1992 5.3 Marelli - Stall after warming up and odor

Steve - I replaced the coils and so I will check those after I check the amps. I am going to replace the rotor and cap. It makes sense for the next stage of the car’s life. I’ll also escalate figuring out best way to get fuel pressure checked.

I have not checked the exhaust back pressure.

The car has not suffered a Marelli meltdown. I’ve had the car since 42k miles and it ran well unitl 53k miles when the car first spluttered, which led to my vee cleanout etc.

Gary

Gary:

The key-word I used in the earlier post was “partial”. If there was a misfire on the A-bank that was unnoticed for some time, the converters on that side will be compromised. With cars this old, there is a poor correlation, if any, between the chance for this happening and mileage.

Double and triple check everything - new parts (probably aftermarket does not always mean good parts)

To be certain, pull the crankshaft and the flywheel sensor and clean them too.

Slight delay due to telephone company cutting my gas line (they’re rolling out fiber optic) :hot_face:

  1. I am revisiting everything in sequence. Re-read the Marelli section of the book as well.
  2. I changed the CPS in late 2017 (about 1k miles ago). It was faulty (low ohms). Flywheel sensor tested fine. I am going to re-test both tomorrow.
  3. Coolant Temp Sensor ohms verified.
  4. I switched ignition modules around. No impact on what is happening.
  5. I pulled the distributor cap and rotor. Photos attached. No doubt these are original. As stated I have a new Marelli rotor and a gasket on hand but did not purchase a cap.
  6. I believe its the B bank (drivers side US) that is not running good. It seems to sputter at the exhaust. The A is smooth. Both streams are hot. So it may be missing but the whole bank is not dead.
  7. After warm up the vacuum shoots to zero and the car would stall if I left revs at 2,500 or so. I’m not running long.

I have a video of when the car was running good the other day. So this seems like a loose connection or something that temporarily worked for one day. I’m looking everywhere but no luck yet.

I will have to switch the coils next I guess.

Regarding aftermarket parts - yes I agree. I have learned after many years to lean on reputable suppliers like SNG, MCL, CW etc. Although on the coils I bought SMP Intermotor.

Gary

A couple of observations I would like input on:

  1. The 92 has both an ignition ECU & a Fuel (Injection) ECU. Both have vacuum lines to them:
    Ignition ECU
    MAPS Pipe to Fuel ECU
    Fuel ECU

a. Fuel ECU vacuum pipe is connected to the center of the cross pipe at the rear of the intake manifolds (in front of firewall).
b. I believe the narrow red pipe at the rear of the A bank is connected to the Ignition ECU.

I have tested vacuum does hold at both locations.

I want to test the wiring from the CTS to the Ignition ECU. That’s where I understand it connects according to the ROM supplement for 92 and later models. The pins on the ECU schematic are 6 (CTS Earth) & 19 (CTS). I intend to check continuity on the wiring and see if the ohm reading at the pins changes when I apply temperature to a good CTS (outside of the car in a non running mode).

The ROM does not show the pin connections for the fuel ECU. I am not at all clear as to what each vacuum is used for in startup and running mode. The ROM is a hair confusing.

Under the section on 6.0s it clearly says that a FF13 code is related to MAPS vacuum and is generated from the Injection ECU - “Looks for fluctuating MAPS vacuum signal and vacuum ‘v’ throttle position”.
I cannot see FF13 referenced elsewhere so my interpretation is that the 5.3 works the same way and that the FF13 code pointed directly to a vacuum problem detected at the Fuel ECU. Quote “Looks for fluctuating MAPS vacuum signal and vacuum ‘v’ throttle position.”

I understand the FF13 is the end result and the loss of vacuum can be caused by faulty CTS/Flywheel Sensor/wiring etc. rather than just vacuum leaks.

If I check pins 3 and 13 for the same OHM reading as the connector at the firewall (.680) that should validate the wiring from the connector to the ECU.

Likewise 1 & 2 for the CPS.

It would be nice to see pin connections on the fuel ECU too.

Input?

Gary

Bottom of the page:

The 6.0L is slightly different with regard to a.) but b.) is the same – the thin red tubing goes to the passenger side footwell, where the Marelli ECU is.

0L

Good link Steve. Not sure why I had not seen that. I will have to digest fully over a beverage or 2. :joy:

Perhaps the most important thing I spotted was the following related to the early Facelift model 26CU:
“It should be noted that the factory repair manual contains many discrepancies in the section devoted to the 26CU system”. No wonder the ROM reads like scrambled gobbly gook in the supplements.

