Save The Jaguar XJ6 And Make Them Run

I now want to tell everyone how to make the Jaguar XJ6 manufactured from the early 1970s through the early 1980s start and run.  I had previously not wanted to tell very many people, so that the price of these cars would continue to stay very low, and I would be able to buy many more of them, hoard them for myself, and save them.  I now recognize that I won’t have the opportunity to do this, so I had better start telling everyone the easy fixes for these cars to get them to start and run, so that they all don’t end up getting destroyed.

The Jaguar XJ6 that was manufactured from the early 1970s through the early 1980s was one of the most frustrating automobiles ever made.  It was a luxury British automobile that was imported to the United States, and it had a high price new at the dealer.  Very wealthy people bought these cars because they were beautiful and elegant in their body styling, paint, trim, interior, and ride.  The interior was mostly leather with wood trim.  The seats were leather, the doors had leather trim, the dash board was fine wood veneer.  The gauges were high quality and craftsmanship Smiths gauges.

Wealthy people bought the Jaguar XJ6, knowing that these cars had a reputation for breaking down frequently.  You had to be wealthy to buy one new, not just because of the initial purchase price, but also because of the constant costly repairs.  After several years of ownership, the wealthy people would sell their Jaguar at a greatly depreciated price.  This was partly because they were ready to get a new car, and partly because they were tired of the unreliability of the Jaguar and having to get it worked on frequently.

The second owner of these Jaguars that the wealthy people sold after several years of ownership, would typically be lawyers and business people that were not as wealthy.  The second owners of these Jaguars, typically had more repair costs than the original owners, and they were much less able to afford these frequent repairs.  These Jaguars quickly went to their third, fourth, and fifth owners, often being sold at very depreciated prices out of desperation.

This is a typical story:  In the small town in Florida where I was from, my mother, father, and sister had seen advertisements for Jaguars in magazines like Architectural Digest, Vogue, and Cosmopolitan.  In driving to Daytona, Orlando, Jacksonville, or Miami, you would see Jaguars.  My mother, father, and sister wanted one.  There were a just a few Jaguars in the town where I was from, and there was one person who worked on them, Donnie Taylor.

My father was somewhat aware of the unreliability of Jaguars, and he talked to other Jaguar owners who verified their unreliability.  The local mechanic Donnie Taylor, had done several engine swaps, where he replaced the Jaguar engine with either a Chevrolet Monte Carlo engine, or a Corvette engine.  My father told the mechanic Donnie Taylor, to look for a good used Jaguar XJ6 for sale, to go and perform an inspection, and to bring it back if it was a good car.  When the engine became a problem, he could do the engine swap.

Donnie Taylor drove to Miami and brought back a 1978 Jaguar XJ6L.  The car was white, with a red leather interior.  This was 1982, I was about thirteen years old.  This car seemed strange to me, because of the red leather interior, and it was so low to the ground.  I looked through the paper work in the glove compartment, the initial purchase price was $19,000 in 1978.  I believe that Donnie Taylor got this car for $6,500 to $8,000, which is tremendous depreciation in four years.

My father loved driving this car.  It was a beautiful car.  It was very fast.  Not the same kind of fast like my father’s Plymouth Road Runners, but a quiet and smooth fast.  At first everything worked on this car, including the air conditioner.  After having the car about a year and a half, the car had been a little trouble.  When the air conditioner quit working, and Donnie Taylor could not fix the air conditioning, and some other things that were wrong with the car, my father was disgusted at the car and at the mechanic Donnie Taylor.  He parked the car in the yard, no longer caring what happened to it, like many Jaguar owners.

As I turned sixteen, I wanted to get a car.  I wanted an Audi 5000, a Toyota Supra, a Jeep CJ7, but aside from the money, my parents pictured me crashing each one of these cars.  The Jaguar had been sitting in the yard for about a year, and one day I took the keys to the Jaguar XJ6L, and I started driving it.  I thought that I was going to get yelled at and stopped, but I didn’t.  Now that I think about it, my father might have thought that this would be a good solution for this car, for me to crash it.

I was the only person in my high school with a Jaguar, and only one of about four people in the town with a Jaguar.  This was not really a good idea.  I was too young and immature for this car, I didn’t crash it, but almost.  I went all through this car trying to see and figure out what was wrong with it.  I won’t go through everything now that went wrong with this car.  Because I really liked this car, my father spent more money on getting it repaired, with a British mechanic in Daytona.  After about a year and a half, this car was too upsetting to me, so much was wrong with it.  My father and I traded it in for a newer, nice, clean four door Volkswagen Rabbit, with everything working on it.  This was a very good and appropriate car for a high school kid.

