14CUX Fuel Cut Off On Overrun?
Discussion
ChimpOnGas said:
Out of idle interest, can someone confirm if the 14CUX ECU cuts fuel on overrun?
According to this reference it does;http://www.v8register.net/FilesRV8WN/160921-RV8-di...
Page 2, 3rd paragraph.
Interesting, thanks for that 
So, what it says is...
"On the overrun the ECU shuts off fuel flow to reduce fuel consumption. Road speed is monitored to enable a smooth transition either
back to driving or idle. If the road speed signal is incorrect or absent idle speed control will become erratic and the engine will tend to stall when the car comes to a halt."
If that's correct that TVR road speed signal box is doing more than helping the ECU manage stepper motor function, whats also interesting is what happened with the XJS when Jaguar introduced the AJ6 3.6 engine, the automatic cars worked fine but the manual version had serious issues associated with fuel cut off, so much so Jag were forced to remove the feature on manual cars, this was done at dealerships as an official and documented factory recall.
This gets me thinking about where TVR got their ECU's from and the car these 14CUX ECUs were really designed for, namely the automatic Range Rover. With all this in mind and from what I've learned from mapping my aftermarket engine management system, if I was still running the 14CUX and it genuinely does have fuel cutoff on overrun, I would do everything I could to remove this feature on my manual gearbox TVR just as Jag were forced to do with the manual gearbox XJS.
Here's an interesting paper that discusses all this....
http://jdrenne.free.fr/Docs/Over-run%20Valves%20an...
I'd encourage people interested in all this to read the full thing, but for those who just want the key part of interest, I've put a copy/paste below:
Over-run cut off is the name used to describe an ECU function where fueling ceases if the throttle is closed above a certain
engine speed. It provides a small fuel economy gain and avoids a rising HC emission problem as the high vacuum misfire threshold is approached. It is usual for cut off to only become active above a certain engine speed (typically around 1500 r.p.m.)
and to de-activate when speed falls to some lower speed (i.e. 1100 r.p.m.).
A simple cut off function is quite acceptable on a car with the inherent damping capacity of an automatic transmission but not so when fitted with a manual gearbox. The point when the fuel reinstates could then cause a noticeable jerk and increase of speed. If the car happened to be slowing on the right amount of gradient the rise of engine speed could be enough to reach the cut off speed when there would be another jerk as the fuel ceased again. The car could then oscillate between the two states thereby creating an unpleasant situation for the occupant(s). Lifting the upper threshold eats into the regime where cut off is beneficial so is not really an answer.
For these reasons all factory manual XJS V12s had ECUs without over-run cut off.
When the AJ6 3.6 engine was introduced in the XJS in 1983 the manual version immediately attracted strong criticism for an unpleasant oscillatory condition when slowing - resulting from the inexplicable mistake of having an ECU with over-run cut off. There was a hurried program of ECUs being modified in dealerships to delete the cut off function.
So, what it says is...
"On the overrun the ECU shuts off fuel flow to reduce fuel consumption. Road speed is monitored to enable a smooth transition either
back to driving or idle. If the road speed signal is incorrect or absent idle speed control will become erratic and the engine will tend to stall when the car comes to a halt."
If that's correct that TVR road speed signal box is doing more than helping the ECU manage stepper motor function, whats also interesting is what happened with the XJS when Jaguar introduced the AJ6 3.6 engine, the automatic cars worked fine but the manual version had serious issues associated with fuel cut off, so much so Jag were forced to remove the feature on manual cars, this was done at dealerships as an official and documented factory recall.
This gets me thinking about where TVR got their ECU's from and the car these 14CUX ECUs were really designed for, namely the automatic Range Rover. With all this in mind and from what I've learned from mapping my aftermarket engine management system, if I was still running the 14CUX and it genuinely does have fuel cutoff on overrun, I would do everything I could to remove this feature on my manual gearbox TVR just as Jag were forced to do with the manual gearbox XJS.
Here's an interesting paper that discusses all this....
http://jdrenne.free.fr/Docs/Over-run%20Valves%20an...
I'd encourage people interested in all this to read the full thing, but for those who just want the key part of interest, I've put a copy/paste below:
Over-run cut off is the name used to describe an ECU function where fueling ceases if the throttle is closed above a certain
engine speed. It provides a small fuel economy gain and avoids a rising HC emission problem as the high vacuum misfire threshold is approached. It is usual for cut off to only become active above a certain engine speed (typically around 1500 r.p.m.)
and to de-activate when speed falls to some lower speed (i.e. 1100 r.p.m.).
A simple cut off function is quite acceptable on a car with the inherent damping capacity of an automatic transmission but not so when fitted with a manual gearbox. The point when the fuel reinstates could then cause a noticeable jerk and increase of speed. If the car happened to be slowing on the right amount of gradient the rise of engine speed could be enough to reach the cut off speed when there would be another jerk as the fuel ceased again. The car could then oscillate between the two states thereby creating an unpleasant situation for the occupant(s). Lifting the upper threshold eats into the regime where cut off is beneficial so is not really an answer.
For these reasons all factory manual XJS V12s had ECUs without over-run cut off.
When the AJ6 3.6 engine was introduced in the XJS in 1983 the manual version immediately attracted strong criticism for an unpleasant oscillatory condition when slowing - resulting from the inexplicable mistake of having an ECU with over-run cut off. There was a hurried program of ECUs being modified in dealerships to delete the cut off function.
This also talks of fuel cutoff on overrun.....
http://www.legionlandrover.com/manuales/service%20...
It states the following....
"When the throttle is closed, overrun fuel cut off or idle speed control may be facilitated dependent on other inputs to the ECU."
After sourcing their 14CUX ECUs from the auto box Range Rover, could it be TVR never learned the lesson Jaguar did in 1983 with those manual gearbox XJS'?..
http://www.legionlandrover.com/manuales/service%20...
It states the following....
"When the throttle is closed, overrun fuel cut off or idle speed control may be facilitated dependent on other inputs to the ECU."
After sourcing their 14CUX ECUs from the auto box Range Rover, could it be TVR never learned the lesson Jaguar did in 1983 with those manual gearbox XJS'?..
This may have some impact with those on the cux if everything isnt spot on with that road speed sensor box thing.
BUT
Even with an aftermarket ecu with the addition and benefits of a crank sensor i still will get shunting if in the 14.1 afrs. When i say shunting it will pootle along in 4th or 5th at 1000rpm no problem but put it in 2nd at a constant light load 1500rpm and it will start. The only way i can stop it is to run it at 12.8 afr then its fine.
I'm currently on a Kent 218 cam which i was hoping would have allowed me to get the afr's back into the low 14's but clearly not in the "shunt zone".
So for me the biggest issue is still the cam / plenum combination. To run at 14.3 afr, which is allegedly best for emmisions on old RV8, at low rpm, i think the plenum requires a pretty standard cam for all to be tickety boo.
BUT
Even with an aftermarket ecu with the addition and benefits of a crank sensor i still will get shunting if in the 14.1 afrs. When i say shunting it will pootle along in 4th or 5th at 1000rpm no problem but put it in 2nd at a constant light load 1500rpm and it will start. The only way i can stop it is to run it at 12.8 afr then its fine.
I'm currently on a Kent 218 cam which i was hoping would have allowed me to get the afr's back into the low 14's but clearly not in the "shunt zone".
So for me the biggest issue is still the cam / plenum combination. To run at 14.3 afr, which is allegedly best for emmisions on old RV8, at low rpm, i think the plenum requires a pretty standard cam for all to be tickety boo.
FWIW it has been shown that the 14CUX code does some very clever sums when monitoring the direction of the throttle position pot and its rate of movement, and eventually adjusts fuelling accordingly. If there is a rapid movement towards zero (throttle closed) and when certain other conditions are met the code runs its 'deceleration' or fuel cut-off strategy, which, ultimately, can result in there being no injector firing pulses at all. The 14CUX code was also designed to know (if the required resistor is in circuit) whether there is an auto or manual box in the system and adjusts values in accordance.
Land Rover documentation states that the 14CUX deceleration strategy was incorporated to protect the catalytic converters, to save fuel and thereby control emissions.
Obviously, 14CUX cannot adjust ignition timing as part of its fuel cut-off strategy as seen in more modern ECM systems.
As for completely removing the deceleration strategy from the code, not one for the faint hearted I think!
Land Rover documentation states that the 14CUX deceleration strategy was incorporated to protect the catalytic converters, to save fuel and thereby control emissions.
Obviously, 14CUX cannot adjust ignition timing as part of its fuel cut-off strategy as seen in more modern ECM systems.
As for completely removing the deceleration strategy from the code, not one for the faint hearted I think!
The 14CUX does have an overrun fuel cut and the high rpm start point can be set by changing prom offset $137 as follows
$04 (4,500rpm)
$06 (4,000rpm)
$07 (3,400rpm)
$08 (3,000rpm)
$09 (2,700rpm) TVR Precat 430 and Precat 430BV
$0A (2,500rpm) TVR 400 CAT
$10 (1,600rpm) TVR 400 Precat, 430BV CAT & 500 + Land Rover 3.9 & MGR
For example, I’ve set mine to $04 (4,500rpm) so my engine keeps fuelling when lifting off below 4,500 rpm plus I’ve leaned off my fuel table's top row to improve the overrun sound track. On the other hand, a heavy Land Rover towing a boat down hill would want maximum engine breaking.
When the overrun fuel is cut it is reinstated once the rpm drop below the set point at prom offset 138 and 139, road speed is not taken into account and only used to switch between open and closed loop idle control. A lot of TVRs idle issues are exacerbated because TVR only increased the closed loop idle speed and didn’t change the open loop idle speed to match.
$04 (4,500rpm)
$06 (4,000rpm)
$07 (3,400rpm)
$08 (3,000rpm)
$09 (2,700rpm) TVR Precat 430 and Precat 430BV
$0A (2,500rpm) TVR 400 CAT
$10 (1,600rpm) TVR 400 Precat, 430BV CAT & 500 + Land Rover 3.9 & MGR
For example, I’ve set mine to $04 (4,500rpm) so my engine keeps fuelling when lifting off below 4,500 rpm plus I’ve leaned off my fuel table's top row to improve the overrun sound track. On the other hand, a heavy Land Rover towing a boat down hill would want maximum engine breaking.
When the overrun fuel is cut it is reinstated once the rpm drop below the set point at prom offset 138 and 139, road speed is not taken into account and only used to switch between open and closed loop idle control. A lot of TVRs idle issues are exacerbated because TVR only increased the closed loop idle speed and didn’t change the open loop idle speed to match.
Excellent Steve.
Here's five potential points of learning to recap....
1. A richer map in places where the engine is forced to run too lean has potential to help
2. The disabling of fuel cutoff or at least raising it's RPM point might well be a smart move
3 The implementation of an igniting timing strategy that comes a bit closer to what people with aftermarket ECUs tend to run, might be of help
4. The ruthless and determined irradiation of all vacuum leaks to include a close study of the crank case ventilation system is a good idea
5. Making sure the TVR road speed signal box is actually working correctly is to be considered
Perhaps after ensuring all the ignition components are in tip top condition, the charging system is working as it should, and the car has good clean earthing points.... the above five key areas are elements every 14CUXer should focus on if he wants a smoother TVR?
Just a thought you understand.... well five thoughts actually
- Do you think there's anything in the 'auto gearbox ECU used with a manual gearbox car' theory, as per the above Jag XJS evidence?
- Does the fuel cutoff really use an element of road speed signal, or is it exclusively TPS?
Here's five potential points of learning to recap....
1. A richer map in places where the engine is forced to run too lean has potential to help
2. The disabling of fuel cutoff or at least raising it's RPM point might well be a smart move
3 The implementation of an igniting timing strategy that comes a bit closer to what people with aftermarket ECUs tend to run, might be of help
4. The ruthless and determined irradiation of all vacuum leaks to include a close study of the crank case ventilation system is a good idea
5. Making sure the TVR road speed signal box is actually working correctly is to be considered
Perhaps after ensuring all the ignition components are in tip top condition, the charging system is working as it should, and the car has good clean earthing points.... the above five key areas are elements every 14CUXer should focus on if he wants a smoother TVR?
Just a thought you understand.... well five thoughts actually
Gassing Station | Chimaera | Top of Page | What's New | My Stuff