Good morning all!

I've recently got my hands on my father in law's XJS 3.6 Manual 1985.
It's in need of full restoration, but while the sun is still shining in England I thought I'd use it up until the winter, where I can then stick her in the garage and get to work.

It drives well while under half to full throttle, but has a terrible over-run shunt while slowing down in gear at junctions. It also smells of very strong fuel, so I'm assuming it's running rich.

After some research on the internet and on the sticky how-to's, it looks as though a lack of vacuum causing the ECU to over-fuel, due to the age of the engine. It's only done 59,000 though.....

I've found a guy online who can modify the ECU so you can adjust it to suit the level of wear on your engine.
XJS 3.6 Richness Problem | aj6 engineering
Has anyone else had my Jag's symptoms and fixed it with modifying their ECUs?

I'm wondering if there is anything else I can try, like the TPS. However with the richness, I'm guessing it's the ECU fault.

Any help would be much appreciated!

Many thanks.

 

Roger is the man, clearly.

My only experience with the 3.6 is in the XJ40;s, and they were all Auto.

BUT

The TPS is #1, as it sits UNDER the throttle body, and the oily goo that is breathed into the throttle body by the engine, creates the "Jaguar Black Goo" This runs down the throttle shaft, gravity is to blame, and straight into the TPS. The TPS then goes hissy, as its now contaminated. Some respond well to a solvent spray clean, some do not.

3 of my '40's,, and both the X300 units were fine after a solvent clean. 2 were still AWOL, so new required.

Good luck.

Not wishing to mess you around at all, but the XJ40 Tech Section will have HEAPS of info on that engine. and its oddities.

 

Thanks Grant.

I've done some research and learnt that the ECU has an actual vacuum line that attaches direct.
I thought it would be wise to check the pipework holds vacuum well before pulling anything around, so I've ordered a cheap vacuum pump to give the system a test.

The vacuum line that comes from the boot floor goes into a tree. One end goes into the ECU, and the other end goes up behind the ECU into a plastic dome. Do you know what this does? I've attached a photo.

I'm hoping there's a vacuum pipe that simply needs replacing to restore good vacuum. If not, I'll try taking the vacuum from a different source as explained in AJ Engineering's link.

Many thanks!

 

Ashley,

No idea on hthat dome, never saw it on the '40's, but they really gave NO trouble at all.

Sounds a very similar system to teh V12 ECU sensing.

Less vac = more fuel.
More vac = less fuel.
That is VERY simplistic of course.

I had a V12 here, years ago, long before Computers and WWW, etc, and no matter what we did, overfuel, crap running, was the normal.

In utter frustration, I connected an oil syringe to the ECU spigot, all I had laying around, pulled the plunger, which applied vac direct to the sensor inside the ECU, helper started the car, and I could get it simply to run sweet as a nut, with that syringe supplying vac. I could vary the fuel "mixture" very easily.

Eventually found a wad of fire extinguisher foam in the line under the car from the engine. Guess it had an engine fire at some time. Owner fessed up when confronted, Removed that wad, car was, and still is sweet as they come.

I agree, a new vac line from an uninterrupted source on the engine wouild be a simple zero$ test.

Also, V12 ECU again. INSIDE that ECU is a short length of vac hose that connects that external spigot TO the MAP sensor on the PCB. These hoses can split, same result, inaccurate vac reading at that MAP, crap running. 2 of my XJS's did that.

Good luck, it will something simple, always is.

 

Thanks Grant, I'll whip apart the ECU tonight and check the internal vacuum pipe.
I'll report back my findings tomorrow evening once I test out the vacuum pump.

I like the oil syringe idea! If I can't get a strong enough and reliable vacuum from the engine, I could always get a little 12v vacuum pump and hook it up to the ECU haha!

 

The T line to the "dome" is to get vac to the Rochester valve. The Rochester valve is part of the fuel tank venting mechanism, ans it both releases fuel tank internal pressure and also lets air it back into the tank, depending upon the situation. The Rochester valve requires a vac source to operate in the "let air back in" mode, and that is where it gets it from.
https://www.jaguarforum.com/showthread.php?t=36730
and
Jaguar Rochester valve ~ Jaguar Vanden Plas "The cats meow"

 

I've tested the vacuum at the splitter in the boot. It's approx. 15HG.
While I was there, I also plugged the pump direct into the ECU. I set the pressure to 15HG and it started fine, as expected.
I then slowly increased the pressure to around 19-20HG and the engine died.
I then set it to 17HG and drove down the road. Of course, didn't get very far as the engine died from lack of fuel as soon as I put my foot on the throttle.