I learned a lot about the 1978 Jaguar XJ6L from having it in my family from about 1982 to 1987.  I haven’t described everything that went wrong with it, how frustrating and upsetting the car was, but it was.  I could not believe it when my father bought a 1983 XJ6L Vanden Plas in about 1992.  I was about 23 years old, and a mechanical engineering student at the University of Florida.  I asked my father, what was he thinking, didn’t he remember everything that went wrong with that 1978 XJ6L, didn’t he remember just parking it in the yard for about a year because he got tired of the repairs and no one could fix it.

When I came home, I drove the 1983 XJ6L about fifteen miles to the beach and back.  I was worried the whole time that it was going to break down.  When I got about five hundred feet from the house, it quit running, and I had to push it the rest of the way home.  This car broke down a couple more times like this when my father was driving it, only in a more embarrassing way.  No one in the area was capable of fixing it.  After a couple of years of sitting in the yard, my father gave the car away for free.

When I was in my mid thirties, I began buying older vehicles.  I had a couple of newer vehicles that were my daily drivers and were reliable.  The other vehicles that I was buying were just hobby cars that I would work on myself, and there was no hurry in fixing them either.  I had a 1967 Porsche 911S that was currently running very well and needed just a little work, and I had a 1976 Jeep CJ5 that was currently running very well and needed just a little work.  I saw an advertisement for a 1978 Jaguar XJ6L for sale, the same car that I had in high school, that I had had so many problems with, and I had to have it.  I was going to outsmart this car this time.  I had a degree in mechanical engineering, and I could figure out anything mechanical now.

Enough of the background and story, getting to the point, from owning the first 1978 Jaguar XJ6L for five years, the second 1978 Jaguar XJ6L for twelve years, and from the 1983 Jaguar XJ6L that my father owned, I have learned why these cars don’t run, what is wrong with them, how to easily and inexpensively fix them, make them start, and make them run.

What I have learned, is that the engine and transmission are not bad, they are not bad at all, they are actually very good and well made.  The biggest problem with these cars is the electrical system, and just certain parts of the electrical system.  The second biggest problem with these cars is the fuel system, and just certain parts of the fuel system.

The mechanics that wanted to replace the engines in these cars with Chevrolet engines, were just wanting to replace an engine, electrical system, and fuel system that they did not understand, with one that was more simple that they did understand.  There was nothing wrong with these engines, they were good well made engines.

There are many Jaguar XJ6 cars from the 1970s and early 1980s that have very low miles on them, less than or near 100,000 miles, because they were not running for most of their lives.  The engines on these cars are probably in good condition.  It was the electrical system and fuel system that kept them from running, not the engine itself.

Most of the electrical components on Jaguars from the 1970s and 1980s were supplied by the British manufacturer Lucas.  Lucas electrical components in the 1970s and 1980s had a reputation for being unreliable.  After going through the wiring many times in my Jaguar, the wiring itself does not always appear to be pure clean copper, but copper mixed with some other metal that makes it a little bit more susceptible to corrosion, and a little more brittle.  Even though when you start getting into this wiring and examining it, and it is a little corroded here and there, this wiring will still do its job, don’t get freaked out by the wiring.  It is the connectors that you need to concern yourself with.  There are two main fault locations for the connectors that I will explain below.

The wiring itself, you only need to look and check the outer casing to make sure that it is not worn through or chewed through by mice, and shorting out.  The wire connectors are the big problem.  Take some time to just look at the wiring to see where it is all going. Don’t worry about the wiring to the headlights, turn signals, and horn for now.  One of the main fault locations that will keep a Jaguar from starting, is the wire connections at the starter relay, the male to female plugs aren’t mated tightly, and they are slightly corroded.  Just below the rear edge of the hood, at the top of the firewall, there is a small starter relay box attached to the firewall.  With the battery cables disconnected, and with making a written sketch of what color coded wire goes where on the starter relay, take off the starter relay and look at the condition of each of the plugs, and spray them with WD40 to get rid of any moisture and corrosion.  Look at the condition of each of the wire connectors that plugged into the starter relay, take some needle nose pliers and gently squeeze the female ends a little tighter to ensure that there is a tight mating.  Spray a little WD40 on both the male and female connectors to get rid of any moisture and corrosion.  Install the starter relay back where it was, and make sure that each of the wires plugs in tightly, where they are supposed to go, not some other way.

