The car in question is a 1991 XJS 5.3 with 85000 miles. New plugs, injector harness and fuel lines about 3000 miles ago.

The problem started with a stumble when turning left. Progressed to stumbling in straight lines and has gotten progressively worse as we have been working on it, at this time it will no longer idle. We have Cleaned the sump tank and filter, replaced the fuel filter. No real change. Removed and cleaned the front and rear crank sensors Both read 650 ohm's with no steel in front of them and when steel is put in front of them they change about 20 ohm's, briefly, then return to about 650 ohm's. I noticed the front sensor was magnetically weaker than the back so I ordered a new one and installed it, no change. We have 13.5 volts at the coils. Removed and cleaned the cap and rotor, nothing significant there, no change. Checked all connections. The water temp sensor needs to be replaced according to the test methods I found online, I put a 670 ohm resistor in place of the sensor, it didn't change the issue. When it would idle, holding one hand over each tailpipe I could feel the stumble in both pipes. The stumble feels like it is missing on 2 or 3 cylinders in a row. It is very abrupt. It seems to shut down completely for a couple sparks and then starts firing again. I am going to do a fuel pressure test tomorrow. After that I'm looking at checking the woodruff key in the front of the crank.

Anyone have a suggestion about what I can check before I start taking the front apart?

 

I am not familiar with the fuel and ignition systems on the 91, but I would try using propane, carb cleaner or something similar to provide fuel to the engine and see if it runs better with extra fuel provided. If no better, keep looking at ignition but if improved, you know you need to look at fuel pressure, temperature sensors, MAP or MAF sensors. Testing fuel pressure is a good direction to go.

 

In this order, what about maybe:

• clean and test the injectors,
• change plugs and HT leads,
• check the low tension lead to the coil as the connector /wire breaks in the rubber connector cover,
• swap coil,
• swap amplifiers

Greg

 

Regarding the amplifiers... Volvo uses the Bosch version on nearly all fuel injected and cat'ed 4 cylinder models (240, 740, 760, 940, 960) from around 1988-1998... They cost a fraction and are still available... Like here:

http://www.skandix.de/de/fahrzeug-teile/elektrik/zuendung/zuendgeber/zuendmodul/1008286/

 

Being Marelli, you have very systematically covered most common issues.

A couple of other issues I have dealt with on a '91 in Manila by remote control are:

Vac hose/s to the ECU units is blocked/split/whatever.
Patch lead that connects the Marelli ECU to the "fuel ECU' in the boot is having a female moment.
TPS is simply dying.
The multi pin plugs of the ECU's are grubby, and a clean with lemon juice and reseat will do no harm.

The woodruff key is a good thing to check, as they do shear. Usually has an associated "knocking" noise when working loose, but not always.

That fuel pressure test is certainly needed.

 

For what its worth, I've said for quite some while now, that anybody with a Marelli car, needs to check the spacing of the front and rear sensors. My car was way out. They are really little magnetic generator. A magnet loses its magnetic ability under heat. If the "sensor" is too far away from the flywheel, or the three fingers on the damper, even if it shows good, the A/C signal it sends is weakened. I put mine as close to .020 as I could, and it runs perfectly. It took me quite some while to track down the problem, but that was it.

 

Aren't the ignition modules in the rear right of the trunk? Turning left could knock them around and fiddle with the connections possibly. Worth double checking anyhoo

 

Ignition modules are on the radiator support. I really can't imagine why the car would only show a problem when turning. The only thing I know of, that is near the steering rack, are the O2 sensors. And they aren't part of ignition on a 91 XJS.

I will stick to the spacing on the front and rear sensors, IF the wires, plugs, cap, rotor and plugs are good. Then again, maybe a coil breaking down?

 

First off, Thank you all for your input. I have checked through most of the items listed, only one thing seems odd. I hooked the pressure gauge to the line connected to the first pressure regulator, just the fuel line into the gauge. When I turn the key the pump comes on and the gauge pegs out, 100psi+. Then when I start turning the gauge out of the fitting, very slowly of course 100psi and all, I get bubbles, lots and lots of bubbles like foam. I'll find the leak and purge the system in the morning. if that gets it running great if not I'll get back to the items you have listed. Thank you again for the pointers.

 

Is the Jaguar pump even capable of 100+ lbs? Can this be a blocked return line?

