I haven't been on here for a while now, which, believe it or not, is mainly due to the XJS running just fine, and it still is... but I have a problem with a 1994 Daimler Double Six (XJ40/XJ81) that maybe some of the Marelli guys might be able to assist with. What follows is a post I made to the XJ40.com forum:

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

 

Other possibilities (I think) are coolant temperature sensor and auxiliary air valve problems.

 

 

Off the top of my head it is the one in the left hand water rail at the front; easy to change but make sure you get the right part.

 

Hi dstary

If the AAV (auxiliary air valve) is sticking then it can cause the Engine to start Hunting with a rise and fall (oscillation) in the Revs

This Engine Hunting Syndrome almost always goes away when the Engine Warms up but can cause problems with Starting

You may be able to free it off with a squirt of penetrating Oil but do be Careful when removing the rubber Air intake hose

As the heat from the Engine can make them very fragile

Going by what you have described this is a very likely cause of your problem, which is often brought on by standing around and general lack of use

A very simple squirt of Oil might be all you need to fix this problem as the Piston gets stuck in its Tube which causes the Engine to 'Hunt' until it warms up

The Nut on the side of the AAV is the Idle Adjustment but probably best left well alone, assuming that your 'tick over' was ok beforehand

You could have a look at the CTS (Coolant Temperature Sensor) to make sure the connections are OK although if it is faulty, then the engine may not start at all



The AAV (Auxiliary Air Valve) a Sticking Piston can cause the Engine to 'Hunt'



The CTS (Coolant Temperature Sensor) located near the Oil Filler Cap If the CTS is Faulty then the Engine may not Start

 

The AAV is still available or at least available as a reproduction from forum vendors. Though there are several different ones, so be wise and make note of your current part number

 

Btw., if it was a ballast or a coil, it would run like a POS at all times. Same for dizzy, sensor or plugs. It MAY be, that your engine is only firibg on one bank. But then you would see if clearly in the exhaust. The engine would still run smooth, as the 6 on the other bank still run smooth. Not like a V8 or V6 where half off makes the engine run like a bag of nuts

 

The Marelli cars (XJS at least) have 2 CTS, one on the LH bank for the FI and one on the RH bank for the Marelli, the one on the RH bank is a correction input so will not prevent from starting but could have the car thinking it is warm when it's cold.

ECU ignition timing can cause hunting idle if the idle timing is varying.

Check the connections on the 2 CTS and also the dreaded white wire connector. NOt familiar with the saloon, but my XJS has a connector under the LH intake manifold in the V that used to go to the Lucas ignition module, Jaguar just joined the white wire here from the Lucas FI to the Marelli.

Also check the VR sensors and connectors.

 

There is also a vacuum connection to the Marelli module, I'd want to make sure all the vacuum lines are in good shape.

To be honest the coil resistances seem a bit high to me, did you zero it out to null the effect of the test leads on the meter when reporting those measurements? The earlier Lucas ignition single coils were 0.5 ohms, but I don't know the Marelli spec.

I replaced my coils and the car did run better. I put in aftermarket coils and they were amazingly cheap.

 

Thanks to all for the advice. I’ll tackle the AAV first, and have ordered replacement coolant sensors. I’ll report back with my results when I get another couple of spare hours to work on the car. Until then, I’ll be XJS-ing to and from work, and there are much much worse ways to commute!

Dave

 

 

I see you've found the issue but I'm wondering how old is the wiring in the ignition system? If its all original that might be a concern in the future. I had a very similar sounding issue and after replacing literally everything on the Ignition system it turned out to be a failing Coil wire plug (Pico/metripack 150 if anyone is interested)

 

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