Fuel Issues

Ricky B

Ricky B

Wiiide Open
Joined
Mar 20, 2005
Location
LKN - Tha Dirty Mo (Mooresville, NC)
Been trying to fix my Dad's old 66 f100 for ever now . . . can't seem to figure out what's happening.

has a 390, stock, 4bbl intake and holley 600cfm carb, carb fairly new, couple years old

anyway it keeps loosing fuel pressure, will run fine for a while then just randomly loose fuel pressure, then just randomly will come back again

replaced the pump and filters . . . no help

pulled all the lines apart, blew em out with air compressor no obstructions . . .

checked the fuel tank, no junk floating around that could possibly be clogging the pickup

said screw it , put a brand new electric pump, with regulator and fuel pressure gauge inline, set it to 6psi, let her roll . . . .

runs for a while, then randomly starts loosing fuel pressure, runs fine again, then there she goes again, you can hear the elec pump cavitating too . . . wtfreak is going on




any ideas . . . I'm pretty sure there is nothing between the tank and the carb that I haven't replaced or checked more than once at this point . . . whats going on?


Thanks
 
Sounds like possibly the float in the carb is hanging up, in the closed position. Will hang up, use what's in the bowl, then it's out. Not a lack of pressure, but a lack of fuel
 
Gas cap isn't an issue for this year model

:confused: what if its the wrong one. or got pluged up somehow ? this will create a vavume over time.
 
upnover said:
Sounds like possibly the float in the carb is hanging up, in the closed position. Will hang up, use what's in the bowl, then it's out. Not a lack of pressure, but a lack of fuel
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burrellsjeep said:
Yep, then the float will drop back and the fuel returns and your back working until it sticks again.
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Would that cause the pressure to actually drop off (I got a gauge on it) and the pump to sound like its cavitating?


upnover said:
Easy to check that, run a hose from fuel pump into a gas can
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GubNi said:
Had the same problem and the pick up line in the tank was rotten and mostly clogged.
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yea well like i said I popped the sending unit off the tank several times to check the pickup, looked clean as well in the tank . . . also blew compressed air through the lines to make sure nothing was in em - but I'ma try the fuel line into a gas can thing, didn't think bout that, will at least tell me if the tank or fuel line coming from it is the culprit probably
 
Yes it would, Because its stuck it acts as a plug in the line.

Example of what I think it is

apeswiki.com_images_0_07_Carburetor.jpg


When the float reaches the top of the carb the jet shuts off the supply of fuel from the pump, Then float sticks and the fuel in the bowl burns off and the floats will slowly fall back down allowing fuel to return to the bowl.

At least that would be my guess. Of course I am basing this on my experience with smaller engine carbs and not a 4 barrel carb.
 
JR, my dad has a 1974 f-100 that had the exact same issue. It turned out to be a rotten line. It has to be somewhere between the pump and the tank. That's why the electric pump cavitated, it started sucking air. Does it happen when the gas tank is full? Or when the tank is half full, etc... My dad had to replace the guts in the tank and the problem was fixed.
 
When the float reaches the top of the carb the jet shuts off the supply of fuel from the pump, Then float sticks and the fuel in the bowl burns off and the floats will slowly fall back down allowing fuel to return to the bowl.

At least that would be my guess. Of course I am basing this on my experience with smaller engine carbs and not a 4 barrel carb.
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Hmm, cool didn't know float could do that, problem is I already took the carb apart to see if there was any junk in it, everything seemed to be moving/functioning, and it was super clean inside . . .

JR, my dad has a 1974 f-100 that had the exact same issue. It turned out to be a rotten line. It has to be somewhere between the pump and the tank. That's why the electric pump cavitated, it started sucking air. Does it happen when the gas tank is full? Or when the tank is half full, etc... My dad had to replace the guts in the tank and the problem was fixed.
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Tank is at like 1/4 fuel level, think he filled it to 1/2 at one point but didn't make a diff . . .

I mean if the tank is bad wouldn't it look rusty and have stuff floating around on the inside?

Like i said i'm gonna try the gas can thing here shortly
 
Are you sure that on of the rubber lines is not collapsing?
 
Caver Dave said:
Just blowing air through the line(s) won't show you a leak in the SUCTION side (between pickup & pump)... only that there's no blockage.
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Yea, its been set to the side now anyway, both my dodge and my brothers dodge are both broken so were gonna fix them first, but I'm thinking its a line, the tank looks super clean

lugnut said:
Are you sure that on of the rubber lines is not collapsing?
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nah, I'll have to double check that, but I mean I don't think so
 
Sice you've go through everything from the tank up, stick another carb on it and see what happens. I had alot of fuel issues with the 460 in my 74, took me awhile to figure it out.
 
bh1075 said:
Sice you've go through everything from the tank up, stick another carb on it and see what happens. I had alot of fuel issues with the 460 in my 74, took me awhile to figure it out.
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Had alot of problem with a 351w/boss in my 73 f 100....rebuilt the carb and everything and was still having a problem...threw a different carb on there and i aint had a problem since. I know carbs are exspensive i went through the same stuff replaced this after this and bthen that. but that is what did it for me..i had a holley 4 bbl. threw on an edelbrock and aint had a problem since. donno how you feel about edelbrock but its worth a try
 
I absolutely hate all things mechanical


Finnaly had time to work on this thing again, So literally everything has been replaced in the fuel system in the ford now and guess what . . . still doin it, so I go back to the vaporlock thing - let it idle till it starts to get hot hot and loose pressure . . . take a garden hose spray the fuel lines and lookie there, pressure springs back everytime i spray her off

So I go around and make sure no lines are touching anything hot, buy hose sleeves and heat tape and cover the crap outta the lines and pump . . . . still heating up and loosing pressure the lines are all away from anything hot except of course the hot air off the fan . . . which I think is whats doing it, but its not avoidable, close the hood on the old steel 66' body and the engine bay becomes an oven even though the motor never gets hot

think it would help if i took the mechanical fan off and put like a Taurus fan on there to get more airflow?

Or should I keep wrapping the everlovin you know what out of the fuel lines with heat tape and sleeving stuff?

or other ideas? the amount of heat the dang engine bay retains when its runnin and the hood is closed is retarded . . . but then again its kinda odd that this is a 66 and has never had this issue it whole life untill about year ago


Once again I hate all things automotive :kaioken:
 
If its truly vapor lock I have been told by many old times that wrapping the fuel lines wont help. They all swear clipping wooden close pins to the fuel line is the best cure.

I went back and read the thread again. So fromback on the first page it looks like it will run fine from a gas can? If so that eliminates every thing on the pressure side of the fuel pump look at the suction side. Replace any rubber fuel line. I have had more fuel line problems in the last couple years than I have in my whole time of wrenching. Keep the length of rubber line to a minimum. This will help keep your fuel cooler (helping prevent vapor lock )and also help to ensure that the fuel pump isnt sucking the line together.
 
Wooden clothes pins on the line goin to the carb. Old hot rodder trick for curin vapor lock and it works. The clothe pins act as a heat sink. Also replace all rubber fuel line even if they look good on the outside. Could be colapsed on the inside and if nothing else its cheap insurance.
 
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