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Thousand Dollar Supercar's 1988 Jaguar XJ-S 3.6


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12 hours ago, Thousand Dollar Supercar said:

There's now only one guy left in Auckland who hates himself enough to work on classic Jags for a living,

Actually LOL’d at this (seems my self-flagellation of choice is mid 2000s Audis)

Good luck with the finding and exterminating the gremlins!

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Very brave indeed opening up some of Lucas' finest! I know the XJ40s of that era had some issues with dry solder joints in their various boards.

I opened up my dash cluster as the rev gauge wasn't working, only to be duly terrified and put it all back together again. Need to get some soldering practice on something less delicate. Oh, and discovered that the rev gauge can be 'cured' by a gentle tap on the top of the dash each time it starts, which hasn't helped motivation to get to the bottom of the issue.

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14 hours ago, BiTurbo228 said:

Very brave indeed opening up some of Lucas' finest! I know the XJ40s of that era had some issues with dry solder joints in their various boards.

I opened up my dash cluster as the rev gauge wasn't working, only to be duly terrified and put it all back together again. Need to get some soldering practice on something less delicate. Oh, and discovered that the rev gauge can be 'cured' by a gentle tap on the top of the dash each time it starts, which hasn't helped motivation to get to the bottom of the issue.

My previous car was an Alfa Romeo, and the speedometer got progressively more intermittent due to a bad connection between parts of the instrument cluster. It took me years to discover that, because I had figured out a temporary solution to kick-start the speedo using a seemingly unrelated button on the dash. ;) 

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2 minutes ago, Thousand Dollar Supercar said:

^ Cool, a mighty Iron Lion...
My XJ-S has been with an auto electrician for some time, and I'm finding it difficult to source a second-hand ECU. There's no particular hurry though - I drive fuel's nice reliable Mitsubishi now. :P

 

oof, whassit doing / not doing?

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1 minute ago, h4nd said:

  oof, whassit doing / not doing?

The spark signal from the ECU to the ignition module deteriorates with engine speed and throttle angle. The engine still runs normally in normal driving, but the ignition problem seems to be creeping further down the rev range over time.

I found a small amount of water seemed to have leaked into the ECU, and maybe that has done some damage, maybe not... The reason for involving the auto electrician is that I wanted to rule out what seemed to be electrical noise on the 12V supply / from the alternator. My oscilloscope seemed to show this noise being absent at idle but increasing as I got to the engine speeds where the misfire would begin, and I thought maybe the noise could be 'confusing' the otherwise-functional ECU. I hoped the auto sparky could tell me whether the noise was normal / could test the alternator safely without blowing its regulator or something.

I told him not to hurry, as I'm back and forward from Australia for work at the moment. I haven't checked in on him lately.

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Hum. Crank sensors on Alfa 156s like to die in a similar way to what that sounds like. Start off by working ok when cold, but losing the top end once the engine's warmed up a bit. Then the range of revs they work at progressively shrinks until they just stop working altogether.

I know crank angle sensors are one of the first things to check under 'won't start' conditions on AJ6s. No idea how much they are these days, but might be a reasonable thing to swap around anyway.

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1 hour ago, BiTurbo228 said:

Hum. Crank sensors on Alfa 156s like to die in a similar way to what that sounds like. Start off by working ok when cold, but losing the top end once the engine's warmed up a bit. Then the range of revs they work at progressively shrinks until they just stop working altogether.

I know crank angle sensors are one of the first things to check under 'won't start' conditions on AJ6s. No idea how much they are these days, but might be a reasonable thing to swap around anyway.

Yes, I’ve heard that the crank angle sensors on AJ6s are known for instantly and completely dying. I can’t see anything wrong with the signal from mine though. Its pulses get a bit rapid for my oscilloscope at higher rpm, but you can still see enough detail to be fairly confident it’s OK. 
 

If I end up going with an aftermarket ECU as Bigfoot suggests, perhaps that would be a good opportunity to fit an O2 sensor and to get a bit more power via advanced timing (a common AJ6 mod was to move the crank angle sensor because the ignition timing of the factory ECU is too conservative). Lots of work and lots of learning though. Stock ECU replacement sounds easier if I can get one. 

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Ah, probably not that then!

Not sure you'd need an O2 sensor to be confident about pushing the timing forward a bit. Plenty of engines running with slightly advanced timing to no major harm. O2 sensor would let you dial that in though.

If you want to get ambitious, I have it under good authority that 3.6l AJ6s are a set of ARP rod bolts (and presumably the appropriate regrind on the cams) away from 6500rpm ;) 

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9 hours ago, BiTurbo228 said:

Not sure you'd need an O2 sensor to be confident about pushing the timing forward a bit. Plenty of engines running with slightly advanced timing to no major harm. O2 sensor would let you dial that in though.

If you want to get ambitious, I have it under good authority that 3.6l AJ6s are a set of ARP rod bolts (and presumably the appropriate regrind on the cams) away from 6500rpm ;) 

No, I was thinking that since aftermarket ECUs support O2 sensors, that would be a sensible addition for magical self-learning idle and fuel maps? I am not informed on this subject. This XJ-S is the only fuel-injected car I've ever owned.

I bet an AJ6 would feel ready to explode at 6500rpm! I hear that AJ6s have plenty of limitations that keep them from even making their quoted power, and so they have no business doing such crazy engine speeds. :P

3 hours ago, yoeddynz said:

ITBs on an AJ6

"Fully rebuilt, balance & blueprinted engine with custom intake and Jenvey throttle bodies. Custom exhaust headers and system. Link engine management. Engine rebuild and setup carried out by Dyno Tuner Uk Ltd

351 BHP @ the Flywheel"

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Yeah I wouldn't rev a 4.0l that high! Not without significant work. The cast AJ6 cranks start getting nasty torsional resonance around 5850rpm. Later AJ16s with the forged crank can make it to 5950, though my understanding is they start having the same issues a bit beyond that. 102mm stroke will do that. Though that racecar video doesn't look particularly wanting for revs. Wonder what it's actually reaching, and what they've done to the engine.

The 3.6s 92mm crank is much more reasonable though. Roughly comparable to the S38B38's 90mm crank, except it's much stiffer. Ginormous main bearings will do that (76mm!). Ford and Mopar big blocks are bigger, as is that monster 472 Cadillac V8. Cummins 4BTs are as well. Apart from that, I haven't really come across anything as beefy.

I've seen racing 3.6l AJ6s that reliably rev to 8000rpm with Pinto forged pistons. They're monster engines, just in incredibly mild standard tune.

*Like-for-like forged cranks are actually worse for torsional resonance as they're not quite as stiff, though they can tolerate resonance better. AJ16 has lighter conrods and pistons though, which might help.

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