Vapor Lock with FI motor?

wrogers55 · #1 (**permalink**) 06-03-2015, 09:40 PM

Bad experience today. About 90 here in Texas. Got caught on a backroad construction and had to move through a few miles at 10 or so MPH. The motor didn't overheat but it was sure hot under the hood. After about 15 minutes of this, the car started to starve for fuel and slowly quit. I let it cool off a bit and it would start fine and run for a short distance and the same thing over and over. Finally decided on a flat bed tow home.

The motor is a 418 SBF MassFlo EFI and has run perfectly for years. Here is what I know so far from the side of the road only.

1. The fuel pressure gauge at the motor would not move when the pump was on at first then would suddenly build pressure. The pump would change tone from freewheeling to labor and I had pressure and it would fire off perfectly. Tank was half full.

2. While it was shut down and not running, I opened the gas tank filler and could hear "bubbling", like the return line was putting vapor back into the tank and causing bubbling.

By the time I got home, and unloaded, it started right up, had good pressure and I drove it onto the garage with no problem.

I'm going to start at the tank and work my way forward but is it possible the fuel rails got hot enough to boil the fuel and cause the problem? Not sure that would explain the pump spinning up without building pressure for a bit but maybe the return line is involved too. Any thoughts would be appreciated as I start down the fixit road.

Tommy · #2 (**permalink**) 06-04-2015, 04:18 AM

What follows is mostly theory rather than practical help. But maybe it can help you sort this out.

The temperature at which gasoline (a mixture of chemicals) boils varies according to what the producing refinery made. For example, some parts of the mixture may boil at 140 F and other parts at 200 F. If any part boils in sufficient quantities, it can produce enough bubbles to cavitate a pump.

Let's take a ride with a drop of gasoline starting in the tank. It's a hot day so the gasoline in the tank is already a warm 95 F. The electric fuel pump is running and producing heat that is dissipated into the gasoline. After a little while, the temperature of the gas in the tank is 96 F. As the drop approaches the warm fuel pump, it gets even hotter as it is sucked in and pushed out by the pump at 150 PSI of pressure. Raising the pressure significantly raises the temperature at which gas boils, so the fuel is comfortably in its liquid state.

On the way to the engine, the fuel in the line may pick up more heat from the hot road surface radiating up, or hot engine or exhaust components. It might also lose a little heat if the air around it is cooler than the gasoline in the line.

Arriving at the pressure regulator, some of the gas is allowed to flow to the FI system to be immediately consumed, while the rest is sent back to the tank, picking up heat much like it did on its way forward. Arriving back at the tank, this warmed gas mixes with the rest of the gas, warming the overall mixture to 100 F. And then the process begins again.

The point of this Disney-like description is to say that the pressurized fuel in the FI system is not likely to vaporize and be the source of the problem. The temperature of the fuel at the fuel pump likely is the problem. And the source of that problem may be heat that the fuel is absorbing during its round trips from the pump to the pressure regulator and back. If I were working on your problem, I'd look for ways to insulate the fuel lines wherever they are near hot things like roads, engines and exhausts. And if the fitting for the return line is close to the engine, maybe you can move it to a cooler place.

harasn · #3 (**permalink**) 06-04-2015, 04:30 AM

I have had this before on fuel injected cars. I suspect your fuel pump is on the way to failing. It seems that when the fuel through the system is at normal temps, all okay, as soon as fuel temp rises, due to higher than normal under bonnet temps, and a fuel return from the engine bay to the tank, the pump starts to overheat and not pump properly. Let it cool down and all good. Although this is pain, its better than it suddenly stopping as they can also do!

olddog · #4 (**permalink**) 06-04-2015, 05:23 PM

I agree that the fuel pressure (39psi) is too high for fuel to boil in the pressurized lines. Since it has ran well for years, your fuel line are no too close to a heat source.

There is a check valve in the pump to keep the system pressurized when you shut it off. If you heard bubbling, it could have been the check valve leaking or the fuel pressure regulator leaking. It also could be that the pump was hot enough to boil gasoline, if so you are lucky you didn't let enough air in to support combustion, when you opened the cap.

