Hi all.

A long time since I posted so sorry about that just had a lot to contend with in life. I still have the Jag but would like to get to the bottom of this problem. Basically car runs fine up to around 40 miles then it starts being an **** hole with the engine bogging down when putting the throttle down. If you are gentle it keeps going give it to much engine will stall. This started last year notably after fitting a new fuel pump and FPRs. I fitted the old stuff back but it’s still doing it. I fitted a new TPS today as mine is still the original. The car does run better and is more responsive but the problem still persists. Any ideas please.

Rob.

 

First let’s get some details on the car? Year, engine, convertible/coupe?

 

Rob,

Are you getting vacuum buildup.in the tank? When it starts playing up, try pulling over and open the petrol tank and see if you get a sudden release. Also, dId you change the fuel filter when you did the pump?

Paul

 

Well blast me it used to be in my signature. I haven’t been on for a while so something must have changed. 91 pre facelift V12 convertible.

 

Plugged filter sock on the end of the pickup tube in the sump tank. As you drive, it picks up more crap from the bottom of the tank, starving the engine of fuel. When you shut the car down, the extra crap falls off. When you run the car later, the same sequence happens.
Plugged main fuel filter.
Fuel pump wearing out, reducing pressure as it heats up.
A fuel pressure cage on the fuel rail would help sort this out and verify if pressure is dropping as you drive.

Jon

 

My sump tank is clean and I have modified the system slightly. No sock filter but I do have a new inline filter before the pump. When I did all this last year I fitted a new main filter not a genuine Jaguar one. Maybe it’s faulty ? Pumps fine I have a brand new one and it’s the same with that.

Rob.

 

Sorry should have added no vac in tank as have a small hole drilled in cap. I think that might have been a Grant mod.

Rob

 

Hmmm. I'd still check pressure at the rail, with a long enough hose on the gauge so you can monitor it while driving. That would at least tell you if it is a pressure related issue or not.
Have all of the rubber fuel lines on the car been replaced? Engine bay, over rear suspension, trunk/boot? Not unheard of for lines to come apart inside. How about a rubber line kinking as the rubber warms up?
When it loses power, does it start running rough, or just get weak? Maybe the temp sensor for the injection is providing incorrect values to the ECU?
Start with fuel pressure, and see where that leads you.
Jon

 

The story for today goes like this. Fitted the new TPS set it up as per Grants instructions. Reset the throttle
rods etc. Started car nice tick over then went for a drive. Once warm gave it some beans all good. Second gear blast all good not a hint of any problem. Drove around 40 miles turned into my village foot down on the pedal engine cut out. Started instantly and then had to nurse it home on a light throttle. Push too far and the engine bogs just like fuel starvation. Got her home and revved the ***** off it in neutral no problem at all no missing perfect.

Rob

 

Change that filter Robbo. This happened to me on Range Rover and a new duff genuine Bosch filter was the culprit. I actually prefer this type:
https://www.merlinmotorsport.co.uk/p...tails-fac-stf0
with this filter in it
https://www.merlinmotorsport.co.uk/p...ilters-bulla02

So try a new OEM one and if it fixes it, buy one of these after.

 

Hi Robbo

I had the same problem on my DD 'The Merc'

Turned out to be an Ignition Problem: Either the Plugs or the HT Leads

Removed the Plugs and Cleaned them up with a Small Wire Brush (As they were Sooted up)

Then made sure the HT Leads were not touching each other

Result! No more Bogging Down Car ran perfect after that and is still doing so

Maybe its the 2 Plugs under the Air Con Motor, which some Shops have be known to 'Ahem' overlook thinking they were probably ok

Although not so, if you have done all of the Resto Yourself

Just a thought, are you using the 'El Cheapo' OEM Spark Plugs (which is good!)

As the XJS does not like the iridium type

Based on what was happening with my 'Merc' I would put my money on an Ignition Spark Plug/HT Lead problem, rather than anything else

Gotta be worth a 'Shot' I would have thought

Alex

 

Good morning all.

