I have a great running and driving Series III XJ6 except that it has always been hard to start and I could never figure out why. I got a hold of fuel pressure gauge today and hooked it up to the fuel rail. It measured 0 PSI of course for having opened the line to install the gauge which bled any pressure that was in the line.

What I found was that when in Park with the key in the run position, the gauge was still at 0 PSI. I did not expect this. Rather, I was expecting the pump to come on and drive up the pressure. I then shifted into Drive with the key in the run position and found the pump came on and almost immediately pushed the pressure up to 35 psi. I then found the car much easier to start and noticed the pressure bounced around 35 psi during the cranking but immediately dropped to 29 psi once the engine started running. Should some relay have turned the fuel pump on when I had the key in the run position?

I was concerned when I stopped the engine and saw the pressure slowly beginning to drop from 29 ps1 down to 25 psi and eventually 21 psi in the space of 15 minutes. I imagined it would go even lower if I waited longer..

I then repeated the start-stop cycle but this time, I put a clamp on the fuel rail supply hose after I stopped the engine. I saw not drop in pressure after a long time watching the gauge.

Hmmm, this again confused me because I could not imagine where the pressure could bleed off between the new fuel pump and the inlet of the fuel rail. Is there a rubber hose somewhere between the output of the fuel pump and the input of the fuel rail that could be leaking? I never smell gas. I imagine that there is a solid metal line between the two spots. Are these metal lines prone to rust?

 

Normal per design. The pump is not supposed to operate by simply turning the key "on".

The pump circuit has two branches: starting and running. There is a relay and a diode pack.

The pump circuit will operate with the key turned to "start". Then, as the engine begins to fire, a switch in the Air Flow Meter closes to operate the pump.



Most likely tour non-return valve is faulty. These are not available new. You'll have to find a good used one or plumb-in a universal valve.

The valve is item #5 in this illustration:

https://parts.jaguarlandroverclassic.../brand/jaguar/

Cheers
DD

 

Looking into a universal NR valve as I cannot source an original one. I assume I will leave the original in and try to add one in a section of rubber hose in the trunk downstream of it. I don't have the car handy but wonder if anyone knows if the hose sections in the trunk are closest to 1/4", 5/16" or 3/8"

 

5/16 for that section of plumbing

Cheers
DD

 

Amazon

8mm 5/16" Fuel Non Return One Way Check Valve for Petrol Diesel Fuel Oil Water Aluminum Alloy Accessories.

 

Thanks Doug and David for the info. I can now move forward on fixing this.

One interesting observation I made, now that I am building pressure in the fuel rail by selecting Drive before Park to start the motor, I find that starts reqlly quickly when cold but takes 2-3 times longer when warm to start. I wonder if this is somehow related to the cold start valve not being used in the later case. If this is true, is there an easy way to get the cold start valve to open every time I start the car, hot or cold?

 

You could rig a manual switch to the cold start injector circuit.

How much cranking does a warm start take, in seconds?

These cars are not known to be instant starters. Taking a bit longer when warm is typical. Many have tried to identify a repairable fault....without success.

An oddity I've observed over the many years is two-try warm start. Let the starter spin 2-3 times. Turn the key off and then back to "start". The engine will fire with a half-spin of the starter. Don't ask me why. It's just something I've noticed over the years on several of these cars. And it only works on warm starts.

An extended cranking situation might be reduced (not eliminated) with a new battery. The fuel injection system like full amps and volts. The starter motor draws so much current that the F.I. system is starved for voltage if the battery is even slightly weak. I discovered this fix (after a long period of trying to solve a long-cranking problem) many years ago on a Christmas Eve, at 5:45pm, in a snowstorm. Long story!

Cheers
DD

 

"These cars are not known to be instant starters. Taking a bit longer when warm is typical. Many have tried to identify a repairable fault....without success.
An oddity I've observed over the many years is two-try warm start. Let the starter spin 2-3 times. Turn the key off and then back to "start". The engine will fire with a half-spin of the starter. Don't ask me why. It's just something I've noticed over the years on several of these cars. And it only works on warm starts."

My experience is the same. Cold start good, warm restart OK, and Hot restart a real issue. I have asked why also; just to get some sleep, I made-up an answer...
Hot restart, no chance of help from 7th injector... First attempt to start has some of fuel wetting the intake walls, meaning final fuel to cylinder is too lean.
I count to 10 between first and second starter engagements. This lets fuel on intake wall vaporize, providing a richer mixture for engine to catch. I have no idea if this makes sense, but at least I stopped laying awake thinking about it !!!!!

With a new 5/16" check valve (non-return valve) be sure to use fuel injector clamps, not worm clamps.
Rgds
David