Bear's Stumped... - Pontiac GTO Forum
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post #1 of 20 (permalink) Old 05-09-2016, 08:34 AM Thread Starter
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Bear's Stumped...

Ok, I'm about at my wits end over a problem I've been having with my '69, aka 'the Beast'.

Symptoms: Runs great, until I hammer it. Then just about the 1-2 upshift, it quits - I mean flat quits, falls on its face, nothing, nada. At first I suspected ignition, but I've managed to rule that out. It's definitely fuel related. When "it happens", if I pull over and quickly raise the hood, I can see the fuel pressure going into the carb (I have a gauge on the regulator) has all but disappeared. Instead of the carefully adjusted 6.5 psi at the regulator output port, there's 2-3 psi, maybe less, and the need is fluctuating. Whats weird is that when I start it cold, the pressure is rock solid where it's supposed to be --- and will stay that way no matter how long I let it sit idling, until I drive the car and hammer it --- then it happens again.

Fuel system, back to front:
*Factory tank, modified to move the fuel pickup to a rear sump. (I still have the factory pickup (a new one with a 1/2" outlet port that I'm not using) in the tank just for the fuel gauge sending unit, but the fuel pickup itself is blocked off.)
*From the rear sump pickup point, -8 AN braided steel (about 2 feet) to a large RobbMc cartridge filter (100 micron).
*From the filter, probably another 4 to 6 feet of -8 AN braided line routed inside the rear and side of the drivers side frame rail and along the rear suspension crossmember to approximately the point where the factory fuel line would have connected.
*At that point it connects to 1/2" stainless hard line from Inline Tube that follows the factory routing where it appears again out of the same hole in the front crossmember where the factory line would have terminated.
*From there, again we go to -8 AN braided line to the input of a RobbMc 1100 hp mechanical pump,
*From the pump, more -8 AN to another large RobbMc filter connected directly to the input of (you guessed it) a RobbMc deadhead regulator
*The regulator and filter are mounted crosswise just above the water pump. From the regulator, -6 AN to the carb.
*The regulator has a RobbMc fitting for a vapor return line (even though it's a deadhead regulator) which is connected via -4 an to the repro Inline Tube factory style return line, back along the factory routing, and connected to the factory vapor return fitting on the tank via more -4 an braided line.

What I've done so far:
Pulled apart both filters and examined their bronze elements. Both were clean inside and blowing the elements out with compressed air didn't reveal any trash in them.
Rebuilt the regulator with a fresh diaphragm even though the "old" one looked fine and didn't seem to have any leaks.
Installed a second, brand new RobbMc 1100 pump adjusted to provide about 10.5 psi on the output side.
When I've caught it "doing it" after a test run while the fuel pressure is in the basement, I've hurried around and removed the gas cap on the tank to see if it might be a venting problem ---- no change, doesn't help - pressure still low.

I guess my next move is to drop the tank, remove the sending unit assembly and see if I can find something loose inside the tank that could be finding its way to the rear fuel pickup under acceleration. It's a job I don't really want to do, but at this point like I said, I'm stumped.

I guess maybe I could completely remove the filter elements from both filters and see if that changes anything... the only things left after that are the lines themselves.

Anyone ever hear of any AN braided lines that deteriorated inside and plugged up from the ethanol content in pump gas?

Another thought I just had... wondering if having the regulator/filter mounted where I do is allowing them to pick up too much heat from the water pump? I've got them mounted with a rubber insulated strap around the body of the filter and seems like the fuel flow would tend to keep everything cool but still...

Bear

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post #2 of 20 (permalink) Old 05-09-2016, 02:20 PM
 
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Dumb question, Bear...do you have the filter sock on the pickup tube in the tank? Maybe the suction at the tube is sucking up the sock?

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post #3 of 20 (permalink) Old 05-09-2016, 02:56 PM Thread Starter
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Actually, it's been so long since I put the tank together that I don't remember if I installed the sock or not. It's definitely something that I'll check out.

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post #4 of 20 (permalink) Old 05-09-2016, 08:12 PM
 
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Bear, sounds like the pump is sucking so hard and fast it cannot deliver the fuel when you hammer down. As a suggestion before you pull the tank take off one fuel filter and try then the other and try.......it may be to restrictive with that strong pump and two filters and that demand.....maybe just an in tank sock and one in line filter.....

Might just check before you pull the tank......fuel filters that are located on the side rail, say on your pickup are steel lines in and out...
As that pump draws the steel line cannot collapse and the fuel flows.....do you have a flexible line on the pump side of the rear filter?..

That would be a spot I would suspect....if you eliminate the filter there as a test a hose itself may not compress as the filter even a new one has some restriction to flow, and if that's demand from that 1100 pump is real strong....

Not sure just saying a place to check....you will get it good luck...

But it restricts on high demand......flow cannot keep up with demand....if fuel they say that rear filter cannot, then hose just ahead may collapse and pump sucks air and loses pressure.........maybe...
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post #5 of 20 (permalink) Old 05-09-2016, 09:27 PM Thread Starter
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There's not even one single inch of flex line anywhere on that car.... what isn't stainless hard line, is braided steel AN hose - even on the vapor return side. I might just try pulling the cartridges out of the filters just for grins though...






(I know the welds are ugly --- they were nice when I first did them, but they leaked --- so considering all the hoopla and nail biting I went through in order to feel reasonably safe about welding on a *used* fuel tank - twice - no way was I going to not get it sealed and have to do it a third time )

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post #6 of 20 (permalink) Old 05-10-2016, 02:40 PM
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Bear, have you tried disconnecting the fuel line from the carb and running the fuel into a bucket to see how much volume you get in a specific amount of time?
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post #7 of 20 (permalink) Old 05-10-2016, 11:03 PM
 
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What is the engine size, cam size, and carb; I had the same problem using similar components.


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post #8 of 20 (permalink) Old 05-10-2016, 11:05 PM
 
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Also, I would change the 90 degree fitting off the sump to a 135 degree one.


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post #9 of 20 (permalink) Old 05-11-2016, 07:11 AM
 
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Bear, I would suggest just try it with the rear filter and filter housing out and a line spliced in. You could rig up an easy screw in an line, because removing the element may or may not leave a possibilitity of an air pocket or Vacumn when pump pulls hard....

Also think young' suggestion on the 90 degree fitting is a good one....you will get it...you have diagnosed a lot already......

When demand is high and the pump sucks hard fuel cannot be delivered.....it still seems that two filters and a regulator, and maybe a sock on sender unit al combine to make the draw too restrictive....by removing that rear filter for a test some restriction is removed.

From looking at your front set-up don't think that heat would be that great there as your fan is directly blowing on that whole set up....
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post #10 of 20 (permalink) Old 05-11-2016, 12:21 PM Thread Starter
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I bit the bullet and pulled the tank last night. Much to my dismay, I found nothing obvious. I'm having a vague memory of something I might have changed after the fire that I'm going to check out later, to see if it's a real memory or something I just thought about doing at the time. With everything open, I did blow out all the lines with compressed air - they were all free and clean. I took the rear filter apart last night also and reassembled the empty canister with no element inside and blew compressed air through it, then did the same thing with the element installed --- no detectable difference in flow.
With the way I welded that sump on, there's still a chance there could be something inside there that I just can't see - I'm going to try to figure out a way to examine it more closely, but from the parts of the tank that I can see with the sending unit removed, it looks pretty doggone clean.




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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Exhaust, Suspension, Alignment, Fuel Tank and Brakes.

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