Hello.

I'm a beginner. I introduced myself.
I present my problem to you.

The Car: XJS V12 March 1991 Cat.
Issue : missfire or too poor (I don't know exactly if it's missfire.) on bank B. On bank A all it's OK.
When :
- when I start the Engine => bank A Bank B OK
- After approximatly 40 s => regulation start and RPM change a little and the defect arrive.
- in the exhaust you can hear => RRRRRRRR poum RRRRRR poum poum RRRRRRRR poum RRRRRR poum poum poum RRRRRR ......

Conclusion => i Try to find a solution.

New parts
- Fuel Pump
- Regulators pression fuel
- 12 Sparks
- 12 ignition harness
- Power module A and B
- Coils
- TDC sensor
- Coolant temperature sensor
- all injectors have been cleaned => ultrasonic
- new injector harness with new connectors (2 independent harnesses => 1 for A bank and 1 for B bank)
- New lambda
- Air Temp sensor
- New seal for injector

Not new parts but they have been checked
- ECU ingition and ECU 16 => OK I have spare parts
- Distributor Magnetti Marelli => Not new but seems OK
- Catalytic exhaust is not blocked
- ACC OK
- Power resitance OK => because I swichted injector harness (because I separted A harness and B harness for injector) and lambda A and B. The problem remains on Bank B.
- Head cylinder => I removed B bank head cylinder to check valve. => all it's OK
- compression => OK

Not sure
-throtlle potentiometer
- Engine speed sensor => I checked with oscilloscope but i can see all teeth but I can see ondulation (wave) on signal.

But now, I don't know what I can do....

I think the B bank it's too poor.
My sparks are white.
When you check without Lamda sensor (Open loop)=> the problem is the same but it's less marked

When you disconnect depression on ECU the sound of engine change and the fault becomes very weak.

So I need your help!
Thank you.

 

Mmmm, sounds fuel related.

I have NOT done this with a Marelli car, very few where I live. Might make a difference.

 

The fuel is new.

 

I'm sorry, i didn't see attached part.
Yes I try to ajust with the potentiometer, but no change. And it's seems to be correct, because A bank it's OK?

 

I'm facing a similar issue with a '89 Marelli, but the occasional drop-outs occur in bank A. Emission tests are OK. I suspect there is an air leak between the inlet manifold and the valve block causing one or more cylinders run lean when idling. I also have problems getting the idle speed low enough.

Jukka

 

Thank you. I changed my seal between head cylinder and inlet manifold. It's not the solution for my Xjs

 

Have you changed over the amplifers for the ignition, side to side, I understand this is just a matter of swapping the leads over. I know nothing about the Marelli system, but it sounds like ignition to me.
The other thing you can do is to remove the injection leads from each injector on B bank, one at a time, as check if removing one does NOT result in an engine note change - indicating that injector is therefore faulty.
You can also do the same with the plug leads,

 

Hello
Concerning Amplifiers I have Old parts and new parts. I changed side by side no difference. I put an oscilloscope on diag connector to check B Bank coil => result is perfect!

For injectors I check all injecors like you proposal, and all connectors.

The issue is not localized on 1 cylinder, It's on all bank B.
Poor mixture or something else.
But I don't know why the lambda sensor doesn't adjust the mixture? And when in it's open loop, why i have this problem.

 

Have you checked the throttle valves? If they’re not set properly it can cause idle issues. There’s a balance pipe between the two manifolds, but if one is closed too much I’m not sure that will fully compensate. It may be totally irrelevant, but worth setting them both anyway.

 

Since you have an oscilloscope, have you checked the spark on the B bank? I'm wondering if it is spark related, because the distributor is a 2 level distributor, essentially a pair of 6 cylinder distributors stacked on top of each other. Could be a defective cap or rotor? Maybe also try swapping the coils from side to side and see if that makes a difference?

 

Thank you for your answer.
Indeed I always have a doubt of knowing if the ignition works perfectly on the bank B with the famous Cap Magnetti
How do you manage to check that the ignition works correctly for all cylinders with an oscilloscope.

 

Hi Nicolas

Since we are 'Clutching At Straws' here, I would think that the 'Non Firing' of 'B' Bank could be the 'Flywheel Sensor' that is the one that is at the Rear of the Engine

The Crank Position Sensor, which is at the Front of the Car 'Fires 'A' Bank (which is apparently working ok on your Car)

But what Fires 'B' Bank is a derivative of the Spark created by the ECU for 'A' Bank which is where the 'Flywheel Sensor' (The Rear One comes into play)

That's the very simple explanation and so a New Flywheel Sensor (The Rear One) could be what you need, where you may be pleased to know that this can be around half the Price of the 'Crank Position Sensor' (At the Front)

Even if that doesn't solve your problem, it is always handy to keep one as a Spare, as in an emergency situation if the Front Crank Sensor goes down, which will cut the engine in a 'Blink' you could use a Rear Flywheel Sensor as a Temporary Fix to get you home

 

That I don't know, as I don't have a scope! Perhaps measure the low voltage at the coil and see if you can see the even firing pulses? Another trick I have used is an inductive timing light, and put the pickup on all the wires and see if they flash at the same rate. I've found dead spark plugs that way.

 

Have you tried the distributor rotor? Being a Marelli it CAN shorten out. The cap is seldom a problem.

 

Not quite OB. The front sensor is the ignition sensor. The rear for the engine ECU. So the rear one is also on Lucas engines.

 

Hi Nicolas

If a Rear Flywheel Sensor doesn't solve the problem, you could always invest in a 'Noid Light' which are fairly cheap to buy on ebay, as at least that will tell you if the ECU is Firing the Injectors

 

Hi Daim

We will have to see but I think the Rear Flywheel Sensor could be the problem, even if I didn't explain it very well!

 

Does anyone have an idea what the Marelli diagnostics socket signals are? One is the primary coil side from the bank "B" ignition amplifier but two signals are coming directly from the Marelli ECU (ECU pins 20 and 21).

I'm going to get/build a capacitive "secondary ignition probe" to look at the high voltage side of the coils with an oscilloscope. From the waveforms one should be able to see what is going on in the cylinders.

Jukka

 

No crank/flywheel sensors on Lucas v12s Daim.

 

Hello
Thank you for your answers.
I Bought a new 'Flywheel Sensor' this morning.
I will test your proposals at the end of this week.
1- change mixture on ECU
2- new flywheel sensor
3- check cap Marelli
4- try to test with my stromboscopic lamp if all cylinder is OK and if it's not possible to use the harness of the lamp to connect my oscilloscope.

Thank You
Nicolas