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Discussion Starter #1
How can my spider run so well when the tank is full, and then when it's low on fuel get a HUGE pressure in-tank build up, with poor performance? I've read all the pressurized fuel tank threads and tried chasing down all the fuel vapor lines, to no avail. Charcoal tank, check valve, one way intake, mini hoses to and from air filter and valve cover, all good. Changed the rear muffler although whenever I get this issue the gas tank is very hot to the touch. If I'm getting a hot tank and high pressure, what's the cause? Please, Alfisti AIUTO!
 

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There is at least one check valve in the vent system with an arrow to show flow direction. Make sure that it is in the right way. You should be able to blow through it in the direction of the arrow but not in the reverse direction. Another possibility is the the vapor recovery tube that runs the length of the car cannot handle the volume of air and fuel vapor that is trying to escape from your hot tank. You could try venting it at the rear of the car to see if it makes a difference. I think that you need a heat shield of some sort to stop your gas tank from getting so hot.
 

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Does your version have an in-tank pump? (The L-jet versions do.) It is a known issue in our cars that a failing in-tank pump or faulty stepped hose between the pump outlet and the fuel line (inside the tank) can lead to running issues when the fuel level is low.
 

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Discussion Starter #4
I verified the check valve, and the full length hose, both seem to operate normally. I tried drilling a small hole in the gas cap too, to release pressure. I thought about the heat shield, but don't all Alfas of that period have mufflers that pass between the tank and spare wheel well, and no heat shield is ever present?
I've already replaced the step down hoses in the trunk, but was not aware of a known issue with the in-tank pump, which I did have a problem with a couple of years ago. This sounds like a likely probable cause. Are the in-tank pumps still easily available? I'll call Centerline Tuesday and see, thanks for the info.
 

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With the fuel level low, check the output pressure of the in-tank boost pump. It should be about 3 psi. Unlike the main supply pump, you check the in-tank boost pump by deadheading the pump with the fuel pressure test gauge.

After that, probably worth pulling the fuel tank flange/pump assembly out and visually inspecting it. Symptoms would point towards a failing connector hose between the in-tank boost pump and the tank flange.
 

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memorse,

can you tell if it is running rich or lean when it runs badly? If it is lean then it points to the in tank pump (or lack of one). If it is rich it points towards excess pressure in the tank due to heat. IIRC, Alfa switched from a bottom tank connection for the fuel delivery tube to a top connection around 1978. The early top entry cars did not have an in-tank pump and there were problems which which led them to introduce it. I think that there was a technical bulletin to retro fit the pump. My '79 Spider did not have one when I bought it in '99 but it had already been converted to Webers and it worked OK without it.
 

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Easiest check is to remove the fuel tank cap and drive it without the cap when the tank is low. Does this change things? When you have been running it with the cap on and the tank is getting low, stop while the engine is idleing and remove the cap. Do you hear an air intake into the tank when you remove the cap? If you hear an air intake and the car immediately runs better, then you have a stopped up vent to the tank. If the tank vent is stopped up and a vacuum is building in the tank above the fuel level, then this vacuum is fighting the fuel pump against delivery of fuel from the tank.

On cars with high pressure fuel pumps like an Alfa, this vacuum will never become high enough (high vacuum gauge reading) to stop the engine from running. But it can make the engine run rough. On older cars with low pressure mechanical pumps, such a vacuum lock can get high enough (high vacuum gauge reading) to equal the output pressure of the pump. When this equalization is achieved, the fuel stops flowing and the engine shuts down. Removing the fuel cap will immediately allow the driver to restart the engine and continue driving. When this eqalization is achieved, the fuel pump pushes some gas into the fuel line and the vacuum sucks it back into the tank on the pump's recovery stroke. Equal push pull.
 

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Discussion Starter #9
To ljayr, I can't pin point the exact timing of those two events. I changed the muffler to upgrade to an Anza, but it was used and I became suspicious as the tank heat problem grew. I now have a straight pipe with small chamber passing through between the tank and spare wheel well, but the problem remains the same.
 

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Discussion Starter #10 (Edited)
To road trip, thanks, that sounds like a good way to narrow it down, I'll put a gauge on the pump and see, also it's been a while since I've pulled the in-tank pump to visually inspect.
To alfa particle, I can't see signs of rich or lean. No excess smoking from exhaust, spark plugs are only two months old and where tan in color when I pulled them out for inspection. My car has the Spica.
To vf31rhill, it's quite the opposite in my case. Driving with loose cap still results in a lot of raw gas smell, rough idle, and drive after heating up beyond about 45 minutes. However, after driving with cap on with low tank, when removed the cap is often forced out of my hand from the outward pressure, falling to the ground before I can catch it. If, on the other hand, tank is close to full, it will spit gas out while expelling rushing vapors and the tank makes an audible "burp", or buckle sound. After this event, when I go to start the engine, it will have a hard time starting and the low fuel pressure light will flash often several times with a very rough idle, before I can get it started again, fighting to keep from stalling. Eventually the light goes out and the car just runs very poorly again until I can give it a chance to cool down, after which it runs like a charm, for 45 minutes!
 
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