It's called "Dechoke Control System" and is intended to clear the cylinders of fuel in case the engine gets flooded (with fuel!)

However, there's usually enough fuel about to fire the engine if it's in running order, so this test (trying to start) is rather inconclusive. It'll probably start anyway, and then the injectors fire and you're running. You can check the throttle sensor (just a switch, not really a throttle position sensor until 1.8 (and later 1.6?) came along) through the diagnostic connector - ( plug - full details in my manual!!) - via MEN terminal. This should produce a light at the LED if the throttle is neither fully closed nor fully open (ie light only on when throttle partly, not fully, depressed).

It's common for the pedal not to get fully depressed due to the carpet, mats, etc. BTW - this is how the air con gets switched off at full throttle - but of course you S/C chappies can be less concerned about that than us normally aspirated blokes!

If it doesn't work, there's a set-up and test procedure involving feeler gauges and continuity checker at the throttle sensor terminals. (1/00)

Instructions:
Switch off all electrics, and check gearbox is in neutral.
Link TEN and GND.
Use a LED test probe or LED with resistor.
Insert "-" or cathode into MEN
Insert "+" or anode into "+B"
Switch ignition on, but do not start engine.

The light should be out with throttle closed, on with throttle partly open and off again when the throttle is fully open.

MEN is linked to a transistor switch and is normally effectively open circuit. It goes to ground when an indication is present. (If TEN and GND are linked)

BTW - make sure all electrics are off. There is just one indicator, and it will light if the heater blower is on 2,3 or 4, aircon on, HRW on, lights on, gearbox in any gear, etc etc

The Manual is "Use and Interrogation of the Diagnostic Connector" by me (plug as in publicity ;-) ) See donutz! (1/00)

Instructions:

You missed out a step. Disable your immobiliser! Until I sussed this +B was Gnd. :-( Even with the car in my garage is too cold to think out there!

The light should be out with throttle closed, on with throttle partly open and off again when the throttle is fully open.

I get it going out at WOT but at other times I get MEN=Gnd. Does this mean my TPS is never showing us off the throttle? I suppose I should get the colour codes for the TPS wires and buzz it out properly. There is slack on my throttle cable with the throttle released but my dashpot may not be allowing full release.

BTW - make sure all electrics are off. There is just one indicator, and it will light if the heater blower is on 2,3 or 4, aircon on, HRW on, lights on, gearbox in any gear, etc etc

I assume this means automatic gearbox? My manual gearbox doesn't have any effect on MEN, not even reverse. (1/00)

I suppose I should get the colour codes for the TPS wires and buzz it out properly. There is slack on my throttle cable with the throttle released but my dashpot may not be allowing full release.

Possible - or it may just be adjustment of the sensor.. There are three terminals POW, TL and IDL After you've unplugged the connector, looking into the sensor socket, POW is at the top, TL middle and IDL bottom. Insert feeler gauges between the throttle stop screw and the stop lever

0.4mm feeler - Continuity between IDL and TL
0.7mm feeler - No continuity
WOT - Continuity between POW and TL

If you're not getting GND on MEN at closed throttle, you may need to adjust the sensor. Insert the 0.4mm feeler, loosen the two attaching screws, rotate the sensor clockwise approx 30 degrees, then rotate it back until you get continuity between IDL and TL. Tighten screws. Check all operations above.

All this is from the (official) workshop manual. Sorry, I don't know the wire colours (too damn cold to go and check!). The IDL terminal is connected to terminal 1N on the ECU (POW to 2L and TL to Ground)

BTW - make sure all electrics are off. There is just one indicator, and it will light if the heater blower is on 2,3 or 4, aircon on, HRW on, lights on, gearbox in any gear, etc etc I assume this means automatic gearbox? My manual gearbox doesn't have any effect on MEN, not even reverse.

Nope - manual. It's interlinked with the clutch.

Pedal released - light on when in (any) gear, off in neutral. In gear - light on when pedal released, off when pedal depressed.

Mine didn't work either when I was writing my manual, and I found that the return spring in the clutch switch was broken. Well - I had to check everything, didn't I? (1/00)

Mine didn't work either when I was writing my manual, and I found that the return spring in the clutch switch was broken. Well - I had to check everything, didn't I?

