1993 GMC G3500 6.2l fuel system priming?

[Splat]

Hello. I replaced the box-like fuel filter on the back of the engine and tried starting her back up. She cranks but that's it. I picked up Hayne's diesel tech manual. Per the manaul I tried bleeding the line at the fuel filter unit's air bleed but I get nothing coming out. It appears the fuel(or lift?) pump is the mechanical type under the passenger side forward of the engine. I do not see any other fuel pumps anywhere. I was hoping to jump the pump to get fuel up to the filter. Anyone know if there's an electrical pump on the 93 G3500? Fuel tank?!

Related question... if I get nothing out of the air bleed at the filter unit would it be useless to crack the fuel injectors to see if I can bleed them there? Thank you.

[DmaxMaverick]

Welcome to the Forums, Splat! (nice handle)

If you don't get fuel at the bleeder while cranking the engine, either you are out of fuel, pump has failed, or you just haven't cranked long enough. It takes some time to fill the filter with the lift pump. Healthy batteries are especially important.

There are several things you can do, depending on your wrenching skill and available resources. You are correct about the injector lines. No point cracking them if you don't have fuel at the filter. You will just exaggerate the airlock.

Try this:
Make sure the batteries are up to full charge. Weak batteries can kill a starter.
Disconnect the FSO (Fuel ShutOff solenoid) wire on the injector pump, and protect it from grounding. It will be the pink or red wire that connects at the top of the I/P. This will prevent the pump from drawing more air into the I/P. Open the bleeder (remove the bleeder completely to ensure it is open enough). Use about 5 PSI of shop air and pressurize the fuel tank. Be careful to not allow the fuel to spill back out at you if the tank is nearly full. This should force the fuel up to the filter, and prime it. Once you have fuel at the bleeder, crank the engine to confirm fuel pump operation. If you still don't get fuel, the pump is likely bad. A healthy I/P will draw fuel, even if the lift pump is bad, as long as it is primed.

If you get a good stream of fuel while cranking the engine, tighten the bleeder and replace the FSO wire. It should start with a short cranking period. It may run rough for a little while, but should clear up shortly. I do not recommend loosening injector lines to help bleed it. When you get it running, open the bleeder. It should give you a good stream of fuel and the engine should continue to run. If not, and the engine dies, the lift pump is toast.

[Splat]

DmaxMaverick, thank you very much for replying.
I should've noted I did fill the new fule filter up to the point of overflow before I installed it. I realize it could, and pro'lly did, get air bubbles in it while tilting it upright to install onto the filter unit.

I don't own a shop, but I do have an a nice big air compressor in the garage. I thought about charging the fuel tank but never did that before so... little leary about that. After I posted I read around on here and the Web and decided to crack open the lines. I did all of them, hooked everything back up and she finally ran, although shortly. I put the battery charger on the battery and tried again minutes later. She started, ran rough, but she ran for 30-40 seconds. I'm letting her sit now and get the battery charged more and I'll try it again. I think she'll go once the battery gets a better charge. I'll post back the results.

[Splat]

Well, she ran again for another 20-30 seconds and then died. I found that my shopvac's hose fits the gas tank pipe well enough for me to pressure the system. I finally got a nice blast of diesel out of the fuel filter bleeder. I closed everything up and tried again, no go. I think I may now need to rebleed the injectors again.... or should I take off the return line on top of the injector pump and bleed that?

[DmaxMaverick]

Don't bother trying to bleed it any further. Unless you can see a major leak somewhere, your lift pump is bad.

[Splat]

I believe you, but the truck was running fine until I replaced the fuel filter. I admit I tried starting the truck about 10 times, allowing the starter to cool in between attempts, so could that have shot the lift pump?

[NH2112]

I'd ensure you still have fuel at the bleeder when pressurizing the tank, then crank it with the bleeder open to see if the lift pump is pumping. If so, then loosen the injector nuts about 1 turn, reconnect the pink wire on the injector pump, and crank 10-15 seconds at a time till you see fuel dripping. Make sure to let the starter cool for about a minute in between. Once you have fuel at the injectors tighten them up and start with full throttle. There still may be some air in the system so keep the RPM up till it runs smooth.

You could also remove the glow plugs during the cranking, but I generally only do that when replacing the IP due to the amount of fuel that it takes.

[Splat]

Thanks Phil! I'll give that a shot tomorrow.

[DmaxMaverick]

Originally posted by Splat:
I believe you, but the truck was running fine until I replaced the fuel filter. I admit I tried starting the truck about 10 times, allowing the starter to cool in between attempts, so could that have shot the lift pump? A healthy I/P will draw fuel if the rest of the system is leak free. The condition will eventually take its toll on the I/P, but for a long time, you may not even know the lift pump is gone, especially in the summer months when the fuel viscosity stays down. The I/P will draw fuel, but it won't prime itself.

Try this (an extra pair of hands will be helpful):
Take the outlet line loose from the lift pump and connect a length of hose to it. Elevate the end of the hose to above the I/P and check the flow of fuel with the engine cranking. Restrict the flow to check the pump's ability to produce pressure, use a gage if you can get your hands on one. If the pump checks good, come back and we'll start the troubleshooting process.

I've been where you are dozens of times, and all but one, it was the lift pump. Many times, you won't be down on power until you put a serious load on it.

[Splat]

Thanks Dmax. I'll try that as well tomorrow. I'd rather not have to go start buying parts now.. been unemployed for too damn long... but if it needs it... I'll let you guys know. Thanks again.

[Splat]

Thank you, gents, for helping me out. I ensured the lift pump is working, cracked the injectors, cranked the engine, got fuel at all the injectors, tightened everything back up... she RUNS! Hah-hah! smile.gif Thanks much. Now, if I get only stop being paranoid about the lift pump. I'm wondering if I should replace it now since I don't know if it's the original. Then again, maybe I should just leave everything alone for a while. :D

[DmaxMaverick]

Good deal! Glad you got it running.

Mechanical fuel pumps are funny animals. They work until they don't. Original or not, I'd suggest leaving it alone if it's working fine. The chances you'll get a short-timer off the shelf is as likely as yours will fail. How many times have you heard (about any component), "The first one lasted 10 years. Now I have to replace it every year."