Good day all,
I am at witts end. The 1976 XJ12 is getting too much fuel, and my diagnostic capabilities are exhausted.

Long story, so hold on. My 76 has had a hot start issue for as long as I've had this running (about 400 miles now). Whenever the car would get to temp, and I park for 20 minutes or less, groceries or whatnot, the car would not start. I would cycle the key a few times to get the pump to prime, and it would sputter start but enough for me to rev the engine as it clears up. I tried diagnosing this several ways, checking fuel pressure (29-30 PSI on both regulators), AAV (rebuilt with Jag Improver Kit), among other things. When I would leave the bonnet open during that time, the hot start issue was gone, so all signs lead to vapor lock. I started by adding a check valve before the fuel filter in the engine bay, and eventually settling on my fuel pump getting weak. All of my fuel lines have been replaced, both of the fuel tank changeover valves had been replaced and verified for operation, and fuel tank strainers are clear of debris; I've done it all. I did all this about 2 weeks ago.

As I was leaving work on Monday, my original and rebuilt Lucasfuel pump died on me. Thankfully, I was still within the airport gate and could safely park it near my work/shop. I thought maybe I ran out of fuel as I have yet to fix the fuel sending units, but I still had fuel. Replaced the fuel filter again, and verified that my check valve was opening as it should, and it was. I preemptively bought a Walbro GSL392 fuel pump knowing I was going to have to replace mine. While I was waiting for it to arrive, on Tuesday I decided to pull apart my Lucas pump again and see if I could mend it. I was able to get it moving, so I reinstalled it but the pump would not build pressure. The Walbro arrived early on Tuesday, so I installed it in the afternoon. The car would not start, or even sputter. I checked fuel pressure on both regulators as I knew the Walbro working pressure is much higher. They're both set at 29-30psi still, so I decided to numb them down to about 25psi. No luck.

In the process of testing electricals as I thought I had lost spark, I forgot to reconnect the orange grounding wire to the fuel pump relay and the car started with residual fuel pressure, great! It's not electrical! So, as the car was idling on residual, I reconnected the fuel pump and the car instantly stalled. Okay? Odd. As I was reading through forums and my manuals, I thought maybe there was a diode somewhere between the fuel pump relay and main relay that had failed, so what I ended up doing was grabbing a spare motorcycle battery I had and running the fuel pump directly to that battery in the trunk, no electrical connection to the car in any way. Still, it would not run with the fuel pump running, only on residuals.

So! I ordered a Bosh 69414 fuel pump from my local shop as he gives me at-cost prices for parts (I help with his Saab projects, it's a deal that's paid for itself by now). As this Bosch was used in the later cars, it's as close as I could get. The Bosch unit arrived yesterday, Thursday. While I was waiting for it, however, I kept testing the car with the Walbro installed. What I ended up finding was the car was over-fueling. If I crimped the fuel inlet hose before the filter with vice-grips, not totally closing it off, just mostly, the car would start, run, and drive. Awesome, so I have indeed isolated the problem to over-fueling.

The Bosch unit arrived yesterday and I chucked the Walbro to the side, that was a waste of $130 right there. Bosch in, fuel pressure regulators set to 29psi each, the whole fuel system running at a combined 29psi, car would not start. So again, disconnected the ground wire to the relay and the car ran on residual, but curiously, the car would keep running. It's like there was enough gravity feed or vacuum to keep the car idling? Really odd. The second I connected the fuel pump relay, the engine immediately quits. So no I'm getting mad. I cleared all the fuel lines with compressed air, there are no obstructions. I completely removed the right hand pressure regulator and bypassed the return hoses on the right hand side, only the left hand regulator remains and pressure is set at 29psi. Will. Not. Run. Reduced fuel pressure to 20PSI, the car sputters. Removed the AAV again and checked operation, free-flowing air, and a functioning bulb (Thanks Jag-Improver!) thinking I'm not getting enough air. I tested all fuel injectors with my injector tester, all functioning. Now, with one regulator in the system, the adjuster screw all the way out (less fuel pressure), and a vice grip on the fuel inlet, the car runs. For kicks and giggles, I've also tried 2 other regulators I have, no change.

How am I getting too much fuel? I really don't understand what is happening, or how to fix this. My only assumption now is my pressure tester is bad, but all of my 3 regulators at their minimum pressure setting can't control this? The only thing I can think of doing now is adding a stupid ball valve on the inlet and finding the sweet spot...

 

I can only offer a guess. Actually more of a suggestion or reminder.

Fuel flow and fuel pressure are different animals.

Carl

A little fuel at high pressure. or a lot of fuel at a low pressure.

 

Well this is annoying.
I was at a parts store picking up supplies and ran into an old mechanic friend of mine who specializes in restorations. I told him this whole story and his first impression was that the injectors are staying open too long. I didn't think of that, so I went to the British Leyland repair operation manual and looked over all of the sensors for ECU input fearing it was the ECU causing problem, lo and behold, the description for the manifold pressure sensor is "The sensor is connected to the inlet manifolds and converts the manifold pressure at any instant into an electrical signal to the ECU. The ECU uses this signal (modified by the coolant temperature) to determine the duration of operation of the injectors." Bam, there it is.

I unplugged the MAP and the car fired right up! Now I'm on the hunt for a 0280100049 pressure sensor! Coolant temp sender tested good, need to get my vacuum tester at home to check the diaphragm within this MAP sensor.

 

Read all about it here : -
JAGUAR V12 FUEL INJECTION 1975-1980 - D Jetronic / AJ6 Engineering
The writer worked for Jaguar when EFI systems were under development at Jaguar

 

Continuing on a bit on this is the coolant temp sensor. I’ve tested it several times and it always reads roughly correct on resistance readings. I jumped the leads on the harness with the MAP still plugged in and the car started great, but I get horrible black smoke from the exhaust at higher revs, so I’m still running rich, even with a jumped harness.

I’ll be prodding the ECU shortly, but I think it’s wise to replace the coolant temp sensor now as I keep diagnosing. What is installed is part number 0 280 130 014. I’ve found one source to get it from, but it’s incredibly expensive. I’m seeing a few cross references, but the connectors are different. I have no issues splicing in a different connector, but I don’t want to purchase something only for the threads to be incorrect into the thermostat housing, or the resistance output to be different than originally designed.

Does anyone have a source for applicable cross referenced sensors that work?

 

Have you looked at this D jet reference: https://jetronic.org/index.php/en/d-jetronic/102-jaguar
The jag part number for the coolant temp sender is C42166. Looks like it's the standard Bosch range, so you could probably use the later switch from a Series III car with a small conversion harness.