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post #1 of (permalink) Old 07-20-2007, 12:13 AM Thread Starter
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Help me understand FI

Ok, so I ran out of gas.
I had a 5 Gal Can hanging off my tire carrier (Thanks Rio).
I put 2.5 Gal in the tank.
Won't start, not at all, just cranks.
Get the truck, tow back home.
Charge the batteries (Maybe not enough juice to run PCM) - Nope.
Follow Leve's rule, check fuel rail first, air (BTW Thanks JeepDawg), ok hmm.
Cycle thru cranking (pump didn't always run at key turn) and letting air out.
When fuel sprays out, jeep runs.

Ok, WTF? I need to 'burp' the system to get it to run?

No a "yes" won't suffice

Uggh, going on the Jeepers Jamboree in 4 days, and realize I need a cluch too, always before a run, always before a run....

Now where is that bottle of scotch.....

Scott
1985 CJ7 T.H.O.R

"He brought me up out of the pit of destruction, out of the miry clay, And He set my feet upon a rock making my footsteps firm."
-Psalm 40:2
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post #2 of (permalink) Old 07-20-2007, 06:46 AM
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Yes. That's the simple answer.

It gets more complicated than I can help you with, but you do have to burp the system. It won't push the air through the injector(s). So you have to build pressure with the fuel pump (key on and of) then bleed the system. I have in the past been able to crank for 20 seconds, wait 2 minutes, crank 20 seconds, bleed. Repeat. On third try it'll fire, sometimes on the second...but it will run rough for a couple of seconds. The system likes the pressure.

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Originally Posted by thorjeep View Post

Now where is that bottle of scotch.....
Headed to Camp Jeep with me.

A bad weekend of wheelin', is still a good weekend.
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post #3 of (permalink) Old 07-20-2007, 10:03 AM
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Scott, one of the first rules of fuel injection is... don't run out of gas. The fuel pump siphon's supposed to be up, off the bottom of the tank. It ain't always that way. The pump often ends up sucking the crud out of the bottom of the tank when the tank goes dry. That in turn, goes into the pump impellers and jams it, or clogs up the works enough to lower the fuel pressure.

Remember that turning the key to start the Jeep the computer will pressurize the line for 2 seconds. Then it will switch to the CPS to control the spark and the fuel pump voltage. If it doesn't see the CPS, the fuel and ignition shut down. So, if you're getting either spark or fuel pump voltage after 2 seconds in START mode, the CPS is likely working.

So, lets say you've got spark, and no fuel. That pretty much screams fuel problem. Now check that fuel rail for fuel. If it's got air in it, there's a few things that can happen.
  • The pump may not be failing
  • The pump siphon my be suckin' air
  • There's a pinhole in the line
  • The ECM's bad
  • The pump's bad
The pump is the biggest cause of the grief. If it's bad you won't get adequate pressure and the Jeep will not run right. It will miss an lope as injectors/plugs fire.

So, I'd check the fuel pressure first, and see what the pump is running at.. and then perhaps drop the tank and pull the pump. If it's clogging, you may recover it by running Seafoam though it. I've done that and it does work. Just siphon out from a jar of Seafoam and back into the same container. Let it run a bit... but not too long. It's usually cooled by the fuel. Then reassemble and see what the fuel pressure's at.

You may be ending up replacing the regulator and the sender assembly.
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post #4 of (permalink) Old 07-20-2007, 10:22 AM
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If it was running fine before it ran out of fuel -

After filling - keep feet off the pedals!

Turn key on for 10 seconds, do not crank -
Turn key off for 10 seconds.

Repeat the above 2, 3, or 4 times.

Now start it up but DO NOT TOUCH THE ACCELERATOR PEDAL until it's running fine.

It takes several cycles of the 2 second "key on" in succession to get the fuel up through the lines and filters.

Pumping the pedal only confuses the computer and inhibits starting.

It's very rare to have to "bleed" an electronic injection system - a mechanical system - yes.


The electronic systems the injector is electrically operated and opens, letting the air out.

A mechanical system the injector operates by the pressure impulses from the pump - the air compresses instead of opening the injectors. They have to be bled.
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post #5 of (permalink) Old 07-20-2007, 11:16 AM
 
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How you fuel system works...

Nope, got better things to do.
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post #6 of (permalink) Old 07-20-2007, 01:50 PM Thread Starter
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Thanks to Lomodyj, Leve and Rich for all the excellent information!

I know how FI works, just never heard that you have to burp the system. Then again, I have never run the Jeep fully dry either. Now I know this is normal in this case, that is really what I wanted to hear.

Rich, thanks for the info, Yeah, I just tried to start it, and hit the pedal, my bad. I appreciate your advice, I will do that next time and let others know!

Leve, everything is fine in the system, I was just running it down so I can drop the tank this weekend to take a look at it before my trip.

Lomodyj, found the scotch

Scott
1985 CJ7 T.H.O.R

"He brought me up out of the pit of destruction, out of the miry clay, And He set my feet upon a rock making my footsteps firm."
-Psalm 40:2

Last edited by Jim_Lou; 07-20-2007 at 08:18 PM.
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post #7 of (permalink) Old 07-20-2007, 03:57 PM
 
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You are welcome!
That was a 1/2 hour or 45 minutes of my life that wasn't wasted.
I feel good about that.
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post #8 of (permalink) Old 07-20-2007, 04:03 PM Thread Starter
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Scott
1985 CJ7 T.H.O.R

"He brought me up out of the pit of destruction, out of the miry clay, And He set my feet upon a rock making my footsteps firm."
-Psalm 40:2
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