Hi all

This forum section is where I feel home and also where the V12 knowledge is, so you get the question although the car in question is actually an XJ12 from 1986. It's not my car, but I support the members of the Jaguar Club of Denmark with technical questions...
The car struggles to start from cold. Coughing and hesitating. Once started, it idles normally, so I guess it's not just AAV stuck.
The coolant temp sensor returns OK resistance values.
And now to the funny stuff: The engine starts better cold with wide open throttle. How much fuel injection does WOT during start result in ? Could this maybe indicate low fuel pressure ??

 

If you run the car for a bit and then try to start from cold a few hours later does it behave the same way? Or is it only if it’s been sitting for a couple of days or more?

My XJ-S will start easily if it’s been used in the past 24 hours, beyond that it stumbles a bit. What be taken to doing, that I think I learned here, is to prime the fuel system. Ignition on, wait three seconds, fully depress and then release the accelerator, ignition off. Sometimes I’ll do this twice before cranking the engine over.
On my car at least it does loose fuel pressure if it’s been sitting for a while. But if I drove it the day before it’s usually ok.

 

In addition to Gatheth's advice, which I endorse wholeheartedly, I would change the B bank FPR. It is not an expensive part and it might be going home so that it is holding pressure but not that well.
https://www.ebay.co.uk/itm/266621399...Bk9SR8Kij6ycZw

 

Thanks Greg and Gareth

The owner of the car is aware of the priming method and uses it.
The car is difficult to start whenever it is cold, also after just 24h.
I have a used and OK B-bank FPR on the spare parts shelf, I will let him test it.

How about the question about starting with WOT? Will it just pour lots of fuel as if it was started ?

 

WOT gives a longer pulse to the injectors (ie more fuel) which makes me think that maybe the fuel pressure is not building up properly, ie FPR not sealing that well, so it needs more fuel to start it because the fuel pressure is low. In effect similar to choking a carburetted engine. The reason it needs more fuel being the pressure is not building up as it should. Maybe when hot the FPR wakes up a bit.
Of course it could be lots of other things, poor pump, semi-blocked fuel filter, acting up A bank FPR (I advise removing it), fuel lines need blowing through, etc etc!
If this does not fix the problem, it could be poor spark when engine is cold; when was the dizzy cap, rotor, plugs and coils last changed?

 

Thanks, makes sense. We will move forward with the FPR. I also have a pressure gauge, that can be cut into the fuel line to properly check the pressure.
I had the idea, that keeping the throttle wide open could result in zero fuel during start procedure, meaning that WOT could be used to clean/dry the plugs of a flooded engine, but as I started out: It was just an idea...
Plugs are quite new. I know there has been work done on the distributor and/or distributor wiring with the help of a very experienced Jaguar mechanic, so I would expect the parts to be maybe not new, but at least in good condition.

 

Leo,

i did wonder if it could be over-fuelling at cold and therfore the WOT actually reduced the fuel intake? I don't know on the V12 but on my 4.0 apparently WOT during cranking cuts the fuel, so maybe the same?

Does the HE have cold start injectors? Also have you checked CTS readings at cold before cranking?

Good luck

Paul

 

Following Greg.

COLD HE needs a FAT BLUE CRACKER spark to even get the thing interested.
Followed by spark plugs that are gapped CORRECTLY, 0.025" I run NGK BPR6EF only. The "P" puts the spark INTO the bang juice, as the specified WITHOUT the P is a flat nose sparker, so the actual spark is almost up inside the cone on the plug. Remember, the fuel today is NOT the fuel of the 80's etc.
Check its got 12 new sparkers, I have lost count of the ones to me with 10, as both #1 are too hard for workshops.

Check the engine Earth strap fiasco, run a dedicated lead, why not.

More to come when the eyes settlke ddown, oops.

 

This WOT fuel cut-off is also something I think have heard off, but if the HE does it: IDK...
CTS reads 4.5kOhm at cold garage temperature. That should be OK

 

Thanks Greg, always good to have your vote in as well
Plugs should have been changed recently, and as the car is quite some miles from here, I have to trust that all are changed for now. I don't think the owner undertakes tasks like this, and the mechanic he is using has 50 years of Jaguar under his belt and is generally a trustworthy guy, so I would expect him to have lifted the A/C compressor to get to the front plugs as well. He should also know what plugs to use, although his own V12 is a PreHE E-type on carbs.
I think the extra earth lead has been added when we had running issues, which was solved by fixing a wire somewhere between distributor and the ignition amp.
I will ask the owner to check out the plug type.

 

Foggy memory disclosure but I'm nearly positive this is correct.

"Flooding protection"

When the ECU sees both a cranking signal from the starter circuit and WOT it cuts off fuel.

Cheers
DD

 

Leo,

Attached is a Pin Out of the ECU I did years ago, and related to my XJS, and the Aussie market, so NO emissions junk. It has aided me many times, as reading wiring diagrams now is not possible.

This is RARE, but so are we V12 nutters. There is a wire from the C1/C4 terminal of the Starter Relay TO Pin 26 of the ECU. This is a "wake up" signal to the ECU to activate the Injectors, coz the Starter Relay is cranking the engine. The connection at the Relay is the one that mostly goes AWOL, so the ECU has NO idea someone is trying to start the beast. It will eventually "wake up", when is gets the Ign Pulse to Pin 18 via the Shielded wire, but that can be a second or more.
Worth checking, and back probing Pin 26 while a helper cranks it. As I said, rare, but 2 of mine had issues with that wire at the relay.

Memory, HA, luv that *****, WOT cuts the fuel pump during cranking, and the rest is fuzz, sorry mate.

 

Thanks Doug and Grant

My memory also starts to wake up now. Your inputs pulls in opposite directions (flooding or no fuel), but I'm confident one of you will be right

Regarding the starter relay to Pin 26 on ECU connection: Am I right, when I say that turning on ignition shall make all injectors click once, controlled by this signal ?? That will be easy to check.
Regarding the need to cut off the injectors by using WOT: I guess that indicates, that we have a lot of fuel in the inlets before the starter is activated.

The owners feeling is that the car lacks fuel. He contacted med to find out if his car has the cold start injector as on the PreHE. His feeling was that they didn't do their job, but as there is no such injector, we now look for other causes...

 

Nope.

When you have "click" at Ign ON, the TPS is causing issues. With Ign ON, and a sharp opening of the throttle will give that "click", same as a Accell Pump in a Carby.

It will be something simple, but a lot of testing will be needed.

These are a NO foot on any pedal system. All mine, even the PreHE, reach through the window, go to stsrt, and they start, no touching of any pedals.