Bad running, hesitation at revs and air in fuel line HELP

mattbibbey

mattbibbey

Well-Known Forum User
As the title suggests, I have a problem. When I first start up, it's fine. It goes through the revs ok and sounds fine. After a while, 5-10 mins perhaps ( but sometimes much longer), it starts to retard at around 3500-400RPM and won't rev any higher. After a while, the rev limit gets lower. Then it begins to sound as though it's running on only a couple of cylinders and the power is testement to that. Then it will cut out.

This happened to me the other day in a car park in abingdon and I walked home and came back with a spare fuel pump. I bled through the fuel and it was fine getting home. But today it happened again and I barely made it home. I put the choke on (to give some revs) and looked at the engine where I could see air bubbles in the fuel line coming from the fuel filter. (That fuel line is clear in my car) I have changed the fuel filter for a brand new one and the air persists.

I'd really appretiate any help! Maybe you know someone who has had this same problem with their 240Z
 
That sound almost exactly what was happening to mine not long ago. I tried loads of things included changing to an electronic ignition, but the thing that eventually cured it was a new fuel tank (the old one was full of debris) and a flush out of the fuel lines and carbs (and replaced any lines that looked iffy). It hasn't happened since.

HTH
 
So if it is an old fuel tank, how does it cause air to get into the fuel system? I suspect that my fuel lines are old and require replacing. Can I just use any generic fuel line kit and make it fit right?

I'm still open to other suggestions on my problem, although I do plan to replace the tank and lines anyway. It's not going away any time soon haha.
 
You can replace the fuel lines easy enough, just get some piping and a flaring tool to make the ends the correct size. Then attach the relevant connectors and "hey presto"

Fuel tanks often corrode around the centre join (where two halves are stuck together) this will cause fuel leakage, or air intake, depending on fuel level. Tanks are like Easter Eggs, two halves stuck together, although sometimes the easter egg join last longer.

EG. I had a '79 toymota had the same problem, if putting more than £20 fuel in, (which was a lot of fuel in 1997) it would leak from the central seal

EG2. Import Bluebird brake lines replaced with standard copper piping and generic couplings. Ends flared by Jason (My mechanic that Canic)

Hope this helps
 
Agreed. My fuel tank has seen better days and it does leak but, at the moment anyway, only from the point where the filler pipe joins the tank. It's not fitted right. The fuel tank isn't a pressurised system though so I'm not sure what the hell is going on.

It's fustrating to have such a great, iconic and gorgeous sports car that I can't trust to go very far!

Thanks for the advice so far, I'm already trying to find a new fuel tank.

Matt
 
Certainly concur with the rusty tank and/or fuel lines problem ( been there, done that ). Flakes of rust tend to get carried up the hard lines towards the front of the car, and can get lodged in the bends. I cut all the hard lines off my first 240Z, and dissected them to investigate the causes of poor flow. Found long strands of rust flakes had built up inside the lines that no amount of back-flushing, airline pressure blow-throughs or hopeful raking with thin wire had cured.

Cleaned tank, new fuel lines, no further problems.......

mattbibbey said:
....I'm already trying to find a new fuel tank.
Click to expand...

Matt,
I have a super clean 1971-dated fuel tank available if you want / need it.
 
mattbibbey said:
When I first start up, it's fine. It goes through the revs ok and sounds fine. After a while, 5-10 mins perhaps ( but sometimes much longer), it starts to retard at around 3500-400RPM and won't rev any higher. After a while, the rev limit gets lower. Then it begins to sound as though it's running on only a couple of cylinders and the power is testement to that. Then it will cut out.
Click to expand...

Just to add that this scenario rang a bell with me.

What I found was that part of the fuel line blockage was 'mobile', and initial start up and running ( with the 'mobile' part of the blockage still in the line down near the tank ) was OK. Then the 'mobile' part of the blockage was travelling up the hard line - carried by the fuel flow - until it came up against a partial blockage further up the hard line. At that point it would become lodged against the blockage, and restricted flow to a dribble. Turning the car off for a few hours, the mobile blockage would somehow travel back down towards the tank - releasing the flow.

Couldn't diagnose any of this until I took off the hard lines and cut them open........
 
Yeah, these last two posts certainly ring true to my problem but the thing that's making scratch my head is how the air has got into that system? I don't have any fuel leaks other than the one I mentioned when I fill up. Surely there cannot be nearly enough negative pressure for the fuel to cavitate, can there?

I noticed today that when the problem starts, the fuel filter is about half or just under half full of fuel. Not sure what this signifies but it must be relevant??!!?

I will replace the fuel lines anyway because they don't look great. I'll probably go down the route of modern, braided fuel hose.
 
Done and DONE!!!! Last night I spent a couple of hours replacing the fuel tank and fuel lines. I got hold of a very good fuel tank with very light rust on the surface from Alan Thomas (thanks very much). I took it back to bare metal with a softish whizz wheel and then painted it with some kind of nitrocellulose lacquer. Then covered the top half of the tank with Dinitrol 4942M, which the Man truck guy gave me on my camp. I used about 3.5 meters of braided fuel line that I bought from Mike Feeney at MJP Auto. I drove the car today for about 35 minutes and had no dramas like I would usually! It would appear that Albrecht was bang on with the diagnoses. Hopefully the previous problem will not manifest again! HOORAY!!!!!

Matt
 
Right, so I may have been a little premature. There is still some air in the fuel bubbling out of the fuel filter! Not enough to cause me engine problems but it doesn't look right. Often the fuel filter is only three quarters full of fuel but I'd heard that was common.
 
mattbibbey said:
Right, so I may have been a little premature. There is still some air in the fuel bubbling out of the fuel filter! Not enough to cause me engine problems but it doesn't look right. Often the fuel filter is only three quarters full of fuel but I'd heard that was common.
Click to expand...

i have never had a full fuel filter and i have a Redtop electric fuel pump so nothing to worry about there
 
I agree with Moggy my filter always has air in it. When I first had my blocked fuel filter problem I thought that the air bubbles may be the problem i.e. I had an air leak - but no just a blocked filter.

The air is a bum-steer.
 
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