Lumpy '75 XJ5.3C

All, I have a 1975 XJ5.3C which is a very good original car (68k miles) but last couple of years not really smooth running.

I have checked / cleaned / replaced injectors
Replaced engine harnesses
Replaced dizzy cap, dizzy arm, plug leads
Replaced all capacitors in ECU
Coolant temp sensor swapped
Tried swapping coil
Tried swapping ECU for a NOS ECU
Checked fuel pressure with a cheap and simple gauge and seemed OK
Checked timing at each cylinder at warm idle and no obvious spark missing when engine falters for a second or so

Still lumpy idle and not turbine smooth at higher rpm either, accel definitively rough compared to earlier form. Usually it runs quite well just after a cold start. Most often it is the RH bank which is lumpy. The tell tale exhaust pressure against the hand is usually smooth from the LH bank and lumpy from the RH bank. Engine temp OK at all times, idling in a queue on a hot day with AC running and motorway runs for hours all se a nice stable temp with the needle just below “N”.

The mostly RH bank symptom in my opinion excludes the ECU and amplifier as IIRC each output drives six cylinders, three on each side.

Heads have been off and cleaned / checked.

It should be noted that running a slightly later ignition than per book gives more power but a slightly rougher running, no other ill effects to be seen.

That leaves two possibilities in my mind:

  1. General under fueling
  2. Ignition problem mostly related to the RH bank

Can anybody shed any light on this based on your experience?

Have you done a compression check? If you haven’t, you should. If you have, what were the results?


I cannot remember the values but they were within spec.

Have you looked at your MAP sensor?

Well you have covered a lot of possibilities but what seems to stand out is the low mileage. 68 K over 45 years would indicate she has done very little work. Have you tried the “Italian tune up”? It is surprising how this can de-lump a motor.

Tops off for gaskets means cleaned out tops.
Any good tips on simple test for the MAF?

A '75 V12 won’t have a MAF, it has a MAP sensor mounted on the fender well and connected to the intake manifold by a hose. The simplest test, I think, would be to see if it leaks.

It has been a long time but the concludion is that the injectors (currently 1, 4 and 5 RH bank) are not working properly. I have had them all out mote than once and even if I get them to give the correct amount the spray pattern deteriorates quicly amount following shortly. So based on the high price of new ones I will build a new fuel rail using mopar injectors at USD23 a piece. A bit of tuning of the ECU may be necessary but being an old school electronics engineer I am not worried.