Is 15HG at idle low?

Even when the ECU had 17HG of vacuum, it still ran the same. Still a bit lumpy. Making me think it's something else?

I did re-read the AJ6 Engineering page, and saw that he references changing the advance vacuum pipe to the manifold rather than the throttle. I done this, but it didn't make any difference. One thing I did notice, is that when the engine was running, there wasn't any vacuum on the throttle tapping at all?

Any ideas are much appreciated!

 

I have the same car and if vacuum line is not connected - car simply doesn't run. I would leave it connected where it is, it works as designed.

In my opinion these cars are fuel-hungry in nature and they will swallow extra fuel, however luck of fuel would be a problem. I would check spark plugs and after running a bit check them for over-fueling (dark/black carbon deposits mean too much fuel). Fuel smell could be there if one plug is not working. You can check plugs by disconnecting one at a time while engine is running. If it makes no difference when you disconnect - plug/cylinder is not working.
Next I would check fuel filter, if it's old it may be restricting flow causing luck of fuel on higher rpms.
Also, there is a 'volume' on the ecu that adjusts fuel map (it's in the hole, you need longish 'volume knob' to get to it), you can try turning it to leaner position if you have a way to see how lean/reach is your mixture. Usually it is done with oxygen meter (I guess colortune plug could be used at idle as well ) .
Also check where you ignition timing is, I assume you have the same ignition with mechanical advance as I do. Mine was way out of curve ( it still is but not as much ).

Good luck! This car is fun to ride!

 

Thanks NJ2003XJ8, I'll take a look at the ECU adjustment screw this evening. Could be the answer! Just out of curiosity, what's the difference between this factory adjustment screw vs AJ6 Engineering add on screw?

But, I'm going to clean up and re-calibrate the TPS first. I had a look this morning and while it was difficult to see, it was easy to notice it's caked in crud. I've got some solvent de-greaser so I'll give it a deep clean and set it back up.

Couple of questions:
What's the idle voltage for the TPS on the 3.6?
Other than taking off the intake manifold, is there an easy way to get to the TPS for removal? I've got large hands so find it difficult in tight spaces!

I noticed this morning that the engine was almost stalling when operating the power steering. Also, if I'm driving down the road under hard acceleration, then release the throttle and engage the clutch, the rpm can drop down to nearly 0, then slowly rise back to 650rpm. Not sure if that's linked to my issue, or it's another one!

Many thanks.

 

TPS access, from underneath, oil filter removed.

Setting up, just pre mark it place before removing it, and put it back, DO NOT turn ON the Ign with it unplugged. Never done that myself, just got warned about that years ago, and never tempted fate, got enough things to worry about.

There would be a voltage for sure, but I dont know it.

 

At 59K your engine is like new, I really doubt compression due to age is the problem. The ECU screw is free, to other one will screw you for some money lol . I really don't know what they do, I think they reprogram fuel map.
TPS maybe, I did replace mine . They were available: https://www.jaguarforums.com/forum/x...ilable-179203/
can't remember taking oil filter out for replacing, maybe, I had whole throttle assembly out of the car... I believe you set TPS to have 0 voltage at idle. You can check TPS to some extent with ohmmeter. The end-to-end I believe is 5.9 - 6kohm, the sliding should be smooth 0-max.

RPM at idle is set by 'air bypass bolt' next to coolant temp sensor. Easy to find. Coolant temp sensor is important(input to ECU fuel map), you can check it's resistance at different temps, there are numbers for it somewhere on the web.

Anyway, my overall experience is that parts themselves rarely go bad(I replaced TPS and temperature sensor), but at the end my problem was rust in connection to power resistor. You want to kinda know for sure it is broken/problematic otherwise you order and replace it to no effect.

Make sure you have a 'system' for your problem diagnostics and go by that list. First check 'consumables' like spark plugs and filters, then 'tunables' like ignition timing, and then parts in the order they are likely to break.

Hope it helps :-)