Chronic Check Engine Light on and Very Rough Idle/Hesitation

This is going to be lengthy, so apologies in advance.

I have a 1994 XJS 2+2 Convertible 4.0 that I purchased in 2007 with 37,900 miles.
Current mileage - 205,128.
The vehicle has been well maintained and any issues that have arisen were dealt with immediately.
In November of 2023 the check engine light came on and vehicle was idling rough, hesitated under load and running rich.
Because my regular mechanic was unavailable due to scheduling and relocating his service facility, I had to utilize another garage (I had been at this shop before, and they did acceptable work).
Between December 2023 and February 2024, the car went through the following diagnostics and parts replacement:

  • Removed and replaced EGR Valve and gasket. The engine idle remained rough.
    and after 15 miles of test drive the check engine light came back on.
  • Various inspections were made and it was determined that diagnostics were
    needed to identify the problem issue(s). Over the next 7 weeks the following
    Computer Diagnostic Testing - Levels 1, 2 and 3
  • With each diagnostic level completed a different component(s) were flagged as a
    possible problem, and the following parts were replaced:
    EGR Temp Sensor
    Fuel Rail was removed and tested - NTF.
    Fuel Injectors tested - NTF
    Removed and replaced distributor cap, rotor, and ignition wires. - No Change
    Replaced spark plugs - No Change
    Inspected TPS Sensor - NTF (Sensor had been replaced about 6 months prior)
    Removed and replaced Idle Air Control Valve - No Change
    Throttle Control replaced - No Change
    Bench tested Ignition Coil and Ignition Control Module - tested within specs.
    Checked Air and Gas Filters - Good condition -
    (both filters have approximately 7,000 miles of use)
    Over the 7-week period of being diagnosed/serviced the vehicle was road tested
    for exactly 807 miles.
    Check engine light would not go off and idle remained rough, hesitated under load and ran extremely rich.
    Removed the vehicle from this service garage.

March 2024
My regular mechanic had completed his move to the new facility. He did the following under warranty from previous services:
New ignition wire set installed.
New OEM distributor cap installed.
New NGK spark plugs.
He recommended that the Ignition coil and ignition control module be replaced. He felt that even though the modules tested in the acceptable range that due to the age and mileage of the vehicle it “could” be the issue.
Also, he could not run a timing test as the idle is too rough and engine too erratic.
At his suggestion I personally ordered and replaced these parts.
No change to rough idle, hesitation under load and running rich.
However, the engine turnover and ignition is immediate - that is the only improvement.

If there is any advice that can be offered to resolve this issue based on the information provided, I would greatly appreciate feedback and direction that can be relayed to James my mechanic. (I do not consider myself to be a mechanic, I only do what I am comfortable with and leave the rest to professionals).

I am advised by my mechanic that the next logical step is to replace the distributor and to perform another engine diagnostic to clear the engine codes.
Before I go the expensive distributor replacement route I would like to know if there are any issues, setting, adjustment, etc., that I am unaware of and could resolve this problem.

Apologies again for the length of this post.

Thank you,

It seems most bases have been covered , I’m not a AJ6 owner

quick questions :

  • has the rotor arm been changed at the same time as the distributor cover ?

  • when the enngine light is on, what code is shown ? (should be visible on the travel computer when contcat is on, before starting the engine)

Yes, the rotor arm has been replaced - twice, just to be sure.
The check engine code is - FF24

#FF24 # ignition amplifier drive circuit
according to :

so worth checking the circuit and connectors along this ?

also clearing the code and check if it come back the same or not ?
(disconnect the battery for a few seconds, then reconnect)

Thank you, will run this by my mechanic.
Appreciate it!

Has the catalytic converter been checked for being plugged?
Have the mechanic[s] done a compression and or cylinder leak down test?

The cylinders have had compression/leak down tests, and all was found to be good.
I do not know about the catalytic converter. I will bring this up to the mechanic.
Thank you, never even thought about the catalytic converter!

I think this is an AJ6 engine and has a crankshaft position sensor. If so it’s the first thing I’d change.

The TPS sensor was replaced in October 2023. The sensor was also inspected, cleaned and deemed in proper working order. However I will ask that the sensor be reinspected.
Thank you,

Do you know if anyone has altered in any way the distributor body mounting position?

No mention of anything associated with the MAF device.

Hate to say it. but you may need to go back to basics and start all over again.(compression,valve timing ,firing order etc. etc.)

