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Well. I took my ECU out. The area around it was very damp, the carpet was sodden. The ECU casing is rusty, the red connector was literally full of water. To the brim. The red connector was also very wet. The connections all look ok, surprisingly. Is leakage on that side of the car common? I don’t have a sunroof. I’ll give the connectors a clean, try and dry it all out, then see what happens. Rather scary photos attached.
Surface rust on the floor of the kickplate. Perhaps runoff from the ECU casing? The connectors. Red was dripping wet, black was ‘just’ damp. The front of the ECU. Surface rust. The rear of the ECU. Surface rust and horribly rusted on carpet. The Jaguar XJ6’s onboard pool facilities
So, I have a leak to track down and fix, and possibly a new ECU to buy. Would a flooded ECU cause a single misfire? It looks bad, but surely all the electrics would be ruined rather than just the timed spark/injection of one cylinder?
Last edited by tobywood13; 01-30-2024 at 09:12 AM.
Remove the white face of the car side red connector
The socket corrosion is from water migrating down the wires going into the car rear side connector past individual seals on the wires
I’ll have a look. Didn’t you say earlier that the relevant connectors were on the black side though?
On another note, I’ve just put the ECU back in the car and now it’s not firing on any cylinders. Could I have broken it by removing it? I can smell petrol, I have voltage to the coils, the tach is moving up to 200 RPM, but it’s only sputtered on one cylinder once. Seems like ignition isn’t happening?
Took the white faces. Red looks ok, despite its watery habitat.
Appears alright to me, no obvious signs of corrosion. White face similarly clean.
The black looks much worse. Not sure if it’s corrosion, but it’s covered in sticky black stuff.
You can see the sticky stuff here, perhaps some dodgy connectors in this one? More sticky stuff evidenced here.
I think I’ll have to get some contact cleaner and have a scrub at these. In the meantime, I’m gonna try and get compression readings done tonight. Tricky not being able to start to warm up the engine however.
Current status: All coils have 12V, strong petrol smell after cranking suggests injection is happening. Tach moves up to 200 RPM. Still not even spluttering on one cylinder.
I’ve had the battery off and the ECU out twice now, all it’s done has made it go from misfiring to no firing. Connections to back of car side ECU plugs seem ok, they weren’t rough handled when I removed them so nothing should have changed. I don’t know why the car has changed.
This car is defying logic. I’m well and truly stuck with what to do next. It seems I have taken a bad car and made it worse.
Last edited by tobywood13; 01-29-2024 at 03:21 PM.
The power wires " sitting power " to the coils / injectors is from wires external to the ECU
The ECU provides a each timed ground to each cylinders' own wires , the 2nd wire on the coil / injectors
You can push in on the ECU connector wires going unto the connector as it is installed on the ECU case
I’ll try that tomorrow. Far too late and far too frustrating to carry on today. Certainly won’t be driving the Jag on Wednesday as planned, I’ll have to find another car. Thanks for all your help so far, I’m sure I’ll get to the bottom of it eventually.
Without reading the thread again, could there be water in the fuel?
I had a customer with a 1994 XJ40 that lived in a very wooded area and no garage parking. The fuel scupper plastic drain would clog and rain water would fill the fuel cap area and seep past the cap into the tank. (the filler was very rusty and leaked at the cap)
Every time we got heavy rains in the area, she would bring me the car on a rollback truck and I would have to drain the fuel tank and refill with fresh fuel.(at leat a gallon of water in the bottom of the tank)
Cars will run poorly with some water but too much and NOT AT ALL.
Without reading the thread again, could there be water in the fuel?
I had a customer with a 1994 XJ40 that lived in a very wooded area and no garage parking. The fuel scupper plastic drain would clog and rain water would fill the fuel cap area and seep past the cap into the tank. (the filler was very rusty and leaked at the cap)
Every time we got heavy rains in the area, she would bring me the car on a rollback truck and I would have to drain the fuel tank and refill with fresh fuel.(at leat a gallon of water in the bottom of the tank)
Cars will run poorly with some water but too much and NOT AT ALL.
