XJS engine trouble

relating to. my XJS 5.3 HE 1984 without catalyst
I’m having trouble getting the engine for this car to run. I have had some starting problems for a while when the car is hot. Has therefore replaced the following: Ignition pick up. Ignition coil (from old double to modern single coil). Power Amplifier. Rotor. distributor cap. Ignition. Sparkplug.
After this the car started but I did not have time to check if it would start with hot engine. I would now be good at the car and therefore separated all the multisticks in the engine compartments, cleaned them with electric cleaner, then smeared them into acid-free vaseline and reassembled them. After this, the car will only start briefly after which it will stop. When running briefly, black smoke comes out of both exhaust pipes. So it gets too much gasoline. If I remove the wires from the injection nozzles, it can start briefly. It seems to burn the cylinders clean of the over-inflated gasoline. After this, I put wires to the nozzles again and the engine starts but runs very uneasy, can’t keep the engine running, the exhaust smoke is black and the engine gets choked in too much gasoline quite slowly.
Gasoline pressure is fine. I’ve tried to replace all 12 injection nozzles with another set I have I have as a spare. Have tried with another Power Resistor (DAC2044). I checked the shielded cord from the Power Amplifier to the ECU in the trunk. All this without any difference.
The only thing I can think of if I haven’t tried to replace is the ECU in the trunk. I want to hear the opinion of others before I spend a lot of money on this …
I want to reiterate that the engine started out fine before separating and cleaning all the multi-stitch under the bonnet.
I’ve been wondering if this could be the cause ?? Can’t remember if I turned off the ignition before doing this. Can the ECU possibly be damaged if I have disassembled multi plugs without putting the ignition key off?
Hope there is someone who can help me ??strong text

It is possibly your CTS (. Coolant temp sensor) or its connection. This is usually found on the top front of the B side of the motor connected to a cable running near the air filter. The CTS tells your ECU when the motor warms and so reduces the amount of fuel going to the motor. Look up “the book” on testing the CTS.
Trev

Per,
Have you tested the fuel pressure at the fuel rail? Have you tested your two fuel pressure regulators to make sure that they haven’t failed? Have you tested the thermal vacuum valve (EAC5086) on your fuel rail as a possible source of your hot start problems (vapor lock).

Paul

Hi Trev & Paul,
I have now concentrated on CTS and it has produced results. I applied a loop between the two wires on the CTS to trick the CPU into thinking the engine is hot. It had the effect of starting the engine. It was running well but it was not crowded with gasoline. So I’m on the right track. Thank you very much for your help so far :-). I will now check CTS and wiring more closely and will return with the result. I’m so excited to see if the hot engine starting problem is solved at the same time. If not, I might ask for help with this at a later date. Thanks so much for help so far! I hope both of you and your family are well healed and not affected by Covis 19.
Per

Hi again my jaguar friends.
Unfortunately, I’m back again. I’m sure the car gets way too much gasoline. I have checked the wiring between CTS and CPU. Found a bad connection in an old repair which has now been repaired. Have measured both wires through and those on a resistor 6 ohms. Have checked with a brand new cord of approx. same length and the same result is obtained. I must therefore assume that the wires are 100% in order. I have also measured the values ​​of CTS at different temperatures. They were not quite within the right values ​​but also not very far from these. These differences may be due to an inaccurate thermometer, but I have replaced the CTS with a brand new one. Still, the problem is the same, the engine starts cold well but is very start-reluctant when it is hot. It is also clear from exhaust fumes that it is getting too much petrol. The hotter the engine, the blacker the exhaust fumes become. Eventually I can not see more error possibilities than an error inside the CPU?!?! Is there a way to control this. or does it need a specialist ?? Or maybe one of you has another good idea ??

Thanks in advance!

Per

Per,
Have you checked the three items that I suggested on 9 July including fuel pressure, both fuel pressure regulators, and thermal vacuum valve (for hot start issues)?

