Vapor lock challenge

[Joel Heinke]

I'm hoping to get input on solutions to a vapor lock issue. The car is a home built Ferrari 250 GTO recreation with a supercharged EFI LS motor along with a C5 style torque tube and transaxle. So front engine setup with under car exhaust and a 22 gallon fuel cell in the rear of the car. The fuel pump is external to fuel cell, is positioned rearward of the fuel cell and vertically just above the bottom of fuel cell. The fuel lines run on the outside of the chassis (so not near the exhaust) with the feed and return lines entering the engine compartment about 8 inches from the engine with a direct route to the regulator/fuel rails (so above a header but not close to it).

I've had the car on the road for about 3 weeks now and have experienced what I believe is a vapor lock about a half dozen times over that period. At first I thought it was a faulty fuel pump, but after replacement the issue still persists. The fuel lines are all stainless tube plus the hoses you can see above. The fuel lines all flow freely and I've changed the fuel filter even though it still looked new. The cell vent line is not blocked and flows freely. I opened up the fuel cell that's equipped with a "fuel collection box" and foam filled and everything including internal hoses and fuel sock looks brand new with no apparent issues.

A few days ago after fuel pump replacement, I took the car out for a test drive starting with a full tank of fuel. After about 40 miles of driving (a mix of in town and mountain twisty) and on an uphill stretch, the engine hitched a couple of times, quit and wouldn't restart. After sitting by side of road for about 20 minutes, the engine restarted and made it about 5 miles up the hill before stalling again. It stalled again one more time before reaching home.

My hypothesis is that it took 40 miles of driving to heat the fuel in the tank to a point where it reached a temperature subject to vapor lock. I believe the vapor lock is occurring in the 2 foot long suction line between the fuel cell and pre-filter/fuel pump but this is only a guess. I say this because the fuel pump sound changes with a surging sound just prior to the engine stalling. The ambient temperature was mid 80's at the time and elevation about 5,000 feet.

Subsequent to this, I insulated the fuel cell with DEI tunnel/floor insulation (3/16" thick with outside foil cover) and wrapped exhaust pipes in near vicinity of fuel cell with exhaust/header wrap. The 2.5 inch stainless steel pipes do run right under the fuel cell.

After this, I participated in a driving event this past weekend and the engine stalled again on a steep uphill stretch with the same symptoms. It's running fine one minute and then just stalls. After waiting about 20 minutes, it restarted, I headed downhill to the nearest gas station, added 9 gallons of cool fuel, and the car made it home without further issue.

My plan is to wrap the entire exhaust system that goes through the chassis backbone with exhaust wrap. I already insulated the mufflers about a week ago but that was to try to reduce heat transfer into cockpit. I'm fueling the car with premium pump gas. If the additional exhaust wrap is not enough, what then????

 

[Davidmgbv8]

I have never heard of vapor lock in a fuel injected engine though not to say it can't happen. It sounds more like trash in the fuel tank blocking up the pick up. Once you stop it settles back down and you can restart. Now I experienced this on a mechanical fuel injected 280SL The fuel tank was full of rust. Several times over the years in vintage racing especially those running SU's, that they picked up so much track dust because their fuel cell vent line did not have a fuel (air) filter on the tube that dropped below the back of the car. The dust was so fine it made it past the fuel filter and got into the jets and fuel bowls. Paul Baverstock jus found his fuel line was full of the stuff that is supposed to make his fuel cells explosion proof. And saying that when vintage racers fuel cell foam breaks down it also plugs up the fuel filter causing issues.

 

[Davidmgbv8]

Would like to see pictures your car.

 

[Mike Pass]

If your fuel tank is too well sealed it may be developing a partial vacuum as the fuel is used. There may be a very very small way for air to get in and so the partial vacuum will be eliminated after a while. Check there is a way in for air to get in to replace the volume of fuel used. The partial vacuum may be too much for the pump to draw fuel from the tank.
Cheers
Mike

 

[Brian]

Vapor lock isn't a problem with EFI, but pump intake cavitation is. It only takes the tiniest of restrictions to the pump intake and elevated fuel temp to make the fuel boil as it enters the pump, and the fuel pump can't make pressure with vapor.

