Hi: I'm 87 years old and I retired a few years ago from restoring old Ferrari cars, the operative word is old, none of these cars were fuel injected. The last Jaguar I owned was in 1951, a 1947 Mk1V, I recently bought for next to nothing a 1989 Jag XJS V12, It had not run for years, it has Marelli ignition on it. I'm an old "engine" guy and i dragged the car home and finally got it to run on all 12 cylinders. Its running incredibly rich, seems not too bad at idle but once the throttle is opening, it goes way rich and to use an old motorbike term, the motor 8 strokes.

I replaced the Aux air valve and fitted a new coolant temp sensor, Still too rich. I wonder if any of you tech guys can offer a suggestion as to what I might look at next. Thanks B

 

Hi Billy

The Vacuum Pipe to the Fuel Pressure Regulator

 

Have you checked that the injectors are actually cycling correctly and none are stuck open.

 

AND

Check the Vac supply to the 2 ECU units. Less Vac = More fuel, maybe the hose to the boot is plugged with aliens, spiders etc.

The EFI loom is highly suspect, and age is the culprit.

TPS out of range, a real probability.

Read my Sticky on "no start........." at the top. OK, its Lucas related, but the EFI system is teh same.

The Injectors fire in 4 banks of 3, NOT sequential in any way.

1.3.5A
2.4.6A
1.3.5B
2.4.6B
Simple.

There are sensors on the crank, front and rear, but of its running, they are deemed to working just fine.

Dirty connectors, Resistor Pack, ECU Multi plugs, etc etc, all reek havoc with these beasts.,

Then there is the BEER, for you, not the car, and you will need a few slabs, trust us all on that one.

 

The single most important sensor for the EFI ECU on the car is the coolant temp sensor. (Though I see you've fitted a new one.) IF the resistance is too high (due a bad sensor or corrosion at the connector) the ECU will think the car is cold and massively over fuel, sometimes to the point the car will not run. You could literally unplug EVERY sensor on the car except for the CTS, and as long as it has the RPM signal from the amp (Marelli ECU) it will run (badly but it will run). That includes the second most important sensor (that can also make it run rich) the MAP sensor in the ECU. The load signal is taken off of the nipple in the middle of the balance pipe at the rear of the engine and conveyed through a vac pipe to the rear of the car where it enters a small damper reservoir before going into the ECU. If that reservoir splits, or the hose is disconnected, the ECU will assume atmospheric pressure making it think the car is running at full load, and thus over-fuel. It may, in the end, be worth inspecting the CTS connector, OR measuring the Ohm value on the pins at the ECU to see what value the ECU sees at X temperature.

Mechanically a split diaphragm in one of the fuel pressure regulators can cause fuel to be sucked into the manifold via a vac hose. A sniff test can check that. Yes a disconnected vac hose to the FPR can cause fuel pressure to rise, but that would usually cause bad MPG, not super richness. It might be good to check the fuel pressure inside the rail (connected in-between the fuel pressure regulators) to see it remains somewhere between 28 and 32psi (ish). At this age, all those hoses probably need replacing anyway to pre-maintenance against a fire.

Other sensors: TPS - think of this as acting like an accelerator pump in a carb. It's not exactly that but it's good enough for a simple explanation. It also tells the ECU when the throttle is closed. You should have about a 4000Ohm swing on the TPS from open to close. The 5V signal passing through the TPS should voltage divide to measure at 0.32v (about) closed. A bad TPS will often cause the car to stumble or hesitate upon TPS opening as the ECU doesn't immediately know to enrich becuse of that, and instead has to wait for the MAP/Load signal via vacuum. The IAT trims fuel, but just by a little, not NEARLY as much as the CTS. The O2 sensors only come into play after warm up and only influence trim. There is a WOT switch and full load vac switch which do help the ECU produce an extra burst of fuel under such conditions.

The Marelli ECU does have it's own CTS input (separate) but it's not as important as the critical front timing sensor, and secondarily the at-the-flywheel RPM sensor. There's also an air temp switch to retard timing in some conditions, as well as a throttle switch which tells the ECU the throttle is closed and at idle for the idle timing map.

Remember as a rule on this car, the ignition system is separate from the EFI system, and the EFI system is dependent upon the ignition system working. Just that rule will help you in the future.

~Paul K.