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Hi all
I would need some tips how / where to start to find the problem with my XJ12 -76 (electronic fuel injection)
Problem 1. Today
Drove 5min today and the car started to run really bad. Wouldnt rev and lost power. with full throttle it would spit a bit and then rev up. I got it towed home while running engine in idle (Ran quite rough in idle and stinks fuel). Spark plugs loooked ok when I came home.
Problem 2: two weeks ago
I drove 20min -> stopped 30min with closed hood -> When starting again the car ran really bad and wouldnt rev. After cooling down it ran fine home. Vapor lock syndrome?
Other info:
I newly have 95 Octane in the tank. In the winter I had 98.
Bot cases ambient temp around 20degC.
I first started using the car November last year so Have not really tested it much until now but it always ran fine.
Oil pressure and water temp always ok.
My best guess so far is something with the fuel system. Temp or vacuum sensor broken? Mybe not enough fuel pressure? But why would it run fine for 5 minutes?
It's possible that the coolant temp sender is failing. The car will run ok when cold because it needs the extra enrichment, but needs less and less as it warms up. If you can measure the resistance of the sender at various temperatures (such as in a pan of water on the stove) and see what it does.\
It's a standard bosch sender, there are tables of the resistance vs temperature either here or in Jag-Lovers somewhere.
I had an open circuit coolant temp sensor in an XJ6 I had years ago and it did exactly what your car is doing. I fixed the broken wire which fixed the problem but a dodgy sensor could give you the same symptoms.
It would be worth checking the CTS and all the connectors.
If you google ‘Jaguar CTS test’ you will get some good info on how to test your CTS.
Cheers,
Jeff.
Last edited by watto700; Apr 20, 2026 at 12:30 AM.
Fuel smell can mean the injector loom is shorting out and wildly overfuelling, which also gives some of your symptoms. Check if the loom insulation is cracking up.
Location?, the use of HOOD tilts to USA, C temp and 95/98 fuel, dunno.
Is it still fitted with the D Jetronic EFI system?
Is it still fitted with the Opus Ignition system ??, if YES, here, where is the Opus Amp. Factory fitmant was in the V, in front of dizzy, and proved problematic and even fatal, especially after 10 or 15 minutes, when it got heated up in that V location. Relocation was a "thing" back in the day, by complaint only.
EFI loom, YES.
CTS and its wiring, YES.
MAP sensor vac hose, and then the electric plug on the side of that sucker.
Trigger board inside the dizzy, for the EFI system, oh boy, then the magnet in the heel of the rotor, old age is at that now.
"Plugs look OK", that has me heading for Ignition, and Opus issues. As these things soot up plugs without any help from us, and they will NOT burn them clean, if they overfuel at all.
The list goes on, as do the nightmares of these beast when I was much younger. 100's of hours spent on them.
Fill in the blanks, help us to help you.
Last edited by Grant Francis; Apr 20, 2026 at 05:56 AM.
I would still check fuel for water contamination first, it's happened to me several times over the years. Pumped it right in there from the gas station.
Hi again
Thanks for all the tips. I finally have some time to start searching for problems.
Started it up now and got it out of the garage. Still runs rough and unwilling to rev so problem doesnt occur only with warm engine.
More info:
- My location: Switzerland.
- The injection parts are all original as far as I know. Placed on top of the radiator.
- Vacuum sensor was just repaired by Dr D-Jetronic in Germany so that shouldnt be the issue
- I have just last year renewed all isolation tape on the cable harnesses, I dont think I have a short circuit there.
- Temp sensors not tested.
I think the easisest next step is to drain the tank and try some new fuel. I can put it into my -76 Volvo. It will run on anything
Hi again
I drained the fuel and tried with fresh 98 Octane -> no change. Still running rough and unwilling to rev. Difficult to explain exactly how it sounds.
I noticed one thing: When ignition is "on" (engine not runnung) and I press the throttle slowly it "ticks" from engine. Difficult to locate but sounds like injectors. Is this normal? Never noticed it before at least.
Hi again
I drained the fuel and tried with fresh 98 Octane -> no change. Still running rough and unwilling to rev. Difficult to explain exactly how it sounds.
