Vapor lock, accelerator pump or?

Mn mopar · Jul 16, 2018

Ok so im having issues with my 1980 mirada, after driving id say 45 minuets to an hour or so under acceleration over about 10-15% throttle car acts like im almost out of gas ( tank does not have baffles) but i know i have gas in the tank as the gage works, or if it is a hot and humid day will happen sooner then that, now im not sure if im dealing with vapor lock or something else, fyi before i started driving the car i replaced all fuel lines the pump the filter had the carb rebuilt new sending unit and cleaned the inside of the tank so now im coming to you guys yet again lol, one question is if it is vapor lock can i add a low pressure electronic fuel pump by the tank and keep running the mechanical fuel pump or just ditch the mechanical one and make a delete plate for that?

Joe12459 · Jul 16, 2018

Vapor lock sounds likely. The electric pump will usually fix the problem. I'd remove the mechanical pump. You can run the electric pump right through it, but if the diaphragm ruptures you'll fill the crankcase with fuel.

Oldiron440 · Jul 16, 2018

Or a week fuel pump.

Mn mopar · Jul 16, 2018

Can i run both pumps with the electrical as sort of a back up?

Duke5A · Jul 16, 2018

Take the return line and put a kink in it as it comes off the fuel filter. Take it for a drive and report back to us.

Mn mopar · Jul 16, 2018

Duke5A said:
Take the return line and put a kink in it as it comes off the fuel filter. Take it for a drive and report back to us.

Ill give it a shot tomorrow, what will that test tell me?

M_Body_Coupe · Jul 16, 2018

Duke5A said:
Take the return line and put a kink in it as it comes off the fuel filter. Take it for a drive and report back to us.

Duke5A,
Yeah, I'm with 'Mn Mopar' on this one, what are you suspecting?

My thinking is actually quite the opposite. The intent on having the factory return line is to provide an 'escape' path for fuel vapours that may be accumulating in the line past the fuel pump. As such, I would be most worried about this line becoming plugged and preventing any such vapour runoff from occuring. The end result could be what is being described, ie a sudden 'engine goes flat' feeling.

I would suggest that 'Mn Mopar' stick a rubber line on that return line, gently push some air past it and see that the line is clear. Do it while the car is not running and as a test-only procedure. If all is good, re-connect, most likely the problem lies somewhere else.

ZieglerSpeed · Jul 16, 2018

Hi to Milaca! My last girlfriend was from Onamia. I'd run a low pressure electric back at the rank positioned low at the bottom of the tank. The mechanical will work as a fuel pressure regulator. On my street strip muscle cars I would run 3/8 line, a Carter race electric and a Carter race mechanical. Drove those cars many thousands of miles when I would race them -no trailers or haulers. NEVER a fuel problem. Not at 6500 R's either

ZieglerSpeed · Jul 16, 2018

The problem didn't start after the work did it? -double check float settings?

ZieglerSpeed · Jul 16, 2018

take the "O" ring off the gas cap

droptop · Jul 17, 2018

X's2 on the float level.

ZieglerSpeed · Jul 17, 2018

You have really covered the bases

ZieglerSpeed · Jul 17, 2018

Sorry, thought you were MN Mopar

Duke5A · Jul 17, 2018

Long time ago I had infamous vapor lock problem on my Fifth Avenue. This predated my modding the car, so it still had the Leanburn 318 in it. Once it got up to temp you would have to progressively give it more and more throttle to maintain speed until the point where it wouldn't go any further. Let it cool down and repeat...

Changed out the Leanburn to a 4bbl. Involved replacing the intake and ignition, along with tossing a new pump on for good measure. Problem still persisted.

Honestly, I don't know why it fixed it. I just went on for another year with the return line kinked until I dropped a 360 in the car. Stopped using a return after that point on altogether. I always surmised that there was a hole in the steel line somewhere and it could have been sucking air.

All it will cost you is two tie wraps. Fold the rubber return line over on itself as it comes out of the filter and secure it with tie wraps.

ZieglerSpeed · Jul 17, 2018

Ahh, this might not help you, but whenever I replace the rubber lines in the rear by the tank or in the front by the pump, I use the expensive fuelie hose (50 psi)

volare 77 · Jul 18, 2018

It doesn`t hurt to start with the easy stuff first. Make sure all the rubber lines are new. I had this bite me more then once. On my F body I forgot about the short rubber line at the bottom of the fender/rocker area.

Mn mopar · Jul 18, 2018

volare 77 said:
It doesn`t hurt to start with the easy stuff first. Make sure all the rubber lines are new. I had this bite me more then once. On my F body I forgot about the short rubber line at the bottom of the fender/rocker area.

Yeah i did notice that one yesterday when i was under there looking at the lines, think im just going to replace the whole fuel line back to front with the electric pump this weekend

AJ/FormS · Jul 22, 2018

ZieglerSpeed said:
take the "O" ring off the gas cap

Yeah it sounds more likea failure of the venting system. Next time it happens ,pull over and crack the gascap. If you hear a whoosh as the air rushes into the tank , baBam, that's it. Fix the venting,

ZieglerSpeed · Jul 22, 2018

When I went to the Chrysler race seminars, Tom Hoover and Larry Shepard would tell us to get rid of the gas cap seal so we don't suck the gas tank into the 4 barrel

M_Body_Coupe · Jul 22, 2018

ZieglerSpeed said:
When I went to the Chrysler race seminars, Tom Hoover and Larry Shepard would tell us to get rid of the gas cap seal so we don't suck the gas tank into the 4 barrel

...you know, that kind of floors me because all you have to do is run the tank vent line into the charcoal carnister and have a full-time venting situation going...

Now, when it comes to actual carb vapour lock that's a different situation. I was experiencing a hard to re-start problem with my TQ until I plumbed the vent line into the said charcoal box, now all is splendid!

Vapor lock, accelerator pump or?

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