Vapor lock 1990 E350 EFI 460 engine?

When I was a kid in the 50's, probably half the cars on the road had clothspins clamped on the fuel line to radiate heat from the fuel easing vapor lock.

When I was a kid in the 50's, probably half the cars on the road had clothspins clamped on the fuel line to radiate heat from the fuel easing vapor lock.

I have heard of this, and even seen it in a YouTube video, but I find it hard to believe this actually works by radiating heat.

First, wood is not a very good conductor of heat. And even if it was, given the high under-hood temperatures, the result would actually be to wick up heat and put it into the fuel line. This is because heat always moves from hotter-to-colder.

What I suspect is/was actually happening is that wood, being a relatively good insulator was actually insulating the fuel line from the ambient heat in the engine bay. If you clamp a dozen clothes pins onto the fuel line, then all that area is now encased in wood, effectively insulating it.

Carburetor engines often put a phenolic block of plastic between the carb and the intake manifold to try and insulate the carburetor from the heat of the engine.

Steve

Do you know for sure the pump is running? Might try changing out the relay for it. They are a known failure point at higher mileages.
Some shops always replace the relay when they replace the pump.

maillemaker wrote:
Evidently the fuel filter is designed to last the life of the vehicle.

When they started to modify fuel that put lots of accumulated junk in the fuel filters. Changing fuel filters isn't expensive and something that the Ford dealer recommended on my 91. Changing fuel filters would be high on my list when it came to anything that appeared to be fuel starvation - not much downside.

Do you know for sure the pump is running? Might try changing out the relay for it. They are a known failure point at higher mileages.
Some shops always replace the relay when they replace the pump.

When the in-tank fuel pump burned up 5 years ago, it took out the inertial switch and the relays with it. So the relays have been replaced within the last 5 years.

I don't have a way to know right now if both pumps are running or not. I will be installing a fuel pressure gauge shortly.

When they started to modify fuel that put lots of accumulated junk in the fuel filters. Changing fuel filters isn't expensive and something that the Ford dealer recommended on my 91. Changing fuel filters would be high on my list when it came to anything that appeared to be fuel starvation - not much downside.

Agree. After I get a baseline reading with the stock setup, I'm going to replace the fuel pressure regulator, fuel filter, and high pressure pump.

Steve

Just to let folks know how this ended up.

The root of the problem is a failure of the in-tank fuel pump.

I did not think to check it, because I had replaced it just a few years ago.

These early 1990s vehicles were early transitions from carbureted engines to fuel-injected engines. The small in-tank pump was not sufficient to push the fuel up to ~40 PSI needed for fuel injection, so they simply added a high pressure pump in the circuit. The high pressure pump is mounted on the frame rail under the driver's seat.

The way the system works is this: The in-tank pump pumps fuel out of the fuel tank towards the high-pressure pump, which then boosts the pressure up to around 40 PSI to the fuel rail on the engine. The injectors drink what they need to supply the engine at any given throttle/RPM, and the excess fuel not used by the engine is routed back on a return line back into the fuel tank.

When the in-tank pump fails, the high pressure pump is strong enough to suck fuel all the way from the back of the vehicle, through the dead pump, and run just fine - under most conditions.

However, I installed a thermocouple sensor on the return fuel rail just as it leaves the engine compartment. On a hot day, that sensor was reading about 125F. If you stopped the vehicle and sat for 10 minutes, that sensor would run as high as 145F. After you start the engine and fuel gets flowing through the line again, it quickly drops back to around 125F.

I used an infrared thermometer to read the temperature of the fuel tank itself after extended (hours) running at interstate speeds. The tank wall was reading about 110F.

Here is what I believe is happening:

Ethanol-based gasoline starts to boil at around 175F. The boiling temperature of liquids is directly depended on ambient pressure. This is why it is hard to heat water hot enough to cook with on top of Mt. Everest - water there boils at only 154F. This is because the air pressure there is only 4.89 PSI as opposed to around 16 PSI at sea level.

When the in-tank pump dies, the high-pressure boost pump is sucking hot fuel through about 4-5 feet of fuel line, and a dead pump. Since the high-pressure pump is pushing 40 PSI downstream, it is capable of sucking that much from upstream. We can't be sure exactly how much resistance there is to suction so we can't be sure what the actual pressure differential is. But my suspicion is that the high-pressure pump is causing enough of a pressure drop in the upstream fuel line that the fuel there boils. Once it boils, the high-pressure pump cavitates and can no longer pump any fuel. With the engine off for about 20-30 minutes, the fuel cools enough to turn liquid again, and the engine will again run until the situation repeats.

You will see this with a very low PSI reading on the fuel rail. When I finally caught it in the act, I was reading around 11 PSI or less. Less than 20 PSI and the engine won't run much, if at all.

I ended up installing digital ammeters to both the high-pressure and in-tank fuel pumps, with readouts in the cab, so that I can monitor the current draw of each pump independently. I also have installed a digital fuel pressure gauge which picks up from a sending unit on the fuel rail.

Ultimately, the Airtex pump I had replace five years ago had died. When we got it out of the vehicle and I disassembled it, one of the brushes was gone, and the other was just a tiny nub. We replaced it with another Airtex pump, and I still had erratic running.

I finally located a new old stock Motorcraft fuel pump. This completely fixed the problem. Not only that, but because the sending unit was properly calibrated for my tank, my fuel gauge read correctly once again.

This problem is very hard to diagnose because the high-pressure pump completely masks the problem until extended operation at high temperatures. High temperatures with short-distance driving won't trigger the problem. If it's cold out, you may never see the problem (although I finally did catch it with the digital fuel pressure gauge in the middle of winter, with snow at one leg of the journey, after an 11-hour drive from Virginia to Alabama).

Because of the highly intermittent nature of the problem, very few complaints of this nature you find on the internet end up being resolved. Although, I have found a few posts after more intense searching where others have found this same root cause.