Idle speed control petrol engines

[Bongolia]

Long one coming up!
As there has been a couple of queries regarding idle problems on petrol engine and as it Pizzin down with rain I thought it may be helpful to some peeps to cover how to check the idle circuit on the petrol Bongos.
So here goes.....
The images here are for a 2.0ltr but the principle is likely the same for the v6.

When the engine is idling and is presented with an additional load the idle needs to automatically increase to compensate for that load to avoid stalling. When your foot is off the accelerator a switch at the the throttle quadrant tells the ECU to switch to an idle setting and the idle regulation is then dealt with on the Bongo V6 and 2/0Ltr petrol engines by an air valve. In order to do this effectively the ECU needs complete control of the amount of air the engine is receiving. Mazda call this device a BACS unit. I shall incorrectly call this the ISCV from here on.
BACS valve or ISCV image and its pipework.
https://goo.gl/photos/ywNTMv3J3afVTjZu8

When the Bongo leaves the factory a base idle is setup this is achieved by taking control from the ECU and adjusted by the air bleed screw at the top of the plenum. You will not be able to satisfactorily adjust the idle without switching the ECU out of control.
You shouldn't need to do this but I will cover the process later.
Here is the air bleed screw and below it the evap pipe you will need to check later.
https://goo.gl/photos/2VjHftvWfq55ekv76
The most common cause of erratic idle is air entering this circuit from split or leaking air pipes or a dirty or malfunctioning ISCV.
So starting with a series of basic checks for the pipework.

The Evaporation unit.
https://goo.gl/photos/KH72hCVtdCmYFdUj8
On the near side attached to the chassis rail you will see a black plastic canister next to that that is a small solenoid (purge valve with the electrical connetion) this degases the fuel tank when you start up
Gasses build up in the fuel tank for a number of reasons emissions regs require these vapours are held in the system and then fed back into the inlet when the engine starts,this black canister is where they are held.

On later vehicles, OBD2 on, the opening of the purge valve is done by the ECU based on readings it receives from sensors around the engine. So if yours is OBD2 then it may trigger differently.
Prior to OBD2 they are simpler in operation.

Switch on the ignition and listen for a click from the purge valve on the canister
If yes this would indicate the solenoid is working, it could still fail internally (mechanically) but usually if they click they work.
Next disconnect the pipe from the inlet manifold to the evap solenoid, ignition off. Put the pipe from the solenoid in your mouth and suck baby, you should have resistance, while holding the suck get someone to switch the ignition on but not cranking. You should note a momentary loss of vacuum. You can use a vacuum gauge for this but this is more fun just be aware you could get some petrol in yer cakeole.
If you are drawing air from here and cannot get a vacuum then you have an issue with the evap. This is unlikely as they normally fail closed.
Anyhow you can take that out of the circuit for the rest of the checks for now by refitting the pipe and clamping it off, if this resolves the problem the swop out the solenoid after checking for a supply voltage.
To do the rest of these checks more easily you will need remove the center section above the engine the bit that holds the gear selector, careful with the wiring and dont forget to remove the engine bay temp sensor and hand brake etc
Put the gear selector to one side you will have enough space to work and remove the centre section.
https://goo.gl/photos/YkqzThncj42G7wBX8
Once that is out of the way all is revealed!!


Pipework
Locate the servo vac pipe, you can try a suck on here to to check for any leakage from the servo but it needs quite a lug so vacuum pump would be best but if you haven't got one then with the pipe attached clamp off the pipe start the engine and see if there is any improvement. If there is then you may have a leak at the servo or the pipework in between. Dont clamp this pipe if it is nylon. You will have to suck.
N.B likely if the servo is leaking you will notice a hard brake pedal this can be a clue.
https://goo.gl/photos/dz246dAhCLYfEFGu8
Now check all the induction hoses after the Air Flow Meter (AFM) the thing that sits on the top of the air filter
This one has a blue dot.
https://goo.gl/photos/GKCdAc8mdnkTfn2G9
Look for cracks and splits in the main ducting pay particular attention to underside of the duct at the point that it attaches to the engine throttle body at the back of the engine and the elbows (2) as they enter that duct.
https://goo.gl/photos/VxUyZwFnvc7CszrV7
On the 2.0Ltr the ISCV is at the front of the engine the pipework passes along the offside of the block and into a Y branch then onto the main induction duct at the rear.Check that area thoroughly especially where the elbows go into the main duct. You will need to look hard at all this pipework as the holes can be tiny.
https://goo.gl/photos/dz246dAhCLYfEFGu8
See it?
If the elbows are damaged they can be replaced by 9mm nylon elbows like this.
https://goo.gl/photos/8csPTZjaDCtmBAzA8
Glue them into place using PU adhesive not silicon sealant.
Anything that allows uncontrolled air into the idle system is not good so you will need to replace any damaged or leaking pipe work before carrying on.