 

[Joel Heinke]

I have never heard of vapor lock in a fuel injected engine though not to say it can't happen. It sounds more like trash in the fuel tank blocking up the pick up. Once you stop it settles back down and you can restart. Now I experienced this on a mechanical fuel injected 280SL The fuel tank was full of rust. Several times over the years in vintage racing especially those running SU's, that they picked up so much track dust because their fuel cell vent line did not have a fuel (air) filter on the tube that dropped below the back of the car. The dust was so fine it made it past the fuel filter and got into the jets and fuel bowls. Paul Baverstock jus found his fuel line was full of the stuff that is supposed to make his fuel cells explosion proof. And saying that when vintage racers fuel cell foam breaks down it also plugs up the fuel filter causing issues.

The filters and lines are all clean. The cell foam is all intact and has not broken down. The fuel sock on pickup point inside the fuel cell is clean. I can easily blow air through all the fuel lines. The fuel cell vent line is clear and uses a small K&N style filter on it that is clear.

 

[Brian]

2 possible solutions-

1) Intank pump

2) Second low pressure pump that filters and fills a surge tank.

Just a 16" of tube with a couple of bends is enough restriction to cause cavitation.

 

[Joel Heinke]

Vapor lock isn't a problem with EFI, but pump intake cavitation is. It only takes the tiniest of restrictions to the pump intake and elevated fuel temp to make the fuel boil as it enters the pump, and the fuel pump can't make pressure with vapor.

Yes, this is what I suspect is the issue I'm experiencing. I'm guessing the only way to resolve is to keep the fuel cool enough that it doesn't vaporize and cause pump cavitation. I'm going at the heat sources now but maybe need to move the pump inside the tank?

 

[Brian]

That's the OEM's solution, but if you have a nice tank without a big access hole to do it, adding the second pump and surge tank under the car may be easier.

With fuel cell and foam, I would lean towards the surge tank option. Biggest plus of this option is if you have a second pump you have a filter between the tank and surge tank and you're pushing fuel through that filter rather than trying to suck it through a pick up sock with foam around it.

 

[Ian Anderson]

Hi
the return fuel line.
does it return to the tank, and if so is it near the draw off point?
the returned fuel will be hotter than the tank fuel so of it draws the warmer fuel in it will continually get warmer.

Euro diesel cars but a small heat exchange in the return line to remove heat from the fuel.

is the high pressure pump being asked to ”suck” the fuel they really do not like to suck upwards at all so preference Is to feed from the bottom of the tank as opposed to sucking it up to the top of the tank and back down to a low mounted fuel pump.

ian

 

[Joel Heinke]

Would like to see pictures your car.

 

[Joel Heinke]

Hi
the return fuel line.
does it return to the tank, and if so is it near the draw off point?
the returned fuel will be hotter than the tank fuel so of it draws the warmer fuel in it will continually get warmer.

Euro diesel cars but a small heat exchange in the return line to remove heat from the fuel.

is the high pressure pump being asked to ”suck” the fuel they really do not like to suck upwards at all so preference Is to feed from the bottom of the tank as opposed to sucking it up to the top of the tank and back down to a low mounted fuel pump.

ian

The return line goes into the fuel collection box inside the fuel cell. So in effect, the return fuel is going directly back into the fuel intake for the fuel pump. Now saying this, the fuel pump is most likely getting the warmest fuel as the fuel at the edges of the fuel cell is likely cooler than the fuel returning from the engine. I'm guessing the fuel cell people routed the return into the collection box to most effectively draw up all the fuel in the cell but I bet it's a contributing factor to the vapor lock issue.

The fuel cell fill plate is on the top of the fuel cell so the fuel must go to the top of the cell before it goes downward to the fuel pump. There is a siphon action for the fuel going to the fuel pump. I know because I have to break the siphon when the fuel line is removed from the pre-filter on the pump. I've had a decent amount of fuel spillage every time I've disconnected the pump and I get nervous about that.

I've picked up the following potential points of improvement from this thread so far.

1. Locate the fuel pump inside the tank and thus putting all fuel lines under pressure. I'd need to run electrical power through the fill plate but I think that should be doable.
2. Locate the fuel return line away from the fuel pickup point. This is easily done once the fuel cell is opened up by routing the return hose under the foam and away from the fuel collection box.
3. If all else fails, put a heat exchanger on the return line.