 

Dead ended it will, but that really proves nothing as far as rail pressure goes.

You need to "T" that gauge into the same line, then run the engine and observe the pressure readings. About the 40psi mark, give or take, is what you are looking for at the moment.

Then you need to establish "FLOW", and that is simple enough. Place the hose TO the rail into a jar, maybe an added length is needed, and turn ON the ignition, and that 2 second prime you get should give a good amount of fuel in that jar, thus indicating you got flow.

Also mentioned is a blocked return line, COMMON enough. Not blocked, but crushed by errant tyre fitters using the wrong jacking points. Look under the LH side of the car, you will see a metal pipe about 10mm diameter, running the length of the car. That is the fuel return line. Look around the front jacking peg, and see if there is any crush damage.

Then look in the same area on the RH side, as that pipe is the "supply" pipe, and also gets damaged.

 

Found both of these pipes damaged on Paul's "Blue Goose" this week, but not crushed enough to cause a problem. About 2/3'rds flattened.

 

superchargedtr6 my sensors don't have any shims under them, should they? Did you have to grind down the mounting flanges to make them adjustable?

Fuel pressure in the rubber line leaving the fuel rail, before regulator, is 38psi while cranking, it won't start.
Fuel line unhooked from first regulator fuel flow put about 1 inch of fuel in a 2 liter bottle during the 2 second prime.
Inspected the connectors on the computers in both buttresses and the one behind the right kick panel all are clean and dry.
Fuel lines under the car are undamaged.
TPS 6.5k ohms down to 650 ohms at full open.

Going out to check injector pulses now.

 

I think the fuel system is OK. We have fuel pressure and flow. We checked for injector pulses from the harness with an LED and checked the injectors by opening them with an external power supply and watching the fuel pressure gauge. I also rechecked the CTS turns out it is working fine.
So on to the ignition system.

 

I am going out on a limb here but have you cleaned your IACV?

 

I filed the mounting flanges until I got it right. I completely rebuilt my Marelli ignition harness, tracing down its function, and potential problems. I used old school thought of how a magneto works. When NOTHING else made this car run right, not even new sensors, filing the mounting flanges, made it fire right up. Maybe when the cars are new, all new amplifiers, coils, ECU, everything works perfectly. But after time, heat, mean hours of operation?, I figure the tables that are programmed into the ECU on the variables, becomes distorted, causing the ECU to provide faulty signals to the amps. Its really not as bad as it seems, once I dug it up, saw what made it tick, and put it all back.
I will continue to tell anybody that has a Marelli car, they NEED to perform this, almost as you would perform any service. If and when I ever have a "sensor" go bad, I may have to cut some shim stock to space it away, but for now, it works great.

 

Is it normal for the rotor to be able to rotate and raise when twisted by hand? Seems to me it shouldn't.

 

Ignore belowas you said it will not even idle

As the stumble seems to be motion induced, while it could be fuel, I am inclined to think it is wiring harness worn/shorting or poor connectors, somewhere. Would try to move any loom and see if there is any movement and try tying it in a slightly different position. and tryng

 

NO, it should be fixed to the rotor post with 2 allen head bolts.

You need 3 things for your car to start
Fuel
Ignition
and Tach pulse

What SC6 says is good advice if your VR sensors (crank or flywheel) are too fat away from the trigger wheel these will cause problems.

The front sensor is the TDC index. It tells the Marelli (ICU) when A bank pistons are TDC and to spark. The ICU then calculates TDC for B bank. This sensor could cause miss firing.

The flywheel sensor is what the ICU uses for its tach pulse (engine speed). The ICU then passes this TACH signal onto the Lucas ECU in the boot.

Both of these signals run on shielded cable. Once you have checked the gap on the sensors check these cables for damage or corrosion.

If the ECU has no tach it will not fire the injectors. Simmilarly if the ICU has no tach it will not fire the coils, and obviously not pass the tach on to the ECU.

 

Warren, when I rebuilt my Marelli wiring harness, I found that the tach seems to be independent of the Marelli working. The tach feeds off of A bank coil. If the coil isn't getting a signal, then the tach won't show rpm. But the ECU has a independent wire, which is also shielded, that sends a signal to the Lucas ECU. The main three things the Marelli seems to need, are the air temp sensor, and the front and rear sensors. Mine would actually start and idle with the TPS unplugged.