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Ex Lockheed
[Rocket Scientist]




  Hess & Eisenhardt Fuel Tank Vent Problems (Rev C, Oct 2001)


Kirby has published a few of my tips in "The Book". Although the book gives a brief statement of the Hess fuel problems, the article is not sufficiently detailed to help out those who are new to the subject. I have spent many hrs testing, replacing parts, and fixing fuel leaks on my Hess convertible over the past couple of years, and although I know a great deal about the flaws and fixes of the fuel system, I still have not completely solved the problems. In order to assist my fellow Hess owners I will attempt to explain how the fuel vent system functions, describe a few tests, and suggest potential solutions. Come back later, I intend to add more pictures to clarify the article.

Hess Fuel Tank and Vent Description:

The fuel tank on a Hess convertible was modified in order to allow the convertible top to retract flush with the trunk. To achieve this the regular XJs fuel tank was modified by reducing the size of the regular tank, and compensated by the addition of a second tank under the baggage area behind the seats. The two  tanks are connected  by a rubber hose with clamps at the center of the vehicle under the baggage area in order for the two tanks to act as one. Two pumps deliver fuel. The engine fuel  is delivered by the main fuel pump  in a header tank. The smaller tank under the baggage area also has a pump delivering fuel to the upper main tank. Not very elegant! See picture below:

I thought it was only the British that came up with complex designs!!. I have included a complete schematic diagram of the fuel tanks and fuel vent system for clarification, ref the link below.

Click here for a detailed diagram of the Hess fuel vent system

Below are listed some of the design flaws and quirks with this Hess fuel system:

1) Complexity - too much complexity. Two tanks, two fuel pumps, a fuel cooler and two fuel senders only increase the probability of failure, and the probability of leaks.

2) Vapor Lock - the fuel system can suffer problems with vapor lock. On an XJs fuel is delivered at a rapid rate and high at pressure to the fuel rail in the engine bay. Most of this fuel is not used and is returned to the tank, now warmer because the fuel travels thru the engine bay, and is also warmer because the fuel travels thru the two fuel pumps. In order to repair the  tanks on my jags, I removed all the trim behind the seats exposing the fuel tank. On a hot day after a long trip in traffic I can touch the tank behind the seats and tell you it is VERY hot. Hot fuel does not sound like a good idea to me, a lot of vapor is created and needs to be vented. The best way to avoid vapor lock is to keep the cooling system in good working order, another large topic. On one of my cars the fuel cooler does not work because the air conditioning is not working. I have driven the car for two summers (95F to 100F) in stop go traffic without vapor lock problems because the cooling system is in good working order.

3) Vapor Trap - the vapor trap behind the main tank is too low. The picture on the rhs shows the vapor trap as viewed from the boot, the pipes are attached one connected to the tank, and one connected to the Rochester valve. The aft tank fuel sender is shown below for reference. If the tank is filled full, the vapor trap is at the same height as the upper level of the fuel in the tank. The regular XJs has a much better design because the vapor trap is at a higher location at the side of the tank. When the tank is full fuel is forced into the vent pipe and fills the vapor trap with fuel. The vent system now has trouble functioning.

4) Fuel Vent System - the fuel vent system can build up too much pressure before the Rochester valve opens to release the fuel vapor pressure. Jaguar had a recall (Campaign No. 89V168000) to fix this by incorporating a vacuum line that open the Rochester valve when the engine is running. This fixed the problems of too much tank pressure while driving but it didn't fix the problem of high pressure building up while the car is parked. My XJs was modified by the recall, but when parked for a period of time I remove the fuel filler a huge whoosh of vapor is released. The tank makes a crumpling sound on many occasions, this is oil canning the tank! This is DANGEROUS. The tank is severely stressed and will fatigue with time resulting in a tank failure or if you are lucky one of the vent pipe connections is weak point and the owner gets a fuel vapor smell. This fault needs immediate attention. I disconnected the vent pipe at the Rochester valve until a better fix was found.

5) Fuel Vent Pipe - the fuel vent pipe between the tank and carbon filter under the front left wheel arch is too small in diameter. The pipe blocks with internal debris from corrosion. I suspect that this is why the fuel tank sees too much pressure. More details later.

6) Jaguar recall - the fuel tank vent system modification "Campaign No. 89V168000 - Excessive fuel vapor pressure / fuel tank leaks" recall was very complex and at least on my cars the modification was installed incorrectly. Refer to the general tips section for more details. Pictures will be included soon.

Fuel Vent System Testing:

Before a person starts work on the repair and testing of a fuel tank, remember the repair of a fuel tank should generally be left to the professionals because it is DANGEROUS. Never attempt a welded repair on a fuel tank. What I propose here are tests to determine the cause of the fuel problem.  This is a task I relegate to the outdoors, working with fuel vapors in a garage is not recommended. If the testing concludes the fuel tank is leaking (quite common, although touch wood I have been lucky to date), then either buy a new fuel tank or get a  professional to repair the tank. The tank can be repaired with sealant, refer to kirby's book.

Simple Tests - provides preliminary indication of potential problems

A good basic test is to let the car stand in the sun for a relatively long period of time on a warm or hot day without the engine  running. Open the fuel filler cap and you will get one of the following:

(a) A large whoosh with the possibility of a crumpling sound from the tank. This       indicates the fuel tank is building up too much pressure. Fix this fault quickly. In the meantime disconnect the fuel tank vent pipe at the carbon canister connection to relieve the stress in the tank. Technically this is illegal in the US, but is a better solution than rupturing the tank. Don't park your car in the garage with the vent pipe disconnected, fuel vapor leaks out of this connection!

