I am bringing my 1991 XJS Classic Collection V-12, back to life.

Starts up fine, Idle is steady, Take her for a test drive and in about 10 minutes she starts to sputter and loss of power. Apply more gas and she boggs down to a crawl. I pull over, stop the engine, wait a few seconds and restart the engine and everything is fine for about 5 minutes then it starts all over. Once again stop and restart motor, and limp home.

Recently replaced Oil Pressure sending unit.
Fuel pressure sensor
New O2 sensors
new fuel filter
New fuel pump
New Fuel Injectors and hoses
New Spark Plug cables
Sump tank was cleaned.

What have I missed?

 

Looks like you've gone through the fuel side of things. On the ignition side, coils can break down as the engine heats up.

 

this sounds like classic fuel starvation , but as you have said, you have replaced a lot of the suspect parts

i would possibly investigate the swirl tank again , as debris from the full size fuel tank may have found its way in again ?

i had this issue myself, and had to clear the tank twice over two years, i finally replaced the cause of the problem , a corroded fuel tank

BB

 

The strange this is, when I turn off the engine, wait a few seconds and then restart the engine, everything is fine for the next few minutes. Then the loss of power, when I give it more gas, it sputters, so I have to just let it idle and limps home. :-(

 

Had the same thing with my '93 XJS. Started nice then slowly but surely began to bog down, run rough and quit. Went through the whole gas thing and at last start, lots of smoke and I was told the Catalytic converters were plugged up...you might take a look at that possibility...I'm done...parting out '93 XJS convertible...

 

Another simple thing I use to get the head on the right path.

Remove the fuel filler cap.

Obviously a FULL tank is a BAD idea for this.

Go for the same test drive. See if it still happens.

These cars with external fuel pumps rely on the fuel tank venting system to at 100% for the fuel to gravity feed the supply side of the pump, and most are not.

If the car is fine with the cap OFF, the venting system is at fault, and the pump is starving for supply fuel with the cap ON.

If nothing changes, plan B. Exhaust clogged etc.

The few minutes wait when it goes hissy allows the tank to equalise, so away it goes again, coz the pump gets fuel, then the vacuam in the tank builds up, engine goes hissy again, etc.

Worked for me many times.

 

Just got back from another test drive, this time with the gas cap open. Same thing, she runs great for a minute or two, then starts gasping for fuel. I pull over, turn the key off, wait 10 seconds then start the engine and off I go with full power, for about a half mile then she starts to fade again.

Is there a tread on how to check the cat conv.?

 

before doing anything more, it might be worth disconnecting the supply side fuel line at the engine, directing the fuel into a container with a hose, and running the pump (you will need to hot wire it) for 10 mins to be 100% sure the fuel delivery is not the problem. This is a very easy test to do before getting the exhaust apart, which can be a bit of a hard job, engine in car.

Greg

 

the exhaust prob sound very plausible but
I had similar problems when I first purchased my car, I did every thing you have done to no avail, it turned out to be the ecu was in limp mode, if you can try another in it you can confirm good or bad

 

This is tricky, as you're discovering, because so many faults could be causing a similar symptom.

When I had [what seems like] the very same issue on my 1988 XJS V12 the fault ended up being the inlet side fuel pressure regulator. Apparently it was outright cutting off fuel supply....although I'm not quite sure what the exact nature of the defect was that allowed it to happen. I discovered this as I was going through the various possibilities, as mentioned by others here, because the problem (fortunately) worsened to a no start situation.....making diagnosis much easier.

Anyhow..... I simply eliminated the regulator, which is not unheard , and all was well.

You mentioned 'fuel pressure sensor' in the first post. Are you referring to one of the pressure regulators?

Anyhow, lots of good ideas already mentioned so consider this simply as one more on the pile.

Cheers
DD

 

Doug, your are correct I mistakenly called the fuel pressure regulator a fuel pressure sensor. The one on my left hand side was leaking fuel through the vacuum hose and dripping a few drops of gas. I replaced it. The right side fuel pressure regulator checked out (no gas leak).
How do I eliminated the regulator?

As for testing the ECU, I wonder if I could swap out the ECU from my 1988 XJS with the one from my 1991 XJS. Anyone know if they are compatible?

 

I doubt the ECU is an issue.

Yours is a Marelli car, the '88 is NOT.

Here is the write up I did years ago in regard to removing that FPR.

Jaguar V12 HE RH fuel reg removal.doc



Something else to add to the list:

Unplug the vac hose AT the ECU in the boot, now blow thru it. This should be an unrestricted blow, as it connects at the balance pipe in the engine bay. It does get oily contamination in it over the years, and that reeks havoc. Being Marelli, there is also a vac hose to the Marelli ECU, so check the integrity of that one also.

 

Something else that came up a long time ago.

The 1/2" fuel hose from the main tank TO the sump tank begins to break down INTERNALLY, and this slows/prevents fuel gravity feeding TO the sump tank. Age, and the clamping of that hose for whatever, over the years destroys the inner lining.

 

Grant suggested to Unplug the vac hose AT the ECU in the boot, now blow thru it. I did, I could blow thru it although it was but I had to give it quite a blow. I discovered that the hoses ends were a bit crusty so I cut off the tips and reconnected it. Now she is giving me a real hissy fit.

Idles fine, but as soon as I give it gas, it starts grumbling, (sputtering, back fire from engine bay).

I'll check the vacuum hoses to the Marelli ASAP and will report back.

This car sat for more than a decade, perhaps I had better get my hands on some new vacuum hoses.

 

Doug, my problem has worsened too, now my 1991 XJS V-12 starts and idles, but she won't go, as soon as I apply gas, she stops running. If I eliminate the fuel side (RH) fuel pressure regulator, can I leave the (LH) side in place? :icon_nut:.

I would really love to get this car on the road and take for a drive to Yellowstone National Park this Summer.

 

Yes.

In fact, you *must* leave the LH regulator in place.

But the new development makes me wonder about the TPS. Have you seen directions on checking it? If not, we can provide them

Cheers
DD

 

Found some info on testing the TPS. Hooked up the volt meter and at idle she gives me .45 reading, at full it reads 4.86.

 

Good find.

0.45 at idle is too high.

You will need to adjust it down to 0.32-0.36.

The top end is just fine.

I have the write up on that TPS is you want it.

 

Grant is right, of course, but also s-l-o-w-l-y operate the throttle and carefully watch your meter. You're looking for a voltage drop-out....indicating a dead spot in the TPS


Cheers
DD

 

I have brought it down to 0.35. Unfortunately the issue is still with me. s-l-o-w-l-y, I turn, no drops. (Going to run this test on my 88 XJS, idle seems too high on it as well).

Side note:The previous owner had the wrong transmission dip stick in this car and therefore the car was way overfilled with Trans Fluid. Fluid level is corrected, I have a new Transmission modulator to be put on, (not sure if that will help, or if it is even related to this issue, but I thought I would mention it.)

And yes I am interested in the write up about TPS.

I am heading to the mountains for the week end, upon my return, I plan on replacing the Trans.modulator and eliminate the fuel side (RH) fuel pressure regulator.



Thank you everyone, your suggestions and advice is welcome.