Last on the list for the time being is the Pollution Control Valve (PCV) this pushes into the front of the rocker cover.It is worth noting that problems here can lead to gaskets letting by like the rocker cover gasket!
Essentially its a one way valve.
Remove the valve and suck and blow it should allow air one way and not the other. If you have some carb cleaner or petrol it would be a good time to give that a bit of a flush out.
https://goo.gl/photos/2u3EDXNnyYSo3Hp9A
That should be all the idle circuit pipework covered.
I think?
Re assemble and check the running.

The next post deals with the ISCValve.

[Bob]

Another splendid write up.

[Bongolia]

ISC Valve.
In the previous post we looked at the pipework relating to the BACS valve. I called it an ISCV well part of it is but in the Mazda version it has a dual function.
There are two valves in that unit the first is a warm up valve and that is controlled primarily by a waxstat heated by a supply of coolant through two small pipes. The second is the ISCV, this is a shuttle valve that responds to demands placed on the engine by opening and closing just the right amount to allow the correct amount of air into the engine to balance the demand. On the Mazda I believe this shuttle is a two part process one is an electrical one and the other manifold vacuum, I could be wrong on that point but it doesn't really matter for the purpose of the exercise.
Anyhow these shuttle valves have to respond quickly to change and can be compromised by a build up of carbon and shite in the chamber or on the valve itself as well as wear and tear.

So if you have carried out the other checks and you have the cover off it would be a good time to clear the valve out. It can be a bit daunting but as long as you are careful all will be OK.

Some basic checks first before we rip it apart and loose the vital bits.
The first thing on the list is to make sure the ECU is being told that its at idle. So at the rear of the engine where the throttle quadrant is you will see a single wire switch. This is simply an on off switch. First check that the plunger is moving freely by opening the throttle at the quadrant, note any stiffness or jerkyness on the actual throttle at this time, make a mental note.
Next connect up a multi meter set for continuity to the back of the switch probing into the wire. Shut, you should have continuity to ground so open then, no continuity.
If that is not correct change the switch.
The ISCV.
This is energized when you switch the ignition on and goes through a setting up process.
A quick check would be to put your finger on the body of the ISCV and switch the ignition on you should now feel or hear a buzzing as the valve sets up. On a couple of occasions I have heard a loud clicking that seems to come from the plenum on both these occasions I found the shuttle to be sticking so presumably the click comes when the valve finally overcomes the resistance and slaps into the body of the ISCV. Normally you will hear a buzz.
If you get nothing at all check you are getting a voltage to the ISCV, unplug the connection and connect up a volt meter to the terminals switch on the ignition and you should see around 5 Vs. If you have the volts and no buzz then the chances are the electrical component is naff so another valve then.
Might still be worth a strip out though as you will have to remove the ISCV in order to swop it out anyhow.

Removing the ISCV
Each side of the ISCV you will see a small rubber pipe these are the water pipes so you will need to clamp these off before you remove the pipe work. Remove the servo pipe to get better access.
On the front you will find a fatter air pipe this is the one we checked earlier. Release the clip and carefully remove it from the spigot.
Unplug the electrical connection and release the mounting bolts then remove the valve by sliding it forward.
Some pixs.
https://goo.gl/photos/9xrtEfJV5nuUphxh9

So now you can take a look at the valve before stripping it.
You may be wondering what the hell this bit is ?
Well as far as I can make out is is just a blanking screw. If you can remove it with circlip pliers then do so as it will make the cleaning a bit easier but be aware this plastic part has been subjected to high temperatures over a number of years and is likely to be brittle. I have never succeeded in getting one out as I bottle out early on.
https://goo.gl/photos/JbYS8kf7GcCw1Kvm7

Using some carb cleaner fill the chamber around the sprung valve and let it soak for a few minutes.
https://goo.gl/photos/iA5ufzaKMrgUX9iSA
Now we need to disassemble the valve by removing the electrical unit.
This is held by four screws of differing lengths make a note of where they come from.
One of these is an anti tamper screw if you dont have the tool for this grip it with a small pair of vice grips and loosen it that way.
https://goo.gl/photos/4cfHGnFzbjJnM7176

The rest are normal very tight cross-head screws. If these are a bit stubborn then completely support the valve body against something solid and give them a good whack on the end of the screwdriver or safer loosen with vice grips like the tamper proof one.
Remove the plastic spigot from the end of the valve and set it aside.
Clamp the valve body in a vice with soft-jaws so as not to mark the face.
Raise the electrical component away from the body do this gently and check it as you raise it . It is likely that part of the gasket will be attached to the the top use a Stanley blade to ease this off and do not try to remove the gasket as long as its attached to one side or the other it will seal fine. You cannot get these if they are damaged so take care. Else you will manicuring a new one.

