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I recently decided to save an '82 XJS HE V12. The car "ran when parked" but hasn't run in at least 2 year. The previous owner put some gas in it and we tried to get it to start but it only ran for 10 seconds or so, this was long enough for me to see the oil pressure come up, alternator charge and hear the engine not knock. We did discover that the fuel pump was leaking so decided to stop trying to start it. The seller was asking much less than the car is worth in parts so I drug it home.
Since then I replaced the pump and fuel filter plus the hoses in the trunk as well as air filters and oil. I also drained the tank and pulled the fuel pickup screen. The tank and sump look perfect, the pickup screen looks good and flows freely. Now I want to hear it run so that I can take an inventory of what works and what doesn't. The problem that I have run into is that the engine will start and rev a little on it's own, then stumble and die out, pressing the throttle only makes it worse. My gut tells me that this is indicating a vacuum leak.
Does anyone have a digital copy of a vacuum diagram, or any other ideas on what I should look for? I have ordered a repair manual but it will be several days before it gets here.
Crank sensors might be suspect to, sounds similar to what I encountered after finding my car in dry storage for ten years
None on the 1982 V12. They came in with the Marelli cars.
I suspect you need to service the distributor (new rotor, cap, ensure the rotor springs bac when turned 15 degrees), change the plugs and HT leads, clean all electrical connectors from the amplifier to the coil etc, and the resistor pack contacts. Then as a precaution replace the coolant temp sensor, and carefully check the throttle potentiometer for proper voltage increase. Then test for spark as the engine is cranked over on the starter.
After all that ignition on, get someone to floor the throttle and listen with a stethoscope for the injectors clicking. There are loads of other things, most notably fuel pressure to check; but if you are methodical and ask the forum for the info you need to do each stage suggested here, the car will start.
Also, see above the topics list, there is a sticky about this subject written by the Expert Wizard of Oz.
Last edited by Greg in France; 02-14-2018 at 02:25 AM.
Thanks for the tips, I will start down the list. I did discover that the new fuel pump seems to be inconsistent. I ordered new fuel pump relays, just to rule them out. The relays should be delivered today but being Valentines day it might not be wise for me to work on the Jag while ignoring the wife. I'll try them tomorrow, followed but an ignition refresh, temp sensor, throttle potentiometer and more fuel system diagnostics.
Sometimes you just need to do that a bunch of a times. My XJS was parked for 13 years and I had to spent about an hour holding it at high revs, slowly letting it lower and lower until it would idle, otherwise it would just die. There wasn't anything particular part wrong with it, it just had to "work itself out" with things being dirty or gunked up etc.
For example, your XJS may have slightly clogged injectors that will open up after you run it a few times, and its dying because you open the intake up and it doesn't get enough fuel and dies. I'm not saying to not investigate anything else, just that sometimes sitting will do this and they need to be run in a bit when you get them going again.
Idk how this diagram pertains to the pre-HE, but if you spend enough time simply looking around the engine bay, you can find broken/disconnected hoses, I just had to do that on my XJ6.
Sometimes you just need to do that a bunch of a times. My XJS was parked for 13 years and I had to spent about an hour holding it at high revs, slowly letting it lower and lower until it would idle, otherwise it would just die. There wasn't anything particular part wrong with it, it just had to "work itself out" with things being dirty or gunked up etc.
+1
Mine will do this every time I fire it up after sitting for 6 months+, maybe not for an hour but I have to rev it high for ~5 minutes, otherwise it dies.
I found a possible issue. I had my son crank the engine while I watched the voltage to the pump and listened to the relays. The fuel pump is only running when the engine is cranking, it is not priming and it stops as soon as the key is back in “run”.
I followed the “Ya got a HE that won’t start” thread but didn’t see what tells the pump to run when the key is set to run, what am I missing?
I found a possible issue. I had my son crank the engine while I watched the voltage to the pump and listened to the relays. The fuel pump is only running when the engine is cranking, it is not priming and it stops as soon as the key is back in “run”.
I followed the “Ya got a HE that won’t start” thread but didn’t see what tells the pump to run when the key is set to run, what am I missing?
The fuel pump is controlled by the ECU. Specifically, the ECU energizes the fuel pump relay by grounding the orange wire to the relay.
The ECU gets a signal from the starter circuit and 'knows' to turn on the fuel pump during cranking. For running, the ECU get an engine speed signal from the infamous "white shielded wire" coming off the ignition amplifier. Just search 'shielded wire' for info. It's a known problem although, typically, a fault here wouldn't allow the engine to start, period.....which isn't what you're experiencing. Still, it has to be checked.
The fuel pump circuit of the ECU is known for giving problems as well. I'm not aware of any test. Swapping ECUs might be your only choice.
Meanwhile....
Locate the orange wire at the pump relay and use a jumper (or whatever) to ground it. The pump should now run whenever the key is 'on'. See what happens.
Jumper in the orange wire did it. I’ll start researching the white wire and ECU testing.
Great find. Your 1982 V12 will have the 6CU model of ECU. It would be best to replace it with the later 16CU, as used from about 1984/5. This is a direct replacement, no changes to anything at all needed, and it is a far more reliable ECU. If the car is running Ok with the pump jumped, then the shielded wire is fine - for now!
Is there a risk to using the jumper on the fuel pump relay? I would like to be able to move the car and do the normal maintenance but don’t want to cause more damage while I search for an ECU.
Is there a risk to using the jumper on the fuel pump relay? I would like to be able to move the car and do the normal maintenance but don’t want to cause more damage while I search for an ECU.
It won't hurt the car.
Having the pump run whenever the key is 'on' presents a safety risk in certain circumstances, though.
Thanks for all the help, a replacement ECU took care of it. The engine runs now and the pump behaves like I would expect. I'm going to open a new thread to track the rest of the wake-up process.
02-10-2018, 02:32 PM
Vacuum leak?
