Greetings all.
1980 XJ6 4.2 engine.
She has been sitting for a number of years and I'm trying to bring her back to life.
I got her to run last May, but mostly on starter fluid, before she wouldn't even do that.
I pulled the injectors and had them cleaned and tested.
I have good fuel pressure, and was ready to make her come alive this morning, but decided to check the injector voltage.
With a volt meter in the sockets, I cranked the engine over.
CYLS 1, 3, 4, all were less than 1 volt.
CYLS 2, 5, 6 all were 10 volts.
I checked the wiring diagrams to see if there might be some pattern or common connection, but nothing jumped out.
Any clues?
Thanks
Bobmo

 

Just so you know, (apologies if you already know), the injectors work by having a constant voltage supplied to them. The ECU actually earths the injectors via a resistance pack located somewhere at the back of the engine compartment. So you should be getting the same voltage on each injector. BTW the injectors all fire together, twice per engine revolution. So essentially there is no "pulse" to the injectors, the ECU merely earths them for a longer or shorter time depending on how much fuel is demanded.

Fuel injection and the Jaguar XJ6 4.2 Series 3 / AJ6 Engineering

 

For the heck of it, check the voltage with the key 'on', but not cranking. You should get battery voltage, about 12v, on all. If not look for a voltage supply problem. Voltage comes from the 'main relay' and thru the 'resistor pack'. Check connections at the resistor pack. They might be corroded?

Also check the bundle of ground wires at the rear of the water rail. The ECU grounds there, and the injectors are grounded by the ECU, so.....

The injector harness might have a problem; not uncommon. You could open it up for a visual inspect or disconnect it and check the wires for excessive resistance

Cheers
DD

 

Thanks for the tips. I'll check them out this am.
Seemed like an odd coincidence that 3 are good and 3 are bad, so I was hoping there was a connector somewhere in the loop.
Bobmo

 

I don't think it is a coincidence, look closely at the wiring harness layout. All those injectors should be getting 12 volts, so there seems to be a couple of large resistances somewhere along the line from the supply.

 

SHE LIVES....but
So thanks for all your help.
I went through the entire injector system, Ohmed everything and Volt check everything, cleaned everything, and put it all back together with dielectric grease.
Made notes with my readings and was much more systematic than usual. She did fire up on the first try, ran for a few seconds and died.
It took a few more tries to get her running again, but its encouraging.

So with repeat-ability, she will start, and run well for just about 21 seconds and die.
I can pump the gas all day but she wont run past 21 seconds.
I can start her again after a few tries and she will run... for 21 seconds.
A few time she will back fire through the intake side.

My leanings are fuel starvation, but I have pressure, and good flow in the beginning, but cant tell what happens once its running.
So could this be an ECU problem?
pressure regulator problem?
or something else I'm not seeing?

Any suggestions would be much appreciated.
Bobmo

 

Hi Bobmo, when I first got my S3 it had similar problems. The pressure was ok to startup and then she just stopped. It turned out the tanks were full of crap which was causing fuel flow problems. I emptied the tanks and flushed them out as well as I could then I installed 2 extra fuel filters between the tank outlet and the tank swap over valve. No more problems. As an aside the crap that came out of those tanks was amazing, rust paper labels and one of the screens from the intank filter. I removed both of those before I sealEd the tanks back up.
Con
PS are you priming the fuel system by putting it in drive and cranking the motor to build pressure before trying to fire it up?

 

The rusty/contamination gas possibility, as Con244 mentioned, is very plausible....especially since the car had been sitting for a few years.

Remove the fuel filter and empty the contents into a clean jar. What you see will dictate your next steps

Cheers
DD

 

Thanks guys,
I'm headed towards the fuel drain this morning as you mentioned and prefilters are on my list.
I've drained all the old fuel, blew it out, added and drained the tank several times, but there always seems to be some fleck of crap at the pump when I check for supply and flow.
I'm also going to install a fuel pressure tap in the main line to the fuel rail to get confirmation on at least this part of the problem.
Bobmo

 

Are you getting a 'click' from each injector ? This is easily heard using a mechanic's stethoscope, or there are little light things that connect into the fuel injector plug that light up when it fires, (I have one in my garage somewhere). Assuming the engine is cold, if it runs for 21 seconds it does sound as if it's running on fuel supplied by the cold start injector. This, of course, shuts down when the thermo-time switch closes. Incidentally a back-fire strongly indicates inadequate fuel supply, it is specifically mentioned by Roger Bywater in his article on the 4.2 six EFI system.