Multiple misfires with new ECU

Hello everyone, I’m new to this forum. What has brought be here is a problem I really cant seem to figure out; I turn to the enthusiasts for help :slight_smile:

I had water ingress into my ECU which caused it to not start what so ever, so I went and got a new one with matching numbers and all (LNB 1410 CC/3–…AA/13). I also soldered on a new black connector as it was extreamly damaged.

The car now starts, so im greatfull for that, but I have three missfires. Cylinder 2, 3, and 6 (as read from the back of the engine) wont spark. The other three will. I’ve ruled out the coils and the spark plugs.

I really have no clue what to do, I’m not good at mechanics. This is my first car after all. I do have a lot of will power though and will see this trough.

I’m hoping to get any suggestions on what to look at. I might add that there are no mechanics in my area that work with these cars so im on my own in that regard.

For full disclosure: I have a thread on this already on another forum but I really just want as much help as I can get. Thanks!

First thing is to clear the 4 drain tubes for the sun roof as this is generally the root cause of this issue!
IIRC others have rerouted wiring into the original ECU and this has worked.

See: Jaguar XJ6; Jaguar X300; Engine Control Unit; ECU

Scroll to bottom for ECU

I unfortunately do not have the original ECU anymore. Rookie misstake to get rid of it…

Just so we can talk about the correct plugs, the AJ6/16 engines are numbered from the radiator back, this changed at the change over from the XK engine.

Thanks for clearifying, the ones not sparking would be 1,4, and 5 so.

My collant level light is also on, im not sure if it’s related

There is no commonality between those cylinders to cause misfires that I can think of. Can you withdraw the injectors with the fuel rail? I know it can be done on the AJ6 engine as In did it to make sure all the injectors were firing fuel.
Reading your text I am assuming you have the AJ16 engine with seperate coils over the plugs, how did you determine that the plugs and coils were OK?
I pulled the rail and injectors and placed a layer of cardboard underneath and cycled the engine, this way I determined that they were all throwing fuel out. Disable the spark if you go that route.

Yes it’s an AJ16. I swapped the coils and the spark plugs around to see if the issue moved but it did not. I then removed the coils and put the spark plugs in and cranked the engine to see if I could see a spark. I could not. Only in the 3 cylinders that would fire did I see a spark.

So that would lead to a problem with the wires to the coils that are not firing.
You need to start checking for broken wires.

If you have an oscilloscope with a current probe or can borrow one:

You will need to scroll down to coil check.
Good luck hope this helps.
IIRC the voltage on the primary is cut to zero causing the hv peak in the secondary.

You wrote that you replaced the ECU, but also that you soldered on a new black connector. I’m confused why you would need to change the black connector, if you replaced the ECU. Could your problems have bene introduced during the soldering. Please can you explain. If you have suffered water ingress to the original ECU, than the terminals in the harness plugs may be corroded. You may need to carefully clean them or even re-terminate the terminals in the harness connector to ensure a good electrical connection to the ECU pins.

I would assume he replaced the harness connector and that is where the soldering happend.

That is correct Harry. I’ve checked all the soldering connections and they seem fine.

Understood. It is just not normal practice to solder on terminals to wires in harnesses because the solder makes for a very stiff and potentially brittle connection. The terminals are normally crimped on.

I was told by a mechanic that only does work on older jags that I should solder. I figured he knew what he was talking about but maybe that was a worse way of doing it.