This starter relay is mounted at the top of the firewall to keep water from splashing up into it.  Water does splash up into it.  Though water splashing up into this starter relay doesn’t usually kill your engine when you are driving, it will make your Jaguar not start even hours later.  When your Jaguar has not been driven after some wet rainy days, or snowy days, it may not start.  Taking WD40 and spraying this starter relay thoroughly has made my Jaguar start on wet days and snowy days.  On these days, I go ahead and spray the distributor cap, spark plug caps, and starter relay with WD40 when the Jaguar does not want to start, and this works.

The second wiring connector that is responsible for the older Jaguar XJ6 not starting, or stop running, is the “inertia fuel cut off switch”.  On the passenger side, on the right side of the footwell, just below the glove box, is a small cover that encloses the inertia fuel cut off switch.  Remove the cover.  You will see a steel ball about 3/8″ diameter, resting on top of a magnetic rusty cup, this is it.  This was designed so that if you hit a pole, or a car hits your car, the collision will jar the ball off the top of the magnetic cup and break the circuit, so that your electric fuel pump will turn off.  The problem is, most of these cups have become rusty and rusted away, and hitting a bump in the road will cause the circuit to break, turning off the electric fuel pump, and mysteriously your Jaguar quits running.  I believe that this inertia fuel cut off switch becoming corroded and rusted away has been the number one cause for Jaguars to not start, quit running, mechanics to replace the starter relay, and fuel pump, and the car to still have problems.  With the battery cables disconnected, remove the rusted and corroded inertia fuel cut off switch, and replace it with a jumper wire connection, until you can get a new inertia fuel cut off switch.  Warning, if you get into a car accident with the jumper wire in place, your fuel pump will continue to pump fuel unless you turn the ignition key off.

I recommend buying the Haynes repair manual for the Jaguar XJ6 in order to look at the wiring diagrams.  The wiring diagrams change through the years 1977, 1978, 1979 etc.  I found that the 1978 XJ6 wiring diagram did not exactly match the wiring in my 1978 XJ6, but it was mostly correct.  As I stated up above the starter relay wire connections, and the inertia fuel cut off switch being rusted away were the primary reasons that the Jaguar XJ6 engine would crank but not start.  Though it has not happened on my Jaguar XJ6, it did happen on my 1986 Mercedes, there is a gear shift lever position sensor that will not allow your vehicle engine to crank unless the gear shift lever is in “P” or “N”.  These gear shift lever position sensors can become wet, rusty, or corroded so that your vehicle engine will not crank.  When your vehicle engine will not turn over, and your battery is not suspected to be dead, try shifting to “N” to attempt to start, or moving your gear shift lever back and forth a few times back to “P” to start.

About half of the time, the reason why the Jaguar XJ6 will not start or quits running is because of the two main electrical problems that I described above.  The other cause of the XJ6 not starting or quits running is the fuel system.  Mostly, it is the fuel pump being killed by rust flakes from the two fuel tanks.  There are two approximately twelve gallon fuel tanks, one on each side of the rear of the car.  These tanks become rusty internally.  At the bottom of each tank, there is a screw out drain plug that is approximately 1/4″ in diameter.  The fitting that holds the 1/4″ diameter drain plug, will also screw out and this fitting is approximately 1-1/4″ diameter.

The fuel pump in my 1978 XJ6L quit working intermittently.  Part of the problem, I discovered, was the two main electrical faults that I described above that were killing electric power to the fuel pump.  But after I got continuous uninterrupted electrical power to the fuel pump, it quit working, the electric fuel pump was seized up.  Even though the previous owner claimed that he had removed both fuel tanks and had them cleaned, at least one of them was still very rusty inside.  I found this when I drained both the fuel tanks.  You don’t have to remove the fuel tanks, just drain them from the drain plugs, it is easy.  Use thread seal tape on the drain plugs when you put them back in so that they don’t drip.

I went and got the biggest fuel filter that I could find, it was slightly larger than a Coke can. You must, I repeat you must get a fuel filter for use with a fuel injection system.  The previous owner had installed a clear glass fuel filter not meant for a fuel injection system and both the ends of this fuel filter burst off, creating a very hazardous situation.  I got the biggest fuel filter that I could find in an auto parts store because I was aware that this fuel pump moves more than a gallon per minute through the fuel rail and back through the return lines, and that there was a lot of rust in this fuel.  The whole point of this large fuel filter, was to put it before the fuel pump.  The replacement fuel pump was in stock at the local small town auto parts store and it cost $200.  No point in ruining a new fuel pump, the fuel filter absolutely needs to be before the fuel pump.

