1990 K5 5.7L with 92,000 original miles. I started it up last weekend after sitting for 8 months with low gas, about 1/4 tank. I go to start it two days ago and it cranks fine, I hear the fuel pump and it runs extremely rough for about one or two seconds, then quits and it will keep doing this. I will replace fuel filter and coolant temperature sensor tomorrow hopefully. Until then, anybody have any input I can go on? Could it be bad gas? Weak pump? Out of gas even though gauge reads 1/4?

 

What runs rough for a few seconds then quits, the engine or the fuel pump?
If the truck ran fine before you let it sit then I'd put money on the gas. If you let gas sit for more than 6 months it starts to go bad, that's why you should always add a fuel stabilizer if you let it sit for long periods. With only a quarter tank you might be able to dilute the old gas with a complete fill-up of high octane gas, but with the price of gas why take chances on wasting 3/4 tank. The best thing to do would be to empty the old gas out. How you ask? Easy, disconnect the fuel line at the filter and put a length of hose on it to a gas can. Then if you put the key to the accessory position the fuel pump will try to pressurize the system for a couple seconds then stop. Then you must wait a about 10 seconds and do it again. As you can see, this will take forever. So instead of using the key to run the pump you can disconnect the fuel pump connector and hook up your own 12V source to it. The connector is located on the driver's side next to the frame rail just in front of the tank. I forget exactly which pin is the power for the pump but if you take a volt meter and measure the pins on the plug you'll figure it out (of course the meter will only give you a 12V reading for a few seconds when you first turn the key to the accessory position while it's trying to prime the system). Or you can try to siphon the gas out, but I've never been able to get a hose to go past the couple of bends in the rubber filler tube. Hopefully some part of this makes sense.

 

I'm going to assume you mean the fuel pump runs rough. Sitting a while seems to bring out the worst in in-tank FP's. I've had it happen twice in cars I'd let sit then tried to start to sell them.

Actually if it's good gas to start with, the engine will try to start but may not run well.

Either way you need to remove a good deal of the gas in the tank. I've done this by jacking the vehicle enough to put a container under it, remove fuel filter and run the pump to create a siphon and let the gas run slowly into the container.
Dont do this in a garage and obviously dont smoke or light anything around it.

Hint: Another method is to reverse-flow your shop vac, wrap rags around its hose and pressurize your tank through the filler.

 

Crank sensor.

Your getting prime mode but not run mode on the pump which is turned on by the computer which is told to by the crank sensor.

 

Thanks for the responses guys. I messed around with the truck this afternoon and it still would not start, same symptoms as I listed in my first post. Then I said what the heck, I'll throw in a couple gallons of gas, so I did and the truck started and ran great. I let it run for about 15 minutes and I revved it a bunch of times and it never sounded or felt like it was going to die. I also waited a bit and started it two more times and it started right up and ran fine, so now I'm thinking it was just out of gas and the gauge isn't reading correctly.

 

Glad it was something so simple!

 

Got to have fuel to make the wheels go round and round, or a big hill.

 

Thanks for the update. Glad it's working again.

Just in case you or anybody else need this info in the future I thought I'd clear up my previous post. If you unplug the wire harness for the gas tank the gray wire is the power to the pump and the purple wire is for the sending unit. If you connect a 12V supply to the gray wire and ground to any metal part, the pump will operate. I hooked up a 9V battery for a few seconds and it worked dry, don't know if it'd pump gas though. With this plug you can also check to see if the sending unit is functioning. Attach an ohm meter to the purple wire and ground the other lead and you should get a reading any where between 0 and 90 ohms depending on how much gas is in the tank. Also, you could attach a potentiometer to the male side of this connector and check the functionality of the fuel gauge. Just get a 100 ohm potentiometer from radio shack and sweep it through it's range.

 

Thanks for the picture, I'll have to go look sometime and mess with it

 

No 350 TBI or carbed motor has a crank sensor on it. To anybody that reads this thread, there is no crank sensor on your 350 TBI or carbed engine.