Starnge no-start behavior

BTW, this is not a battery or a starter problem, as the engine does cranks beautiful. This afternoon I was taking my 1994 XJS, 4.0L straight six to the smog check shop. After driving about three blocks I realized that I forgot to bring along the registration paperwork. I made a U turn and drove back home. I parked the car on the driveway and left it running for about a couple of minutes to go get the paperwork. Up to this point there was absolutely nothing wrong or strange with the way the engine runs. After two minutes I came out of the front door and realized that the engine had stopped running, although the ignition was still ON. Turned the ignition OFF and then I tried to start the engine again. Well, it did crank but it wouldn’t start. I tried it many times, but it just wouldn’t start. I tried to start it again some three hours later with a cooler engine, but the car just simply will not start. I have no idea why the engine stopped by itself when I left it running. I have no idea what may have happened with the engine. NOTE: Before I came back to the house for the paperwork, I did stop somewhere else for a couple of minutes but the engine continued to run just fine with no problem. What has, then, gone wrong? This engine runs like a brand new XJS, sharp and powerful.

My head was not clear this afternoon. I’ll do some troubleshooting in the morning, but if somebody here has a good tip it will be much appreciated. I’ll post this on the XJ40 forum, as well.
Thank you!

Few things to check with AJ16, slightly less than with V12 fortunately. Starting from the simple ones:

  1. Have you got fuel? Real fuel, not the one shown on the gauge
  2. How long have you tried to crank the engine? 5 seconds? 10 seconds?
  3. Have you tried to operate the accelerator pedal (press it down half-way) while cranking?
    4.It is easier to fill in your profile than giving this answer all the time: have you got automatic transmission?
  4. Can you hear the fuel pump on ignition on (priming)
  5. When was the last time you’ve changed the fuel filter?
  6. Have you got alarm installed? Regardless of the condition
  7. Do you know how to check coil pack
  8. Do you know how to check CPS
  9. Do you know how to check engine temp sensor
  10. Have you got any errors displayed?
  11. Have you done a y recent repairs in the area of the alternator?
  12. Have you check the engine oil level on the dipstick?
  13. Have you tried to run the engine in limp mode?(disconnect MAF with ignition off)

See is tach is showing approximately 200 rpm while cranking, quick CPS check.

Well, ignition issues here are now immaterial because the electronic fuel pressure gauge on my A-pillar and the mechanical fuel pressure gauge on my fuel rail are both telling me that, while cranking, both needles erratically move to about 2 to 3 PSI at best. So, unless it is a funny fuel pump relay failure mode, what we’re dealing with is the worst case scenario, i.e. a defective fuel pump, why? well because it is my understanding that the fuel tank has to come out in order to remove the fuel pump, right? Why couldn’t it be something a little easier, unless the fuel filter all of a sudden became plugged up?

This is the same response that I posted on the XJ40 section because the engines are so similar, although I wouldn’t know if the fuel pump on the XJ40 is, too, inside the fuel tank?

Or, perhaps, the fuel pressure regulator at the rail?

Before dropping the towel - hotwire the fuel pump connectors directly to the battery. Faulty relay can give you similar issues…

In 1994MY engine is an AJ6 not AJ16 so dizzy not coil pack etc

  1. Did you check the inertial cut off on the right side A pillar.

Well, since it appears to be a fuel delivery problem, I decided to go straight to the fuel pressure source. I went and applied +12.75 vdc to the f/pump relay output, pin 87 where it connects to the O2 sensor input wire, pin 2, which BTW is a blue wire and not red/blue like the Jaguar manual says. I could hear the pump running, but my fuel pressure gauge on the fuel rail went up to only 6 PSI and it stayed there for as long as the power was applied to it and not any higher. To make sure that all electricals were correct, the voltage, when applied, went down to 12.6 volts (down from 12.75) and the current that the running pump was drawing was 3.45 amp.
At least this tells me that the relay and the pump are all getting the power they should be getting.

The only other possible issues could be with the F/P regulator. I wonder what is the status of the fuel pressure regulator when the engine is not running (no vacuum applied to its diaphragm. And, maybe the filter is plugged up. I believe that I may have a new filter in my cabinet somewhere.

The pump is running, but it’s not capable of building up to the 40 PSI it’s supposed to with proper power applied. Oh man, change the pump?

Maybe, but offhand I’d be more tempted to change the FPR.

Here, try this: With the pump running, pinch the return hose to the LH FPR. If the pressure jumps to 40 and beyond, replace the FPR. If it still won’t rise, replace the pump.

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Vacuum on fuel pressure regulator is actually supporting the spring dropping the pressure. You won’t get running values with vacuum line connected and engine running,
If regulator is faulty - it will drop the pressure all the time like your probably - just put syringe into the vacuum line going to the regulator and compress it to see if there is a major difference… Test suggested by Kirbert should be done at first.

I’m not sure what all that means. Whatever, you should get maximum pressure with the engine shut off. With it running, manifold vacuum will cause the fuel pressure to drop some.

I’ll try that, thanks K!

The 6-bangers have only one FPR, no?

Yes, the AJ6 engine has only one fuel rail and the one FPR.

K, you just gave me an idea.
I do hear the pump running when I give it good solid 12 volts, it’s just that the pressure at the rail won’t go any higher than 6 PSI. So, from f/p pumps failure history, how often is that they have totally quit working (as in inop, burned, or frozen, etc.) and how often they still run but the pressure is not enough? For some reason I’d say that burned or frozen pumps count for most pump failures, which would point more to some line restriction, wouldn’t you think? (You can just go ahead and call it wishful thinking…)

Fuel pumps usually seem to get noisy when they’re going out.

Throttle open = lower vacuum
Throttle closed = higher vacuum
Fuel pressure regulator like most of the regulators is operater by force of the spring. Spirng force against pressure. Vacuum diaphragm applies force to the spring making the pressure purging point slightly earlier.