I'm hoping someone can explain the Marelli system operation so I can do some diagnosing.

Car is 1992 XJS with 5.3 and I recently noticed a lack of power/ability to rev on the interstate, full throttle in second gear and it wouldn't climb above 4k RPM or so. It wasn't breaking up or stumbling, engine was smooth, just wouldn't rev past 4k.

I suspected a ignition problems, maybe the rotor, cap or coil as those seem to be typical culprits, and also considered a plugged cat secondary to those problems.

I got straight home, it drove fine at moderate throttle and got to work diagnosing. I ordered a rotor and cap, also 2 new coils and while waiting for parts, I took off the stock downpipes and checked the cats, they flowed water so weren't completely clogged, but I could see some spots that appeared slightly plugged.

But here's where I need some education, it doesn't seem like the exhaust was restricted, and if it was partially, not enough to limit the engine to 4k RPM. But since I had them off, I "modified" the cats so they would flow freely. So now I suspected an ignition problem. But upon started the car after downpipes were reinstalled, it idled like normal and I could see that both banks were firing (exhaust was warm on both sides, condensation in the exhaust). So that means that all 12 cylinders are firing. Is there something about the Marelli system where it works normally at low RPM and then fail at high RPM? Seems like a dead coil is a dead coil and a burned rotor is a burned rotor.

Or is this fuel related? Didn't feel like it, super smooth running, even when it hit the limit. But?
Car has a new fuel filter, and has had several tanks of gas through the new filter. Clogged fuel filter? just starting to die fuel pump?

experts and experience please!

 

A really easy place to start would be the fuel filter. Empty the contents into a clear container. What you see will dictate the next steps.

Cheers
DD

 

Here ya go!

left to right:
new paper towel, paper towel after I dumped the fuel and third paper towel is after I blew the filter with compressed air and back flushed with new gas.

looks about as good as it can be. Maybe a restriction on pump pickup?

 

Here is the Marelli Supplement which can explain it better than I can.

 

Thanks! thats exactly what i was looking for!

 

Please be aware that the Marelli wiring is different for the 6.0 than for the 5.3.

 

Resolved!

https://www.jaguarforums.com/forum/x...theory-275527/

 

To be honest, I'm utterly confused. You've been talking about your MY92 and a MY94. The still photo of the pump with the rubber pump module isolator in the video are parts of the XJ40/92-94XJS fuel pump module, but the mechanic is talking about two pumps. The 92 face lift XJS has a lot of changes incorporated since the first use of the Marelli ignition system. The 92 MY XJS was the first year Jaguar use the XJ40 in tank fuel pump plastic module and went full digital on the ignition system. Therefore, almost all previous model year technical introductions, wiring diagrams and supplements should be use as night,night reading material only. Jaguar made significant changes in the fuel delivery system, ignition system, emissions and hot start system, electrical wiring and other changes which are not critical for making the car run when the rolled out the Face Lift. After multiple revisions of the crappy and over engineered fuel pump module used in the XJ40 and the XJS, Jaguar changed to the X300 in tank fuel pump design. Those tanks were zinc coded and had an open steel anti slosh pan welded to the tank floor. Two pumps can be hung off it when needed. This was a simple design and pumps can be fairly easy be changed out. Assuming you're nimble enough to hang out in the trunk. This in tank pump design has been carried through the early 2000 XJ models, afaik.
Now, your sketch in the other post is also suggesting that your' re talking about the in tank plastic fuel pump module. The tube with the O ring you subscribed is part of the fuel return system which gets restricted at the bottom and creates a siphon effect to draw cooler fuel inside the module. A pump that sounds like it's running, doesn't mean it is actually pumping. Don't ask me, how I know. There's also a check valve in the lid of the module and on the pump outlet. In my case, the rubber grommet inside the lid where the pump outlet attaches to and seals are getting old and bleed off pressure. In addition, in my case, the press fit steel cap that hold the grommet in place sheared of the plastic by some previous installer. That was probably the main reason I've got that car with a manual conversion, single pass alu-radiator with tween e-fans for a fairly reasonably because nobody could get it running. Another critical, or maybe critical part, for making those pumps work correctly is the rubber check valve at the bottom of the module. In my case, the edges started to deteriorate as well. Having said that, those housings are NLA and cost a king's ransom on the used market if you can find one and hope it has still some live left. Jaguar should have stayed with the time proven search tank design until the end of the model run.
I highly recommend to acquire Jaguar's Face Lift model year's technical instructions, electrical wiring diagrams, technical bulletins and supplements and read both of Roger Bywaters web pages which I just did, one again. Roger mentions that some of the info in the Shop Service manuals are ironious especially when it comes to the Marelli design.

