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XJ81 starting issue

dstary
Posts: 20
Joined: Fri Feb 05, 2016 11:19 am

XJ81 starting issue

Postby dstary » Mon Jan 15, 2018 10:35 am

Well, it's been a wonderful, trouble-free run in the Daimler for a while now, so something like this was probably inevitable. Long story short, the car is stubbornly refusing to start at the moment, and I would appreciate any advice on how to fix this. Long story goes something like this...

Last Friday, I started the car up, and the idle was rough, to say the least. It slowly rose to fast idle, but was constantly dipping down to almost stalling before picking up again. Gently pressing the accelerator just made stalling more likely. But, I was able to reverse it out of the driveway, and after a couple of minutes all was well - idle was sound, accelerator didn't cause any nastiness, so I took it for a drive without any drama. Smooth all the way, like nothing ever happened.

Fast forward to this morning, in the usual Monday rush to get to work, and the same symptoms as Friday. It took a few spluttery attempts to get the car to idle, and when it did, it was uneven, dipping down to almost stalling again. So I thought I'd leave it the couple of minutes (like Friday) and had a coffee. I entered the (open) garage to clouds of fumes, to me it smelled (and looked) like incredibly rich running. The accelerator wasn't helping, so I decided to turn the car off, and take the XJS instead. Thankfully, that car ran beautifully!

Tonight I get home, after pondering on this all day. My first guess was maybe the good old Marelli rotor (cap was replaced with OEM only 12 so months ago), or perhaps the coils. I get home, and try to start the car - same result as this morning so I turn it off. I remove the cap, inspect the cap and rotor, but bar a few scorch marks on the points, all presented well. No burns, no damage to be seen, so I put it back together. I try to start the car again - it maybe seemed a little better (could be my own wishful thinking), but was far, far from good. I turn it off again. I disconnect both coils, and check resistances - primary resistance on the A coil was 0.8 ohms, and on the B was 1.1 ohms; with secondary resistance at 5k ohms (A) and 4.8k ohms (B). Now I couldn't find any specs to check these against, but given that both are similar, unless both coils have failed simultaneously, I am assuming that they are serviceable. Now the weirdness - I reconnect coils (noticing a little electrical cable insulation breakdown at the coil plugs to the primary connections, but the wires seem good), and try to start the car again.... nothing, no rough idle, no spark at all (or so it seems), just smooth cranking and lots of clattering (maybe the injectors, but I didn't listen closely), with a strong fuel smell from the tail pipes. I wait an hour to make sure it isn't flooded - same. No spark at all.

So, I am over to your collective wisdom, and I'll post this same issue in an XJS forum for those 6L guys. Are those coil plugs a likely culprit (perhaps removing and reattaching them caused some breakage), should I be looking at those ignition amplifiers, are the coils out of spec and have both failed, should I check a sensor (CPS maybe) is it likely to be a fuel issue, or (god forbid) an ECU fault? I should also note no VCM codes throughout this whole saga.

In the meantime, thank god for the simple old 5.3 HE...

Dave
1985 XJS HE (manual conversion)
1994 Daimler Double Six (XJ81)

dstary
Posts: 20
Joined: Fri Feb 05, 2016 11:19 am

Re: XJ81 starting issue

Postby dstary » Mon Jan 15, 2018 11:27 am

Update: it lives! I tried to start it again for the last time tonight. First attempt: rough idle and stall. Second attempt (with a good 5 seconds or so with the starter running) and it came good. Started a little rough but picked up idle and now running smoothly, with just a tiny oscillation in the idle (I’d say 50rpm if that).

So I’m thinking it must be a sensor - which ones should I tackle? If they are easy to buy and fit, I’m more than willing to swap them out “just in case”.
1985 XJS HE (manual conversion)
1994 Daimler Double Six (XJ81)

Starbuck
Snr Member
Posts: 673
Joined: Thu Mar 19, 2015 4:48 am
Location: Melbourne, Australia

Re: XJ81 starting issue

Postby Starbuck » Tue Jan 16, 2018 12:27 am

Hi Dave,

There are many possible culprits, though not that many sensors. It has to be said that (thankfully) as the potential causes of failure become more expensive they are also each less and less likely.

The Marelli V12s use two 'reference' sensors on the crankshaft - one on the flex plate (flywheel) and one on the crank pulley. Both need to work for the ignition to function and both have been known to cause intermittent 'no spark' failures. These are considered primary input signals and if either fail completely the engine will not start.

