Vapor lock in fuel filter?? Hardi pump carb car.

Ok trying to see if anyone else has had this happen. 1977 308 GTB. Has new strainer in the tank, new fuel filter, new Hardi pump, all new lines, with proper sizing. None of the check valves, just plumbed like the euro line routing diagram. So tank to filter, filter to pump, pump to carbs, then return to right tank. If I start the car and let it run all is good. However if I shut it off and let it sit a bit when hot, the Hardi pump races like it is not getting fuel on the suction side. So my assumption is that the fuel in the filter is boiling and the pump is preventing those vapors from getting by. Does anyone else have this? For a diaphragm pump like the Hardi, should I move the stock filter to be after the pump? It seems quite a few people use these pumps so someone must have run into this before I would think. Thanks!


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Thanks. This is interesting. I had tried installing mine this way initially and it left me stranded on the side of the road. As soon as the oil got to temp I had fuel starvation and it shut off until it cooled down. So yours is plumbed tank to filter, filter to pump, and then dead headed at the carbs? Do you have any of the US anti roll over plumbing at your pump or just simple like the Euro cars? I must be doing something different but I’m not sure as to what. Thanks


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Thanks! This is very interesting indeed. I tried plumbing mine like this and had fuel issues. The only difference is I had the evap connected. I wonder if I’m getting a vacuum in my tank causing the pump racing issue. Since you got rid of the evap, how did you vent your tank? A vented cap?


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The only thing is I replaced all the lines in the evap system and cleaned the three way valve. With the valve I can easily blow through it in the direction toward the tank and with a bit of compressed air it opens to blow towards the canister which I think seems right. The issue I have is when the car is off and hot. So that’s why I was thinking the fuel in the filter is boiling from the heat of the engine compartment while the pump is off. On hot start the pump races and takes quite some time to prime, but once it does it slows back down and all seems ok. It’s a very frustrating issue.


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Interesting. I didn’t think the US cars had any way to vent there. I thought only the Euro cars had an ability to vent in the neck. I’ll have to take a look at that on my car.


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Thanks for the help. I’ll have to checkout the overflow option possibly. I did not realize this was on option on a US car. I guess will dig into this sometime this weekend as well as checking out the charcoal canister itself which I believe is original. My car has been a nearly three year restoration process as it was a barn find when I got it, was off the road for thirty years. So I’m always finding little things to go through now that I have all the major things done.


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Quick question for you both. When you added the vent using the overflow on the filler neck, did you simply put a T in the line so the the neck still connects to the tank and then add a hose to that T that drops down below the car?


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My overflow from the filler is plumbed into the large top vent connecting the two tanks. So the only way for me to open this to atmosphere would be to put a T in the line so it still connects to that balancer pipe. Unless you are connecting to something else. But that’s the only fitting on my filler neck


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This is where I added a T and then a hose to vent.


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So quick update. Unfortunately after opening up that vent my problem persists. I can tell the tanks are venting freely now as I blew into the vent I created and there was no restriction and I could feel it exit back out when I stopped. So that is off the table as the source of the issue. I also blocked the return line again and the Hardi pump is much happier. It no longer races and builds 4.5 PSI pretty consistently. I was able to run the car in the garage and let it get both the coolant and oil to full temp and could rev the car freely sitting in the garage. So I don’t think it’s a heat or vapor lock issue as sitting parked in a hot garage I would think would be worst case scenario and it was fine. But I tried taking it out on the road and under load it was a no go. Since it was warmed up i got on it a bit and it pulled hard and cleanly all the way past 7k in the first three gears. I was excited about this but then it fell completely flat on it face and acted like it ran out of gas. I let it sit a minute and restarted it. It was fine a little way down the road and then acted like it ran out of gas. I got it home and let it run in the garage for another 10-15 and it ran fine there. I’m suspecting the fuel filter. However both the filter and the tank strainer are brand new. I was also considering ignition but I wouldn’t think I would get those few nice pulls past 7k if it was an ignition issue. It’s very maddening.


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