Hi, a valid possibility. I don’t think there’s water in the fuel, although it did start playing up after sitting a couple of days after a refill so it was a concern of mine. The drain is clean and the filler area nice and dry. I managed to get the car to idle for extended periods, even if it was reluctant to get started, so I assumed the fuel was ok. Water wouldn’t cause a misfire on a single cylinder, as far as I know.
This issue has developed somewhat from a high idle to a non start to a misfire, and now to a different non start. If I can restore some life to the car over the next few days and get some cylinders sparking again, I’ll probably take it for a drive to run some fuel through and make room for some fresh, extra extra premium juice.
Surely that's too good to be true? My car is a '95 4.0 and this seems to match the symptoms. Doesn't save the ECU, but at least I might be able to fix the leak. I'll check my VIN and see if I can track down this plenum. Where in the car is it located? Interior under some trim, or in the engine bay?
Hopefully this flap is the cause of the leak. Thanks Parker!
Last edited by tobywood13; 01-30-2024 at 09:24 AM.
Right, so I’ve cleaned all ECU connectors, charged the battery, and changed the spark plugs. And she lives! Barely. She’s now misfiring on cylinders 4, 5, and 6. So an entire bank, not just 4 like it was before. Any suggestions why that might be?
I was hoping it might just be wet plugs, however I’ve tested the coils with a spare plug and a spark light and we have no spark reaching any of them, let alone going into the cylinders. Fuel injectors are injecting, so it’s just spark on the last 3 cylinders.
Is there a single connection of the ECU that distributes spark to each of the two cylinder ‘banks’? Or a fuse, relay, earth point, anything like that? As I've had the ECU out a few times now, cleaned it, put it back, perhaps a connector isn't connecting? More cleaning, more firm connection? Thanks for suggestions, we’re gradually getting there with the old girl.
Last edited by tobywood13; 01-30-2024 at 02:50 PM.
From studying the wiring diagram, I have narrowed my search down to these ECU connector pins.
BLACK; Pins 5, 9, 11 - timed ground to coils 4, 5, and 6 respectively.
RED: Pin 18 - feedback from H02 sensor for cylinders 4, 5, 6.
It may also be the H02 sensor itself for bank 2, but main issue seems to be lack of spark from the coils now. Sparks which were present before I removed the ECU, so it surely must be related to that. A strange coincidence that these sparks have quit, when the connectors for the other cylinders are also right there on the ECU plug next to them. Before fiddling with the ECU again, I will test each of the bank 2 cylinders for spark, it's possible they're down on injectors but the strong petrol smell steers me towards spark. My poor catalytics...
If cleaning these pins works, I'll probably return to my 5 cylinder running condition of a few days ago. Or, if I'm really lucky, I will restore all 6 cylinders. Probably not. So, ECU will be coming off again, connectors for these pins will be cleaned, examined, pressed in to the plugs, dried out, then reattached. Hopefully, this is enough to coax some more life out of the coils.
The O2 sensors will not be used until 88 C degrees on the engine coolant Temp sensor , which will be awile until the engine warms up , normal
The O2 sensors will only regulate the injectors in pulse width ( injector open time )
Editing with more info
Keep asking questions
The coil wires can be crossed on the aft 2 coil connectors
The aft coil # 6 is the Light Green / Purple wire
The # 5 is Light Green / Orange
The wire common to all coils is the Whte / Pink as the power " sitting " on the coils
The ECU provides a timed ground to fire the coils
There is a correct aft O2 sensor connection ...................
This can have effects on you regardless if you tinkered with it before , but then only on warm engine
So as this is occurring on a cool engine, we can exclude the O2 sensor from the list of suspects. I’ve checked my coil wiring, the aft two are correctly plugged in.
I’ll have another look at the ECU connectors. I’ll double check to see if there is spark on the rear cylinders by grounding plugs on the exhaust manifold, but I’m confident they’re not sparking. Possibly could be flooded rear cylinders from previous cranking with no starting, but seems too coincidental to occur on the rear three at the same time.
01-29-2024, 12:51 PM