Oh great. Another thing that's broken! :-) Why does the ECU want to know whether you're in gear?! (1/00)

Any idea why the ECU needs to know about throttle release? It seems to catch the revs fine as is.

For idle, it's to provide 'fuel cut' (ie no injection when throttle closed - this is for economy and to provide a smoother overrun (or as the workshop manual puts it - 'prevent engine bucking during deceleration'). Also to bring the Idle Speed Conrol into operation, to cut out carbon canister purge control at idle and it affects timing. For full throttle, it's to cut the aircon and it affects fuel injection amount. (1/00)

Why does the ECU want to know whether you're in gear?

It affects fuel injection amount, idle speed control, carbon canister purge (only purges when driving in gear),and timing is adjusted. (1/00)

But my wife mentioned today that the car has cut a couple of times when coasting to a roundabout on no throttle. Strange coincidence that this happens just when I found a fault. Hmmmm. So I adjusted it.

No way could I get a feeler gauge in there so I did it rather roughly, but it seems about right. I had to use a tiny socket from a "toy" socket set taped to an Allen key to turn the hidden TPS mounting screw. But now I get the right MEN readings. Also I was fooling myself about the gearbox/clutch one. Once the TPS was fixed I could see what it was masking. So let's see how it goes. Ever since we got the car I've had to have the base idle high or my wife complained about stumble/stall. Mazda dealer found no error codes or faults but me raising the base idle fixed it.

Maybe it's always had this fault and I've just been masking it with high idle? Maybe lots of people see stumble due to a badly adjusted TPS? BTW When "blipping" the throttle I can provoke really bad stumble. I seem to remember this is an MX5 trait, but again my high base idle hides it. My readjusted TPS position causes it to happen again. Do I need to adjust the TPS again or am I stuck with this one? (1/00)

I was going to say that although it's a long list the items are pretty trivial at first sight and will probably have a small effect on driveability, emissions and economy, so you probably wouldn't notice and would compensate naturally for driveability issues. However, it sounds as though your idle problems are more significant (even though obviously you do compensate for them).

I always feel that one of the impressive features of (nearly) all Mazda engines is that you can cruise for a long period at very high speed then brake hard (say for French traffic lights - just seems to be a feature of roads in France that suddenly there are lights after a long long clear bit!) and stop with a completely smooth unwavering idle at exactly the correct speed. It sounds to me as if your idle speed control valve is perhaps sticking or somehow not operating smoothly.

If you disconnect it (it's a black two pin connector just under the TPS plug - if you can get to it) with the engine idling (and warm), the valve should click and engine speed should rise promptly to about 1200 rpm. The other test is to check the resistance of the valve at the connector terminals - should be 12+/-1 Ohm. Faults are rare, though. Two other things which are critical for smooth correct idling are inlet manifold air leaks (lack of) and correct fuel pressure. (1/00)

However, it sounds as though your idle problems are more significant (even though obviously you do compensate for them).

Other than the stalling it used to have (fixed with highish base idle - about 1krpm) it's fine. Now I've fixed the TPS I've adjusted the base idle and I'll see how it goes. My previous MX5 used to also show stumble with a quick (1/4 second!) throttle blip - I think that by the time the ECU is reacting to you being on the throttle you're back off it! Does everyone else see this?

I always feel that one of the impressive features of (nearly) all Mazda engines ... and stop with a completely smooth unwavering idle at exactly the correct speed.

Once I raised the base idle I got this! Except lately my wife complained about a couple of stalls. I'll see how it goes. (1/00)

Does everyone else see this?

...... Nope. A well tuned engine should not do it. (1/00)

my car in its stock '90 1.6 115hp days was just as rubbish with regards to idle droop than it is now with a supercharger. .........

I thought we were talking about throttle tip-in hesitation? Idle droop is something very different. But I do think both can happen to NA and blowed engines, and both do not happen in engines that are tuned well. (1/00)

Nope. A well tuned engine should not do it.