Paul

I have checked fuel pressure and it is exactly as it should be. Both fuel pressure regulators are brand new. Thermal vacuum (the one that sits on the rail) also works …

Did they already delete the cold start injectors by then or is there a dedicated cold start circuit of sorts?

Isn’t the V12 ECU sensing manifold vacuum - what if the line got disconnected and the ECU always calculates „full throttle“?

And is it possible that you mixed up two connections? Not the CTS, as this seems right - the ECU goes leaner when the connector is bridged and the sensor itself delivers the right data. And you swapped it with no improvement - as it’s been running before I would n o t change components that might have issues and make it complicated. Stick to figuring out what got disconnected, mixed up or let the smoke out.

David

I do not think I have changed any wires. The only thing I have changed is the repair of wiring harnesses for CTS and airindtke temperature sensor and I have followed the colors exactly. Do you really think it could be the vacuum hose from the ECU ?? I mean, it will not start in hot condition at all, and in that situation there is no vacuum. It only comes when the engine is running. Nevertheless, I will check this immediately tomorrow …
Regards
Per

Per,
David makes a good point about the vacuum signal to the EFI ECU. If that signal is missing or altered through a leak it would cause fueling problems.
The vacuum that the EFI ECU sees comes from a nipple at the rear of the metal balance pipe at the rear of the engine, continues through a section of rubber hose that travels along the left side of the transmission to a metal pipe that goes to the rear of the car, then to another hose that goes up into the right rear of the trunk to the EFI ECU located in the well above the right rear wheel.
One way to test that the EFI ECU is getting a proper vacuum signal is to temporarily install a vacuum gauge with a “T” fitting just befor the EFI ECU. This way you can see the vacuum reading at the EFI ECU with it still connected as the engine is running. A worn hose, or a loose clamp could alter the vacuum getting to the EFI ECU.

Paul

Hello.
I have checked the vacuum as suggested (see photo) and it seems to be in order!?!? I can add some symptoms: When the engine is cold, the revs go up and down until it reaches a certain temperature. But it smells bad right from the start and runs very unevenly. I can not spot anything other than a CPU error!?!? Just do not know if you can check it yourself or if it is going to an expert. You are probably a little tired of me due to my seemingly unsolvable problems :frowning: :slight_smile:

Per

Roger Bywater of aj6 engineering at Bollington in Cheshire, England is the man to contact in respect of your potential ECU problem.

You could always risk buying a used ECU from eBay or similar & substitute it for yours to see if that improves matters.

Rgds.

Andy.

Per,

Re-check all the plugs you disconnected and cleaned. Maybe something didn’t go back correctly…
Did you also remove the big ECU multi plug? Make sure it’s seated correctly and that you didn’t bend/displaced any of the contacts.

The vacuum reaches the ECU OK. This doesn’t mean though that the sensor in the ECU is good.
You could check that it holds vacuum with a vacuum pump or even a big syringe and your vacuum gauge.

Other tings that might be worth checking:

  • Adjust the ECU’s base fuel map a few clicks leaner

  • Throttle Position Sensor. Easily done with a multi meter, you can find the details on Kirby’s book.

  • Vacuum advance and associated plumbing.

Unfortunately, the only way to really dismiss a bad ECU is to try with a known good working one.

As Paul mentioned, could be a problem of the ECU not seeing the correct vacuum.
Any fuel injected car has a fuelling map that is very sensitive to vacuum since the vacuum signal is the input with most effect.
If the vacuum line leaks slightly the ECU will think the throttle is open more than it really is. That will cause excess fuel delivery. Not a very common problem compared with the CTS and a host of other failure modes.

If the engiune was running fine before all the engine bay connectors were pulled apart and refitted then the chances are that points to the problem.
Those moulded Lucas bullet connectora are very hard to pull apart and there is always a chance that a connector wire was dislodged or something else was accidentally disturbed in the process. Time to revisit that area.