Please let me know if there's additional areas of improvement for my issue. Thanks in advance for the input so far.

 

[Davidmgbv8]

Nice build. I see you are running AC. Jag XJ's used to have fuel cooler mounted to the return line from the evaporator to chill the fuel for return to the tanks. Now I do not know if anyone makes something like this or you could find one on ebay or XJ6 scrappers.

 

[Brian]

You have a sock and a prefilter? Using both is a double wammy. Plus the length and turns of the line between pick up and pump, it all adds up to too much suction at the turbine's intake.

With the foam, you almost have to put the fuel back at the pick up or you'll run dry when you get to the last 1/4 tank.

Keep in mind, with the return fuel system, the pump is running flat out all of the time unless you have one of those PWM modules and it takes a little time for the fuel to migrate through the foam.

Beautiful car. If you can find a place for a small surge tank and pump, that would be my fix. Let the surge tank return to the main tank, and the engine return to the surge tank. Put a 100 micron filter between fist pump and surge tank, and 10 micron after the second high pressure pump, no prefilter for the low pressure, just the trash sock.

 

[Joel Heinke]

You have a sock and a prefilter? Using both is a double wammy. Plus the length and turns of the line between pick up and pump, it all adds up to too much suction at the turbine's intake.

Ahhh, this is exactly the type of insight I was hoping to get! Thanks Brian!

While I do plan to move the fuel pump into the fuel cell, that's going to be a more involved undertaking that I'd like to schedule for the winter months.

To continue sorting out the car (only 1,500 miles on it so far) and enjoy it some more in the short term, it sounds like I can potentially suppress the stalling issue some by removing the element from the pre-filter. It's a billet Aeromotive unit so this can easily be done in an hour or so. I originally put the pre-filter in place because the Aeromotive fuel pump instructions were insistent about it being needed and I didn't know about the sock on the in-tank pickup. Live and learn....

 

[Pat]

What a stunning car! It must be really frustrating to have these issues.
I always try the simple stuff first.
Have you tried ethanol free fuel? Ethanol really lowers the boiling point and it wouldn't take much Arizona summer heat to bring it to boil/vapor lock in the extended fuel lines from the tank to the engine. You may want to get some insulation on the lines from the tank forward. Also, did you open the gas cap when you were vapor locking? If you heard a hiss from air entering the tank, you may have a tank vent issue as mentioned above.

 

[Brian]

If you have pressure in the tank, it's helping you and not hurting you. If there is ANY vacuum, big problem. Remember, all you have getting fuel to the pump intake is the air pressure in the tank. Since you have a supercharger, I would almost recommend adding a check valve from the intake to the tank so any time you make boost, you put some pressure in the tank to help the fuel to the pump... if you want a down and dirty fix. There are some obvious safety negatives having a pressurized fuel tank. I've found that returning hot fuel to the tank heats and pressurizes, but if you're driving from a full tank to 1/4, you have to get some air in there to relieve the vacuum.

 

[Terry]

I ran into a similar event, with a short drive and it would buck and die, but the car had a hard time starting too, I had to run heavier wire and removed all the electrical plug connections, I think I changed out the pump with one that had more flow too, are you using the factory LS pressure regulator/ return? And if so is it for something like the LS9

 

[Joel Heinke]

I ran into a similar event, with a short drive and it would buck and die, but the car had a hard time starting too, I had to run heavier wire and removed all the electrical plug connections, I think I changed out the pump with one that had more flow too, are you using the factory LS pressure regulator/ return? And if so is it for something like the LS9

After having the engine stall from this issue, the engine would start fine after waiting about 20 to 30 minutes for the fuel to cool down. Until the fuel cooled, the engine wouldn't restart at all. I could tell by the sound of the fuel pump pre-start if fuel had cooled enough for restart or not.

Yes, this engine still uses the factory LS fuel pressure regulator even though the fuel rails and all the fuel lines are not GM but aftermarket parts. The pressure regulator is mid to late 2000's vintage and I believe came on an LS3. I don't know and don't have the reference to tell you if it's anything like what is used for LS9.

 

[Terry]

check your cam and crank sensor wires, if they are to close to something like right on the block or very close to the exhaust it creates a ton of resistance and the ecm thinks the engine isn’t running and will shut off the fuel, also a bad , weak cam or crank sensor will do the same