My recent 160+ mile shakedown cruise when very well but one problem did crop up.

On three separate occasions the engine died while driving. In each case the situation was identical:

- Hard acceleration (like 75% or more throttle opening) in first gear, then sudden closing of throttle
- Engine dies instantly; no sputtering or 'fading away'. It just flat-out stops as soon as the throttle is released.
- Tachometer drops out instantly
- Engine restarts instantly (just click into "N", turn key, restart, resume driving)
- Engine fully warmed


I'm fairly certain that I have loose wire/connector that is responding to the movement of the engine so my first course of action is inspection and jiggle testing of all relevant wiring.

The die-out is so instantaneous that I really don't feel I have a fuel system issue. But....?

Any additional ideas?

Cheers
DD

 

Bugga.

TPS dropping too far on the sudden closed throttle movement???, not sure it a dead out would happen, but maybe.

Coming from 75% hard accel to zero, would see a huge vac surge "hit" the MAF inside the ECU, maybe enough to stall it out, coz we know Hi vac = less fuel (shorter injector pulse).

Fiddle factor, good move.

 

Thanks Grant.

Fortunately I have a known-good spare ECU to swap out, if needed.

The TPS is new so I'm less suspicious of that.

Getting back to the ECU.....

Item #1 in this pic is the vacuum canister thingy that is mounted in the vacuum pipe immediately ahead of the ECU.....

https://www.jaguarclassicparts.com/u...pipe-5-3-litre

.....Jaguar calls this a "reservoir".

Hmmm. If it's a reservoir (I thought it was a filter) then I have to wonder if it's there to sort of cushion/buffer the MAP sensor from sudden vacuum changes?

I don't know how to test it. I'll do some experimenting with a hand-held vacuum pump, I guess, and see what happens.

Cheers
DD

 

Doug
You know the ECU closes the injectors down under these circumstances until the revs fall to below about 1400 rpm. I wonder if the ECU for some reason is not starting them up again? Maybe worth a connector clean all through, including the resistor pack, ECU and injector loom plugs?
If yours is a Lucas car, there is not a lot of relative movement possible between the amp and the coil, but the low tension feed to the coil could be the culprit. The copper does fracture inside the OEM coil connectors.
Greg

 

Doug
Mine ran like a dog with that canister in the line. I say dump it. Mine did not have one originally, but a spare ECU I had came with one and I tried it.
Greg

 

My understanding of that "cannister" was a moisture trap in that vac line, not sure on that one, just what I was told very early in the V12 days.

Others stated it was a pulse damper for the early HE, so I reckon 6CU cars.

Knowing what I know now, it would take some SERIOUS moisture build up in that line all the way from the engine bay to actually get to the ECU.

Blocked/restricted vac pipe under the car I have seen a few times.

Mine had it, it cracked, NLA, went without, no issues with the 16CU, but the 6CU was a tad doggy.

The suggestion from Greg re the EFI shut off and restart is very sound.

 

Interesting supposition. The buffer is tiny, and likely doesn't affect any vacuum changes bigger than a second. Tiny rapid variations, like a misfire, would be suppressed. I'd suspect the canister was leaking if it made any pronounced effect on performance.

But I may temporarily remove mine to see if it alters behavior. I Shouldnt make any assumptions.

 

Ok, good, if you know the engine runs well *without* the reservoir that simplifies things for me. I can just remove it and see of anything changes.

Might be interesting to cut one open and see what's inside

Cheers
DD

 

Mine crumbled, and it vacant space inside it, nothing at all.

 

Hmmmm. Worth pondering; I hadn't thought of that at all.


Hey mister...there are no dirty connections on MY car ! Quelle horreur



Which I'm hoping would be revealed with some jiggle testing.

During the conversion process I repaired/replaced all wiring that showed any visible defect. However.....who knows what lurks beyond what can be seen with the naked eye?

Cheers
DD

 

Thanks for all the fast and useful replies.