This large fuel filter installed before the fuel pump, the new fuel pump, draining the rust out of both fuel tanks, removing the inertia fuel cut off switch and replacing it with a jumper, and addressing the starter relay connections to make sure they were tight and free from corrosion, made the Jaguar XJ6 start and run every time since then.  However, another very common Jaguar problem had to be fixed, involving the fuel system.

There are two fuel tanks, one on each side of the rear of the vehicle.  On the 1978 XJ6 there is one fuel pump in the bottom of the trunk, on earlier versions, there used to be two fuel pumps, one for each tank.  There is a switch on the dashboard, where you select which tank you want to pump from, the left tank, or the right tank.  If you select right tank, the left tank feed valve closes, the left tank return valve closes,  the right tank feed valve opens, the right tank return valve opens.  You essentially have four different fuel valves which have to electronically switch positions.  It was a common problems on Jaguars, that the wrong return valve would open or close, and the fuel pump would move fuel out of one tank and pump it back to the other tank which might be completely full and fuel would come gushing out of the fuel cap, a dangerous situation.

There were other common combinations of fuel valve failures on Jaguars, such as the fuel valves failing to allow you to change over from a completely empty tank to a full tank.  In order to eliminate once and for all the various combinations of fuel changeover valve failures, some of them very dangerous, the previous owner got some fuel hose, off the top of my head it was approximately 1/2″ diameter fuel hose that fit perfectly on the fuel tank nipples, and he connected both tanks together, with a “T” fitting in the middle.  The fuel pump was fed by one end of the “T”.  Once he connected both tanks together with this fuel hose, you could go to a gas pump, fill one tank, and the fuel would cross over to the other tank, filling them both at the same time.  When the fuel pump was pumping, it was pumping from both tanks at the same time.  The fuel pump changeover valve that allowed you to select left tank or right tank, was thrown in the garbage.  The fuel pump now always pumps from both tanks at the same time.

Where does the return fuel go, to the left tank or the right tank?  I didn’t know, and it didn’t matter because both tanks were now connected together.  I started to consider, what if both of the fuel return valves closed, not allowing fuel to return to either tank?  So I removed one of the rear wheels to allow access to the fuel return valve for that tank, and I removed the valve and replaced it with straight hose.  Now, with both tanks connected, with no fuel changeover valve at the fuel pump, the fuel pump pulls fuel from both tanks at the same time, and now with one of the fuel return valves removed, the fuel can always return to one tank, and in essence both tanks because they are connected, there is no longer any where for the fuel system to get shut off by faulty valve operation, like many Jaguars used to have.

After all of the work above was completed, I could drive the car without any problems.  There were no engine problems, no transmission problems, no power steering problems, no alternator problems, no starter problems, no water pump problems, no lights not working, no windows not working, no gauges not working, no brake problems.  No leaking of any fluids.  The only thing that didn’t work was the air conditioning.

Everything worked fine, and the car was thirty years old when I did the work.

Again, something I need to point out, if a Jaguar XJ6 was brought to a mechanic with a fuel system problem, it would have been very difficult for the mechanic to diagnose what was going on.  Was it a starter relay problem that was killing the fuel pump, was it a bad inertia fuel cut off switch that was killing the fuel pump, was the fuel pump bad, was the fuel bad, was there debris in the fuel, was the fuel changeover valve making the fuel pump pull from an empty tank, was neither fuel return valve opening preventing fuel from flowing, was the correct fuel return value not opening causing fuel to go back to a full fuel tank and preventing fuel from flowing?  With the difficulty in determining what was going on, which might not happen every time, replacing the fuel pump might not solve the problem, and the car owner would be irate when the car broke down again after just coming from the mechanic.  This is why these cars got sold off by the original owner after several years, the subsequent owners could not afford the repairs, these cars became parked in people’s yards, and the value of these cards depreciated greatly.

The XJ6 was an attractive good looking car to begin with, and it still is.  They are good, solid, well built cars, with good engines and good transmissions, they just need the electrical and fuel system corrections that I described above.



12 thoughts on “Save The Jaguar XJ6 And Make Them Run

  1. Thank you, very good post with good ideas, Perhaps armed with this info I can trouble shoot my fuel pumps not working in my 1982 xj6 as there seems to be very little info on these cars with carburetors to be found on the web.

    Liked by 1 person

  2. Can you answer some questions we have on the above repairs you did to your 1983 xj6?
    We are doing the same repairs to our 86 xj6 and my husband had some more detailed questions to ask you!


