E30 Zone

Hi all,

I've been trying to find the source of a high idle on my M20B25. I have checked all the vacuum and intake hoses for leaks, and everything is fine.

I've replaced the blue temp sender, re-checked my TPS for continutity at both closed and WOT, and it's OK (and adjusted so the microswitches are opening and closing in the right places).

I've also had my AFM apart to move the sweep arm onto virgin resistor track.

When I start the engine, it idles at about 1000rpm. But it stays there, regardless of engine temp. I was led to believe that once the engine was warm, it should drop to around 700 - 800 rpm.

Could my ICV be shot? I've tried to diagnose this off the car as per the Bently manual and many online guides - checking the resistance of the coils (which is way off - 70-80Ohms on centre pin to either outer pin, 100ish across the two) and supplying 12v to either side to see it 'snap' open and close - which it does.

It also hums when connected to the harness in the car and the ignition is turned on - with the valve cracking half way open and staying there.

Does the valve 'throttle'? Should it open to different positions depending on the temp and the signals from other sensors? Or is it an on off thing?

Is there anything else I could do to check it before I shell out for a new one?

Thanks

Jason

www.greatoldone.co.uk

Resistance of ICV - measured at the ICV
Continuity on TPS - measured at the TPS

Throtte stop is adjusted as per the Bently specs - I can just slip the correct feeler guage past the butterfly valve.

You need to check the TPS is actually switching, and check its resistance, and that of the blue plug, at the ECU.

ICVs can benefit from a healthy dose of Carb Cleaner though.

It's had so many doses of carb cleaner that I think it's now got a taste for it...

I will check at the ECU plug after work.

No - mine is an early '86 engine, Motronic 1.0 (2 row ECU connector)

Wiki to the rescue!

http://www.e30zone.net/e30zonewiki/inde ... tronic_1.0

Thanks!

Blue temp sensor - pin#13 and ground, should read ~300-360ohms when at operating temp, ~3000ohms when cold - CHECK

TPS - closed pin#2 and ground, WOT pin#3 and ground, both should read continuity (0 ohms) when active - CHECK

speed sensor - between pins #47 and #48 should read ~550ohms - Hmmm, not sure on this. Took a reading between Pin 8 & pin 27 (Crank speed sensor 1 & 2), and got approx 990ohms

icv - pins #33 and #34 should read ~40ohms - CHECK (which is weird, as when I last checked it was way out of wack. It's bang on now, 20 ohms on each individual coil, 40 across both, at the ICV and at the ECU. Must have had my tester on the wrong setting last time.

Could this be down to an incorrect setting of the idle screw on top of the throttle body? There is a screw under a black cap to the right of the throttle when your looking down the intake at the butterfly? The idle sticky for motronic 1.0 at the top of the forum mentions it, I believe.

[Off Topic] - why is there an output for injectors 3&4 (pin 14) and injectors 1&2 (pin 13), but nothing for 5&6?

Ok, some more diagnosis. I thought it best before I start buggering around with idle screws etc.

According to the idle sticky, I should be getting these values from the ECU to the ICV plug in the engine bay:

* between the inner pin and ground there should be battery voltage
* between any outer pin and the inner pin there should be around 10V

Here's what I get:

Pin 2 (middle) to Pin 3:



Pin 2 to Pin 1



Pin 2 to earth / ground



So, according to the sticky, there is either something wrong with the ECU, or the wiring harness - and I've ruled the wiring out as I have continuity from pin 1 and pin 3 on the ICV plug to the correct pins on the ECU plug.

Could it be the ECU? Everything else seems to be fine bar this idle issue. Here's the ECU code



I believe it's motronic 1.0, due to the 2 rows of pins.



Ta,

Jason

If I remeber correctly the idle circuit pof the 073 ECU was its week point. Can you get a temporary replacement ECU to try out ?

You may be better off looking for an "081" ECU which is fully compatible but an updated version.

I'll start looking for a replacement ECU - I don't have a source for a temp replacement, so I may as well see what I can find on eBay.

Thanks

Yes - I put one up yesterday.

http://www.e30zone.net/modules.php?name ... c&t=225982

So, anyone got one?

Received a spare 073 ECU yesterday (Thanks HairyScreech!) and plugged it in. Ignition on, measured the voltages at the ICV plug, and bugger me they're exactly the same as with my original ECU.

I have continuity on the lines from the ECU to the ICV plug, so it can't be the harness, right? Or could it?

One thing I did notice - with the new ECU, with the engine running, unplugging the ICV to measure the voltages and then plugging i back in caused the engine to start oscillating between approx. 1500rpm up 2500rpm and then back again. When I turned it off, and tried to restart, it wouldn't. I had to plug the original ECU back in to get her to fire again.

Update - I have tried to follow the motronic idle setting sticky, with no luck

The idle bypass screw is completely shut on the throttle body, so unscrewing causes the idle rpm to rise.

Likewise, I found that the bypass screw on the AFM is completely open - so much so it just spins in the socket. I had to take the AFM out to get the screw seated again so I could wind it in. And when I do so, the idle speed rises as well.



Help? As I've stated in the posts above, the blue temp sensor is ok, the AFM is on new carbon track and the TPS is good at rest and at WOT, and is correctly adjusted. Both the cranks sensors are good accordin to willnz.

Further update - I have sprayed the ICV with a liberal dose of carb cleaner (you could eat your dinner off it now).