Find a lot of downloadable information on this website:
JagRepair.com - Jaguar Repair Information Resource
Rodger Bywater's new website:
https://www.jagweb.com/aj6eng/
Rodger Bywater's old website:
https://www.jagweb.com/aj6eng/index.php

Hope this will help some to keep 'm rolling.

 

Sorry to muck up the post with the comparison of '92 and '94 fuel systems. I only included the above video because of a comment they made about an o ring and I thought it sounded like the problem I had.

And your description of the fuel pump module is bang-on, I noticed the check valve on the bottom of the pickup and at the outlet after the fuel pump, and I had the problem with the grommet/seal at the top of the pump (as well as an ill-fitting 0-ring on the right side of pickup in drawing above).

 

And given your experience, is there any reason I can't abandon/discard all of that and simply have a pump inside the tank sucking fuel off the bottom without any of that contraption?

 

I ended up installing a tank out of a 95 after the rad shop stripped off the paint on the outside too. I told him not to, duh! I've tried different pumps and modules only to find out having fueling issues. I also stripped and cut out the bottom of the plastic module and strapped another pump to it which shortly burned up because of small particles getting through the filter. The steel tank is hard to clean due to the anti slosh panels. My car seems to have spent most of it's time in a arid environment but still accumulated surface rust and one gallon full of hard tar like bits.
Here are some visuals of what I've been talking about in post #8:


Sheared off fuel pump seal retainer.

Pump seal with retainer with sheared off plastic.

Aging pump seal.

Aging fuel pump module strainer on a spare pump. This was already missing in mine.

Deteriorating rubber check valve and venturi jet to the right.

Location of check and venturi assembly. The larger ring is where the pump is locked in.

Top part of the module with pump and return screen mounted. The O-ring Andy was talking about.

Bottom without screens. This is where the fuel enters.

Another failure point is the electrical connector to the module. At least this is how it was on my car.

Another bad part is where the pump wires connect to the module housing. Jaguar made multiple revisions on those pump modules for the XJ40 and XJS. Duh!

Here's a hack from a previous owner.

My hack attempt before I moved on to a 95 tank.

More hackery ...

...more...

...more...

Another view of the aging pump module top assembly with return filter and venturi removed. There's a check valve in the return line.

Bottom assembly.

The anti slosh tank out of an XJ40 with pump holding bracket and return line. This would have been my next attempt to fix this problem beside of having a tank made with expensive aftermarket fuel pump assemblies. Since I have the 95 XJS tank installed another issue showed up. The OEM rubber isolators for the pump have been turning into sticky rubbery goo. This also has happened with a brand new aftermarket pump module for my 92 Range Rover Classic. I'm not sure what is here at fault.

 

Thanks for those pictures, you seem to have wrestled with the exact same problem. Right now, I have the pump sucking straight from bottom of the tank with the plastic pump holder being that, a holder only. I removed all the plumbing associated with it. I suppose there could be the side effect of the pump wearing prematurely because it is running hotter without the plumbing but I tried to resolve the leaks exactly like you and didn't have any luck. and I'm justifying the decision that submerged fuel pump technology has improved in the past 30 years.

 

Do i get it right, that this particular problem could be identified first by watching fuel pressure at the rail? i have same problem with my 92, doesnt run above 3k rpm and shooting from tailpipe. Im looking in different areas, such as ignition and TPS, and i 6 months ago i have taken pump housing out to inspect it, didnt find anything specific, replaced the filter and thats it, mb i miss-looked that problem, but pulling tank out and getting inside is not fun, so i would rather check fuel pressure first.

did your car experience this problem on idle as well? or only at load?

 

My symptom was it wouldn't rev above 4k RPM, but it was felt like a floor mat was stuck under the throttle, didnt sputter or seem like it was running out of fuel.

here is another thread i started about the problem. It's kinda confusing because I'm talking about ignition problems which then turn into the fuel problem resolution.

I can't definitely confirm this, but I think the problem only showed up after i ran the tank to a 1/4 which exposed the (leaking) seals in the stock housing. So, if your tank is low, fill it up and see what it does. I'm not saying that having the leaking seals covered in gas will solve the problem, thats just a theory that you can test.

https://www.jaguarforums.com/forum/x...theory-275527/