My 1984 XJS with its third engine, a 1986 screamer that is the best of the lot so far, has stranded me several times with an unusual symptom.

Out of nowhere (which leads me to an electrical, timing, ECU or advance system possibility), the perfectly-running cat drops rpm, begins to sputter, and backfiring can be heard inside the exhaust manifolds or the CATs.

The only way to keep the engine alive is to pump the gas peda (which results in the backfiring), and the car loses power radically. If you stop and attempt to let it idle, it just dies, and usually will refuse to re-start at all, although it will fire and run for a few moments at about 300 rpm, chugging until it dies again.

At first, my mechanic denied there was any problem or at least was unable to diagnose it because of the intermittant nature. However happily, it has now gotten worse and seems to occur mostly when the car warms up--although this morning, cold, it refused to start at all until heated by the sun.

So I have an all-or-nothing situation. One moment I am ripping along in pure 12 cylinder nirvana, and at any moment the gremlin can strike, reducing me to roadkill.

If anybody has any idea about what to look for here, or has had a similar problem, assistance would be greatly appreciated!

Thanks,

Lou

Submitted by fccm.sw@cox.net on Tue, 08/03/2004 - 21:12

Lou, I gather that this means you had pressure on the line in the trunk.
This means absolutely nothing as to whether you have pressure in the fuel rails. There are two pressure regulators involved here and either one failing can cause pressure to be wrong in the rails.
You need to check the pressure in the rail and it needs to be in the neighborhood of 45psi.

My pump was working well enough to get my rail up to 45psi (sometimes) but had no hope of keeping up with a running V12. so my pressure would fluctuate from substandard to non existant. The engine would start sometimes, even run (kinda) (the lean backfire then shut off) It all seemed very confusing. I just had to remeber to remain focused on a fuel delivery problem.

As others have said, check your fuel rail pressure and leave the gauge on while attempting to start/run your engine.

mike

Mike Smith
90 XJ-S Convertible
79 MGB LE

Submitted by cleavefamily@c… on Tue, 08/03/2004 - 21:02

All fuel injected cars are very sensitive to fuel pressure because it takes pressure for the injectors to work. Volume should never be confused with pressure. Suggest you take Art's advice and have pressure checked.
Stew Cleave
JOCO Chief Judge
'69 E-Type 2+2 and other LBC's

Submitted by fastxb2003@aol.com on Mon, 08/02/2004 - 23:03

Thanks gentlemen, I replaced the fuel filter today.

I can testify that there is fairly good fuel pressure at least on the supply side (engine side) of the filter, judging from the spray of fuel that hit me in the face when I pulled the line off that side (I was never one to obey warnings on the side of things; they are just put there by lawyers anyway, right? ;-) This of course was with the ignition off and having been off for 24 hours, so it would seem to be maintaining pressure on the supply side.

Before attaching the new filter, I placed the gas supply line into a glass jar and switched the key on, just for a moment. I got a good supply of gas into the jar, and it was clean.

After replacing the filter, the car still refused to start. It eventually fired a few times, but that was all, same as before.

So today the cat hissed and spit at me; at least this is a sign of life.

1984 Black XJS V12
On 3rd engine - 1986 motor
(This one takes off like a Scalded Cat)

Submitted by silver007@shaw.ca on Mon, 08/02/2004 - 01:22

Art Dickenson.
1983 XJS GT "Silver"
Pacific Jaguar.ca
Hi , apparently debris clohhing this inner filter can lead to a burned out fuel pump, I would check out inner filter, and while itis out you may wanna take rhe tank out and give it a de rusting, wev used to take a counyed number of nuts and bolts and place them in the tank, put in some paint thinner, or other non corroseive thinner, and shake the Dickens out of the tank, it worked on the Brit bikes, and if you are strong enough should work for a Jag, you may need another guy to assist as these tanks are larger,
Initially it sounded to me as a gas shortage, when you have it together again, open one of the fuel rail RETURN connections,the return side is on the passenger ( North American cars) side. Possibly you could attach a pressure gauge and make sure you are getting about 35 PSI .
If you still have a problem I would remove a spark plug and see if there is spark, at the same time note if the spark plug is dry or wet, and also what colour the electrodes of the plug are, they should be chocolate brown ( milk chocolate that is )if they are white it means lean mixture, not getting enough gas
Best Wishes, Art

Submitted by fccm.sw@cox.net on Mon, 08/02/2004 - 00:54

Lou,

I have to agree with Art, If you got that much crap out of the filter I would certainly worry about the pickup screen. Did you get good fuel flow when you disconnected the filter? (I forgot to mention that you needed someway to plug the hose from the tank) I believe that the screen is in that little "Sump Tank" under the battery. It's my understanding (my problem turned out to be the fuel pump) that you have to drain the tank to remove that. Hopefully some one else knows for sure.

Mike Smith
90 XJ-S Convertible
79 MGB LE

Submitted by silver007@shaw.ca on Mon, 08/02/2004 - 00:45

Art Dickenson.
1983 XJS GT "Silver"
Pacific Jaguar.ca
Hi, there is also apparently some kiinda filter inside the gas tank, take a look at that item while you are there, Art

Submitted by fastxb2003@aol.com on Mon, 08/02/2004 - 00:28

Well, I took off that filter and back-blew it with compressed air...you wouldn't believe the rust and crap that came out of it! But being Sunday late, no parts stores open, so after rinsing and cleaning it several times, I put it back in place. The motor problem is still present, but at least I know that tomorrow a new gas filter is going to go there.

1984 Black XJS V12
On 3rd engine - 1986 motor
(This one takes off like a Scalded Cat)

Submitted by fastxb2003@aol.com on Sun, 08/01/2004 - 19:38

Thanks Mike! I don't think this vintage has ABS...the chassis being a 1984 model I don't think came with it.

Thanks for telling me where the fuel filter is! Is there more than one, another one upstream, or just the one behind the spare?

1984 Black XJS V12
On 3rd engine - 1986 motor
(This one takes off like a Scalded Cat)

Submitted by fccm.sw@cox.net on Sun, 08/01/2004 - 19:00

Are you sure you are hearing the fuel pump? The ABS Pump (up in front of the passenger seat) runs for a few seconds and (at least in mine) masks out the fuel pump pretty well.
The fuel pump is in the trunk right under and inboard of the battery.
The fuel filter is behind the spare. Remove the spare tire and it is that aluminum beer can looking thingy.

Mike Smith
90 XJ-S Convertible
79 MGB LE

Submitted by fastxb2003@aol.com on Sun, 08/01/2004 - 16:27

Thanks, Mike! I'll have that checked for sure. It DOES act like fuel starvation, but what got me off this track was the fact that when I tried to start it yesterday AM, it wouldn't fire but I smelled raw gas from the exhaust. So I figured it wasn't likely the fuel system.

I can hear the pump running, and build up pressure and stop, as normal.

Where is the fuel filter on this thing?

Thanks Again!

1984 Black XJS V12
On 3rd engine - 1986 motor
(This one takes off like a Scalded Cat)