Separate the two parts.
https://goo.gl/photos/eB7DjoQbzrWfLKM18
Here is the shuttle valve with the springs attached you do not need to remove these they wont be in the way.
(Be aware that in the base of the body is a small white plastic pin make sure you dont lose this if you remove it take care when rebuilding the valve that it seats into the socket and the right way round (its easier to put it in the spring and carefully reassemble it that way) you can see the build up of shite around this valve as well as the wear on the piston, give this a nice soak and carefully remove the gunk from the valve with a toothbrush.
https://goo.gl/photos/fbbfX8CxxkkuBpiS9
Now turn your attention to the sleeve where the valve slides look at the wear here unless its stepped then it should be OK.
Clean the body with carb cleaner as well as the mating surfaces of the device.
N.B. you will find what appears to be an O ring the first one I did of these I took that out and replaced it with a new one but it would not run at all well so my assumption is that it is a hard plastic O ring and not soft. When I refitted the old one the problem went away.
https://goo.gl/photos/L2cthQwp8euNVUxJA
It just needs to be clean of carbon build up dont get tempted to polish anything they are quite hardy.

So providing you haven't lost or busted anything put it back together , remember about the plastic pin.
Use a gasket varnish on the facings apply with a small paint brush I use Wellseal for this purpose butter both faces and leave it to set up for a minute before assembly. Replace the screws in the correct order and really tighten them use the vice grips for the last bit of the turn but take care not to snap the heads off!
https://goo.gl/photos/kcj4fHa5aaxT3sMQA
Thats that, now reverse the removal procedure and refit to the van using varnish on the faces as you did with the device.
Thoroughly recheck everything you have done and when you are happy start it up.

When you first start it you will likely hear a dieseling as you rev the engine so don`t rev the engine hold it on a gentle idle for a couple of minutes. It is caused by the carb cleaner being drawn into the engine it will clear after a bit but sometimes it can take a while. Bring the whole thing up to working temperature and see what its like now.
Dont forget to top up for any coolant lost..
Allow the engine to cool overnight and re start in the morning when its a little warm switch on a load like the AC and make sure the load is compensated by the ISCV. It should be idling when warm at around 750 to 800 steadily. If it is steady but running very low or too high then its likely someone has been at that idle screw so you will need to adjust that. I will cover that in the next post.
Good luck.
Any errors in the above are due to an overindulgence of the Bells.
They made me deaf you know.

[mikeonb4c]

Fantastic guidance and these should be got into a fact sheet so the work doesn't get buried in old posts.

Mind you, I'd thought Bongos were fairly simple beasts. But after reading this I'm starting to get wistful about my 1958 MG Magnette. Crank handle, feeler gauges, socket set (i still have it were all I needed to sort most stuff.

[ade33]

Superb post(s)

[Flanners]

I was unable ever to find the conclusion to this post/guide on how to switch out the ECU and then re-trim the air screw on the TB to obtain a perfect tickover speed. It was obvious my idle screw had been messed about with as the idle fluctuated when warm, I had tried to cure this problem by turning the screw but could not get rid of the slow rise and fall of revs in Park/idle.

To adjust the tickover speed the ECU needs to be prevented from interfering so I used a paper clip as a jumper between the GND and STI (ten) pins on the OBDII port in the passenger footwell, I then with the engine running adjusted the idle screw slowly until I obtained a perfect and smooth tickover at 800rpm. I then shut off the engine removed the jumper and restarted for a perfect non fluctuating stable tickover.

[Bonneville]

Thanks Flanners (and Bongolia), excellent stuff. I too came to that point in this thread but my idle miraculously fixed itself after all the other work on it so didn't need to reset it.
I found the biggest factor was a proper ignition timing job. Mine was a degree out and, with the new rotor arm and cap, the idle came up to where it should have been.
Also worth a look is the cam timing. These will run in any of the 4 positions on the cam wheel and all produce different results.

https://mazda626.net/topic/34678-camsha ... ce-retard/

I've not tried it yet as I like the torque where it is.