I think my steps will be....

1) Inspect and jiggle test for a possible wiring fault. If no joy.....

2) Remove canister/reservoir. If no joy.....

3) Swap out ECU

Cheers
DD

 

Further to this something just dawned on me.

The 1400 rpm "turning on" of injectors seems abrupt. I wonder if this might be a clue that supports your theory?

Cheers
DD

 

Forgive your humble servant, O Mighty One!

 

Just a note....

Not to sound maudlin but I can't help being impressed with the amount of support, useful information, and shared knowledge Jag lovers have via discussion boards such as this. Especially XJS owners. Much more so than many other hobby car forums I've been part of.

Consider this thread. In just a few minutes good ideas came rolling in...including things that probably would never have dawned on me. Whenever a problem comes along it seems that I always learn something new from others. I doff my cap.

I am quite sure that, without the internet, far fewer old Jags would be on the road today.

Cheers and thanks to all

DD

 

Absolutely agree. And in my case it gives me the courage to get stuck in, knowing that such a safety net is in place.

 

Cheers to that Doug. I've on two different occasions passed on some interesting car purchases because when I went looking for the relevant forums, they were either not DIY friendly or the volume of posts were too small to make the archives valuable.

I can say for certain, if it weren't for all the helpful advice and information of other XJS owners, I would have sold off the XJS at least a dozen years ago.

 

I believe that is an important clue. Wouldn't that indicated that the electronic ignition system has been shut down? The engine didn't stop rotating, only the signal cut out -- that either the ECU sees a condition to shut it off, or some other fault is cutting it out.

Given that it is repeatable bug, loose connections or bad contacts seem unlikely. Is it a possibility that the ECU isn't getting all right info at the right time? What I mean does the ECU require A,B, and C to make the right decision in a certain circumstance, but in that particular situation it's only getting A and B? I don't have the schematics in front of me, but it might be worth reviewing what are all the key signal inputs to the ECU under your scenario where it fails.

The only other thing I can think of is does that situation cause a vacuum line to collapse?

 

Mac is correct if it was loose connections then the problem would most likely not be as repeatable as you mentioned. He has a good point with the Tacho.

To test this theory pull the fuse to the main EFI relay with the engine running, I have never tried this but as the ignition uses separate supply I would not expect the tacho to do drop instantly, worth a try.

 

Right, as it pretty much eliminates a fuel supply problem. If the pump was giving out, or the filter clogged, or a pressure regulator was faulty...something like that....the engine would splutter for a moment of two and the tachometer wouldn't instantly go to zero.

Yes, specifically the primary side of the system such as: ignition switch, wires to the "-" and "+" coil posts, wiring to/from the amplifier....something along those lines.


More specifically like the key was turned 'off'



Since the ECU depends on a trigger signal from the primary side of the ignition system it (the ECU) would shut down simultaneously, yes.




Accelerating hard and then releasing the throttle creates a fair amount of engine movement. I'm envisioning a faulty wire or faulty connection reacting to that movement....shifting a tiny bit each time that movement occurs. Similar to, let's say, a loose connection at the back of your radio that causes the radio to cut out every time you go over railroad tracks or that certain bump near your house.


Certainly possible, yes. This could shut down the injectors and cause an engine stall.


Which is possible, especially since the scenario involves a sudden transition from low to high vacuum. The MAP sensor in the ECU operates off a vacuum signal so there's a possible tie in. However, the result of no vacuum to the MAP sensor is a very rich mixture....which would typically cause a spluttering and chugging. Not so sure that it would instantly kill the engine.

Hopefully we'll all know what went wrong in a day or two!

Cheers and thanks for the input

DD

 

I can envision a poor connection giving a repeatable fault


Worth a try, yes. And I agree that the tach is not likely to drop out instantly. The tach does not operate off the ECU. However, both the tach and the ECU rely on the primary ignition system to operate.....which is an easy place to begin an investigation

Wish me luck

Cheers
DD