Jaguar XJ-S. Manual - part 64

 
 

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Wrap some foam around the metal filter so it won’t rattle and clang against the floor or battery support.  I used the 1/2” 
foam tape used for sealing doors.  After encircling each end of the filter with foam strips, I added some electrical tape to 
hold it in place. 

 

FUEL PICKUP - SALOONS:  Saloons -- either XJ12 or XJ6 -- don’t have a surge tank, but they nevertheless have the 
same problem with clogged fuel pickups, one per tank.  Without having read this book, Chuck Sparks came up with a 
remarkably similar fix as that described above, fitting two generic fuel filters in the lines between the tanks and the 
selector valve. 

 

FUEL SUPPLY MONITORING:  From a fail-safety point of view, the surge tank pickup situation is bad.  Regardless 
of which pickup screen you have installed or how often you clean it, there is really no way to know if it’s plugged until 
you notice the symptoms -- and the first symptoms are likely to be low compression readings in a couple of cylinders! 

Of course, a plugged pickup isn’t the only possible cause of lean running under hard throttle.  A failing fuel pump, an 
obstructed fuel line, or a fuel filter overdue for replacing are other obvious candidates.  Whatever the cause, the result 
will be the same: if you run it hard, you’re gonna be buying new pistons. 

Oh, c’mon, the problem can’t be that serious!  How come other cars aren’t burning pistons at full throttle all the time?  
Well, obviously because their fuel systems weren’t designed by Jaguar engineers.  On most cars, nobody ever even has 
to think about the fuel supply; it’s just there, and it works.  The concept that an engine could run lean because of a 
plugged fuel pickup strikes owners of other cars as laughable. 

Of course, you could resolve to never drive the car hard -- but why else would you own a Jaguar?  A better solution 
would be to have some indication that the fuel supply is holding up before you burn pistons. 

Two methods come to mind.  The first would be to install a fuel pressure gauge or sensor in the rail.  Robert Dingli 
says:  “I bought a VDO fuel pressure gauge for about Aus$40 and connected it to the fuel rail where the cold start 
injector was once supplied.  I believe any pressure gauge designed for hydrocarbons would be suitable and other brands 
sell for much less.  My gauge is mounted in the engine bay as I am paranoid about high pressure fuel entering the cabin. 

“There are a couple of things to note about connecting the gauge: 

 

-  use high pressure fuel line and fittings. 

 

-  don’t mount the gauge on the engine as vibration will kill it. 

 

-  use a restrictor in the line as the pressure fluctuations will also kill the gauge.” 

If you have the Digital P system, ideally you’d use a gauge that reads the difference between the fuel rail pressure and 
the intake manifold pressure.  Since the fuel pressure regulators are referencing manifold pressure, this should result in 
a steady reading at all operating conditions and any variation is a sign of trouble.  If you use a regular guage that 
references ambient atmospheric pressure, the fuel rail pressure will go up and down as the intake manifold vacuum 
varies, so you will have to monitor both and do a little math to make sure things are operating properly. 

For most intents and purposes, a pair of air/fuel meters (see page 36) is a better idea than a fuel pressure gauge. 

 

SURGE TANK DRAINING:  In theory, the easy way to drain the surge tank is to pinch off the line from the main tank 
to the surge tank and run the pump, which will pump the fuel through the fuel rail and back into the main tank.  You 
don’t even have to hotwire the fuel pump relay; you can just start the engine and let it run until it stalls. 

Good idea in theory, but in fact may not work too well.  As described below, the return line from the engine doesn’t 
simply go into the main tank, but rather goes through the tank and is directed into the vent connection for the surge 
tank.  Running the pump will circulate the fuel from the surge tank around and back into the surge tank, and you will 
have to run the engine until it burns all the fuel in the surge tank.  

 
 

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So, pinch off the vent line too?  Bad idea, because then the pump would pull a vacuum on the surge tank and cavitate 
long before it empties the surge tank -- unless your surge tank has been fitted with the JLM 2163 vent kit mentioned 
below, it which case this might work fine. 

Forget that method; there are other perfectly workable methods that are very easy.  The intended method of emptying 
the surge tank is to clamp the main line from the main tank, and then remove the rubber plug in the floor underneath the 
surge tank and open the drain in the bottom of the tank (see page 244).  Since the vent line actually opens near the top 
inside the main tank (as opposed to where it looks like it goes) you only need to make sure the fuel level is below that 
level so it sucks air instead of fuel.  Just plan on this job when the tank is less than half full.  Of course, you can do it 
when it’s full; it’s just that you will first have to drain the main tank down to the level of the vent before the surge tank 
will empty. 

Another way to drain the surge tank is to pinch off the main line from main tank to surge tank, disconnect a line from 
the fuel filter and direct it into a gas can sitting in the trunk, and remove the fuel pump relay and apply a jumper 
between terminals 30 and 87 in the socket to run the pump. 

Of course, one way to drain the surge tank is to drain both the main and surge tanks; that way you don’t have to pinch 
off any hoses.  See page 244. 

