hi there, I'm hoping someone might have an answer to this question: when cold, the car starts and runs like a dream and it continues to do so until you turn it off. However when trying to restart the warm engine it is a no go. Half a dozen guys at a classic cars restoring shop have cracked their brains over it and given up. I have gone through the workshop manual forwards and backwards and am no wiser.
The car has a Lucas fuel injector amplifier and no crankshaft sensor. Injectors clicking as they should.
The car has been parked up for more than a year now...

 

Some diagnosis required:

1. when it is hot, is there a spark?
2. when it is hot, are the injectors firing petrol (gasoline)?

Start there and come back to us! There are guys on here who will, I guarantee, get it going as long as you work steadily and methodically through their tests.

 

Thanks Greg, I am taking confidence from your reply and I will go at the engine in a moment (leaving the hammer aside, promise) and share the diagnosis shortly.

 

Alright. I started the engine from cold and let it run for 3 minutes turned it off and it won't start again as usual. I have a good spark. The injectors however only squirt a faint hint of fuel barely any trace on the tissue and the spark plugs are dry. Turning on the ignition the fuel pump works and cuts out after a few seconds as you would expect and the injectors click pressing the accelerator.
I don't have a fuel pressure gauge but from what I can see there is not much of a pressure coming through to the injectors. Any suggestions?

 

pat,

I hope this is not a red herring but the early XJS uses a Lucas Opus ignition system. These were prone to failing when only a few years old, particularly on V8 Rover engines. I had a 77 Rover 3.5 and it would start and run fine from cold. Would occasionally cut out when hot and would almost never start from very hot. That's typical of the Opus distributor ignition module failing. If so, you can sometimes potentially improve things by slightly closing up the air gap. But realistically once they start failing, you need to find a replacement module.

As I said, I hope it's not a red herring, but it's worth checking if you have an absence of spark after it's left the coil and failing to get through the distributor.

Good luck

Paul

 

Pat,

Greg just rang me, and said I needed to look, oops.

NO hammers, Whisky, hell yes.

I have heaps of scribbles in the shed on the PreHE had too many over the years.

I will go looking and come back here, maybe an hour or so, L:OTS of uinfo to wade through.

Hang in there, these are SIMPLE beasts.

MEANWHILE.

Inside the distributor is a trigger board, more later, some are 3 wire, some are 4 wire. Have a look down the rear of the distributor, and you will see a "ribbon" type mini loom, tell me the number of wires, 3 or 4 please.

Back soon mate.

 

Thanks Paul, the ignition was already swapped for a modern one and is working fine. The guys in the restoration shop used to rebuilding classic jags who also replaced the ignition system cracked their brains over the not-running-when-hot and couldn't figure it.

 

Cheers Grant,
as I mentioned to Paul the ignition known prone to issues was recently replaced (couple months ago), the original Lucas ignition system was replaced by a more modern one. The old had 3 wires, the new has 4. The spark coming off the plug is a strong blue. If it is not completely out of timing in my humble opinion I'd put my money on fuel delivery being an issue 🤔 Anyway, the sledge is nearby, an Irish repair is still an option...

 

Okay, here's some more food (for thought):
I started the (cold) engine and let it run for approx 10sec, turned it off, repeated it. The 3rd time it just about and reluctantly started, the 4th time no more starting, whatever you try.
In my experience it now takes about ½ to 1hour before I can start it again.

 

OK,
Here we go.

That 3 or 4 wire, 4 in your case, has ZERO to do with spark or lack of. It is 100% fueling related.
That Injector test after shutdown is iffy at best. the system ONLY fires the last 6 Injectors that operated at shut down. When you do the throttle rest. If you pick the correct Injector, whoopee, but since its 4 batches of 3, lotto type odds really.

That 4 wire is the EFI trigger Board inside the distributor. The 3 wires are as they were on the old 3 wire unit. The 4th is an Ignition 12V feed. Shortcuts come into this, and many use the +ve of the coil for this 12V feed, WRONG. The coil is a ballasted system, so it sees 12V ONLY when the starter is cranking the engine, then it reverts to 7V +/- for running. That 4th is a Red wire from memory, so trace it out, and ensure it is NOT on the coil. There is a 12V Ign Feed on the Ballast block.

Next is the CTS (Coolant Temp Sensor). Located just aft of the B Bank (LH) thermostat housing, and has 2 wires in a plug. They go AWOL, and HOT start is a non event if that happens. Unplug it, bridge the terminals IN the plug. This "fools" the ECU into thinking the engine is at operating temp. Loss of signal from that sensor is a DEAD fuel system on the D Jetronic and the later P Digital.

Paul is spot on with the Opus nonsense, and Jaguar made a loom to relocate that unit out of the "V" where it was overheated. Opus is SPARK only.

Next, the rotor inside the dizzy. The heel of that rotor has a magnet imbedded in it. This magnet activates the Hall Effect Sensors in each end of the Trigger Board as it passes over the top. Magnets lose their "thing" when they get hot. If that Trigger Board was replaced, the matching Rotor should have come in the kit. The shape of the rotor is different, round or square, and I forget which went with what Trigger Board, and never understood the reasoning, as I always thought a "magnet is magnet no matter the shape", dunno. Remove that rotor, make sure its a magnet, and then use a hair dryer, heat it up, ensure it still remains a magnet. NOT silly hot, just heat it up. They rarely give grief, but I have had 2 go weak when heated.