There is also a coolant temperature sensor, and an air temperature sensor (as per the Lucas system) but these are secondary signals and the engine should still start even if either sensor is not working correctly.

The ignition module sends a reference (speed) signal to the fuel injection ECU - this also relies on the crankshaft reference sensors.

Check both 'A' and 'B' banks for spark. If both are out there is a good chance it is one of the reference sensors. On the 5.3 and 6.0 it tends to be the rear (flywheel) sensor that fails first. The front can go too of course. On the 5.3 both are interchangeable, unfortunately on the 6.0 they are very slightly different.

The only way to check if both reference sensors are working correctly is with an oscilloscope, otherwise you can swap them for known good units to test the theory one has failed - otherwise take 'pot luck' and purchase new sensors. Note that the reference sensors have a required air gap of 0.2 to 0.8 mm, and failure to shim them correctly will result in a broken sensor tip at one end of the scale, and an intermittent or 'no signal' at the other end.

The coolant and air temperature sensor resistance can be checked with a multimeter at specific temperatures to see if they are working correctly.

The biggest favour you can do yourself as the owner of one of these is to the get the Brooklands print of the factory Jaguar XJS workshop manual and supplements, as it covers the Marelli system on both 5.3 and 6.0 versions (and fuelling) in great detail. The version I have has a Brooklands publication number of AKM3455.

For all its faults the Marelli system is actually pretty sophisticated. The older Lucas system on the pre 88-1/2 HE has its advantages too, but also a number of disadvantages - particularly as the years advanced and more 'electro-vacuum' controls were required for emissions and smooth running!

Good hunting,

Starbuck
'94 XJ12 (and other toys)

spottroadgarage2006
Posts: 132
Joined: Thu Aug 23, 2012 9:27 pm
Location: Dunbar Scotland

Re: XJ81 starting issue

Postby spottroadgarage2006 » Tue Jan 16, 2018 8:49 pm

Hi I had a similar problem with my xj81 and replaced both flywheel and front pulley sensors and checked all the ignition parts without any success. I finally gave up and loaded the car on a trailer and took it from Scotland down to David Marks. David went through the car looking at every thing that could cause ignition problems, he spent a good five hours with meters and circuit drawings before he diagnosed a dry joint in one of the footwell fuse boxes. It wasn't evident from any voltage checks possible with the fuse boxes in position and needed the boxes to be removed and then opened to expose the internal diode connections . Once resoldered the problem was solved and has never reoccurred. I would suggest that if all else fails get hold of a known good fuse box and do a swop. David did a great job solving the problem and did say he hade seen a similar problem in xj40 six cylinder cars but this was the first he had experienced in the V12 engine XJ81.


Best of luck Ivan

Rok_Dr
Snr Member
Posts: 296
Joined: Mon Sep 15, 2014 5:15 am
Location: Perth, W Aust

Re: XJ81 starting issue

Postby Rok_Dr » Tue Jan 23, 2018 7:01 am

Hi Dave

Nice to hear from you despite the starting problem. It’s an unfortunate fact of life when you own classic cars that these "character building" issues will appear from time to time.

I see Starbuck has posted with her usual well considered expert advice, so you are in good hands and I‘ll be interested to hear what the underlying cause was in the fullness of time.

Which reminds me I should post a brief update on Lady P’s blog, as she and her garage companions have also given me some "character building" frustrations in the last few months which are thankfully sorted…… I hope :)

Cheers

Steve
12 - 1994 Jaguar XJ12 "Lady Penelope"
6 - 1986 Alfa Romeo GTV6 Grand Prix
5 - 2003 Discovery 2 S Td5 "Charlie"
4 - 2002 Nissan Pulsar ST "Iggy"
0 - 2006 19'6" Windsor Genesis "The Castle"
Image

dstary
Posts: 20
Joined: Fri Feb 05, 2016 11:19 am

Re: XJ81 starting issue

Postby dstary » Mon Feb 05, 2018 11:28 pm

I managed to get the car started (smoothly!) last night, thanks to a little bit of penetrating oil in the original AAV. I do now have a spare AAV on the shelf though, so if it plays up again I’ll just replace it. I also have two coolant and two crankshaft position sensors on order.

I’ll hopefully get a decent drive of the car this week, and I’ll let you know how I go.

