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Discussion Starter · #1 ·
I have a 62 mercury comet 170 I6. Drove fine after purchase for a few weeks. Did about 200 trouble free miles. Suddenly car started starving for fuel after driving at a consistent speed. Promptly got worse and worse until it would only idle, and the fuel filter sporadically bubbles empty, then refills, and repeat. Replaced fuel pump (twice), no change. Replaced rubber lines, no change. Replaced entire fuel line from pump to tank, no change. Replaced sending unit, no change. Everything from the tank to the carb is brand new. It usually runs fine for a few minutes but bubbles are visible upon startup and get significantly worse after a few minutes. Ive checked every connection, replaced every clamp, and do not know where else to turn. Always starts fine and idles fine even after running a long time on a hot day so i dont particularly suspect vapor lock. I have a video but i cannot attach it. Thanks guys!
 

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Discussion Starter · #3 ·
Yes i have checked that. I actually need a new cap lol its so vented that you can jiggle it around in the locked position, and on a full tank, gas comes out of the fill tube upon acceleration lol. Definitely does not create a vacuum in the tank and breather is not clogged either
 

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That’s pretty much what mine did when the screen in the gas tank clogged up from junk in the tank. I had to drop the tank and clean it out.
 

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There must be a restriction or a leak "somewhere".

You say you have replaced "everything" that could contribute to the problem....

... so time to start diagnosing.

Suggestions...

Disconnect fuel line from tank to pump, at the pump. Does fuel flow freely?
Can you blow (gently) in that line from pump to tank and hear air bubbling in the tank?

Same with line from pump to carb. Blow thru both ways, any restriction?

Kinks in line somewhere? Rubber or steel lines.

New rubber hoses, but maybe collapsed somewhere?
Shaved a bit of hose on install over the steel line?

Suspect crappy aftermarket fuel pump? Maybe 2 in a row NFG?
 

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Discussion Starter · #7 ·
I did drain and rinse out the tank with some gas when changing the sending unit. Had some crud but old filter on sending unit was not clogged, and both are now new. I did blow through the line and yes the tank does bubble. Both fuel pumps did seem to have no problem sending out a steady stream of gas when the out end was open. I have not checked the line to the carb but since the bubbles happen before the pump, and it doesn't leak and runs great otherwise, i dont suspect anything being there. I have the original steel line in tact with new rubber on the 2 inches between the tank and like, and like 6 inches from line to filter and pump. Ran a secondary rubber line all the way from tank to pump and filter just to see if maybe there was a leak i couldn't find in the steel line. Did verify none of the rubber is collapsing on either line. About to try mounting the electric pump at the tank. If there is any leak, i should be able to see it, and if it's percolating, the extra pressure might help. Otherwise I'll try to run a vapor separating filter and a return line, but would rather not tap back into the tank and risk losing some pressure. Good ideas? Thanks you guys, too. I really appreciate your expertise and ideas.
 

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Suddenly car started starving for fuel after driving at a consistent speed. Promptly got worse and worse until it would only idle, and the fuel filter sporadically bubbles empty, then refills, and repeat.


This does not sound like vapor lock to me.
 

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Discussion Starter · #9 ·
Well, update: after changing the Sending unit, the bubbling is still there and the car still occasionally hesitates, but it has gotten a lot better. Hasnt stalled out on me for about 50 miles, but i can tell the fuel pressure still drops every now and then when the filter gets bubbly, because it hesitates when hitting the gas randomly, and occasionally cuts out on the freeway for a second or 2 while driving at constant speed, but it always comes back and resumes working fine. Im going to get a pressure guage and might still try the vapor seperator, but for now, non ethanol fuel and all the other parts have gotten me back on the road. Daily driving it for now so ill keep yall posted on how it does. Thank you all for the advice!
 

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I have a 62 mercury comet 170 I6. Drove fine after purchase for a few weeks. Did about 200 trouble free miles. Suddenly car started starving for fuel after driving at a consistent speed. Promptly got worse and worse until it would only idle, and the fuel filter sporadically bubbles empty, then refills, and repeat. Replaced fuel pump (twice), no change. Replaced rubber lines, no change. Replaced entire fuel line from pump to tank, no change. Replaced sending unit, no change. Everything from the tank to the carb is brand new. It usually runs fine for a few minutes but bubbles are visible upon startup and get significantly worse after a few minutes. Ive checked every connection, replaced every clamp, and do not know where else to turn. Always starts fine and idles fine even after running a long time on a hot day so i dont particularly suspect vapor lock. I have a video but i cannot attach it. Thanks guys!
I have a 67 stang with the 200 had same issue to make a long story short, ended up replacing to a rebuilt carb. All good now. One other thing may be the type of hose clamps on your hoses should use a constant pressure type. Screw clamp leave a flat spot and cause a leak or vacuum to suck in air.
I have a 62 mercury comet 170 I6. Drove fine after purchase for a few weeks. Did about 200 trouble free miles. Suddenly car started starving for fuel after driving at a consistent speed. Promptly got worse and worse until it would only idle, and the fuel filter sporadically bubbles empty, then refills, and repeat. Replaced fuel pump (twice), no change. Replaced rubber lines, no change. Replaced entire fuel line from pump to tank, no change. Replaced sending unit, no change. Everything from the tank to the carb is brand new. It usually runs fine for a few minutes but bubbles are visible upon startup and get significantly worse after a few minutes. Ive checked every connection, replaced every clamp, and do not know where else to turn. Always starts fine and idles fine even after running a long time on a hot day so i dont particularly suspect vapor lock. I have a video but i cannot attach it. Thanks guys!
I have a 62 mercury comet 170 I6. Drove fine after purchase for a few weeks. Did about 200 trouble free miles. Suddenly car started starving for fuel after driving at a consistent speed. Promptly got worse and worse until it would only idle, and the fuel filter sporadically bubbles empty, then refills, and repeat. Replaced fuel pump (twice), no change. Replaced rubber lines, no change. Replaced entire fuel line from pump to tank, no change. Replaced sending unit, no change. Everything from the tank to the carb is brand new. It usually runs fine for a few minutes but bubbles are visible upon startup and get significantly worse after a few minutes. Ive checked every connection, replaced every clamp, and do not know where else to turn. Always starts fine and idles fine even after running a long time on a hot day so i dont particularly suspect vapor lock. I have a video but i cannot attach it. Thanks guys!
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