The EFI loom, running down each side, at the bottom of the "V", is old and suspect now. They short internally, and maybe? that is happening as it gets heat into it.

The fact it runs fine until shut down is the main focus of what I am suggesting. You still have spark when hot, so nothing on that side.

BUT, that 3 minutes is not really HOT, so I will keep thinking.


That Injector test when hot is iffy at best on th

 

Subject to Grant's advice, which we all bow to, a few more questions:

1. Have you replaced the coolant temperature sensor?
2. Have you replaced the fuel pressure regulator?

A faulty FPR is always a possibility, and a faulty CTS, somehow playing up when it gets a touch of heat, is also a candidate. The later (HE) engines will not run if it is faulty, and the pre HE (which you have) might well be similar.

 

OK, that is not a good thing for the V12. They HATE starting, move a few feet to get another car out, and shut down.

That was drummed (clipped the ear) way back when I was with Jaguar over that one. Carby cars, no issue, EFI cars will flood, sadly.

We were "instructed" to think long and hard before starting a V12, and if the decision was made to start one, it MUST be left running until the Temp Gauge just left the COLD mark before shutting down.

Spark plugs sooted up are the main result, as well as the excess fuel.

PITA I know, but thats what it is on these. The HE is a SMALL amount more forgiving, but not much. I treat them the same.

 

Ok, that's some food for me, thanks lads, I'll be working down the list and see what's coming out of it. I know Grant and I appreciate not to do the starting-stopping under normal circumstances, but from what I see on the spark plugs taking them out there's no sooting - if they were going black, I would consider that good news as there would be fuel...

 

Pat,

Another thought since a Batfone call.

Your car should/will have Cold Start Injectors.

2 in total. One each side in the Inlet manifold, inboard of the throttle body.

They leak, dribble more correctly. I disconnect them electrically and mechanically. Leave them there to fill the holes, or make a plate to cover the holes.

They are not needed.

Cold starting without them is still simple.

They will bleed off fuel from the rail/s very quickly. It mostly ends up in the air filters, due to the angle of the dangle.

 

Ballast box, the coil is connected to this


​​​​​​Right, have ticked off some from the list-
the coil is connected to the ballast box, that is according to the assembly manual that came with the set.
Did the trick with the paper clip on the CTS, to no avail.
Can't check the pressure on the fuel regulators because I have to organise a gauge for that but obviously I do have the right pressure once I have it going, isn't it so?

What I can't get my head around is the fact that I can start the engine cold and then go driving for an hour or longer with no problems but as soon as I turn it off and then try to start,it won't. I would have to wait for it to cool down properly.
... it's gone beyond the issue of a SAS (simple and stupid)

 

Will check that pronto.

 

Disconnected the two cold start injectors. No change.

 

... I'll take Grants advice and leave the hammer be ... for today. But here's to another day wasted on British engineering 🍺

 

Forgive my recap, as I am not yet sure about the answer:
When it will not restart, is there definitely a fat blue spark?

If yes, then following a steady path is required, but the problem MUST be fuelling. Therefore, I would ensure the basics first, i.e.:

1. The pump may be going funny when stopped and restarted - worth hot wiring it as a test
2. the 1/2 inch supply to the pump from the main tank to the sump tank may be partially blocked or failing internally, and a slight vacuum in supply occurring - this is quite common and is a must replace item
3. the sump tank under the battery may be full of crud or its internal filter blocked, or its pipe out to the pump,
4. the feed and return lines may be blocked or pinched
5. tank breather may be blocked - test by leaving the cap loose
6. the lines to the filter, or the filter, may be blocked or blocking somehow when pressure is relieved.
7. If you disconnect the line to the fuel rail, after the filter, in the no start condition, and ask a helper to turn on the ignition, guiding the fuel away from the engine with a tube, you should have a HIGH pressure stream of fuel into the rail when the ignition is turned to on.

If all seems good uafter all this, then the injectors are not firing. Once this is known we turn to Grant!

 

Greg / Grant,

Despite owning a pre-HE, I'm almost ashamed to say that I don't the exact manner in which some of the components operate.

However, the way that I've read Pat's description of the symptoms makes it feel that there can only be a couple of components that potentially are causing the problem.

IIRC, Pat, you said that there was spark at the plugs when the car was refusing to start when hot. Therefore, unless somehow the timing moves way off when trying to start (Which I can't see being possible?), the problem must be fuel. But as the car runs fine up until it stops, then my naive assessments is that it seems to narrow down to a few things that ca only exist at startup after a hot run:. And I guess all of these would be verifiable by observing injector spray on say, 4 different cylinders at startup (therefore covering both cylinder banks)? In no particular order:

- Is there a vacuum being created in the fuel tank and supply line, during a run which doesn't stop the car running but then precludes it from pulling fuel on the hot startup routine. The vacuum could then slowly dissipate via filler gap or piping after stopping until it's back to normal for next cold start? Maybe opening the filler cap before restart could identify if there's a vacuum?

- Is the filter blocked or fuel pipe collapsed during hot running, thus preventing hot start until they gradually relax for next cold start?

- Does the ecu pick up latent heat after a run thus not operating properly for injector firing, until it's cooled down again?

- Does the fuel rail pick up latent heat after a run, thus vapourising the fuel and precluding a hot start? Watching the injectors whn churning the starter might verify this?

- Does the CTS play more of a role in starting than it does in hot running? Maybe, like the engine poistion sensor on an AJ16, it's not important to be optimal when running but critical when hot starting?

Just random thoughts...

Paul