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Old 12-16-2006, 10:58 AM
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My truck wont start. 91 serria 350 tbi

i just went out today to move my 91 gmc serria with a throttle body 350(180,000 miles on the clock but not sure if its a original engine) that is my daily driver and it wont start, it turns over fine but wont fire up. it doesnt even try to start it will just crank and crank.

first thing i checked was the spark and it is fine. i checked engine codes and nothing comes up. i tried pouring a little bit of gas down the throttle body and it trys to start up but wont go.

i filled up the gas tank on thursday on my way home and then i drove it a short distance yesterday about 10 miles and parked it untill this morning when i tried to start it.

i think its a fuel problem but i dought its water in the gas as i drove it yesterday no problem.

guys have some advice on what to play around with?
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Old 12-16-2006, 11:10 AM
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you should be able to hear the fuel pump when you turn the key on if not check the relay under the hood if you hear it run then look on the fuel line going to the throttle body.There should be a schraeder valve "looks like a valve stem" on the supply line. Cycle the key on and off "dont crank starter" then remove the schraeder valve cap and depress the valve. Watch your eyes and probable dont have a smoke in your mouth . Fuel should spray like hell. If you have a fuel pressure gauge thats better but if it just trickles out or doesnt come out at all thats your problem.check the filters also.hope this helps. good luck
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Old 12-16-2006, 11:49 AM
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ok its got a good fuel pump fuse. i think i can hear a little tick from the gas tank after the ingition is turned on but not positive. i can feel/hear the fuel pump relay.

i looked and doesnt seem like there is a valve on the fuel line unless it is under the truck somwhere.

im just going to go pick up a fuel filter and pump to try.
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Old 12-16-2006, 12:27 PM
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I dont think there is a schrader valve either.

You can see if theres pressure at injectors by grounding blue or green wire with key on, the injector should start squirting fuel.



Possible Problems

empty gas tank

fuel filter plugged

pump relay is bad

fuel pump is bad

regulator spring is broken

no power to injectors

bad ground side connection on injectors

wiring problem somewhere

computer injector driver is bad

something simple it always is




Fuel Pump Circuit
Fuel pump is powered through a relay. With key on engine not running the pump will only run for a few seconds before computer turns it off, if vehicle has a fuel module(only used in large trucks) the pump will run for about 20 seconds then shut off. The oil pressure switch is part of the pump circuit it acts as a back up to the relay. If the relay fails engine will have a hard start condition, if relay & oil pressure switch both fail there will be a no start condition. Pump can be forced to turn on by applying 12 volts to terminal G on ALDL connector; this will bypass the fuel pump relay.




Injectors
With connectors off injector resistance should be 1.4 ohms. With key on and engine not running there should be no dripping from injector.

Injector Wiring
Red 12 volts when key is on
White

Dark Green ground side of circuit controlled by computer
Dark Blue
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Old 12-16-2006, 12:28 PM
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You should definately be able to hear the fuel pump. have somone listen right at the fill tube with the cap off. It should hum for 3-5 seconds to pressurize the system.
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Old 12-16-2006, 12:59 PM
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i just tried listening into the fuel neck as someone turn the ignition on and i cant hear anything so im going to go ahead and pull the gas tank off and replace the pump.
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Old 12-16-2006, 01:09 PM
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Before you do that use a test light and check the wires leading to the pump.You may need a hand turning the key because the power only comes on for a few seconds. It should be a gray wire but Im not sure on your truck. It cant hurt. good luck.
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Old 12-16-2006, 01:52 PM
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If you have to change the pump with a full tank of gas it might be easier to take the bed off and remove the pump, rather then draining the fuel and dropping the tank. I have had the same problem with my Suburban, I think we got it fixed at a shop for $300. New pump from auto zone is $60-$230.
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Old 12-16-2006, 03:00 PM
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i just finished draining alteast half of the tank into all of my jerry cans, i have a ATV jack with a flat platform so im hoping that will work to drop the tank. i will try the test light to the pump as well i didnt even think of doing that
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Old 12-16-2006, 03:15 PM
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i thought my haynes manual would of said how but it doesnt, how do you remove a truck box? pull tail lights wiring out and bed to frame bolts?
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Old 12-16-2006, 03:22 PM
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post topic

these are wonderful ideas and with no disrespect,my 90gmc 350 last week woudn't start for some reason & i changed the plugs. they were wet .? i didn't notice that you said you checked them .


justin.
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Old 12-16-2006, 03:43 PM
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Before you drop the tank check the elecrical. More people have changed pumps only to find out they had a bad ground or some other elecricl issue.
If it is the pump make sure you buy a good one or you will be doing it again soon. I personally like Airtex brand, I have had excelent luck with them. They are not cheap but they work.

Good luck, that is a pain in the butt job.
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Old 12-16-2006, 03:51 PM
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This just happened with my uncles blazer, pump wasnt coming on, so we were looking at the fuel relay, figred out which wire sends the power down to the pump, put 12v on it and the pump kicked on, so it was just a bad relay. 12 bucks later we didnt have to drop the gas tank, id check it out.
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Old 12-16-2006, 04:41 PM
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i pulled 1 plug out when i checked for spark and it was dry.

i liked the idea of pulling the box off instead of the gastank that is still half full but i was only able to get out 3 bolts out of 8 total? a bunch of them are blocked by things like the leaf spring mount and i cant get my impact in there to get the rusty bolt out.

i looked for the fuel tank wiring and found it but to get to the ends i have to ethier pull the box off or drop the tank it looks like?

if i drop the gas tank about how far can i bring it down before i do any damage to the fuel lines?
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Old 12-16-2006, 05:07 PM
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Truck won't start

The tank will drop about 6 inches. I as many others recommend you pull the bed. I know the rusted bed bolts are sometimes a PITA however the fuel pump IMHO is the worst of the two. The fuel lines are sometimes impossible to put on.

TBI injection does not have a Schroeder valve . You have to disconnect the fuel line and install the gauge. MFI w/Fuel rail has a Schroeder valve....russ
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