 

SURGE TANK DRAIN UPGRADE:  Somewhere along the line, the drain plug in the surge tank changed from 
C41803 to CBC4518.  The newer drain plug is magnetic, while the earlier item was a simple plug.  If you are working 
in the area and find no magnetism in your drain plug, you definitely should opt for a replacement; any particles that 
stick to that magnet are particles prevented from plugging the pickup screen. 

The 1988 Jaguar Parts Catalogue lists only the first part number, but later editions apparently confused things a bit.  
Wally Magathan says, “According to my parts manual, RTC9900CA, Jan 1989: on page 2D 05, there is a diagram of 
the sump tank, with the drain plug part annotated as having been replaced by a magnetic plug.  One puzzlement, 
however: the diagram is indicated to pertain to cars equipped with 3.6 engines only.  There is no diagram labeled for 
use with 5.3 engines.”  Kevin Darling responds:  “This puzzled my local Jag dealer as well.  But the RTC9900CA parts 
book is wrong, since both my 1985 and 1990 V-12's have the sump tank.” 

Supposedly the CBC4518 plug will fit all the earlier cars. 

If you’re cheap, of course, you could always simply drop a magnet into the surge tank while working on the pickup.  It 
might be more challenging to clean, but it should do just as good a job of keeping ferrous particles off the pickup 
screen. 

 

SURGE TANK VENTING:  The vent line from the surge tank doesn’t go to the vapor separator, as one might expect.  
Instead, it connects back into the main tank about 2” above the bottom.  However, this is not a simple opening into the 
tank.  Inside the tank, this fitting continues as a tube that makes a couple of bends and terminates with an open end 
pointing upward a couple of inches from the top of the tank. 

Meanwhile, the fitting for the return line from the engine compartment is at the bottom left rear corner of the tank.  
Inside the tank, this fitting continues as a tube that runs across the rear of the tank -- there are actually a couple of clips 
brazed to the inner surface of the tank to hold the tube in place -- and then bends upward and terminates with a 180º 
hook that pokes its end down into the opening of the pipe from the surge tank fitting described above.  The connection 
of the two pipes is not liquid-tight; if you tip the tank over and pour a liquid into one of the fittings, it will leak out of 
the connection within the tank.  But it doesn’t appear to be a very loose fit; the tube from the return line appears to 
nearly fill the opening in the end of the tube from the surge tank vent. 

In other words: the fuel returning from the engine compartment passes through the main tank but really goes directly 
back to the surge tank, with a leaky connection inside the main tank to allow air to bleed out or something. 

Mike Morrin provides a possible explanation for this Rube Goldberg:  “Let's say you are nearly out of fuel, with a little 
fuel still sloshing around the bottom of the main tank.  You park, with the engine running on the side of a road with a 

 
 

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lot of camber so that all the fuel runs to one side of the tank.  The main outlet from the tank is now above the fuel level. 
 The engine can keep running (or restart) for as long as there is fuel in the surge tank. 

“With the arrangement you describe, the engine can keep running until all of the fuel in the surge tank is burned in the 
engine.  If the return pipe just went to the main tank, the fuel circulated by the pump would all end up in the main tank, 
and the surge tank would be empty in a minute or so.” 

Apparently something about this system didn’t work as intended, though; in Issue No. 2 of the XJS Bulletin, Ray 
Ingman reports that Jaguar issued a kit to provide an additional vent for the surge tank, JLM 2163.  The kit includes a 
replacement pickup tube that features a vent connection.  A line from this connection is simply teed into another vent 
line up near the vapor separator. 

Ingman describes the incentive for this fix as a “problem of fuel vaporization within this tank under adverse, e.g. hot, 
climatic conditions.  This results in flat spots and hesitation in engine performance.” 

 

SURGE TANK VENT CONNECTION:  There are three threaded fittings on the fuel tank in the ’83 coupe.  The main 
drain line from the tank to the surge tank requires a 13/16” wrench and the return line from the front of the car requires 
a 19/32” wrench -- both readily available at most hardware stores.  However, the fitting where the surge tank vent 
connects to the main tank requires a 1/4” Whitworth/5/16”BSF wrench. 

 

Fuel Pump 

 

FUEL PUMP CONTROL:  On both the Digital P system and the earlier D Jetronic, the fuel pump is controlled by the 
EFI ECU via a relay in the trunk.  The ECU contains a circuit that determines whether or not the engine is actually 
running and shuts off the pump if it has been motionless for more than a coupla seconds.  You can observe this circuit 
working by turning on the ignition without starting the car, and listening to the pump run; it should shut off in a couple 
of seconds. 

Note: there have been some reports that owners have gotten the idea that they are supposed to wait until the pump stops 
running before starting the car.  This is not the case; the fuel pump control circuit is only provided as a safety device.  
When starting, the key should be turned from off all the way around to start promptly. 

The fuel pump control circuit is unreliable in the 6CU ECU, and sometimes will fail to keep the pump running more 
than a coupla seconds regardless of whether the engine is running or not.  Typically, the starter is engaged, the engine 
fires, the starter is released and the engine quickly dies -- misleading the mechanic to believe that the problem is related 
to coming off the starter circuit.  Of course, like most electronic failures, it can be intermittent -- runs perfectly for a 
while, then dies, then inexplicably runs perfectly later. 

