Keep loosing a piston !

[Flappers]

Hello all, I have come to a crossroad with my Double Six. I have had a recurring issue with loosing a cylinder on the right bank, as i sit in the car. I have had it on several occasions back to a local "Jag Man" who has replaced the leads with high performance ones and replaced all the plugs on 3 occasions. He suggested doing a compression test on the last plug replacement and the results were as follows:

....A.........B...
1 160......155
2 160......160
3 130......160
4 160......160
5 150......160
6 160......160

So,on the last occasion where it wont fire the cylinder on the drivers side I replaced the num 3 plug and it then ran on all 12. I need to add here that more often than not, the car wont fire on all 12 from startup, I need to push the rev's to 12.000, hold it there for 5 min or so until I hear the last cyilinder fires up.

So, I've been putting off having to face whats now needed, I started her up a few days ago, and I felt it missing again, felt diffrent, i felt the exhasts and now its missing on the other bank ! So .. I tested all the plugs by removing the lead on each until i hear no change in engine note and nothing. I couldnt determine what cylinder was at fault, it fealt it ran rougher on each plug lead removal. I could be wrong, but I'm at my wits end with it, and its a bloody nightmare trying to get at anything. I do love the car and its not going to be sold, I just need to get a bit of advice on the best way forward. She's done 98000 from new, full documented service history. I have been advised by a friend of a company in Wales who rebuild for about 6k, but they just want the engine, so it means a new loom, new auxiliaries etc. My local chap says he will take the engine out, rebuild it from the ground up for about 2.5k, but I just struggle to trust that ... Perhaps the other option is to get the heads off ?.. I dunno, bit lost and I just became a new Father to twin girls, so money is now tight, and as soon as I mention the Daimler to my partner ... well lets just say its a face I aint keen on seeing

Just to surmise, wont allways start on all 12, the plug from that 130 cylinder on bank A ( if im even looking at A & B banks correctly ) is sooty more than wet with oil. Nearside bank now missing, all plugs seem to be firing.

Please help if you can

[mike020150]

Hi Sam,
Maybe it's me but I'm slightly confused by your description of which cylinder is giving the 130psi test result. Is it no 3 from the front on the LH bank as viewed from the driver's seat and as listed in the compression readings in your post or is it on the RH bank? Either way, the first thing to do is to repeat the compression test both dry (engine hot, throttle wide open) and after the addition of a little engine oil to the bore or bores being tested. If the reading after adding oil moves to the same level as the others (eg 160psi in the original test) the problem is a damaged bore/piston/ring. If the reading is largely the same, the problem is valve related. At a practical level, compression test readings within 10% of one another are acceptable so, yours are not by any means disastrous. Not holding the throttle wide open or a battery flagging during a test of 12 cylinders can all introduce errors to individual readings. Guessing from the readings that your car may be a pre HE model, also raises the prospect of the Lucas Opus ignition system being at fault and causing misfires. The Opus system, in particular the ignition amplifier/pick-up and the air gap setting are well known for causing erratic misfires. Hence, after bottoming the compression test issue, I'd suggest the next most likely culprit to investigate would be the ignition system. Mike.

[Flappers]

I'm also unsure of what side A & B are. I was just handed a scrap of paper with that written on it. I'll need to go back and ask the mechanic. Is there a standard, ie A offside and B nearside ? Anyway I'll have this chap re-do the test as you have explained. I'll post back here once it's complete. Thanks Mike

Ps. The plug I changed was on the offside bank 3rd from the bulkhead. I'm unsure of pre He, it's a 1989 Double Six.

[mike020150]

Hi Sam,
'A' is the RH cylinder bank which explains the confusion. Since it's a 1989 car, my guess was wrong about it being pre HE so ignore my specific comments about the pre HE ignition system. Getting both wet and dry compression tests remains the best start point. At least then you will be able to know whether you have to go inside the engine to look for a cure or whether the problem is external (ignition, fuelling etc). Mike.