1992 5.3 Marelli - Stall after warming up and odor

(Steve) #21

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.


(Geekay) #22

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.

I’ll ping Roger for some clarification.

(Geekay) #23

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.


(Steve) #24

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)

(Geekay) #25

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

FF13 Code Facelift XJS.pdf (41.8 KB)

(Steve) #26

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)

1992 Marelli - Flywheel Sensor - Fuel ECU Engine Speed
(Geekay) #27

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

(John) #28

Where are you located Gary?

(Geekay) #29

South Carolina. Near Greenville.

(John) #30

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

(Geekay) #31

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.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #32

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

(Geekay) #33

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?


(Geekay) #34

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.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #35

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.

(Paul M. Novak) #36

You are wrong. The attached picture shows the red vacuum line leading from the rear of the right intake manifold to the Marelli ignition ECU in my wife’s pre-facelift 1990 XJ-S convertible.
Why did you think the pre-facelift Marelli cars did not get a vacuum signal?


(Geekay) #37

Kirbert & Paul Of course all the Marellis have the vacuum line. Duh! Senior moment on my part. The addition of the diagnostics codes is totally unrelated. I was on totally the wrong frequency. Duh…for a 2nd time.


(Paul M. Novak) #38

No worries. I just wanted to avoid any misinformation in the email string. I am glad we are all on the same page now. :slight_smile:


(Steve) #39

You probably are not reading everything that is posted here, I explained earlier what is what and posted a picture (annotated)

(Geekay) #40

This is what I was missing. Has the overview I needed and a much easier high level read than the Rom. Also locates the “Rochester Valve” referenced in the troubleshooting flow chart.

See - Jaguar XJS 1992 Technical Introduction