I agree the pump is most likely your problem, but more importantly for safety reasons it must be checked first thing. I would make absolutely certain it is good before turning the key on again.

After changing my first fuel pump, I was stunned by the design. I work in process control for a chemical company. If we ever used technology like that around a liquid that was only half as volatile as gasoline, we would never get out of prison. The only thing I can figure is that they rely on the fact that there is not enough air in the tank to support combustion.

D-CEL · #5 (**permalink**) 06-05-2015, 10:22 AM

I would say its the pump or regulator.

Jason

Luce · #6 (**permalink**) 06-06-2015, 05:20 AM

My money would be on a filter or restriction between the tank and pump.

Hats off to Tommy. Excellent illustration of the mechanisms that can send you off the trail looking for the real problem.

Also, do you still have winter gas in the tank? they formulate it differently and winter fuel is much more likely to vaporize prematurely at the pump inlet.

Tommy · #7 (**permalink**) 06-06-2015, 05:43 AM

Luce,
The problem appears to be heat related, so it is not likely the filter.

A failing pump could be the problem, but it may also be that an old, well worn pump is running hotter than in years past. When the combination of hot environment and hot pump combine, vapor lock in the pump can occur.

Snake2998 · #8 (**permalink**) 06-06-2015, 08:20 AM

Tommy has it right - however problem is with the pump which doesn't like handling fuel that's too hot - answer is to have your tank as full as possible in really hot weather to act as a decent heat sink and also a fuel cooler (similar to oil cooler) on the fuel drain-back side well away from the engine compartment and possibly a new fuel pump if the other two don't cure the problem. Insulating the fuel lines in the engine compartment will help as well.

wrogers55 · #9 (**permalink**) 06-06-2015, 08:51 AM

I think I may have it, although it could still be the pump, and it was acting just like an old fashioned vapor lock. First it has been running in the same configuration since '06 with zero problems, but in northern California and never really stressed by heat. Second it all happened after a forced long idle and slow move through some back road repair, and it was really hot. No motor overheating but I have a good cooling system and oil cooler and that has never been a problem.

However, once I got it home and it cooled down, it's all back to what was normal, runs fine again. The pump sounds normal again and the pressure is normal via the gauge again.

So, I think it may be the aluminum fuel filter being heated up by the header, with little or no air flow that day. Maybe it was pushing vapor out of the filter under pressure to the fuel rails and when I could get it to run it just kept heating up again and he comes the surge and die deal. Not sure but the filter is only 3 inches or so away from the header tube and the line runs right along there too.

Could still be the pump because I don't know why it would change noises and not pump up pressure when it's hot but time will tell.

The picture is of the filter by the header tube on the driver side. Years of being OK but clearly not a good idea for Texas heat no matter what.

Thanks for the all the info, great educational stuff for everybody.

g8rnbft · #10 (**permalink**) 06-07-2015, 08:47 AM

Its a "going bad" fuel pump. Vapor lock does not happen with a relatively high pressure electric pump. Intank pumps rarely fail "all at once". You have been warned, the next warning might be the last.

Luce · #11 (**permalink**) 06-08-2015, 06:52 AM

The "Vapor lock" is happening as the fuel get's hot, it vaporizes at the pump intake, and EFI pumps can't pump vapor... at least not at 43 psi. Any small restriction to the pump intake can make the pump cavitate, and hot fuel REALLY exacerbates the problem.

I was commending Tommy because it's a series of situations that contribute to the situation rather that a singe component failure. It could be fixed by either not getting in traffic in the heat, removing or reducing the restriction between the pump and fuel, or running the fuel through a coil of tube packed in ice before putting it back in the tank. You could even pressurize the thank with 20 PSI of air to raise the boiling point of the fuel prior to the pump, but that has a boat load of other hazards associated it.

It may be that insulating the fuel lines under the hood is the best solution if all is well now. Or if cosmetics are more important, just stay out of the traffic.