I have a 1988 XJS V12 HE which has gone through a major rebuild. The engine suffered a cracked cylinder head and I obtained 2 new heads which have been gas flowed and had Fast Road Cams fitted. An ECU to match the cams has also been fitted. The problem is that the car will start and sounds really good but quite quickly it starts to hunt. The revs rise and fall from 1000 to 1400. If left to heat up this does stop and it ticks over as it should. To stop the hunting I can disconnect the Throttle Potentiometer or remove the air valve at the back of the left hand air box and simply block it with my finger. Both stop the hunting. But the engine warmed through also stops it. However left to warm through the engine starts to misfire and will eventually stop and will not restart. If I remove all the spark plugs and clean and refit them the engine starts easily but all the problems are repeated. Its like the motor is running on full choke and eventually strangles itself. I have fitted a new Auxiliary Air Valve, air valve at the back of the left hand air box, distributor cap, innards, leads, plugs, checked the fuel pressure which holds at 2.5bar, except when hunting and it fluctuates with the revs rise and fall. The vacuum pipes to the ECU have been replaced, the injector rail has been removed and checked, all appear to pulse without leaking under pressure, the fuel return pipe was choked and the charcoal filter removed and by-passed, new water temperature and air temperature sensors fitted.
This is a brief account of the troubles if anyone has an idea of what I can look at next please let me know.

 

There are 2 water temperature sensors, one for the gauge and one (CTS) for the ECU; if the latter is going flaky it can cause all sorts of problems.

 

Is this ECU setup for the cam some aftermarket deal that may be looking for a different resistance reading from the temperature sensor? Jumper the coolant sensor and see if it affects anything.

 

Typically the hunting is due to too much air coming in from.....somewhere. Typically the AAV but you've already replaced that. Anyhow, more air = higher engine speed. Higher engine speed with the TPS showing closed throttle will cause the ECU to cut-off fueling at xxx-RPM. The engine speed then drops, fueling resumes, engine speed goes up again, repeat repeat repeat.

Plugging the air intake reduces incoming air so the engine speed drops below the fuel cut-off range. Unplugging the TPS removes the "throttle at idle position communication to the ECU


The AAV closes and incoming air is thus reduced



And the plugs are black, I presume?



Air valve at back of left hand air box? Not ringing any bells.....but I'm tired . Please elaborate.


One thought regarding the cams....

Have you taken a vacuum reading? And doe the reading at the ECU match what you're seeing at the engine? Often high perf cams result in low engine vacuum. The pressure sensor in the ECU would interpret this as additional load/demand and increase fueling. This may or may not have been considered when the ECU was re-worked

A leaky vacuum hose to the ECU would cause the same problem -- ECU pressure sensor seeing low vacuum

Cheers
DD

 

You do not say if the ECU is a modified Lucas or aftermarket.

Hunting idle like this can be caused by a couple of things

Cold enrichment is not enough for the bigger cams.
IAC or AAV is sticking open at some point - my car does this until warm.

 

Thanks for all the replies. The original water temp sensor was checked and a resistance figure obtained. The sensor was replaced and the new sensor had as near the same resistance. I've now changed that and am on the 3rd sensor. The AAV is new but I have removed it and carried out the hot water cold water test and it does open and close. The ECU was re-mapped by Roger at AJ6 Engineering a well respected expert on V12's. I did think I had cracked the problem when I checked the vacuum pipe from the Balance pipe back. I found part of the metal pipe at the back had corroded internally and was choked. I replaced it with rubber vacuum pipe but as yet I have not obtained a vacuum reading. I 'll work on that shortly.

 

That would/might possibly lead the ECU to think the throttle was wide open, and thus go into max fuelling. Also, have you checked that the full throttle enrichment switch is functioning correctly? On Lucas ignition engines, at least, this is bolted to the cabin end of the A bank manifold, and it is controlled by manifold vacuum. If the vac falls below a certain level, full rich fuelling takes place. Check that the switch vac pipe is connected and not leaking, and that there is power to the switch in one of its two wires, and that the switch closes (it should be open when vac is applied) and makes the circuit when vac is removed. 18 below