This is a fairly easy job, the ISCV or Idle Speed Control Valves on the Cosworth YB are prone for carbon contamination, and suffer from being poorly set up.
Our guide will help you clean, test and set up your old Idle valve and make it as good as new, aswell as showing how to do a road spec ALS mod.
There are 2 types of valve, the one used here is a later 4x4 unit, I will get images of a 2wd Valve when i see one next, but the process is very similar but only have one internal spring.
The Strip Down.
To strip the valve is very simple.
Remove the locknut (Usually 4mm)
Then the plastic retainer will come off along with the outer spring.
Next the piston will come out, this may need a bit of carb cleaner of a gentle tap to remove.
The inner spring will then come out.
Remove the O rings from the outer body.
Be aware the springs are light, but under tension, so be carefull not to loose them.
Lay all parts out in the order they were removed so we can determine if the valve was correctly built last time.
(you will be amazed how many i see put together incorreclty!)
The Big Clean up.
Its often easiest to use a small bath to clean and soak the parts of the valve. I use a little ultrasonic bath here to fully clean all the parts. but in most cases this will not be available, so a small container just big enough to hold the inner parts can be used and filled with something strong enough to break down Carbon, like Carb Cleaner, or Brake cleaner.
Leave the parts to soak for 30mins, while giving the pot a shake every 5 mins to dislodge the carbon.
Do not soak the Valve itself.
The valve needs cleaning internally, so again carb cleaner is ideal, fill the inner cylinder, shake, and continue until its internally clean.
I then tend to use a fine grade of wet and dry paper, wrapped around a nice fitting socket.
This can be pushed into the Valve, and rotated to clean the cylinder wall. Dont go mad and increase the cylinder bore size, just enough to ensure the cylinder is clean and smooth.
One more clean with carb cleaner to remove any further depostits should see the valve itself all cleaned.
By now the other parts should be nicely soaked and ready to be removed one by one and cleaned with a small brush, before being dried with a clean rag.
Pay close attension to the piston, try to get right inside to clean all the dirt out.
Lay the parts out again as we did when stripping.
There are two springs in the ISCV, inner and outer. It is vital these spring rates are correct and they are the correct size.
If you have lost or found broken springs, dont just throw in any spring you find tin the bottom of the tool box, it wont work! If you need parts i advise finding a dead vailve from somewhere and stripping it for used parts.
You will notice one spring is stronger than the other with a thicker coil.
The stronger spring is the INNER.
Was it before? I see lots of valves with the stronger spring on the outer which prevents the valve operating correctly!
The Test and the Build up.
Firstly pick up the empty ISCV body. Give it a shake and listen. If the ISCV body rattles with no components fitted then there is a good chance if the valve even does work, it wont for much longer. Its a sign the internals are working loose, and usually a sign they are past there best. If it does rattle, you can continue with the build and test, but try to source a valve for replacement.
Now you are ready to build the valve.
This inner spring can now be added into the Valve body.
Now for the piston. Gently drop the piston into the cylinder.
Which way around does the piston go?
Well look at the piston, it has a deep side and a shallow side. The deep side goes into the Valve and sits on the Inner spring.
You should be left with a valve with a piston floating.
You can now test how good your cleaning job was, using a small punch or probe, gently push the piston in allow it to return on spring tension, do this a few times, feel free to use a small amount of lubrication here if needed.
DO NOT use WD40! this evaprorates and leaves a sticky grease like residue which will prevent the valve operating freely. Use a good grade very thin oil, like 3 in 1 or machine oil.
If this is not smooth, go no further untill it is, go back over the steps of cleaning and honing if needs be, but this must be smooth or the valve will never operate correctly.
Once you are happy the valve is smooth in opertaion its time to do a bit of trickery on the inner spring.
The springs tend to go weak with age and cycles, and sink, this means the valve passes air when it shouldnt and opens to easily. so with the piston sat in the valve body i aim to see the piston around 2mm under the top face of the valve body, reomve the piston and stretch the spring, or contract the spring to get this height correct.
The piston has a small scribe part way down its body, aim to get this scribe line exactly inline with the top cut out in thre valve body air port.
Scribe line shown above
Scribe line can sit exactly on the top of the Air Pass cut out indicated.
The piston will sit around 2mm under the top of the valve body once this is done.
Once this is done and you are happy with the spring height, give the piston a few pushes and work the inner spring, it should push the piston back to its optimum height each time!
Then your almost there, refit the outer spring, refit the plastic retainer, and apply a small amount of thread lock tho the thread, put the nut back on and tighten. Do not go mad and wind the nut all the way down, just take it down a few mm, leave around 3mm of thread showing above the nut.
Finally some more lubrication can be applied through the air ports on the side.
You can now replace the o rings, after cleaning and inspecting them. Replace if necessary and ensure there are two!
Dont go mad on the lucknut!
Thats the build up and base setting done.
Now you can refit and test. If you have the means you can happily bench test these valves.
Reset the base idle.
As the valve has now been recalibrated, the car will need to be run up to temperature, unplug the ISCV and reset the base idle to 900-950 rpm. With the valve then plugged back in the base idle should remain stable.
This comes down to how the car is set up, I can only give you bold outlines here as its mapping and set up dependant, you need to be happy the car is correctly set up, and all calibrations are correct.
Issues of the valve not operating enough or increasing idle enough will mean the spring tension is to strong, re- strip and compress the inner spring.
Trial and error untill its correct.
Issues of the idle increasing when the valve is reconnected with the enigne hot are a sign the inner spring is too weak, stretch/replace untill the idle remains stable. Trail and error untill correct.
Remember to also try the valve operation when the car is cold, see if the idle speed is niciely increased.
TPS Calibration has a large impact on ISCV operation, The weber ECU opens the ISCV as soon as it sees any throttle angle, and also if the TPS is too far into negative figures the ISCV operation becomes extremly irratic.
Road Spec ALS Mod.
The purpose of the ISCV is to control idle speed, this valve can also be used to pass greater amounts of air when using ALS, but if you go mad, or use the "top hat" valve, you will start to loose the valves ability to progressively control engine speed, it becomes an "on/off air bypass valve, and as soon as the ECU comands the valve to open even slightly the revs will increase massivley, causing it to be totally useless as an actual ISCV.
We can make a comprimise though, still allow a progrsssive idle control but also increase its flow rate for ALS.
We need to be aware of the drilling shape of the valve body, you see the cut out has a small "lump" cut away from it, where we line the piston up with earlier, this is to allow the ecu to command the valve to open, but not make the RPM jump by adding too much air, its a progressive bypass.
So in order to keep the valve functioning as an ISCV we can keep the progressive drilling but enlarge it to flow more air.
Using a dremel, or similar you can neatly file the port size to a more triangular shape, increasing the flow by around 50% but beacuse its still progressive and starts small the ecu will still be able to control idle speed. Once the ALS is active and the valve is pinned flat out it will open the whole port and flow lots more air.
Something along these lines will keep the valve operating as an ISCV keeping a nice cold start.
If you want to pass even more air, you can drill extra holes in the side of the valve body, but ensure they are BELOW the top height of the standard drilling. So as the valve begins to open for small idle speed increases these drillings are not opened. The will operate only when the valve is opened by large amount.
After drilling or filling you must ensure the piston is smooth in the cylinder! hone and clean as required.