Hmmm. If others agree that a throttle blip doesn't cause a momentary drop in revs (more pronounced the more leccy gear is turned on) then I wonder what to check next? I'm ordering the ECU book from Donutz so I can test all the sensors. The idle behaviour is totally unchanged with the supercharger, and to be honest isn't a problem, but I guess I'm seeking perfection. :-) (1/00)

I thought we were talking about throttle tip-in hesitation?

Nope. If I keep leaning on the throttle everything is fine. This is a very, very tiny blip of the throttle. Just tap your foot very quickly and barely more the throttle at all. (1/00)

Just tap your foot very quickly and barely more the throttle at all. ......

And then the revs go down, and that is not normal. (1/00)

You perhaps do get a slight 'pause' on a s/c car but isnt that due to the sebring bypass actuator switching?

I'm talking about a blip far, far to short and gentle to trigger the sebring bypass. (1/00)

Tip in is analogous to a 'flat spot' on a carby engine where the accellerator pump is duff. The accellerator pump dropped a squirt of raw fuel into the throat to overcome the flat spot; i.e the point at which the main jet took over from the idle jet. On a fuel injected engine the mixture profile is programmed to anticipate this problem.

Fuel injected engines use air control as the idle medium rather than fuel, as with the idle jet in a carby.

I can say I do not appear to have this "tip in" problem on my engine. I'd suggest that you have the computer's appropriate switches checked and adjusted if necessary (possible???). Alternatively check the gasket on the idle speed control valve on top of the inlet manifold for air leaks. (1/00)

Although it's a long list the items are pretty trivial at first sight and will probably have a small effect on driveability, emissions and economy, so you probably wouldn't notice and would compensate naturally for driveability issues.

Further: Since "fixing" the TPS the car has been very hard to drive as it stalls when coming off the throttle at junctions. The ECU doesn't seem to be catching it at all. I have also seen the ECU taking the idle from 2k down to almost zero and then back up in a periodic way. None of this happens if the TPS is adjusted such that the ECU never sees idle - it then idles fine and "catches" falling revs without problems.

I've readjusted timing and base idle and it's still pretty bad. I guess now I need to check for vacuum leaks and do some basic tests on the ISC. But temporarily I'm very tempted to "break" the TPS again! (1/00)

Maybe something else to check - clogged fuel filter or dodgy fuel pressure regulator - get the pressure tested.

On my first mx5 the blocked filter caused a significant droop in pressure when the throttle was first applied. This was observed as a hesitation.

One trick I learnt from the fuel injection guy at the time was that you can easily increase the fuel pressure from stock (ie for those super charger guys) without any cost by removing the regulator and "squezeing" it a bit using a suitable press. You will see that there is a metal can fitted to the body, the centre part with a depression in it. This can contains a spring with acts against a diaphram. The spring pressure sets the fuel pressure at which the bypass will open. At the factory thay calibrate the regulator by squezing the can until it's to spec. So just squeze the centre depression some more to get the desired effect - voila!! (1/00)

There are pages and pages of troubleshooting poor idling in the official workshop manual.

Hints for "Idle moves up and down":

Fuel cut at idle - air valve not closing after warm up and idle speed reaches to fuel cut speed (?) (NB - the air valve is the one on top of the manifold airbox, in the middle. It should be completely closed after warm up)
Fuel injection amount fluctuating - bad contact point in airflow meter.
Air leakage of intake air system
Poor spark
Mixture too rich - evaporative emission control system malfunction
Low engine compression

And for "Engine stalls on deceleration" (My physics master used to say "no such word. it should be 'retardation' ") Idle speed control malfunction
Fuel cut control malfunction
Engine feedback control malfunction
Idle speed misadjustment

And there are step by step procedures for each - which don't necessarily match the hints. For example, one of the steps in the "deceleration" bit is to disconnect the oxygen sensor to see if it makes any difference.

All I can say from here is that something isn't right. If the problem is triggered by the TPS (as it sounds) could it be the evaporative emissions (carbon canister) system or (more likely) a stuck idle speed control valve?. It sounds a little bit as if you've had idle speed controlled by the throttle butterfly rather than the ISC. (1/00)

It'll be obvious when you find the problem.....................

