Hi all,

I am a new member, please go easy on me

I have a Jaguar XJ12, 1988 model. It has been about a 1 year project getting this car running and all the bodywork sorted out. I am now looking for advice on the more refined elements of getting the car running correctly.

The problem I am having at the moment is that the car will not enter Over Run Fuel Cut Off (ORFCO). I.e if I am driving at 80km/h at tip out on the accelerator pedal, the car will not cut off the fuel. I therefore have very little engine braking, and poor fuel economy. I will add, that when I say it wont enter ORFCO, it might do it for a day reliably, then not do it again for quite a while, as in days or weeks. My car uses an ECU from the same year, so it is a 16CU. I have checked the TPS and it is set at about 0.33V at the throttle closed position, within the 0.32-0.36V requirement. I have also connected a vacuum gauge to the engine and routed it to my dash so I can check what sort of manifold pressure it is at, it is going to about 21-22 inch-hg when tipping out at 80km/h. More than enough vacuum. I do have 82 deg C thermostats installed, so thought that engine temp may not be getting hot enough, I drove the car until it was warm, I pulled over and wired in a variable resistor that I set to a temp that would simulate 90 deg C, alas, still no ORFCO.

Any thoughts on what else could be disabling ORFCO?

 

My suggestions after too many of these, over too many years.

OK,the TPS is et at the idle range, BUT, is it dropping into that range when the pedal is released. I doubt it.

Too many times the simplest is the culprit, and these are SIMPLE cars, full stop.

Remove the 2 rods, and cable, 4 nuts, and Lift the capstan, meter the 2 wires, you will need an analogue meter for easy ready. Open SLOWLY, watch the needle rise. Nice and smooth is mandatory.
Release the capstan drum, and note the retirn to your setting, and being analouge, you will need to best guess it. If its OK, read on.

The throttle rods MAY be out of adjustment, and I will assume the spacer was used behind the idle stop and drum ear when adjusting them? Cold engine, not so critical, but as things heat up, oops.
The throttle discs are actually returning to the idle 0.002: prest gap, and not sticking slightly.

There are some stickies about all this at the top of the XJS Section.

Over run valves, on the front of each Inlet manifold. PITA. Jag deleted, refitted, deleted etc over the life of the V12. I remove and plate them, no thought at all. NOW, that made a HUGE difference to all of them.

 

Thanks for the response Grant.

I did have an issue where the Capstan was not returning all the way back to the stop. As such it would sort of stop at random distances from the stop. I have since sorted that out and it stops at the same position every time. I think the best test it to wire my multimeter directly to the TPS wires and monitor the voltage on tip out while I am driving. Make sure its always going back in range. I will add that I am checking it randomly now at cold and after a drive and it is always within the range. So I actually do not think this is the problem, but could be wrong....

I have not played with the linkages, but they do not seem to be preventing the capstan from returning to the stop. I will look into finding a linkage adjustment procedure and have a go at it.

 

Here ya go.

Beer o'clock, time for some reading.

 

Hi Grant, I came across your TPS sensor calibration earlier. It is very helpful thanks. I have set the throttle discs recently, for unrelated reasons and I know they are correct. I also checked the linkages last night and I am confident they are set right. Regarding the ECU fuel pot, is this something that would effect the ability to go into ORFCO? I actually think the car is a bit rich at idle due to the smell, so this is worth doing, but as far as I know, ORFCO is only triggered by engine speed, TPS voltage and coolant temp? Are you indicating that fuel trim might disable ORFCO?

 

OK, just woke up, te joys of old age.

INSIDE the ECU, is a short Vac hose. It connects the spigot on the side TO the MAP sensor attached to the tickery. They split, RARE, as RARE as I can politely say on here.

I have had 2 over the years, and the car is Rich and stinky.

82 stats the correct length, and with a bleed hole at 12 o'clock are just fine.

The CTS (Prime fueling signal for the ECU), B Bank stat backing casting, is a common culprit, Bosch 0 280 130 026. Cheaper than chips. Used on many cars, Commodore, Ford, Toyota, etc etc. Not worth testing, fit a new one, AND check the wiring inside the plug, they break, grow Green Goo, and that sends a fuzzy to the ECU, and it fails to read fuzzy. Running V12, unplug the CTS, DEAD V12.

Just because, these are called a V12, they are not, They are a Double Six, and the firing order is the gimme. Always think of them as 2 engines on a common crankcase, simple.

 

Thanks Grant. I think I figured it out though.

What I did was I ran cables from the TPS connector to my dash. I then connected my multimeter to these cables and monitored voltage of the TPS sensor (yellow and red wires). I paid attention to the voltage on tip out. It was going to say 0.6V then very slowly going back to my set value of say 0.33V. This was why it was always measuring correctly when checking it after driving, by the time I got out of the car, connected my multimeter, it was back to reading 0.33V. I had a play with the capstan and determined that it was actually the accelerator cable being sticky. I put some gun oil down the accelerator cable and what do you know, it works fine now.

 

Goodo.

Simple cars these V12's, something I have said for too many years.

 

Good work!

I had a cable problem like that on my V12. I was chasing a high idle problem. It would stick only at the last 2-3 millimeters of travel.

Cheers
DD