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post #1 of (permalink) Old 01-11-2000, 01:03 AM
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Fuel starvation?

I am new to this thread but have been referencing previous entries
trying to solve failures I have been experiencing. Any specific
analysis would be appreciated as I am exhausting my own ideas.

The truck is a 78 Bronco with a 460 swapped in by a previous. The fuel
pump is mechanical and is located right on the block. The truck has
been missing much like you
would expect from fuel starvation. It tends to occur only at highway
speeds but not exclusively. As you ease off the pedal the engine begins
to run smoothly again but faced with a hill or acceleration the problem
will generally arise again. It has been an issue here just since it got
cold and snowy but i do not think they are related.

I generally am a fan of replacing entire systems on old vehicles just to
stop the guessing game but in this case I have gone one step at a time.
First I replaced the fuel line front to back and the filter. Seemed to
be fixed but once on the highway for 45 minutes it started again. Next
I tested the fuel pump on the bench and it seemed to be fine 4 to 5
pounds pressure. I put a new one in anyway thinking it was well worth
it if it worked. Nope. While I was at it the tank seemed due so I
replaced it while I was in there.

I did start with the notion of vapor lock and ran the truck without a
cap checked for leaks etc. but this did not seem to work.

The engine does not ping when this occurs. As you push the throttle the
engine failure increases. Generally the problems seem to arise after
the engine is warm or has been running on the highway for some time,
although not always. Finally it is worse under a load situation such as
a hill or acceleration.

The only thing I haven't replaced is the intake that reaches into the
tank itself.

Any ideas?
Finally the engine has been stripped of some of the pollution and vacuum
hoses but these do not seem to be the issue because plugging them does
not solve the engine problems while it is occurring.

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post #2 of (permalink) Old 01-11-2000, 11:18 AM
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Re: Fuel starvation?

A plugged catalytic converter can give you those symptoms.

post #3 of (permalink) Old 01-11-2000, 12:35 PM
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Re: Fuel starvation?

Just "Blue Skying" some ideas here, don't replace anything without testing it 'cuz of me!
I assume that there's no computer on it, only an ignition module, carb is a big 4 barrel, and ignition system is in good shape.
1) Converter partially plugged as Norm Speed mentioned - Easy to test with vacuum gauge by running engine at high (~2500RPM) idle for a few minutes to see if intake manifold vacuum falls off vs. time due to pressurizing exhaust system.
2) Fuel Pump - What is pressure mounted on engine? (thinking if cam lobe for fuel pump was rounded off, press. would be low installed).
3) Crud in tank floating around and getting sucked up against "sock" filter on tank pick-up - I had terrible trouble with a Olds 455 once because of that. But you have new tank, and if the pick-up was crud-covered, would think you would have noticed.
4) Fuel Filter - Is it an in-line between pump and carb? The big one with hose fittings? Picking up heat from the block? Could try insulating the outside of hose with some insulation with reflective-foil on the outside. I have used pipe-wrap with a foam core, heavy foil on outside. Not fire-retardant, so keep off of the exhaust manifold!
5) Carb Venting - Was there a carb bowl vent that's not connected or plugged now? (If heat from engine vaporizes gas in the bowl, it needs to get out, or it can fight against the pressure of the fuel pump trying to pump liquid gas into the bowl).
6) Heat in general - With a big-block in there, is radiator larger to handle? Or is it really cooking? And any fuel line subject to high temperatures can become a gas generator. My big-block Olds had a 3 port fuel pump - 1 in, 2 outs. 1 out was a fuel bleed to a line back to the tank. This cycling of fuel kept the pump cool (relatively speaking). Worked great, would not vapor lock even idling for 30 minutes at 98 outside, stuck on expressway @ 0 MPH, with A/C on!
7) Gas tank and/or gas cap venting - No, you covered that by trying without the cap.
8) Here's an idea all the way at the other end of the spectrum! What about carburator icing? Is there a working air pre-heater, and a working flapper valve and temp controlled vacuum in the air cleaner? Tube connection from pre-heat exchanger on exh. manifold to neck of air cleaner in good shape?

These are a few things that came to mind quick. I'll add to them if I think of more in time.
Everybody else, join in if you've got some ideas!

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post #4 of (permalink) Old 01-12-2000, 02:13 AM
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Re: Fuel starvation?

Float height measurements is about the only other thing that I can think of. The other thing is point gap. Does it have points? Does it run fine accelerating heavily from low speed?

[img]/wwwthreads_images/icons/cool.gif[/img] MG48 - 88 Bronco
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