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Discussion Starter · #1 · (Edited)
Hi, so I am on with sorting out my new XJ-S.

One issue Ive got is that the idle isn't stable. I initially thought it was the ECU; because this:


Perfectly describes a lot of my problems (other than I don't have high CO emissions). Anyway, when I removed the ECU lo-and-behold it had a sticker on it indicating that it had already had not one but two trips to AJ6 Engineering!

I got in touch with AJ6 Engineering again, and Roger Bywater said he'd test the ECU and make sure its operating okay, I don't live to far away I so I whipped it round and its now with him. But I am not optimistic that this is the fault. So I am now going through the motions of checking other faults with it.

From where I am standing, if a car is running rich that means there is either too much fuel or not enough air. So my investigations have taken me to ensure that the throttle is operating correctly.

According to this page:

http://www.jagweb.com/aj6eng/lucas_efi.php

the Pot's voltage should range from 0.32V to 4.5V at full closed and full open.

But how can I actually go about measuring this? The pot is pretty hard to access; does I need to expose a bit of the cables and stick a multimeter on there - surely this has to be done when the engine on, or just the power on but engine off?

Thanks for any advice. Also any other advice regarding the richness is greatly welcome.
 

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I guess you answered your question about whether the ECU mod would fix your drivability problems.

The pinout is the basically the same for the 8CU and for the 6CU as used on the V12, the only difference being the latter has an extra injector driver output, for the 2nd bank of 6 injectors.

You will need to scrape off a bit of insulation, or get some insulation piercing probes, or otherwise get a connection to the throttle pot wiring. The engine doesn't need to be running. One thing you need to check is whether the voltage increases steady and gradually as the throttle is opened - what can happen is the carbon track wears through and you get intermittent connection. Then the voltage jumps around and could cause driveability issues. From memory, apart from knowing when the throttle is closed, the main thing the throttle signal does is allow the ECU to detect when you suddenly put your foot down, and it temporarily enriches the mixture - a bit like the throttle pump on an old Weber carb.

I personally think those ECU's are quite hard (and time-consuming) to fully test. Really you would need to measure the injector pulse width vs vacuum, at various engine speeds. At the time I knocked up a rough and ready rig using a mityvac + vacuum gauge then looked at the pulses on an oscilloscope. Trouble is, I never knew what the readings would have been for a known. good unit. Although it was useful for mending duff ECU's.

That system is called the "P-Digital", and whilst the base fuelling is determined from a look-up table in a form of memory chip, all the other adjustments such as cold start, transient enrichment etc, are done by analogue means and as such are subject to errors should electronic component values drift over time (which they do). Then there's the pressure sensor itself, although that would be quite easy to statically test.

It sounds like you are in pretty much the same position I was. I actually did modify a spare 8CU, to have an adjuster which reduced the overall fuelling - the car still ran badly as was thirsty as ever. Unfortunately both ECU's are long since sold.

If it's running that badly, perhaps you've nothing much to lose by considering a mega-squirt? Although you may struggle to ever get that to drive as nice as a correctly operating factory setup - you sure would have some fun though!
 

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Actually rather than mess with a mega squirt, you could always fit the later 9CU system from an XJ40 3.6 or later 6-pot XJS? Same procedure as the work I did with the 4.0 engine swap, but using your existing motor? Not a huge job and would solve all your problems.

As you've probably gathered, I spent ages trying to bottom the issues I had with the 8CU system, looking back I should have just binned it from day 1.
 

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Discussion Starter · #6 ·
Thanks for the input. Definitely going to keep those options on the table - I might have to pester you about those options in more detail later.

My ECU is actually with Rodger at AJ6 engineering, it turns out it has already been modified - so its just getting checked to ensure it is fault free.

I've disconnect the TPS and can tap the wires and read with a multi-meter. I am hoping if I connect a 5V supply then I can make a reading without the car being functional as it has no ECU.

I'll report back if I can make a TPS reading today.
 

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You could check whether there are any breaks in the track by measuring resistance between the pins as you turn the sensor.

Feel free to ask any questions you like, especially if you go down the 9CU route. The more I've thought about it, that route offers a chance of guaranteed success - whereas you could be messing forever with megasquirt, to get it just right, for road use. The only possible issue now is sourcing the parts, especially now Andy Harvey and Graham (grublogger), are no longer breaking whole cars for spares.

I do wonder if that 8CU system perhaps always was a bit "sub-optimal", even back in the day. You get the feeling it's been a stop-gap solution, whilst they were still developing the (far better) 9CU system.

What's the rest of the car like, have you got any pics etc? It's always exciting getting a "new baby" isn't it! They are fun with a manual box too, I often wish I'd converted the V12 I have here, when there were still plenty of 3.6 manual donor cars about.
 

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Discussion Starter · #8 ·
Oops, I thought I'd replied to you yesterday.

The car is rough, to be honest. And I am still finding things. But it's a case of putting one foot in front of the over and getting closer. It's currently not in a good position to take photos and I can't move it because AJ6 Engineering still has the ECU and I've got the fuel tank out.

I keep find new sections of rust. I figure the worst of it is as fellows.

Sills Right and left (I've got new replacement panels)

Driver side rear arch (I've got a used replacement panel here)

And the boot/under the fuel tank. I am on with this now. This sort of includes the rear right hand wing.

Unfortunately circumstances means I have to work outside and quite frankly the weather has been awful around here, we get a few nice hours here and there but interspersed with rain which is very disruptive.

Back to the ECU I did ask Rodger at AJ6 Engineering about replacing the engine and the ECU. He seemed to think that changing the ECU would be a big job and that the best thing to do would be to get this engine working with this car with this ECU. Second to that he suggested the 4.0 with this ECU - but he did say it wouldn't be as good of a match.

I didn't dig into why changing the ECU would be more work. I guess I can ask him when I go back to picking it up.
 

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I don't fully agree with Roger. Having done the job myself I can honestly say it really wasn't all that difficult. You need to be reasonably competent with vehicle electrics, which it sounds like you are. I reckon I'd probably spent nearly as much time faffing about with that 8CU system, as I did doing the whole conversion. Not to mention 3 or 4 years of spoiled jaguar driving experience.

Roger's opinion might be based on costing out the time involved, which doesn't really hold true if you are doing it yourself on a hobby basis. Of course when I did it, the XJ40 components themselves were not all that old - my donor car was barely 10 years old.

In terms of time though, I don't recall the job dragging on particularly, even allowing for me totally cleaning and repainting the engine bay, whilst the engine was out. I had the whole job done over a few weeks worth of evenings, with a few weekends - although back then I never had more than one day at the weekend due to family commitments.

I know what you mean about keeping on finding more bits of rust though. My first XJS (also 1984) was like that. What's yours like on the rear lower quarter panels, adjacent to the silencers? I had to make up repair panels for those, also the front/rear of the sills as well as a split in the boot floor. It was good under the fuel tank though, and the arches weren't too bad either.

I'd say not to fit a 4.0 engine unless you plan to upgrade the EFI to a 15CU as well. I cannot overstate how much better my car drove after that swap - the guy who eventually bought it off me liked it too :)

PS check out ebay item 303777982397, looks like the car might be pretty complete too
 
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