If you’d like to test this circuit to determine if this is your problem, simply remove the fuel pump relay in the trunk (the 
one without a red mark on it) and put a jumper into the socket connecting terminals 86 and both 87’s.  This will allow 
the pump to run whenever the ignition is on.  If the car now runs perfectly, the fuel pump circuit in the ECU (or the 
relay itself) was the problem.  AJ6 Engineering, Corsaro, Beckman, or any other place that repairs ECU’s should be 
able to fix this problem for far less than the cost of a new ECU. 

If you’re willing to risk the odds that you will be in an accident where you are knocked unconscious, a fuel line is 
severed, and something ignites the leaking fuel, you can leave this jumper in place permanently -- but a better idea 
would be to cut the orange wire from the 85 terminal in the socket and connect it directly to ground and reinstall the 
relay.  That way, the relay will continue to handle the heavy current to the fuel pump instead of pulling it through the 
ignition switch, as is done with the 86-to-87 jumper.  Note also that if you happen to have a leaky injector, turning the 
ignition on without starting the engine can fill a cylinder with liquid fuel, causing considerable damage the next time 
the starter is engaged. 

You might also consider upgrading to the later 16CU ECU.  See page 294. 

 
 

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FUEL PUMP POWER SUPPLY:  Elsewhere in this book there are lengthy discussions of problems with voltage losses 
in the original Lucas wiring.  The electric windows, the windshield wipers, and the headlights all commonly suffer from 
low voltage due to high-resistance connections or simply undersized wiring.  Why should the fuel pump be any 
different. 

The voltage to the fuel pump, unfortunately, is a far more serious concern.  The fuel pump provides a constant flow of 
fuel.  The engine takes what it needs, and the excess is returned to the tank.  As long as the engine is at low power, most 
is being returned to the tank, and a reduction of pump capacity would go unnoticed.  When the throttle is floored, 
however, a fuel pump working at low voltage won’t provide enough fuel to keep up, and the engine will run lean -- 
precisely when the EFI system is in “open-loop” mode and desperately in need of enrichment.  The inevitable result 
will be burnt pistons, in short order.  What’s worse, it’s quite possible you’ll spend thousands of dollars rebuilding the 
engine without ever figuring out why you burnt pistons -- and hence may do it again the next time you floor it. 

Now, today, before the next time you hammer that gas pedal, go out there and measure the voltage across the terminals 
on the fuel pump itself.  You’ll have to remove the spare tire, the trunk carpet, and a couple of panels to get at it, but it’s 
not difficult. 

At idle, the voltage should be at least 11V.  Better yet, have someone hold the engine at 2000 rpm.  At that speed, the 
voltage should be at least 13V. 

If it’s not, fix it.  For analysis, you can use the same VOM while you’re there to determine where the voltage losses are 
-- in the ground connection, in the fuel pump relay itself, in the wiring from the battery to the fuel pump relay, in the 
wiring from the fuel pump relay to the pump.  In the XJ-S, all these items are within a couple feet of each other in the 
corner of the trunk.  In the saloons, the battery is up at the front of the car, so testing each part of the circuit is a bit more 
involved. 

Better idea: just forget the testing and replace everything.  Run a new 12-gauge wire directly from the battery post to 
the fuel pump relay.  Run a new 12-gauge wire from the fuel pump relay to the fuel pump.  Run a new 12-gauge wire 
from the fuel pump to ground.  Make sure you’re providing good connections to the fuel pump relay socket, not just 
splicing into the existing wires near the terminals -- the existing connections where the terminals are crimped onto the 
wires may be the problem!  If the voltage at the pump terminals still isn’t what it should be, replace the fuel pump relay 
itself -- that’s the only thing left it could be.  All of this could be done in perhaps an hour for ten bucks worth of wire, 
connectors, and a relay.  Money well spent. 

 

FUEL PUMP NOISE:  A noisy fuel pump is a very common complaint.  The two most common causes, though, are not 
the pump’s fault.  The first is a problem in the mounting.  The pump is supposed to be mounted in foam rubber and 
connected at both ends with flexible hose so that it can vibrate without transmitting sound into other parts of the car.  
However, its near-vertical mounting often allows the entire pump to slide downward through its mounting until the 
outlet barb, within the hose, is jammed hard against the trunk floor.  This allows the vibrations to be conducted directly 
into the structure of the car.  To prevent this, the author wrapped a piece of soft rubber around the pump and held it 
securely to the pump body with a hose clamp.  The pump could then slide downward only until this rubber contacted 
the mount, and could not slip any farther.  Problem solved. 

Ed Sowell has a related problem:  “Mine was really noisy for a while. Then I found some jerk had put a floor jack 
under the boot floor pan, pushing it so that it ran into the pump. Got quieter when I pounded the floor back down.” 

The other likely cause of pump noise is a plugged pickup or other restrictions on the suction line to the pump.  See 
FUEL PICKUP on page 249. 

 

FUEL PUMP OVERHEATING:  Joe Bialy warns against running the pump with an empty tank, or otherwise with no 
fuel in it:  “The fuel is the only means of cooling the pump windings.”