If it helps everything is fine once the car is warm. And this morning it was also fine with it cold but it started behaving badly after 5 miles. :-( (1/00)

or (more likely) a stuck idle speed control valve?.

It could be this. I think it's the ISC I can hear buzzing - should I be able to or is something else buzzing? (1/00)

And this morning it was also fine with it cold but it started behaving badly after 5 miles. :-( .......

This could be a faulty air valve (on top of the intake) which is responsible for raising idle speed when the engine is warming up. Or the coolant temp sensor on the back of the engine. Or lots of other things .... (1/00)

Take it off and give it a damn good clean using carb cleaner. Its just a solenoid that opens and closes a bypass valve. The ECU sends a square wave and varies the duty cycle to control the amount it opens - effectively it's acting as an integrator, adding up all the bits of force appled during the active part of the cycle and translating it to an average displacement.

The buzzing means it's getting a valid signal. That fact that it's clearly audiable means there's some obstruction I should think as I can't hear mine. (1/00)

Yep - I think we're getting somewhere now. You shouldn't be able to hear buzzing - it does very much indicate a stuck ISC solenoid.

You'll need to drain or block the coolant pipes to it, disconnect everything a take it off.

After cleaning, it should be fully open with no power applied. If you can supply a varying voltage, you should be able to close it (maybe not completely) gradually and smoothly. (That's not an official test, BTW)

They recommend you to use a new gasket on reassembly (between the valve and the throttle body)

Whilst you've got this far, it's probably worth checking the air valve (connected to ISC by a water pipe). It's non-electrical - just a wax capsule. Test it by cooling it to 0 deg C, mark the shaft then warm with a hot air gun to see that the shaft moves. (This is an official test!)

Again, new gasket recommended. (1/00)

... I have got a buzzing ISC valve but cannot remember if the if this is post completely different ECU to the rest of you...

.. and cold weather first then heating the wax capsule with an air gun ... you cannot blow past it when it has warmed up (follow passage from inlet side) ... think of it as a choke on the old carb set up. (1/00)

The ISC is the thingy under the throttle body. However, it either controls or takes a sense from the gizmo plumbed into the top of inlet manifold. There is a specific test for that item as well. Measure valve postion, then place in a freezer for 15-30 mins then attack with a hair dryer and measure the valves movement.

Doing such a test obviously also requires a new gasket so two birds are killed with one stone. (1/00)

Idle speed control malfunction
Fuel cut control malfunction
Engine feedback control malfunction
Idle speed misadjustment

Idle speed is adjusted fine. All the injectors sound like they are injecting. No intake leaks found either with eyes, ears or a few squirts of WD40. Disconnecting the ISC causes the revs to rise a fair bit and more of a hiss than a click to come from the ISC. This weekend I'll test all the sensors and voltages and see how it looks. Adjusting the TPS back to where it was has made the car drivable again! (1/00)

You shouldn't be able to hear buzzing - it does very much indicate a stuck ISC solenoid.

I did some more listening, but now I'm really not so sure. I guess I can whip it off and strip it down. I had to fit it to the dummy throttle body as part of the s/c install so I know about the cooling pipes and gaskets. I guess this is one for the weekend! (1/00)

The ISC does its thing by controlling the air flow. I suppose the theory about the revs dropping too quickly is possible - that is why there is a throttle damper (dashpot), to give the ECU chance to "catch up".

The lever (on the throttle quadrant) is supposed to contact the damper piston at 2500 rpm +/- 100 rpm as the revs drop from 4000. How about trying to set it so that the dashpot comes into play sooner - eg at 3000 rpm? Adjust the screw on the bottom of the dashpot piston, and check with a feeler gauge as the revs fall.

I guess if you set the idle high it just stays high - ain't no free lunches when you start modifying for performance, and this type of driveability is an issue when they are trying to make the car "a good Corolla" as well as a good sports car. (1/00)

I still can't fix this idle problem. Once suggestion is that the extra load of the supercharger "surprises" the ECU as revs fall very quickly. .....

Hmmm, I thought I had learned here that a Supercharges puts no extra load whatsoever on the engine??? ;-)))) .......

