Well I completed my winter project which was to remove the down pipes and empty the rolled up "restrictors" at the top of the down pipes. After starting the car I could tell there was an exhaust leak seeming to come from the left bank, car ran terrible under load and "popped" from the leak. I pulled it in, cooled it down and checked the nuts which were not tight as they could be. I snugged everything up and started the car and this time under load it doesn't seem like the exhaust is leaking but it's running horrible even under idle ( initially the idle was good but under load was bad). I'm dumbfounded that every time you try to do something positive on these cars when you wrap it up it runs worse! Any ideas as to the cause?

Double check your oxygen sensors to make sure they are connected (electrically). Did you use any sealant or cleaners that could have damaged the sensors?

Mark

All I did to the O2 sensors was remove them then put them back on. It's behaving as it did last year when it out of the blue wouldn't idle or run right then eventually stooped starting all together ( thought it was a bad CTS connection). I'm thinking of replacing the O2 sensors just for good measure but I hate just throwing parts at something.

Re-trace all your steps, carefully. I'm betting you disturbed 'something' topside. Broken wire, vacuum hose....

This isn't a criticism. It's very hard to work on a V12 without inadvertently touching lots of 'stuff'.

Cheers
DD

Yeah, been there done that! I've been back over everything 3 times and still the same. I'm going to put a new CTS and plug on it for grins as I have a new set on the bench.

Doug, I've went over everything and changed out the CTS, checked all injectors and plug wires, vacuum hose etc. Since tightening up the left down pipe it's slightly improved but continues to run rough and "pop" intermittently out of the left exhaust pipe. It will make exhaust leak sounds under load and run crappy but again "better" since I snugged that side up. It runs the same O2 sensors plugged in or unplugged. Also if I hit the "enrichment switch" on the capstan it increases the idle. My question is do you think a leaky doughnut gasket on that side would cause it to run as it is? ( I had much trouble getting that left one up and in properly) I've ordered 4 new ones and really don't relish the idea of dropping it again but if I must I must. Any and all input is appreciated.

May be nothing but l gutted the cats on my range rover HSE some years back and the end result thereafter was a very "tinny" sound when listened to up front, almost sounded like an exhaust leak but was not.

My cats were already gutted a few years back. It's not tinny, it's a "pop" sporadically. The part removed is in the pix, it's above the cats at the manifold to down pipe connection.

Is it possible that an exhaust leak could be allowing excess oxygen in to the o2 sensors causing over enriching at idle?

Yeah, just to be safe I re dropped everything, I'm going to put 4 new crush gaskets on ( doughnuts) and two new O2 sensors just to eliminate any doubt.

I had to put all new exhaust manifold gaskets on my car to get it right. What a job....

yeah, unfortunately I'm having to do it twice! I've got it down to about 20 mins total time to get both down pipes dropped after its off the floor.

Not the down pipes. The actual exhaust manifolds. More like 20 minutes per nut. I bet I put 20 hours into that job. Looking back, not sure it wouldn't have been better to have pulled the engine, lol.

Ouch, I'm hoping the new doughnuts and sensors do the trick. I've had enough lying under this car for awhile.

I have owned several XJS's. By far, changing the manifold gaskets was the most frustrating job I've ever tackled. And that includes a full rear cage overhaul. i.e. rotors, calipers, diff seals, etc....

No joy😞. New O2 sensors, new down pipe crush doughnuts, new CTS and pigtail, even put on another ignition amp. Still stumbling, low idle, backfiring. Checked all electrical connectors and vacuum lines. AAV is pulling air as it should. Idle comes up when enrichment switch by capstan pushed. Fuel pump is humming away in boot.

JT
Could you please post a summary of exactly what the car is doing now? I am not clear if it is just an idle problem, or if the car is still refusing to run under load, and what the main symptoms are. My sympathies, but it will be found - eventually!
Greg

Greg, it's both idle and under load. It runs fine if one increases the rev's in park but it stumbles and hesitates in drive and at idle. It starts up fine with a few revolutions yet idles at about 400 RPMs and has mild backfires and lopy / shuddering idle.

I've mused over the problem. Removing the 'strainers" or leaking donuts or even manifold gaskets should not have anything to do with
rough running. I can't envisage extra o2 entering the exhaust pipes and messing with the sensors. Pressure in the pipe precludes that.


The engine is missing on one or more cylinders. Masked by adding revs. But, 400 RPM idle sounds just a tad low to me. Too much load for the engine to sustain smoothly.


Or, the mixture has gone lean or rich? Adding fuel as you mention suggests it is lean. Irregular fuel pressure. Stumbling injector/s?


Fouled spark plugs, fuel or oil? Or just worn out, gap too wide.


Dirty and/or corroded distributor cap? Leaking HT wires. Northern light show at night under the bonnet?


Back to the tail pipe. Popping there? Stinky rich exhaust? Even to the point of black smoke? Or lean, and I can't think of a symptom for that!!


Access to an exhaust "sniffer"!!! A "friendly" SMOG shop?


Sorry for the hodgepodge of ideas, I just typed as they came up.


Carl

Sorry if you have already posted about the following, but could you tick off which of these things you have done already?

1. Checked/replaced dizzy cap and centre graphite electrode
2. Replaced HT leads and plugs (correctly gapped to 25 thou)
3. Checked dizzy spindle is not jammed and springs back when turned
4. Checked/replaced dizzy internal pickup, starwheel condition, and condition of twin wires from dizzy bottom to the amplifier side plug
5. Checked vac capsule is functioning as it should
6. Checked connectors and wires to coil + and - (wires can fracture within the OEM connector rubber covers
7. Checked for a big powerful blue spark at the plug end of the HT leads
8. Checked ignition timing? If so how exactly did you do it?
9. Checked and cleaned injector resistor pack plug and socket
10. Checked loom to coolant temp sensor on B bank thermostat casting
11. Tried replacement CTS.
12. Checked that CTS and air temp sensor plugs not accidentally swapped
13. Checked the first foot or so of the shielded wire from amp that leads to the ECU. That is strip and ensure the inner wire is NOT earthing to the shield. Check that the shield is earthed to the loom at the amp end of the wire.
14. Check you have not put a screw through the firewall low down (eg to secure the US passenger side front wheel baffle). This can hit the loom to the ECU which is the other side.
15. Checked injectors are pulsing and flowing the correct amount of fuel when fitted to the car
16. Checked the injector pulses are quickening when the car is revved
17. Checked fuel pressure in the fuel rail
18. Have you verified the vac line from the tapping at the centre of the cross pipe across the rear of the motor that balances the two intakes and ends up at the ECU will hold vacuum?
19. If you remove a vac pipe that goes into the inlet manifolds (eg gearbox modulator, crankcase breather sprung thingy on B bank air box, or any other capped off inlet manifold spigot) does the car rev up?

If you are not sure the best way to test all of these items, just post as between us all someone will know!
On the symptoms you describe: Does the car rev in Neutral just as well as it revs in Park? Does the car run Ok in Reverse or just as badly as in Drive? Can you hold it OK on the accelerator at (say) 1,000 rpm?
Greg