I will definitely change the filter and see how that goes. I just find it odd that the car plays up after 40 odd miles. You would think if the filter is faulty it would misbehave all the while. OB plugs and leads all replaced when I got the car so maybe 2000 miles ago. Std NGK plugs fitted.

Rob.

 

Changing the filter is a cheap way to eliminate that possibility, Robbo. Why, in my case, it caused it was that a fault in the paper element inside the filter caused it over a few miles to gradually bunch up and plug the outlet, although it could flow enough at low fuel demand it choked the flow at higher demand.

A question: does the engien stop dead, or does it sort of dy gradually? If it stops dead, then ignition is more likely to be the culprit. Have you tired 102v straight to the coil positive and testing that?

 

The scenario on Saturday was like this. Driving back to my village and before the turn it’s down hill so coasting and very light throttle. Came to a stop to turn into the village put my foot down and engine cut out slip into neutral and started the car instantly carried on to home not far less than a mile car bogging all the way home if you tried to give it gas. Revs up in neutral it’s fine. To me this feels like fuel starvation.

Rob.

 

Should have said I can make the engine cut out by putting my foot down if I ease off runs fine and also ticks over fine.

Rob

 

Agreed. So i suggest:

• 12v hotwire to the coil (just to be sure),
• new filter,
• check the TPS for broken wires, not engaged with the capstan spindle properly, and correct setting as this if off means the ECU gets no signal to increase pulse width at the injectors
• is the B bank FPR working?

 

New filter next week. TPS is new fitted yesterday. Set at 0.34v idle and smooth up to 4.65v max. FPR is original but I have new and it was the same with them fitted. 12v to coil not sure on this as my car is Marelli and 2 coils. I assume I can check the TPS signal to the ECU plug. Can you let me have the correct pins to check please.

Rob

 

The signal from the TPS goes to pin 7, see attached. The diagram can be expaned in Word tab 'view' to make it more legible.

 

Hi Robbo

(1) Blocked Air Filter making your Car run rich, next time She Starts to 'Bog Down' take the Air Filters out and see if that makes any difference

(2) If She starts to Bog Down, pull off the road engine running and see if you can hear the Fuel Pump running consistantly

(3) Blocked Injectors: You may be able to introduce some Injector Cleaning Fluid (Carb Cleaner) into the Fuel Rail

(4) Failing Fuel Pump Relay and or Main Relay: As you know very easy to Swap out on an XJS V12 they are inside the Plastic Box by the Drivers Side Gas Strut on a (UK) Car in the Boot/Trunk

(5) Blocked Exhaust, not allowing Exhaust Gases to Exit Fast Enough (But no Cats on a UK Car, so Probably not that

(6) To Keep the Fuel Pump Running for Testing Purposes: Orange Wire to Ground

(7) Coolant Temperature Sensor if not working Correctly this can cause Over Fueling and potentially 'Bogging Down' Problems You could try the Paper Clip Trick

(8) Dreaded 'White Shielding Wire' The White Wire is joined at the LHS of The Air Intake and goes to Pin 24 of the ECU and is Grounded at the ECU End ONLY not both ends or you will have Fueling Probs

(9) Crank Position Sensors (I had to file out the fixing holes on mine to get the Correct Gap

(10) The one at the Front Fires 'A' Bank this input is used by the ECU to Fire 'B' Bank and is a derivative function of the Flywheel Sensor at the Rear
The Rear Sensor Monitors the Engine RPM for the ECU The Front Sensor Triggers the Injectors and the Spark Gap for Both Maximum of 0.042 pref Less!

(11) Blocked Vacuum Pipe Causing Over Fueling

You know all this Stuff anyway, though it might help someone else

Alex

(11) Silver Box down by the Headlight Nacelle, may need the Plug and Socket Cleaning 'Shiny Bright)

 

Whats the story on that? This is good info to have.