. I had one on my pickup and one on my 91 Firebird which is also TBI.

With the actron adapters though, they use a special proprietary thread on the scrader valve though so you absolutely HAVE to use their gauge. No short cuts here ... I already tried. By the way, you should have 8-13 PSI per GM spec. I would say if you have roughly anything more than 9 PSI you're probably fine because my old 89 pickup's pump didn't go out complete, it just made the engine run REALLY lean and stalled out and backfired and all kinds of nasty stuff ... a new pump jacked the PSI up to 15 and it ran better than ever afterwords.

When I had tested my fuel pressure, the kit I rented from O'Reily's had a TBI adapter included.