  3. hello! We are working on doing the above repairs you stated above to our 86 Jaguar xj6, we had some specific questions my husband wanted to ask:
    How do you disconnect the button on the dashboard that switches over the tanks?
    Does the one return fuel return valve need to have its electrical connections connected, we are doing exactly what you suggested in your above article.


    1. Amynest2014,

      I did not disconnect the fuel changeover button at the dashboard, because if you press this button on the dashboard, it allows the dashboard fuel gauge display to show the left tank fuel level, and pressing this button again causes the fuel gauge display to show the right tank fuel level. Do not disconnect this button on the dashboard.

      In the bottom of the trunk, once you disconnect and remove the fuel tank changeover valve, cut the bare electrical wire ends off even with the wire insulation casing, and then use electrical tape to wrap the end of each electrical wire INDIVIDUALLY, so that there is no chance of an electrical arc from one wire to another, or one wire to nearby metal. Once each individual electrical wire end is wrapped with electrical tape, you can tape the no-longer used electrical wires together in a bundle, and tuck this bundle back away from any fuel lines.

      When you remove a rear wheel to gain access to the fuel changeover valve located there, you are going to remove this changeover valve, and just use a continuous hose connection now, there will be no valve anymore. For the electrical wires that went to this valve, cut the bare ends of the electrical wire off flush with the wire insulation casing, then wrap the end of this electrical wire with electrical tape INDIVIDUALLY, so there is no chance of an electrical arc with another wire or nearby metal. When you have cut the bare ends of the electrical wires and wrapped them INDIVIDUALLY, you can tape the no-longer used wires together in a bundle, and tuck this bundle end away from the fuel lines.

      I was lazy in only removing one of the fuel return line changeover valves in the wheel well, I would recommend removing both of the changeover valves in the fuel return lines in both wheel wells, because to tell you the truth, I had noticed on long drives that the fuel seemed to be returning to one fuel tank more than the other. Though with the center bottom of trunk fuel changeover valve replaced by a Tee Fitting, where fuel flows freely from both the right and the left tank at the same time, these two tanks do level out fairly well between each other.

      Liked by 1 person

  4. Hello!
    Our xj6 is a 1986 vanden plas, we did exactly what your article stated, but our car will not start. We replaced both fuel tanks, with new fuel tanks, removed all the fuel changeover valves, and put the filter in before the fuel pump, put new fuel pump in too. The car will turn over , like its trying to start but is not starting- any suggestions?


    1. Amynest2014,

      Did you replace or bypass the “Inertia Fuel Cut-Off Switch”? In the 1978 Jaguar XJ6L, the Inertia Fuel Cut-Off Switch is on the right side of the passenger side foot well, just below the glove compartment. There is a small cover. Once you remove this cover, there is a metal ball that is resting in a magnetic rusty cup. In a vehicle accident, this ball being jarred loose from the top of the cup, breaks a circuit, which prevents the fuel pump from running. With time, this Fuel Inertia Cut-Off Switch becomes so rusty, that it will not let the fuel pump run. For the time being, you may need to use a wire jumper in place of this rusty Fuel Inertia Cut-Off Switch.

      Amy, get in the car and try to start it, with your husband at the trunk with his hand on the fuel pump, listening and feeling for the hummmm of the fuel pump running, is the fuel pump running when you are starting the car?

      Also, there is a start relay plug with about six wires going to it, right a the top of the firewall between the engine compartment and the cabin, just beneath the hood when it is closed, spray this start relay plug with WD40 to get rid of moisture and dried alloy flakes that are preventing a good electrical connection in these wire ends. The wire connections here are often a problem for starting.


    1. Earl,
      No, I don’t personally have any suggestions about the power steering. There is a mechanic in Houston, Texas that has worked on every type of car and truck there is for the past 50 years, he is very knowledgeable, and he makes about two YouTube videos per day covering different kinds of repairs, offering tips, suggestions, and solutions. Look up Scotty Kilmer on YouTube, go to his videos on his home page, and look through his hundreds of videos, he probably has a couple of videos on power steering, and he probably has some videos on “stop leak” products.

      O.K. now that I think about it, yes I do have a good suggestion. There is a product sold in some good auto parts stores called “ATP-AT-205” that can be used for power steering leaks. I have used this AT-205 on a truck front axle seal leak, and it reduced the leakage by more than half. What this product does, it re-conditions and restores elasticity to rubber parts and seals, helping them to once again function like they are supposed to.


    2. I had the same leak. my jag had been in storage for 15 years. I took he R&P to well known repairer. he replace all the seals & rotate the 180 deg as the rack can wear on the connecting teeth even standing still


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