Boiling fuel?

[240ZMan]

Some time ago I posted this thread

http://www.classiczcars.com/forums/showthread.php?t=10041

but never resolved my issue. Basically whenever the under hood temperature rises, my engine begins to stumble at idle. This happens at moderate temps, such as today when the air temp was about 60. If I let the engine idle for a few minutes the problem begins. Today I discovered that when the problem occurs, I can see bubbles coming out of the fuel filter mounted on the right fender. (The filter is clear plastic.)The fuel level in the filter is almost at the bottom of it. I'm running an electric fuel pump only (no mechanical), although I used to have the same problem when I used the mechanical one.

If I leave the hood up for a few minutes the bubbles go away, the fuel level in the filter rises to the top, and the engine idles fine. I'm pretty sure now I have a vapor lock type situation, but what has me confused is what is heating the fuel on that side of the engine. I read about people in Arizona driving their carburated Z's in August without vapor lock, and I'm having problems in March in Colorado!

FWIW, I've confirmed that the fuel return is indeed working as I can hear the fuel coming back into the tank if I remove the fuel cap and put my ear down.

This problem has been with the car since I got it almost 2 years ago. Since then I've put a new motor in, rebuilt the carbs with Z-Therapy bodies, changed all the fuel hoses (not the metal lines though), and the problem remains. I even bought a second set of carbs on eBay and the problem remained. At this point I'm thinking it must be a fuel system problem, but I am out of ideas of what to do to prevent the fuel from boiling BEFORE it crosses over the enginer and gets to the really warm side!

Oh, since my original post I've also put a new 3 row radiator in, and replaced the fan clutch. Other than this issue I don't have any heat issues.

Any suggestions?

[PrOxLaMuS©]

well.. you could do maye these idea's...

go out and buy a temperature guage.. and tap into the line.. or at least as close as you can before the fuel enters the line.. maybe you can see how hot the fuel is before it enters the carbs..

OR.. get a fuel pressure gauge...

naturally as any liquid gets hotter.. it expands.. especially if it literally boils...

so if the pressure is too high.. or above factory specs.. it could be the fuel pressure regulator.. or the fuel is expanding or heating up too quickly..

that's weird its acting up in March here in Colorado.. it's been soo damn cold lately.. finally cleared up today though...

anyway.. let us know what happens

[240ZMan]

I did run with a fuel pressure gauge spliced in just after the filter for a few weeks. The pressure seems to stay the same, about 2 - 3 psi all the time. It's a mechanical gauge and I wonder if there are spikes that it doesn't pick up. I'm not running with a pressure regulator.

I toyed with buying one of those IR temp readers, but for nearly $100 I was hoping to avoid it, not that I wouldn't like to add it to my tool collection And assuming it confirms the fuel is boiling, I'm back to the question of why does it happen on realitively cool days?

[PrOxLaMuS©]

hmm.. i'm very curious about this too..

what kind of fuel grade are you running? I doubt despite the "octane" they would have diff. boiling points... but very strange..

anyone know?!

[240ZMan]

I've had the problem with both regular and premium gas across different brands as well.

I did a little looking for the boiling point of gasoline and it turns out there isn't one particular temperature. Gasoline is a blend that is different for each batch. Some of the components with a lower BP start to vaporize as low as 104 degrees F. That doesn't mean you're going to have vapor lock at the temp though. There wasn't any discussion I found relating octane rating to BP.

[2-4-T-Z-MAN]

It would suprise me if the bubbles you are seeing are a result of fuel over heating.

I wonder if the bubbles are air bubbles being sucked in at some point between the tank and filter.

You've replaced all but the hard lines. If they are the original lines, they could have a small pinhole drawing in air.

Do you still have the orig. mechanical fuel pump?

[240ZMan]

The problem was there when I used the mechanical pump, and when I changed to the L28 I switched to an electric. Problem stayed the same.

But your thought of gathering bubbles from a leak is interesting. I'm not able to come up with an explanation for how they get there though. The electric pump is back at the tank and if there was a pin hole in a line I'd expect it to leak fuel which isn't happening. I'll guess the problem would be between the tank and the pump? And why would increasing the temp under the hood cause this? If I open the hood everything returns to normal within a few moments.

FWIW, I've tried leaving the gas cap off but that didn't make any difference either.

As I said, I'm stumped :stupid:

[ezzzzzzz]

I don't see how you're experiencing vapor lock and boiling petrol. My thoughts are you have an unvented or partially unvented gas tank. When the problem occurs, immediately go to the back remove the gas cap. If you hear a sucking sound then there is your culprit. Or have someone remove the cap while you watch the fuel filter. I'd bet the problem clears almost immediately. The only other issues would be the flex fuel lines from tank to pump or a clogged pickup tube in the tank. Try driving around in conditions that cause the problem but with the gas cap loosened and see if the problem occurs.

[240ZMan]

I have tried removing the cap and driving around with no difference. Sometimes there is positive pressure in the tank (the lid pops up a little) but I've never noticed a vacuum.

[2-4-T-Z-MAN]

You're correct about the pump pushing to the front, it would push gas out if there was a leak after the pump.

When you say that you open the hood and every thing returns to normal, is this at idle or are you driving?

At idle an air leak would not necessarily show up as it would under load.

[240ZMan]

If I let the engine idle parked for 5 - 10 minutes (more or less depending on how warm the day is) the problem appears. When I open the hood the problem goes away within a few minutes. I've definitely confirmed it's a heat issue as during the winter it rarely occurs, and in the summer, it's quite often.

This problem has been with me since I've had the car (about 1 1/2 years). Since then the entire engine and carburation system has been replaced with the exception of the intake manifold and fuel rail.

Once the engine is much above 1k rpm the symptoms go away. My current solution is to lean the carbs as much as I can without killing the drivability and keep the idle speed above 1k rpm. Even then it doesn't idle smoothly when the underhood temp goes up but at least it doesn't stall.

[Curtis240Z]

Have you tried insulating the fuel rail? I also used phenolic spacers on the fuel rail supports that are directly touching the fuel rail.