I used to have to replace my TPS twice a year. Kept one in the trunk at all times because you never knew when it was going to go. When the car hit 200k, paint was gone, interior was shot, I parked it. Put probably 120000 miles on it and it was still going strong, very reliable… except that TPS.

The distributor has not been removed or messed with other than new rotor, cap, etc. since I have owned the car.
As far as the MAF, I was told it was in working order. However, based on the feedback received, it may be time to replace the MAF anyway.
Thank you for your input.

Steve - there is cleaner for the MAF unit which may very well help, before replacing the unit - ask your mechanic to maybe clean it and then test again - the cleaner spray that is made just for the MAF is what to use - Tex.

Tell your “so-called” mechanic to stop replacing parts!!! These engines are not that difficult. Where are you located?
You obviously have a good spark. I assume that has been checked.
Has the fuel pressure delivery been checked?
Dismantle the air intake hose and clean the throttle body, make sure there are no splits in the rubber hoses or vacuum lines.
When it is idling, you can listen for air leaks, could be coming from the intake manifold.
The distributor does not need to be replaced.
Has anyone reset the potentiometer? this is a simple task that probably should be done.
All of the sensors and modules have a testing procedure, seems like your mechanic is playing a guessing game at your expense.

1994 XJS 4.0 Check Engine Light and Rough Idle Issues

To all who have responded to my inquiries over the past months regarding the chronic issues with the check engine light and rough idling, thank you very much!
I have investigated all suggestions and taken action as appropriate but have yet to resolve the issue.
As I stated in a past post, I do not consider myself a mechanic, so I do what I can and when necessary, rely on the professionals.
I have been chasing this issue since November 2023, and I have yet to find a resolution. Please allow me to recap the issue and actions taken to date 07/05/2024, and apologies for the length of the ensuing post.

Mileage when issue began 104,751.
Current Mileage - 105, 450

Check Engine light came on with error code FF24.

Vacuum testing done 3 times once with dye, 2 times without - No leak identified.
Performed Levels 1, 2 & 3 Systems Diagnostic Testing - No Specific Issue Identified
Removed and replaced fouled plugs with NGK plugs - Twice
Replaced Ignition wires to plugs - OEM
Replaced ignition rotor and cap - Twice - OEM
Replaced TPS
Replaced Ignition Coil - OEM
Replaced Ignition Coil Module - OEM
Replaced EGR Temp Sensor, EGR Valve Motor, EGR Vacuum Sensor EGR Base and Gaskets.
Removed Fuel Rail and Tested - Tested to Specs
Tested all Fuel Injectors - Tested to Specs - NTF
Replaced Temperature Sensor
Replaced Throttle Idler Switch
Disassembled and cleaned Mass Air Flow Sensor.
Removed Throttle Body Assembly, Cleaned, Confirmed Operation and reinstalled.
Catalytic Converter tested with electronic sensors - car road tested for 100 miles in city traffic and freeway driving - NTF - all readings within specs.
Compression tested cylinders - All within spec and good compression.
Added a BG Engine Performance Gas Additive - No Improvement to engine performance.
Latest Diagnostic test shows no new issues other than engine is running “Lean”.
Prior to November 2023, MPG was 16-18 City and 21-23 Freeway/Highway
Now mileage 8-9 MPG regardless of City/Highway driving.

An aside note - If battery is disconnected and reconnected after about 5 minutes, check engine light goes out but engine runs rough. After about 5 minutes, check engine light comes back on.

As I stated in my opening, when the check engine light came on in November 2023, the Error Code was FF24.
For a brief amount of time after working on the engine, the check engine light did go off, but the engine continued to run rough. After about 30-miles check engine light came on again - this time Error Code FF44.
Rechecked work performed and connections road tested and Check Engine light returned, and Error Code reverted back to FF24.
Last week went back recheck fittings hoses, reattached battery and road tested.
Check engine light returned with an Error Code of FF44.

I no longer know what to do to resolve this issue.
A mechanic who I have great trust in and regularly services and repairs Jaguars, Range Rovers and Rolls Royce, said this YOYO code issue has him stumped as diagnostic information for this model year does not provide definitive guidelines for the 1994 model year XJS as opposed to a 1996 model year XJS.

Any insights the collective may have in potentially resolving this will be greatly welcomed.

Once again, apologies for the length of this post.


“Computer diagnosis” w/o an OBD II system ? :confused: I’ve SO wished my '94 had one, so I could do just that … instead of being limited to those two dozen or so J.D.S. error codes … :angry: