Discussion
Just one more thing in the seemingly endless quest to get this damn thing to run properly.
The overun valve (Bosch item sat on the front of the right hand Vee piped from in front of the butterfly to the back of the plenum - I assume supposed to be opened by the ecu on deceleration to let some air in to help burn off any excess fuel) has packed up and is stuck in the open position causing a very high idle and lean running.
I have removed it and blocked the pipework - does anyone know whether I can leave it off or will it have any real detrimental effects ?
The crackle and pop has increased significantly which is quite nice actually but aside from any environmental issues is there a reason this valve was fitted ?
The overun valve (Bosch item sat on the front of the right hand Vee piped from in front of the butterfly to the back of the plenum - I assume supposed to be opened by the ecu on deceleration to let some air in to help burn off any excess fuel) has packed up and is stuck in the open position causing a very high idle and lean running.
I have removed it and blocked the pipework - does anyone know whether I can leave it off or will it have any real detrimental effects ?
The crackle and pop has increased significantly which is quite nice actually but aside from any environmental issues is there a reason this valve was fitted ?
It's either an over-run valve (no electric feed) or an idle stabilisation valve (electric feed) - aka stepper motor.
The over-run valve does more or less as you say and draws the fumes from the crankcase under certain manifold depressions. The ISV allows a controlled amount of air through, under ECU control, to fine-tune the idle speed. If it gets gunked up of course it sticks, causing some bizarre running symptoms depending on where it sticks.
In theory the car will run OK-ish without it, but you may run into starting or poor idling troubles.
Ususl solution (no pun intended) is to clean in paraffin or similar.
Ian
The over-run valve does more or less as you say and draws the fumes from the crankcase under certain manifold depressions. The ISV allows a controlled amount of air through, under ECU control, to fine-tune the idle speed. If it gets gunked up of course it sticks, causing some bizarre running symptoms depending on where it sticks.
In theory the car will run OK-ish without it, but you may run into starting or poor idling troubles.
Ususl solution (no pun intended) is to clean in paraffin or similar.
Ian
wedg1e said:
It's either an over-run valve (no electric feed) or an idle stabilisation valve (electric feed) - aka stepper motor.
Ok - now I am not sure what it is. I have a hotwire Disco V8 and the electrically operated stepper motor on that is on the side of the plenum nearest the bulkhead.
On the TVR there is what I am assuming is the cold start valve in that position - no electrical connections to it I assume it operates on a bi-metallic strip.
At the front there is this electrically operated Bosch valve - looking through the pipe stubs you can see a disc that rotates that has a hole of varying profile cut into it so it would seem that it is designed to control idle speed rather than assist on overun.
However the injection system is the flapper type and I thought they had no automatic idle control with only the hotwire versions with a stepper motor. Also if this is for idle control where is the overun valve I have seen mentioned or does the stepper perform two functions ?
Can anyone throw any light on what this valve is / should do ?
chunder said:
Ok - now I am not sure what it is. I have a hotwire Disco V8 and the electrically operated stepper motor on that is on the side of the plenum nearest the bulkhead. Sounds like where the over-run valve is on the flapper system - a sort of dome-shaped alloy block, with a black pipe leading into it.
On the TVR there is what I am assuming is the cold start valve in that position - no electrical connections to it I assume it operates on a bi-metallic strip.
Cold start valve should be on right-hand side of plenum chamber (viewed from driver's seat) - it will have an electrical feed.
At the front there is this electrically operated Bosch valve - looking through the pipe stubs you can see a disc that rotates that has a hole of varying profile cut into it so it would seem that it is designed to control idle speed rather than assist on overun.
This is the Auxiliary Air Valve - it is open when the engine is cold but gradually closes as it warms up, either by the electrical feed or by the warmth from the engine. It admits extra air to raise the idle speed during warm-up.
However the injection system is the flapper type and I thought they had no automatic idle control with only the hotwire versions with a stepper motor. Also if this is for idle control where is the overun valve I have seen mentioned or does the stepper perform two functions ?
Can anyone throw any light on what this valve is / should do ?
On a flapper fuel system there is a valve attached to the back of the plenum chamber with no electrical contacts and a single 3/4" thick pipe attached to it.
This is the overrun valve. Its job is to operate when the vacuum in the plenum chamber is lower than it would normally be at idle.
It takes its air from in front of the throttle butterfly near where the crankshaft breather feeds into the throttle body.
If you blank this off you will get high vacuums on the overrun and you will suck lots of oil down the valve guides.
On both my cars (1985 390 and Rover VDP EFi) this valve can be adjusted if removed, there is a spring and a small nylock nut (M4?). I suffered from the valve weakening with age and opening under normal idle (idle speed shoots up and stays up, diagnosed by squashing the pipe to close it) so I tightened the nut a bit until it didn't do it and then sealed it with a blob of paint.
Russ
This is the overrun valve. Its job is to operate when the vacuum in the plenum chamber is lower than it would normally be at idle.
It takes its air from in front of the throttle butterfly near where the crankshaft breather feeds into the throttle body.
If you blank this off you will get high vacuums on the overrun and you will suck lots of oil down the valve guides.