(b) A small whoosh indicates the tank vent system may be OK, fuel vapor could still be leaking if the pressure can build to a higher level.

(c) No whoosh indicates the owner probably has a leak in the tank or fuel vent system. Further testing will show if the tank is leaking or the vent pipes.

More Complex Testing - refer to the schematic diagram in the above link

Fuel Tank Integrity - The first test is to ensure the fuel tanks do not leak. Remove the spare wheel in the trunk, and disconnect the fuel vent pipe between the tank and the vapor trap. Connect the vent pipe from the tank to an air line. Using a tee connection connect a pressure gauge with a range to about 10 psi. Apply air pressure and pressure the tank to about 1.0 to 1.25 psi. If you hear oil canning, stop the application of air. Now remove the air source and plug the connection. Record the pressure and record the pressure again a few minutes later. If the pressure remains the same you have a good sealed system. 

If the pressure drops you have a problem! Because the Hess has two tanks you cannot automatically assume which tank is at fault. Unfortunately now things get more complex. To determine which tank is leaking you will need to remove most of the trim behind the seats.  Remove the pipe connecting the two tanks (of course ensure the tanks are as near empty as possible. Fuel vapor is dangerous, conduct this task in a well ventilated area. Now pressure test each tank independently to find out which is leaking. I found that the tank under the baggage area was leaking at the joint for the fuel pump plate. The sealant deteriorated, being hard and cracked after ten years. I removed the screws and applied a new gasket and sealing compound. A new rubber hose was also installed connecting the tanks. The tanks were pressure tested and I was set. I was lucky enough to have to spend just a few dollars. If the tank leaks then the tank must be removed and replaced or fixed. Some vehicles had problems with abrasion between the tank and fasteners. This could be a repair you could fix yourself using a bonded repair kit available from Pep Boys etc. If the fault is at a seam then you are out of luck, and will need a professional repair using sealant on the inside of the fuel tank or a new tank. Good luck finding a new tank!.

Vent System Integrity - Now check the fuel vent pipe between the tank and the Rochester valve under the front LHS wheel arch. Disconnect the vent pipe at the Rochester valve and the vapor trap in the trunk. Apply compressed air and ensure the vent is not blocked. Seal one end of the pipe and apply compressed air to the other end of the vent. Tee in a pressure gauge and ensure the pipe is not leaking.

Rochester Valve - Apply air pressure to the Rochester valve at the connection for the fuel tank. The valve should open at about 1.0 to 1.50 psi. You will need an accurate pressure gauge. The second test is to apply a negative pressure to the smaller connection at the valve. This should open the valve representing the open condition when the engine is running.

Fuel System General Tips:

Low Fuel Tank Level - do not let the fuel in the fuel tank become too low in a hot climate. The fuel in the tank is a heat sink for the hot fuel returning from the engine bay. Low fuel levels greatly increases the risks of vapor lock on a hot day.

Overfilling The Fuel Tank - do not overfill the tank. The fuel spills into the vent system spewing raw fuel into the carbon canister and reducing the effectiveness of the vent system.

Cooling System - keep the cooling system in excellent condition. A poor cooling system results in a hot engine bay, vapor lock, and the potential for much more expensive problems, such as dropped valve seats.

Fuel Cooler - Keep the air conditioning working to allow the fuel cooler to do it's job. Not a necessity, at least for me.

Jaguar Recall - check that your vent system is plumed correctly. My jaguars had the connections at the Rochester valve and carbon canister, locations "A" and "B" were reversed. The vent pipe from the fuel tank was connected at "A", and the PCV vent was connected to "B", refer to the above link for a diagram of the vent system.

Fuel Filter - the most common place for a fuel system leak is at the fuel filter pipe connections. The fuel filter is located behind the spare wheel in the trunk (boot). Check the fuel pipe  connections on each side of the filter while the engine is running..

Fuel Vent Modifications:

This task is still being conducted on my two Hess Vehicles. When complete I will report on my success or failure. The modification are only proposed at this time and require verification.

Vapor Trap - find a better vapor trap and locate the trap in a higher location. Not easy but!

Fuel Vent Pipe - replace the fuel vent pipe running under the car to the carbon filter. The pressure drop along the length of pipe is causing the tank to build up a higher pressure than the designer intended. The Rochester valve opens at 1.0 to 1.50 psi. I have tested my valves several times. The tank starts to oil can above 2 psi. If the Rochester valve worked as intended all would be fine. The valve opens at the specified pressure but the tank is at a higher pressure due to the pressure drop in the pipe. My next test is to verify this by placing a pressure gage at the tank vent outlet and the Rochester valve location. If the gauges do not read the same value I will have verified my theory.

Rochester Valve - find a Rochester valve that opens at a lower pressure than 1psi. I pressure tested the aft fuel tank removed from the vehicle. The tank deformed like a balloon at 0.5 psi.This effectively means the structure around the tank is restricting deformation. The flat top is not sufficiently strong or stiff enough to contain the pressure in its own right.  I have not had time to search around but I should be able to find an alternative valve at the scrap/breakers yard. Any input here would be appreciated.

In summary I have had to spend a lot of time testing to find the cause of the fuel vent problems, but once armed with the data, the solutions can be quite an easy fix. Nearly all Jags seem to have problems with either fuel tank integrity or problems with fuel vapor escaping and causing a gas smell in the passenger cabin. The Hess is just somewhat worse than other jaguars because of the additional complexity of the fuel system. 



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