People say they adjust base idle above 1krpm and this fixed it. If the idle is this high how does the ECU pull it down lower? Can it? Surely the ISC can only add air - or does the ECU also play with the timing? .....

AFAIK, the ECU does also play with the timing to regulate idle speed (not absolutely sure about this, though. (1/00)

Hmmm, I thought I had learned here that a Supercharges puts no extra load whatsoever on the engine??? ;-))))

I'm told about 20hp at max rpm, but there must still be something at idle. The odd thing is that if I adjust idle with the throttle butterfly (TPS adjusted to not report IDLE) it's *much* better. I suspect a failing O2 sensor, but I guess I could have an intake leak still.

AFAIK, the ECU does also play with the timing to regulate idle speed (not absolutely sure about this, though.

I've noticed that when you remove the TEN-Gnd wire the timing becomes more jittery. (1/00)

The lever (on the throttle quadrant) is supposed to contact the damper piston at 2500 rpm +/- 100 rpm as the revs drop from 4000. How about trying to set it so that the dashpot comes into play sooner - eg at 3000 rpm? Adjust the screw on the bottom of the dashpot piston, and check with a feeler gauge as the revs fall.

Getting to the throttle is difficult. To rev the engine I need to push the wheel with a screwdriver. I can get a fingertip to the dashpot, but that's about it.

I guess if you set the idle high it just stays high

Well, guess again! I put the TPS in "truth" position and wound the base idle up high - 1200rpm. The idle now drops to 1200rpm and then after a few seconds the ECU seems to say "ooo, tad high" and it drops to about 900. So it looks like it can pull the timing to drop the revs lower. And if you think about it pulling the ISC cable causes idle revs to increase! No time to recheck my timing tonight as I had to jetwash the wheely bins (strewth, how exciting and glamorous!)

ain't no free lunches when you start modifying for performance, and this type of driveability is an issue when they are trying to make the car "a good Corolla" (to quote Bob Hall) as well as a good sports car.

Perfect idle with a sebring supercharger is possible, and I'm not far off, it's just that I unknowingly took a wrong turn and now need to backtrack. I'll get there! If the TPS hadn't been wrongly positioned I'd have seen this issue straight away and been FORCED to fix it rather than just fiddling in my "spare" time as I am now. (1/00)

Manual idle screw sets base idle if at stock idle rpm.. the ISC then adds/subtracts around this ..
i.e. maybe -10% + 25%.

So if you put idle above the 850 rpm you are disabling the ISC function a bit.. I say a bit as the ISC is closed above 1,500 rpm and then wide open below if TPS is a idle to try and help 'catch' the falling engine speed.. it will then subtract all it can when the rpm fails to fall because the manual idle is holding it up. Does that make sense? ..

this is all theory from the way the Link ECU works .. it makes sense as a system.. but has no proof! .. is throttle plate initial opening correct? .. not sure on damper .. but is it correct stiffness/rate? (1/00)

it will then subtract all it can when the rpm fails to fall because the manual idle is holding it up. Does that make sense?

Yes, and it matches what I now have. Idle at 1400rpm (blower and lights on as dark morning) then dragged down to 900ish after three seconds. I'll back off the screw until it settles at about 1k under all conditions but I suspect this will be over several days. But at least I now have the car driveable with the TPS set correctly.

I suspect the only problem is that setting the idle with the s/c fitted is different to setting it without. There is stuff in the s/c instructions on this, but I decided I knew how to do it properly. :-) (1/00)

Remember my idle that was fine (ish) as long as my TPS was positioned incorrectly so that the ECU didn't see the idle problem?

Well, upon the advice of a few other supercharger owners, I have now set my base idle to about 1100 rpm and everything seems fine. As the revs drop it hangs around at 1000 ish for 2-3 seconds and then the ECU leaps in and pulls it down to 900ish.

I need to drive it under a greater variety of conditions, but hopefully stalling at roundabouts won't happen again. Watch this space.

I adjusted the idle partly with the idle screw and partly be restricting my filter->s/c body bypass with an aquarium airline restrictor. This seems to have also got rid of the slight lean condition on tip-in. Yipee! (1/00)