On both my cars (1985 390 and Rover VDP EFi) this valve can be adjusted if removed, there is a spring and a small nylock nut (M4?). I suffered from the valve weakening with age and opening under normal idle (idle speed shoots up and stays up, diagnosed by squashing the pipe to close it) so I tightened the nut a bit until it didn't do it and then sealed it with a blob of paint.
Russ
Thanks guys I am beginning to understand.
So the cold start valve is not a valve but is the injector into the plenum that operates on a thermotime switch independent of the ECU.
The overun valve is on the side of the plenum at the back nearest to the drivers seat and is operated mechanically.
The Bosch valve located independently of the plenum at the front is the auxiliary air valve and is controlled by the ECU.
So there is no idle speed control valve or stepper motor on a flapper type system, the only control aside from base idle adjustments being the aav (which I guess is what I have seen referred to as the cold start valve in other threads).
Armed with this info it sounds as if the problem is being caused by two faults - I found an intermittent short on the wiring to the aav causing erratic operation last night and the overun valve must also be opening before it should as even with the aav blanked off revs still occasionally rise at idle.
Thanks for the help - will let you know when resolved.
So the cold start valve is not a valve but is the injector into the plenum that operates on a thermotime switch independent of the ECU.
The overun valve is on the side of the plenum at the back nearest to the drivers seat and is operated mechanically.
The Bosch valve located independently of the plenum at the front is the auxiliary air valve and is controlled by the ECU.
So there is no idle speed control valve or stepper motor on a flapper type system, the only control aside from base idle adjustments being the aav (which I guess is what I have seen referred to as the cold start valve in other threads).
Armed with this info it sounds as if the problem is being caused by two faults - I found an intermittent short on the wiring to the aav causing erratic operation last night and the overun valve must also be opening before it should as even with the aav blanked off revs still occasionally rise at idle.
Thanks for the help - will let you know when resolved.
I'm not sure that the aav is controlled by the ECU.
I am at work now and haven't got my books here.
I think that the AAV is controlled by a bi-metallic strip inside itself and by the thermotime unit. The thermotime unit is near the engine temperature sensor (front of the inlet manifold) and looks very much like it. The thermotime unit gives power to the aav depending on the water temperature and time.
The cold-start injector gives extra petrol into the plenum during cold start conditions (and also during the first 12 seconds of cranking). This is controlled by the ECU but most owners leave theirs disconnected.
Russ
I am at work now and haven't got my books here.
I think that the AAV is controlled by a bi-metallic strip inside itself and by the thermotime unit. The thermotime unit is near the engine temperature sensor (front of the inlet manifold) and looks very much like it. The thermotime unit gives power to the aav depending on the water temperature and time.
The cold-start injector gives extra petrol into the plenum during cold start conditions (and also during the first 12 seconds of cranking). This is controlled by the ECU but most owners leave theirs disconnected.
Russ
rus wood said:
I'm not sure that the aav is controlled by the ECU.
I am at work now and haven't got my books here.
I think that the AAV is controlled by a bi-metallic strip inside itself and by the thermotime unit. The thermotime unit is near the engine temperature sensor (front of the inlet manifold) and looks very much like it. The thermotime unit gives power to the aav depending on the water temperature and time.
The cold-start injector gives extra petrol into the plenum during cold start conditions (and also during the first 12 seconds of cranking). This is controlled by the ECU but most owners leave theirs disconnected.
Russ
Quite right - sort of. The AAV gets a 12v feed as long as the fuel pump is running. So when you start from cold, the AAv is open, allowing extra air to go with the extra fuel provided by the ECU knowing the coolant is cold and the thermotime switch causing the cold start injector to add a few seconds of extra fuel spray. Then the heating element in the AAV starts to close the valve, reducing the air and lowering the idle speed.
If the engine is warm and you try to start it, the heat from the manifold 'soaks' through the cast alloy body of the AAV and has already closed the AAV blade, so you get correct idling conditions.
The only reason it apears that the AAV is'controlled' by the ECU is that for some reason the earth return for the AAV is taken from the main power ground of the ECU. Probably just simplified the wiring; there is no controlling circuitry on the drawings I have.
The Thermo/Time Switch only controls the Cold Start Injector. On many TVRs, you can take the T/TS and CSI off and throw them away - the engine will happily start without them.
Ian
Thanks again for the advice - we now have a definitive description of the operation of these items.
Cold start injector from thermo time switch (disconnected)
Aav from ignition or ambient heat (short rectified and can now see disc spin and air gap close when voltage or heat applied - now closes all the way)
Overun valve mechanical (now adjusted to open under higher vacuum)
Still idled slightly high (1200) and no further air leaks - suspect AFM set up which is now being looked into as at idle moving the wiper that is positioned by the flap just by a hairs width brings the revs back to 1000.
Cold start injector from thermo time switch (disconnected)
Aav from ignition or ambient heat (short rectified and can now see disc spin and air gap close when voltage or heat applied - now closes all the way)
Overun valve mechanical (now adjusted to open under higher vacuum)
Still idled slightly high (1200) and no further air leaks - suspect AFM set up which is now being looked into as at idle moving the wiper that is positioned by the flap just by a hairs width brings the revs back to 1000.
Speed Matters | Wedges | Top of Page | What's New | My Stuff