So, I ran a little test - I disconnected and capped the vac lines to the FPRs, ran her from cold to normal temp, waited thirty minutes, and tried to fire her up again.
From cold, ran totally fine without the FPRs connected. After 30 minutes, she started right up and stayed running, but the idle bumped back and forth from 500 to 750. I could also definitely hear a miss, as if one of the plugs wasn’t firing. Cleared up after a few minutes.
some injection cars don’t have two FPRs… has anyone modified the Jaguar to a different system
Jeff, on the unstable idling, I do not know if it helps, but some ECU models had an input to disable the lambda feedback when idling, to avoid the fluctuation in rpm . The inhibit signal comes form the switch in the gear level and is associated with N or P.
Best
Rui
Oddly (or not?) my V12 idles smoother in D or R in the closed loop. The open loop base fuel map must be slightly off, whereas the O2 sensors correct the mixture better at idle. I also have a SuperEnhanced ECU, so…
No idea why, but the automatic gear box can work as a damper to the engine rpm fluctuations, when a gar is engaged. That is, if the ECU were not receiving the inhibit signal idle would be more smooth with a gear engaged.
Yes, but if I force the ECU into closed loop while in P or N (by pulling that plug) it idles smoother too.
I’m talking subtle…in open loop, the A bank has the ever so occasional miss, but it’s more like a half-miss. I’ve tried everything to correct it. I just live with it.
Go to the archives and type in “Removing Fuel Pressure Regulator.” An interesting discussion will appear in the first post, from some years ago. If you read it you will learn a lot about the operation of the two regulators. Some owners have removed the first regulator. I am not one of them!
Engine needs to be running for the pressure to increase or decrease. Been there, done that, Even installed a permanent fuel pressure gauge.
https://www.jag-lovers.org/snaps/snap_view.php3?id=1362255765
If a hot start is the issue, the FPR is not the likely culprit. Pressure is lost when the engine is not running and as I said earlier, that check valve or a leaky injector is a suspect.
Hot start problems were rampart on the pre-HE XJS. Initially Jaguar headquarters in New Jersey recommended modification of the fuel rail to reduce the multiple paths that the fuel could take back to the fuel tank. The later cars were modified by the factory with a redesigned fuel rail and the ability to raise the pressure in the rail at shutdown via manipulation of the second fuel pressure regulator. I went through this process while chasing hot start problems on my 1978 XJS. The first thing I would do is a leak down test of the fuel rail to see if it was holding pressure.
The following links might provide some insight as to how I solved my problem.
(https://bernardembden.com/xjs/railcomp/index.him)
(Redesigned V-12 Fuel Rail Fabrication)
Recognize your car is the later model but the causes and trouble shooting still applies
thanks Dave… I did read that as you suggested… very enlightening discussion on FPRs operation and removal… btw I know some of you owners have installed louvres to vent out heat … how do you feel about the results
I have not done louvers on the XJS, but one problem is that the fresh air intake is at the base of the windshield. Hot air out of the louvers, into the cabin.
I don’t know whether the hood vents make much of a difference when driving, but they make a dramatic difference getting rid of the underhood heat when stopped, and the engine is shut off.
I have two thermostats wired in parallel to turn on the aux. electric fan. One is the OE unit in the cooling system, and the other reads the air temp. above the “Vee”. After a good run, and then stopping, the electric fan will run a few times, and then stop. I never have a “hot start” situation, and the louvres are forward enough that A/C operation does not suffer. I can also control the fresh-air/ recirculate flaps with a switch, should the need arise.
Another update…
First, I do have a newish Bosch fuel pump and filter I installed about a year ago. I also don’t have a check-valve on the fuel line for what that’s worth.
So, last night I ran another test. I left the A bank FPR disconnected and reconnected the B bank. Ran to normal temperature, let sit for 30 minutes, and tried to start. Result was back to not wanting to start at all. Turning the key and it’d jump to life at about 1500 RPMs for about a millisecond and then died. Tried a couple of times same result. Then went ahead and worked the gas to keep it started and let it run until it cleared up, took a drive, and then set the timer to 30 minutes. During that time, I disconnected the B bank again and when the 30 minutes pinged, I tried to start again. Result was a rough start, but didn’t die, kicking to life with a 1500 RPM surge and then settling to stuttering between 500 and 750 with the same slight misses. Essentially the exact same results as the very first test I ran.
I plan on running the same suite of tests over the next couple of days just to validate a consistent process. I want to confirm if it’s predictable behavior with the B bank connected and disconnected. Also, I think I’ll pull out the trusty stethoscope to figure out which piston/s might not be firing and causing the slight miss. Not sure if the injector/s are a problem as I would expect if they were clogged or failing that I’d hear it sometimes when she’s purring nicely.
At this point it does sound like a fuel pressure problem of some kind during hot starts - something in the system is allowing the “vapor” lock condition.
Dave, do you have louvers on an XJS? Pics please? Very interested to see those!
Find out if your fuel pump has a built in check valve or not. My Walbro did not, which gave me terrible hot starts. Since putting in a check valve of my own after fuel pump, my hot starts are much better, although I’d still like to improve if I can.
How long should the fuel system hold pressure after shutdown?
Hi Bob, Finding a place that could stamp louvres, and then all the work of stripping and painting was not something I wanted to do- especially as my paint is in pretty good shape. So, over to plan B.
I found some aluminum louvres, meant to be rivetted to any Jeep of your choice. I trimmed them to remove the rivet holes, and shortened them a few inches, and then shaped them to be a perfect fit on my hood, which has a very slight convex profile. I primed with heat resistant paint and then topcoated with Arctic Blue. After measuring several times the hood was cut, and 3M tape was used (which was applied to the louvres first to color the edges of the tape, when sprayed.) They have been officially tested in excess of 100MPH, and overall, I am very happy with them.
Most people that comment when the car is out don’t have any idea they are not OE.
BTW, I have two pressure gauges on my engine- one on the rail, and one reading the pressure before “A” bank regulator. The fuel pressure on the rail will bleed down over several hours. The fuel pressure between the pump and first regulator, 15 minutes ago, was showing 26 pounds, and my car was stored in late September, and not started since.
Normally the Bosch pump has a check valve.
This doesn’t mean that it works though… adding a check valve is dead easy and costs nothing.
What part number(s) are these secondary check valves please?