I noticed one thing: When ignition is "on" (engine not runnung) and I press the throttle slowly it "ticks" from engine. Difficult to locate but sounds like injectors. Is this normal? Never noticed it before at least.
Tick may be detonation, not good. Timing or mixture issue? Others to say though, I can't comment much on the v12. How did the fuel look? Any obvious water?
Tick may be detonation, not good. Timing or mixture issue? Others to say though, I can't comment much on the v12. How did the fuel look? Any obvious water?
No, this happens when engine is not running. Only ignition on.
Ah, I didn't read well. I'll bet the tick is the throttle position sensor or switch, but again, lots of linkage and switches on that beast, let others say. Prob normal though. But you're gonna have to start a systematic approach now with help from Grant and others to isolate the cause. 1st rule, assume nothing (it's new etc...), verified data only. Patience.
Ign ON, engine OFF, open the throttle SLOWLY and there is 9 or 10 CLICKS from the Injectors. That means the Throttle Switch is communicating wth the Brains.
OK, new fuel, no change, bugga. I would change the fuel filter, and when its all disconnected, attach a suitable hose to the incoming hose and point it over the guard, and have a helper switch ON the Ign.,\. The 2 second fuel prime "should" squirt a good distance. This a very basic test for pressure and FLOW, specific numbers matter not for now.
Please take the common sense precaution with this a Fuel goe BANG at the wrong time if care is forgotten.
Next I would remove a few spark plugs, and note their condition. Sooted up, a Pre HE common issue os overfueling. If that is what you get, clean, or replace all 12, easy on a PreHE.
Check for spark integrity. Use a spare sparker in any lead and lay it across the head nuts and crank/start the engine. NEVER do this with ANY sparker not installed, as the fuel spary from that cylinder with BBQ the car and you quicktime.
One of mine had similar issues, and eventuall the EFI loom was the culprit, NOT what I wanted to find, so one wire at a time, it was renewed. 30 years on, still OK.
More later, I am playing with the X Type, ET is residing in of the modules somewhere, and a box of matches is looking sooooo good right now, haha.
Tick may be detonation, not good. Timing or mixture issue? Others to say though, I can't comment much on the v12. How did the fuel look? Any obvious water?
3) inlet air temp - 355 Ohm @ ~18degC. - Probably ok
4) Injectors resistance: Should be 2.4 Ohm @20degC
Measured 11 of them 2.5...2.8 Ohm.
Injector 5B: 4 Ohm. <-- Problem?
Pinged all injectors with 12V and they tick ok.
5) Fuel pressure. Not yet tested. Pressure gauge ordered.
Test run May1:
Idling engine til warm. Running ok as far as I could tell. Idle 800rpm, Oil pressure middle of gauge.
Few hundred meters test run: ran ok
Oil pressure falls to minimum at gauge when warm. Will test again when engine has cooled down. Oil level ok.
Summary:
Seems to run fine suddenly which is terrible news. Will do a longer test run with back-up following car.
Why is oil pressure sinking? Oil pressure sensor temperature sensitive?
Air Temp Sensor is only a trimmer, and does very litle, still good to look
The one high injector is a concern, matbe. They are a very tough Injector, compared to later stuff. I have used the "tap, tap" on them and the HE engines many times.
Simply run the engine, take a small spanner, 14mm or so, and go along the Injectors and SENSIBLY tap, tap each one on the METAL section. It has cleared many "iffy" Injectors for me. I have never replaced, or had cleaned any Injector in any Jag to date. The PreHE has rather niosy Injectors compared to HE etc, but they are what thay are.
Oil pressure dropping, NORMAL, and it will be 98% the Smiths sender unit, they be fickle right out of the box. We kept 20 or more in stock when I was with Jag Spares. They gemerally last about 10 years. The idiot light sensor is set to light up at about 8psi, so worry not. The oil pump on the V12 is a monster, and moves a LOT of oil at crazy pressure. The weak links are the 3 o/rings inside the engine on the suction and supply plumbing, and engine out is the easiest way to sort, BUT, worry not, they will not cause an engine failure, just that low oil pressure HOT, if the sender is deemed OK. One of mine is over 800K kms, and never been opened up, and yep, that gauge wanders a bit, so I stopped looking at it.
XJ12C-76: engine running bad - tips needed
You know where I'm going... Not uncommon at all.
