Hey everyone, I have a 67 500 F model with an unusual fuel issue. Seems 'air' is getting into the line and bogging the fuel pump. I'm stumped after I 1) replaced the carburetor, 2) replaced the fuel pump, 3) checked the gas cap for air flow, works fine, 4) checked the sending unit, its a recent replacement and in very good condition, 5) checked inside the fuel tank, spotless clean like new inside, 6) replaced the entire fuel line from the tank to the pump and from the pump to the carb.
After all that I am still having an issue of air in the line. The car runs fine down the road but then suddenly without warning will start to 'buck' on and off like the engine has the hiccups. The only visible problem is always the same and that is air in the fuel line. Other things I did to the car were change the head gasket because the old one was bad and leaking, adjust the valves, replace the old distributor with an electronic unit, replace ignition wires and plugs. I'm at the point where I don't like driving the car any more. Any one have any ideas what this insanity is about? its just to simple of a system to be giving me this much trouble. BTW I also have several 66 cabriolets. Thanks. Bone from NY

Check base of carb and spacer for flatness. Spray some carb cleaner around base of carb and run the car. If the cleaner gets sucked in that is where the air is getting in

Hi Bone and welcome.

How are you diagnosing that you have air in the fuel line? You say it is something visual?

Tony

Probably has transparent fuel line from pump to the carb. I have the same issue with bubbles but no problem with operation except that sometimes fuel line gets empty, fuel returns back when engine is off but I think that I will overcome this when I put a new gasket set on my carb. Are you sure that those hiccups are not electrical? Is it backfiring also? I will recommend a new fuel pump even if the one you have is new.

I immediately thought the same thing as Paolo. The Bakelite spacer at the bottom of the carb is prone to warping.

gordinir8 said:

Probably has transparent fuel line from pump to the carb. I have the same issue with bubbles but no problem with operation except that sometimes fuel line gets empty, fuel returns back when engine is off but I think that I will overcome this when I put a new gasket set on my carb. Are you sure that those hiccups are not electrical? Is it backfiring also? I will recommend a new fuel pump even if the one you have is new.

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That's what I was guessing but it's perfectly normal for it to be bubbling away like a cauldron. If you have a fuel filter sometimes it's empty, sometimes it full, sometimes it's like a bubble bath so it isn't anything to get overly concerned about.

I to am thinking possibly Electrical? I hope Bone hasn't gone off on a tangent and it is something electrical like a dodgy coil breaking down internally.

Someone wrote a good quote the other day. I can't remember it fully or who it was but it was something along the line of 80% of fuel related problems turn out to be ignition faults!

Tony

I had a dodgy ignition switch that caused similar symptoms

1500500, thanks for the note, the base of the carb is also brand new and I was extremely careful about placement due to it being in near contact with the head gasket. The air I am referring to is in the fuel line which I did install a clear line to see what was going on. I appreciate you input.

paolo66, typically I use a small propane bottle, carb cleaner would just make a mess. If the propane gets sucked in it will speed up the motor which is an easy diagnosis. Thank you for your input.

Vitesse, Gordinar, yes it is a clear fuel line, at times it is totally empty and the car runs like it is running out of gas.
I will add this, the vales are in need of a reset. You can hear a small popping on occasion when the car is at idle, it is the sound of the back fire into the intake. The old carb had so much carbon built up that I was surprised it was still in running condition. I am taking the head off this weekend for a valve job to be done and the only other electrical thing I could replace is the coil which I will also be doing this weekend.
I know it can be the valve but I wanted input on this fuel issue, seems there should be no air in the line. Its to simple of a system, 5.8 litre tank, sending unit, 3 1/2 meters of fuel line, pump, line, carb. Too simple lol. Any way thank you for your time and input.

Luxe, even if it was the spacer, how would that allow air into the fuel line? At that point there is a vacuum there that would prevent air from going backwards not to mention the stop valve in the float bowl.

Hmmmm I wonder, I will have to take that apart and check it for operation, I did think at one point it could be electrical. Thanks for the note.

Don't get hung up on air in the fuel line. Like others have stated it is a thing for the 500. I for instance have a filter installed right after the sending unit near the tank. During operation that clear fuel filter is 3/4 full of air. If you are convinced it is a fuel delivery issue, as all the hosing has been changed, the only other possible culprit would be the mechanical fuel pump.


I mentioned the spacer because if it is warped, under acceleration the engine will act as if it is starving for fuel as air is sucked under the carb. Several people here have complained of a similar issue due to either warped spacer or warped carb base. Hope you find the cause soon.

I had a similar issue, mainly it would try and stall on low revs or idle.
First off I would check your valve clearances. If they are too tight it will cause the engine to try and stall.
If that is all ok then if you changed the pump then its worth while stripping it apart to make sure there isn't any dirt or small piece of rubber from the hose blocking the filter. Easily done when refitting a hose.
Did you check the length of the push rod that drives the pump? that gasket size is important. If there is not enough sticking out you wont get the full movement of the pump.
Also check the small filter on the end of the fuel sender pipe.
What type of fuel hose did you use? if its too weak it could be collapsing. Have you checked there are no kinks in the pipe?
Failing that its either carb related or ignition (loose connection)

As this seems to be such a common problem, could be why Fiat introduced the fuel return pipe from the carb back to tank on the 126?

Peter

How does the fuel get out of the tank? If the pipe just exits at the bottom, this should not present any issues, but most will have an internal pipe lifting the fuel, then exiting nearer the top into the flexible line. Could the internal pipe be cracked?

There is a filter in the sender unit

66cabriolet said:

I am taking the head off this weekend for a valve job to be done and the only other electrical thing I could replace is the coil which I will also be doing this weekend.

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If you take the head off and find some spare time please take some pictures and post them perhaps on a separate post, it is good to have lot of info for the newcomers like me.

Yes sir I am aware, I did take the unit out as an initial instinct, the unit appears to be 'newer' than the original, the tank is pristine clean and the unit screen is clean. Still, I bought a brand new one I will replace it with for my own sanity lol.

Franko, yes I checked everything you mentioned and no issues with the sender, the fuel pipe (cavas) and the pump pumps allot of gas.
I have set the air in the line issue aside for the moment and am focusing on the head valves and the electrical. I know the intake valve on one of the cylinders is leaking, it spits back into the carb at idle on occasion and the clearances are all good (checked twice).
Yesterday oI took the car for a 2 hour drive. In that time the problem happened just a few times so I tried a different tactic, I actually shut the key off and for 2 seconds and then turned it back on, the problem for the moment went away so, I am going to go through the ignition system starting at the source, the battery lol. I will chase the wiring all the way through the ignition switch to the coil. It is simple enough and will take little time to determine the problem if there is one. I am thinking of just running an entirly new wire from start to finish to rule out grounding issues too.
The distributor is a brand new '123' type electronic, but the problem was there before I replaced it so that is a no brainer. Spark plugs and wires are also brand new. Wire to the coil from the key switch I will replace. I will also take the key switch out, apart and assess for wear.
Fun car but man does it suck up time to tinker with lol.