Boiling fuel?

[240ZMan]

The car used to have the dealer installed insulation and the problem was present. I've since removed it, and have temporarily used some pipe insulation. Didn't make a difference. FWIW, I don't think the problem is heat on that side of the engine. Although I have a header now, I have a very large heat shield protecting the carbs. And I had this problem with the stock engine, exhaust, emissions, etc. before.

[Bambikiller240]

There should not be sufficient heat on the right side of the car to cause fuel to boil. If it is boiling fuel the first thing I'd want to know is WHERE is the heat coming from to cause this. (there isn't any exhaust manifold, or pipes on that side of the motor)

Are you SURE that the bubbles are fuel boiling?? I wiould suspect it to be air in the lines (possibly from a pin hole in a hose or line) rather than boiling fuel.

[240ZMan]

This discussion has me thinking it may not be boiling of the fuel. I had originally thought it was because:

1) the problem only occurs when the underhood temp rises

2) I saw bubbles in the filter

3) opening the hood and letting things cool makes the bubbles and the problem go away.

I agree that I can't offer a good source of heat on that side of the engine. But I also can't explain why air would be sucked into the fuel system prior to the filter only when the underhood temp goes up. I know I'm missing something here - just don't know what it is!

Still stumped!

[PrOxLaMuS©]

you could get an "injector blower" from the 280ZX's ... so the "blower" can cool off your fuel rail and carbs!

Might be worth a shot.. cheap at junkyards

[beandip]

This discussion has me thinking it may not be boiling of the fuel. I had originally thought it was because:

1) the problem only occurs when the underhood temp rises

2) I saw bubbles in the filter

3) opening the hood and letting things cool makes the bubbles and the problem go away.

I agree that I can't offer a good source of heat on that side of the engine. But I also can't explain why air would be sucked into the fuel system prior to the filter only when the underhood temp goes up. I know I'm missing something here - just don't know what it is!

Still stumped!

This is VARY strange . I doubt vary much that it is air from a pinhole because under pressure or not you would see some leaking , or at the vary least a stain or wetness. What is weard is that it goes away when you open the hood. Thats the part that has me stumped. Insulation on the rail is after the fact,so that wont help. even if the pump was marginal , the cool air shouldent effect the flow ,could there be something wrong with the fuel filter that is blocking the flow when heated? I am grasping at straws here. Gary

[Bambikiller240]

Earlier this year I had discovered a bad fuel hose (between the sender and the hard lines, that short run of hose). It only leaked when the engine was running. "Cheap arse Kragen fuel hose".

[240ZMan]

I tried an experiment today. I took an old electric fuel pump and connected it to the input of the fuel filter. I put the other end of the pump into a 5 gallon gas can. In other words, I took the fuel tank and the lines leading up to the it out of the equation. I let it idle for 20 mintues and not once did it have any issues.

After I connected the regular fuel line back up immediately it didn't want to idle well. Also before it's suggested the problem is with the fuel pump, the one I used for this experiment was removed from this car because it was too loud, and I had the same idle problem with it when it was installed.

Carl, I changed the fuel line you described but unfortunately for me, it didn't make any difference.

So I figure it's either the hard line between the fuel pump and the filter, or it's the tank. What I can't explain is how the tank could be causing this problem. Remember, if I leave the hood up, the problem never appears.

Does this new information give anyone some idea of what is going on?

Thanks for staying with me on this one.

[240ZMan]

Forgot to add that taking the fuel cap off doesn't make any difference either.

[2-4-T-Z-MAN]

I would say that your experiment eliminated any possibility of vapor lock.

I would continue looking at the hard line and the connections between the tank and the filter. After all your experiment shows the problem is in that area.

Don't know what the deal is about the hood.

[240ZMan]

I ran another experiemnt with the output of the fuel filter run into a gas can. The flow is plenty strong and smooth. I didn't measure the rate, but I can't imagine it wasn't enough for the L28.

I put the fuel pressure gauge back in just after the filter. Normal pressure is a rock solid 2 psi. Blipping the throttle will show a very small movement. When the problem shows up I see the guage jumping up to 3 psi and the movement is timed with the stumbling of the engine. It appears that something is causing the pressure to spike briefly and this causes the engine to stumble. Again, only when the under hood temp is high.

I also spent some time under the car looking at the routing of the hard fuel lines. This gave me an idea for a new theory. The idea is that the spiking of pressure I'm seeing is fuel percolation (not vapor lock) caused by the fuel being too warm. I suspect the fuel is getting heated some as it flows along the drive line tunnel and then with the hood closed and the engine idling for a few minutes on a warm day, the fuel is just getting too warm.

But why for my car and not others in warmer climates? I'm wondering if the return flow is too slow and thus the fuel spends too much time being warmed. I have tried blowing through the return line from the engine compartment and I can blow into it, but it takes a lot of effort. Also, if I put my ear to the fuel filler when the problem is occuring, I can hear the return fuel splashing into the tank, and it's not a smooth flow, but rather a spurt, then a dribble, spurt, etc.

So my question becomes what is the best way to clean the return line without removing it from the car? I'm thinking of disconnecting it at the fuel tank and blowing carb cleaner into it from the engine compartment. Any other suggestions?

[2-4-T-Z-MAN]

At this point anything is worth a try. I'm running an L28, in Florida and have not had the whole problem you're dealing with.

I came to my conclusion about the pin hole as I did have that issue once.

I'm scraching my head on this one.

[beandip]

If it was the fuel moving too slow through as you said , then why is the filter only part way full , unless the pressure of the pump in not adiquate and it cannot over come the pressure in the rail . Due to the heated fuel boiling. 2psi is lower than what my pump puts out. I am getting 4 psi. You said that you installed a different fuel pump and it dident make any difference ? Is this right ? What was the pressure that pump was making ? Just for information, the fuel pump from a carbed RX-7 is a quiet pump and mine pumps at 4 psi. I got mine at the bone yard for $15.00 , if you come across one grab it and try it , if it makes no different at least you have a spair. I am thinking it's the low pressure of the pump that cannot over come the pressure in the fuel rail caused by the fuel heating and expanding . When you open the hood it cool the fuel line to the point the weak pump over comes the pressure and fills the filter and every thing smooths out. Again I am trying to make sense of this and this is the only logical thing I can come up with. Gary