Thanks for all the advice!

Dave
1985 XJS HE (manual conversion)
1994 Daimler Double Six (XJ81)

Starbuck
Snr Member
Posts: 673
Joined: Thu Mar 19, 2015 4:48 am
Location: Melbourne, Australia

Re: XJ81 starting issue

Postby Starbuck » Wed Feb 07, 2018 2:52 am

Hi Dave,

The AAV won't have had anything to do with your 'no spark', but they are notorious for causing other starting maladies and loping idle during warm up. Our second XJS had a sticking AAV and it started fine, but then about halfway through the warm up would suddenly start to rev and drop, rev and drop, until finally warm enough that the sticking AAV would 'pop' closed and all was fine. We replaced the AAV and that fixed the loping idle problem.

A common warning sign that the AAV is stuffed when buying a V12, is unusually high idle when warm. What happens is they are hard to start so mechanics open up the bypass screw to allow enough air to pass that the car will actually (just) start cold, but then the hot idle is much too high - often 1300 to 1400rpm. The cars then 'thump' into drive and reverse (and try to take off!), and they burn their transmission fluid in very short order (and ruin clutch packs) because it is like holding your foot on the accelerator and keeping your foot on the brake every time you come to a stop.

I have seen people rebuild V12 AAVs using the bulbs from thermostats, and I have seen one person make a 'manual' valve to replace the AAV, but we fitted a 'cheap' (relatively) aftermarket AAV to the XJS that we got from a reputable Jaguar parts seller, and that AAV behaves perfectly.

Don't ignore the fusebox suggestion from Spot Road either - the XJ81 is just as susceptible to dry joints in the fuse boxes as the XJ40 is. In fact it's arguable that just like replacing caps and rotors on Marelli cars, and rebuilding the centrifugal and vacuum advance on the Lucas V12 cars, that if you haven't done it yourself and know it has been done - assume it hasn't been done and just do it. Once you have replaced the sensors and if everything else checked out, if you still had spark problems my next suggestion was going to be the fuseboxes before you start suspecting ignition amplifiers or the Marelli control unit itself.

Oh, and as I previously mentioned make sure you are super careful air-gapping those new sensors when you fit them!

Thanks for the kind words Steve. I sincerely hope Lady P is going well over in sunny WA.

Best,

Starbuck
'94 XJ12 (and other toys)

ChrisW
Distinguished Mbr
Posts: 1515
Joined: Sat Apr 14, 2012 6:05 pm
Location: Whatlington, East Sussex

Re: XJ81 starting issue

Postby ChrisW » Wed Feb 07, 2018 10:33 pm

Something that contributed to my XJ12 starting problems was the small hose to the AAV from the inlet manifold. The inner lining of the hose had collapsed, not apparent from the outside but severely restricting the airflow to the AAV which had been over adjusted to compensate. There's a picture halfway down this page.

viewtopic.php?f=33&t=6881&start=320

The other main contributory factor was broken and missing nitrile bushes in the throttle linkages causing poor synchronisation between the banks. The throttle back stop adjusters had been maladjusted to compensate.
http://www.springballchange.co.uk
'93 XJR
'93 XJ12
'73 MGB Roadster
97 Range Rover Autobiography

dstary
Posts: 20
Joined: Fri Feb 05, 2016 11:19 am

Re: XJ81 starting issue

Postby dstary » Fri Feb 16, 2018 11:14 am

Thanks for all the advice once again - these sensors have now arrived, and I think I’ll tackle them first. While there certainly could be a failing wire, or (shudder) a dry solder joint, at least this way I can eliminate the sensors.

Back to your collective wisdom though, without having had a good look at how they are fitted yet, how am I to shim them to the correct airgap? The sensors are not adjustable on their brackets, and by the look of the workshop manual I’d have to use washers between the mounting point and the sensor bracket - is this the case? Also, is it simple enough to get shims between the sensor tip and the teeth? Or do I need some other measuring device? And what is the correct gap? My manual has each at 0.457-1.067mm?

Once again, thanks!

David
1985 XJS HE (manual conversion)
1994 Daimler Double Six (XJ81)

dstary
Posts: 20
Joined: Fri Feb 05, 2016 11:19 am

Re: XJ81 starting issue

Postby dstary » Fri Feb 16, 2018 11:16 am

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1985 XJS HE (manual conversion)
1994 Daimler Double Six (XJ81)


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