I see the 26CU has a coolant temperature on pin 5. The Ignition ECU also shows that on pin 19 (from the sensor). I know the I-ECU sends the speed over to the F-ECU. Need to understand more how these ECUs function for future troubleshooting. Confused.
:tired_face:

I’ll ping Roger for some clarification.
Gary

Well…no work on car today but some reading of ROM/CD.

The ROM has a write up on the FF codes that were introduced with the Facelift models.

My initial understanding was that the vacuum line (red) to the Ignition ECU is used by the latter to check threshold pressure readings below 1000 rpm (must be below 80 Torr/3.1Hg) and below 1500 rpm (must be below 550 Torr/21.65 Hg). I hope my conversions are correct.

But that would mean the main the Ignition ECU has a pressure sensor?

ROM Quote - “This OBD function carries out 3 checks on the MAPS signal pipe and vacuum signal”.

There is a flow chart schematic for tracing the source of the code.
a. Pipe damaged/plugged?
b. Vehicle ran out of fuel?
c. Fuel pressure under load correct?
d. Is flow restrictor fitted in vent pipe from fuel tank to Rochester valve?

When the code is set the MAPS function is not used. Fueling is controlled by Throttle angle (TPS signal) and engine speed. O2 sensor feedback is also disabled.

But the last entry in the flow chart says “Replace Fueling ECU”.

It’s clearly important to check the vacuum on both ECUs and get rid of the code as a first step.

My speculation (always bad) that the CTS or Flywheel sensor could relate to the FF13 code were wrong.

Gary

For posterity, here are the FF codes and the corresponding troubleshooting diagram.
I am not 100% sure about it, but from what I remember, the Lucas fueling ECU (36CU for the 6.0L and the 26CU for the 1992 5.3L) is the one that sets/stores the DTCs

6lv12-1994-alldata-36cu.pdf (155.2 KB)

That’s different to the ROM on the 5.3 (26CU).

FF13 Code Facelift XJS.pdf (41.8 KB)

The PDF I attached in my earlier post is generated from ALLDATA (the terminology there is different - PCM vs ECU), the codes are the same though.
The one you posted is a copy from the Jaguar manual but very hard to read (low res)

It’s pdf - just increase resolution to say 400 pct.

Where are you located Gary?

South Carolina. Near Greenville.

Ah, too bad, I’m in New Hampshire. Got a 92 also and it’s great to have another XJS to swap parts.

Here are images of the 2 ECUs referenced above on the facelift models. Searching through the archives this same question about the FF13 code and the 2 vacuum lines has been asked. I’ve not found a definitive answer as to the specific function each one enables. I speculate the red vacuum line (“signal line”) is used to check / set the diagnostic code if vacuum is out of spec., as it only appeared on the diagnostic enabled facelift models. (Meaning there is a diaphragm in the ECU).

Solutions often are CTS/CPS/Flywheel Sensors faulty or faulty wiring/connections. Fuel pressure /Low fuel also appears on the FF13 Troubleshooting flow chart.

At this point is seems logical to make sure the lines are clear, vacuum pressure is the same at the ECUs as the origination points and that the ECU’s hold vacuum when a pump is directly attached to each.

Interesting that the parts listing at Jag Classic Parts does not show the vacuum connection on the Fuel ECU but it is there.

At this point I need to install the rotor and cap and re-connect plug leads and then check all vacuum lines to ECUs and sensor connections/wiring/ohms and go from there.

Fuel pressure rig needs to be developed too.


How did this notion arise? It’s an ignition ECU, it has to sense vacuum in order to control timing.

Not sure I follow - the pre-facelift models did not have an ignition control module with a red vacuum line, correct? “Notion” relates to the functions of that red vacuum line and how FF13 is set, as opposed to references to “MAP sensor” in the Fuel ECU - 26CU.

2 Map Sensors, 1 in each ECU correct? Make sense or are we on different frequencies?

Gary

Found this description of the Ignition ECU on the Volume 2 Supplement on the CD. It relates to the 6.0 but should be similar on the 5.3.

ALL Marelli ECU’s must have a vacuum line. I dunno if it’s red or not, but they simply cannot provide the equivalent of a vacuum advance function if they can’t sense vacuum.