Whilst I had the ICV off, I noticed that the metal elbow that slots into the throttle body was very loose. As where the ones for the Servo vacuum take off. Looking at them they seem to have been press fit parts when new - but over time they've worn down and can slide in and out quite easily.

To ensure that there's no air leakage here, I've bonded the elbows in place with some JB Weld. They can't move, and there's little chance of a leak there now.

I've fitted it all back together, and started the engine. It is now oscillating back and forth between 1000 rpm and 1200 rpm, hot or cold. Stopping the engine and just turning the ignition on, I have power to the ICV as before - but the damn thing isn't humming like it used to. Still has the correct resistances across the coils.

With the engine running, unplugging the throttle switch causes the engine to surge until you plug it back in - and it goes back to jumping back and forth again. Unplugging the AFM cause the engine revs to die back to about 850 / 900 but it struggles to stay there.

Any ideas? I'm beginning to wish I'd just left it be...

Right - I believe the ICV is dead, as there is voltage at the plug, the lines have continuity - but no buzzing from the body of the valve.

I left it overnight just in case giving it another shot of carb cleaner had somehow flooded the interior of the valve actuation motor. What's puzzling is that I get the right resistance readings when I stick my multimeter on the terminals at the ICV and at the ECU.

I've also been round with the carb cleaner on all the hoses and sources of possible air leaks, and haven't found any...

I've tightened up the rocker cover, as admittedly that was a bit loose - but not that it wasn't clamped down.

I've had the AFM off again, and shot carb cleaner though it to ensure the flap is moving and not binding anywhere, and that the air bypass pipe in the bottom isn't blocked.

Starting it up, she starts to swing back and forth between 1200 and 1000 rpm as before. Unplugging the temp sensor or the AFM stops the jumping back and forth, and she holds steady at around 2000 rpm. Plugging them back in causes the the jumping around to start again. As stated before, the blue temp sensor is brand new, and is showing the correct resistances cold and warm - at the sensor and the ECU.

What I have noticed is that by leaving the air filter off, and cracking the AFM door open more than the air flowing by it should do stops the jumping about and the revs die down to an acceptable level... Could the flap be tension be too high? Could it have been increased by a previous owner?

Any advice would be welcome, as I'm just going around in circles at the moment. Thankfully not at 1200 rpm...

Almost everything you're fiddling around with is tricking the engine into running rich. By unplugging the blue temp sensor the ECU will default to a rich fuel map, and my forcing the AFM door open the engine will think more air is coming in, and will increase the fuel accordingly. When you do this and you say the revs "die down", are they low (700rpm) or just stable (not fluctuating any more).

If you don't get any buzzing from the ICV, it suggests it is dead. Can you borrow a replacement from someone?

Lower than 1000rpm - that's by ear. I don't know for certain as I can't see the rev counter whilst I'm propping the flap open. But it is stable - it's not fluctuating any more.

Winding the bypass screw in the AFM in causes the revs to rise but it still fluctuates between around 1700 and 1300 when it's all the way in. The throttle by pass screw is all the way in.

I don't have anyone around that I can borrow an ICV from - guess it's off to the wanted forum again... Or eBay.

When you took your AFM apart to move the slider onto virgin track, did you mark the position of the spring properly? As you have stated...the previous owner may have in any case moved that spring.

Maybe you need to try a new AFM or at least have the O2 level measured.

Some early m20 motors have the so called cold start relay fitted which according to the Wiki switches a resistor into the blue temp sensor circuit to fool the ECU into thinking the coolant temperature is different.

This may only affect cold starting but have you checked if you have this cold start relay fitted?

Hi,

I didn't touch the spring when I moved the slider arm - All the advice I'd read said to leave it well alone, as 'BAD THINGS HAPPEN' when you mess with it.

My engine is an early high compression M20, but it does not have a cold start relay. It's never had a problem starting, kicks into life immediatly when I hit the starter button. It's once it's started that the high idle, and now the fluctuating idle starts.

Well, I've managed to source another ICV - hopefully that will arrive today.

I've also bit the bullet and ordered replacement gaskets and seals for all the intake side mating faces. All the hoses where replaced not so long ago, and they've been leak tested.

I will not be defeated!

Ok - here's the latest.

Both ICVs show no life at all when connected to the loom, with either of the ECUs I have. When connected to a PP3 battery, the valve snaps open and close on both of them.

Triple checking the harness back to the ECU plug still shows continuity on both of the ICV lines.

Reading voltages at the ICV plug shows 12v middle pin to earth, and 12v on middle pin to either outer pin. On both ECUs

I'm beginning to suspect that both ECUs are toast, with regards to the PWM signals to the valve.

Unless there's something else I can check?

More research done, and it would appear that the Transistors that control the idle valve in the ECU are kaput.

From what I've read, it occurs when the ICV overloads or shorts out - blowing the transistors. Which is interesting, as one of the procedures in the "Tuning Motronic" sticky calls for shorting all three pins together in the ICV plug. I guess this let the smoke out of them!

Luckily, it's a part that's easily replaced within the ECU - just a quick soldering job. I've ordered replacement transistors (BDX53A) and I'll replace them in both the ECUs I have.

Here's the source of the info:

http://www.s14.net/forums/showthread.ph ... cs-inside)

Well, there's your problem:





Opened up both ECUs with the hope of changing the idle Transistors, only to find this horror show - same damage, both ECUs. The traces here are completely burnt away, almost to the full thickness of the PCB. And this is nowhere near the idle transistors...

Balls.

Guess i'll have to start cleaning up the Motronic 1.3 system I have and splice that into the kit-car.