V12 runs too rich

I’ve fit a V12 Marelli engine from 1989 in my E Type.
It was a bit long to engine run as I’ve to adapt lot of things to have this engine in the car.
Now it is done and I’ve got a first small ride yesterday.
My concern is that the engine sounds to not run properly on all the 12 cylinders. Black smoke coming out is very important and all the spark plugs look very black.
I’ve checked all the sensor used to managed the fuel and for now on I’m stuck. I dont know what’s wrong.
Does anyone has an idea ?

One guy with a rich running V12 found that some shop had jacked up his car with a floor jack on the port side and had crushed the return fuel line to the tank. The restriction caused the rail pressure to skyrocket and hence the engine ran rich.

Have you tried more throttle? If it runs better at WOT, there might be a problem with the vacuum line to the EFI ECU.

Perhaps the most common cause of rich running is a bad coolant temperature sensor – or bad connections to it. It’s a cheap sensor, somebody will advise you to just replace it. Or, you can unplug the sensor and jam a paper clip in the connector to short it out. The short will simulate a very hot engine. It might be tough to start cold, but once warm should run OK. If so, new CTS time.

Hi Kirbert,
thank you for your feedback.
About the fuel lines, I’ve restaured them during the work done on the E.
For the temp sensor, I’ve fit a new one on the rear right water pipe assembly.
I’ll check the small tube going to the ignition EFI.
What about the throttle control switch? Can it gives a such issue ? I’ve tried to make it switch on/off with a small depress and It seems to always stay open.

Did the location of the CTS change ? On earlier 80s V12s, the CTS was on the B bank water rail, at the front of the water rail. You indicate you put on at CTS at the “rear right water pipe assembly”.
SD Faircloth www.jaguarfuelinjectorservice.com

Hi SD_Faircloth,

finally I do have 3 CTS on the engine:
-1 from the E Type at the front assembly from the right water rail. This part comes from the E because the xjs one cannot be fit as it too large.
-1 at the front of the left water rail used for the Ignition Marelli ECU
-and the 3d one I fit at the rear from the right water rail.

I realize now that I’ve changed the CTS from the ignition but the one used for the EFI is the old one from the engine.
I need to check it.

Congratulations on getting that engine to run for the first time. I have successfully accomplished two engine swaps in Jaguar XJ6s and know the feeling of satisfaction after getting the replacement engine running for the first time. Well done!

What is the fuel pressure at the fuel rail? The stock XJ-S Bosch fuel pumps are capable of putting out 100 PSI or so but the fuel pressure regulators reduce that to 36-39 PSI during normal engine operations. If your fuel pressure is too high your engine will run rich.


Hi Paul, thank you for your feedback.
I do not have materal to mesure the pressure. However, as I’ve changed the main fuel regulator which is the most important one on the rail, I assume that this part of the problem is good.


At least check the vacuum line to it to ensure there’s no fuel in it.

I went through something similar on my 88 Lucas V12 last year. Spent weeks checking out everywhere, it ended up being a bad ECU.

That’s always a possibility, of course. And for those who just purchased a non-running car, the odds take a giant leap, seeing as how a replacement ECU can be expensive enough that the PO decides to just put the car up for sale.

Easiest way to diagnose is to just swap in a borrowed ECU – if you can find someone to loan you one. Second easiest way to diagnose is probably to ship the ECU off to Roger Bywater at AJ6 Engineering and have him perform some magic on it.

It is always interesting to run your newly purchased car for a few weeks and then discover what the real reason for the previous owner selling it was - something that you don’t immediately notice on a test drive. My first XJ6 had nothing more dreadful than a bad alternator. My wife’s Renault Turbo suffered from a bad, but curable, dose of French electrics. The Alfa had a broken distributer advance spring and clogged up carbs.

I purchased my XJR-S with an engine warning light, and oxygen sensor error, but only on the proviso that the garage selling the car promised to fix the problem. Their first attempts at replacing the oxygen sensors, failed. Their next effort was to send it to a Jaguar “expert” who removed the bulb from the engine warning light and sniped the wire that sent the diagnostic error to the display. I was not overly impressed with this effort and the car was returned to them again. After another round of bogus diagnostics they admitted it was a bad ECU from the start. They had to find, and suck up the cost of, the special Zytek ECU.

Thank you for posting that info. It is not the first time I have heard about “experts” creatively fixing an illuminated warning light by removing a bulb or cutting wires. It is another example of Prior Owners and/or their shops knowing no bounds in their ingenuity and creativity. :wink:

Some of my Jaguars had examples of this, but nothing quite that bad.


There must be a long list of car trade ticks and traps somewhere. Literally rebadging a base model as an S or XL to charge more. Obviously clocking the speedo is common. The classic body work bodges with chicken wire, newspaper and filler. Tin can wired round the hole in the exhaust. You don’t have to look hard on eBay to find delay units that will simulate the start up self test for things like air bags and ABS units to disguise hidden faults. Cheap battery for a 1.0L Ford Fiesta struggling to power a much larger engine. I’ve even heard of someone wiring up the fuel gauge to make it read “F”, and charge a little extra for a full tank of petrol. Could probably start a long thread in the Pub.

I bought my 1988 XJ-S barely running, took a big chance. Once I pieced together everything and started working on it, I determined myself lucky. But it took a couple months to determine exactly how well the car was kept before me.

1st owner had the car for 25 years and took great care of it. But towards the end, I think maintenance was neglected and it started running rough. So they sold it.

2nd owner had the car for a couple years but didn’t know what they were doing. LUCKILY they didn’t screw anything up, they just couldn’t figure it out (didn’t have The Book!) So the car sat for 7 years non-running.

3rd owner had the car for 1 year in the hopes of restoring it. They were about to drop in a Chevy V8, but decided to sell it instead (Thank God!!!)

4th owner is me. This car had a lot of neglect (the mice were starting to move in), but nothing was destroyed and once I got things replaced and restored, it’s actually a very excellent condition car, with only 51,000 miles (bought it at 48000). So I got lucky. Only big issue was rusted fuel tank, but in the end, that wasn’t too bad/expensive.

Fortunately, I don’t think every PO is a shameless scam artist. They’ve always wanted a Jag/Merc/Alfa/whatever but a little clueless on the technical aspect. I don’t expect everyone to be able to work on the car of their dreams themselves, but some healthy scepticism and a bit of prior googling will sure help in the conversations with their chosen expert.

Another Alfa story - there is an unlabelled switch on the dash. I asked what this was for and was told by the PO that when he first took the car into the garage to get an MOT test he was told that it was for the fuel injection pump and that he shouldn’t touch it. I knew that was a bogus story, due the pair of 40DCOE Webers and mechanical fuel pump that were rather difficult to miss in the engine bay. Turns out the PPO had installed air horns and the switch on the dash changed the mode from a regular single tone, to La Cucaracha (illegal on the road in the UK). No idea why the garage told him that story about a fuel pump (he wasn’t the sharpest tool in the box), or just disabled it.

I have got my V12 7 years ago. I did not removed the engine from the car myself. I schould have done it!
If I ve done this myself, i would have seen that there were 2 vacuum hoses on this “Marelli engine”. One for the ignition ECU, and one fof EFI!:scream:
I had connected the ignition one when I installed the engine in the E Type. Not the other.:face_with_symbols_over_mouth:
Now everything is fine!:smirk:

here s what i do if its rich running, turn the knob slightly lean ! Wahlah ,leaner!