Idlespeed motor action when ignition switched on / depressing the gas during start

[Horsebox]

Hi all,

I'm trying to work out why the car manual says to open the throttle slightly when we start the engine. Obviously it helps air into the engine. I've been through all the manuals but can't find a reference as to what exactly the idle speed motor does when the ignition is turned on. I've started my car OK without any throttle, so I guess the idle motor must be open enough to let the air through.

Any modern car doesn't need the gas pedal pressed to start it, and on the few times I've tried with the D it's started fine, so why does the owners manual recommend we do this? I know the D isn't exactly modern, but systems like the idlespeed motor seem to be in all cars now.

Why do we blip the throttle, and what does the idlespeed do exactly when we turn to ignition II?

Cheers!

[David T]

Hi all,

I'm trying to work out why the car manual says to open the throttle slightly when we start the engine. Obviously it helps air into the engine. I've been through all the manuals but can't find a reference as to what exactly the idle speed motor does when the ignition is turned on. I've started my car OK without any throttle, so I guess the idle motor must be open enough to let the air through.

Any modern car doesn't need the gas pedal pressed to start it, and on the few times I've tried with the D it's started fine, so why does the owners manual recommend we do this? I know the D isn't exactly modern, but systems like the idlespeed motor seem to be in all cars now.

Why do we blip the throttle, and what does the idlespeed do exactly when we turn to ignition II?

Cheers!

The manual does say to hold the throttle down when starting the motor. There is no accelerator pump or choke. The manual probably says to hold the throttle down because it starts very rich when cold because the cold start valve is shooting raw fuel in so keeping the throttle plates open allows more air in to dilute the mixture. Most Deloreans (if they are set up and running properly) will start very easily without touching the gas pedal at all. By holding the throttle open a little you are also shutting off the idle motor so it isn't active. The Delorean has a mechanical fuel injection system, it cannot be compared to a modern fuel injected car or an older carbeurated car.

[Bitsyncmaster]

It jumps to an open (about half open) position depending on the temperature sensed by the idle speed thermistor.

If you don't start cranking the engine, I think it slowly opens to full open in about 20 seconds.

[DMCMW Dave]

Normal operation, and I even use this as an ECU test, is to watch the valve and have an assistant turn the key on ("run" position) without starting the car.

The valve will swing immediately wide open (due to zero engine RPM) and then within about 20-30 seconds close almost all the way. No idea why but this is what they all do and also explains the "flare" you often get when starting the car.

[Bitsyncmaster]

Normal operation, and I even use this as an ECU test, is to watch the valve and have an assistant turn the key on ("run" position) without starting the car.

The valve will swing immediately wide open (due to zero engine RPM) and then within about 20-30 seconds close almost all the way. No idea why but this is what they all do and also explains the "flare" you often get when starting the car.

That is what I remember it doing with my car but just last week, helping another owner, I think it opened without starting the car. That owners car does do a fast idle (warming up) for a long time which I've not seen before.

That would explain why the owner manual wants you to step on the gas to start the car (if you waited till it closed). My ECU will not close or open the valve until the engine is running.

[krs09]

When turning the key the "door" should swing open. Mine seemed to be very sticky when turning the key. After taking it off and spraying the valve with carb cleaner and giving it a good shake the door opens MUCH smoother (worth a shot). Ive never had to press the gas to get it started, it starts, runs and idles great without touching the pedal ... I guess thats ok. Should I have to press the pedal to get it started? Is it ok that I don't?

[Horsebox]

Brilliant stuff, thanks guys.

I'm just reading a very interesting book on Bosch injection. In the "testing procedures" section it talks about measuring the duty cycle or dwell of the idle motor, in the same way that we can do for the frequency valve. The idea being that we see what percentage the motor is open at any time, without having to eye-ball it.

I wonder if we can do this with our cars? We can get easy access to the electrical signals going to all three terminals of the idle motor via the while bulkhead connector.

Anyone tried this? What terminals would you put the meter across, the left and right terminals?

[Horsebox]

Just did a basic test for measuring the duty cycle of the idle motor. Seems to work. I didn't go too far as it's pouring with rain, dark and cold, and I was getting carbon monoxide poisoning in the garage.

Anyway, with the dwell (or duty cycle for %) hooked up: Ground at the engine block and positive lead to to terminal 8 of the large white bulkhead connector in the engine bay which connects to the RHS of the idle motor...

Ignition to position II but engine not running: Duty cycle 53% (supporting the idea that the motor half opens in this state)
Engine idling with coolant temp +6C (cold engine): Duty cycle 36% (suggesting the motor closing somewhat to hold the target idle speed)

Might be a useful way of seeing what the idle motor is up to, but need to do more tests. We could prove the test method by seeing that as engine friction decreased as it warmed up, duty cycle reduces as the motor closes. Could also turn on lots of electrics or A/C (if only mine worked!) and expect to see an increase in duty cycle as the motor opens up again to meet the engine load. Could have someone put into Drive (if auto) and hold on the brakes etc etc.

[Bitsyncmaster]

Just did a basic test for measuring the duty cycle of the idle motor. Seems to work. I didn't go too far as it's pouring with rain, dark and cold, and I was getting carbon monoxide poisoning in the garage.

Anyway, with the dwell (or duty cycle for %) hooked up: Ground at the engine block and positive lead to to terminal 8 of the large white bulkhead connector in the engine bay which connects to the RHS of the idle motor...

Ignition to position II but engine not running: Duty cycle 53% (supporting the idea that the motor half opens in this state)
Engine idling with coolant temp +6C (cold engine): Duty cycle 36% (suggesting the motor closing somewhat to hold the target idle speed)

Might be a useful way of seeing what the idle motor is up to, but need to do more tests. We could prove the test method by seeing that as engine friction decreased as it warmed up, duty cycle reduces as the motor closes. Could also turn on lots of electrics or A/C (if only mine worked!) and expect to see an increase in duty cycle as the motor opens up again to meet the engine load. Could have someone put into Drive (if auto) and hold on the brakes etc etc.

Sounds like your getting good readings. I do remember with the stock configuration, the idle motor never will close fully. The stock ECU limits that minimum duty cycle.

[Horsebox]

Sounds like your getting good readings. I do remember with the stock configuration, the idle motor never will close fully. The stock ECU limits that minimum duty cycle.

I'll test it more fully outside, tomorrow, when the weather is better, but it does look like you are correct in many respects re idlespeed namely:

1) idle valve opens to half upon switching to ignition II
2) idle hunting caused by idle motor not having enough torque when nearly closed. (my idle motor was at 36% with a cold-cold engine running just long enough for me to read the duty cycle before having to shut it off or die by poisoning!) No doubt an engine at operating temp will need a lot less air. I'll check it tomorrow.

Dave, what I need is some kind of data logger, so I can record duty cycle, voltage etc in real-time and then graph it so I can see what is going on, any suggestions on that?

I couldn't see any evidence that with the ignition set to position II (but not started), the motor would start to close at some point. I left it in ignition II for a minute or so and got a constant 53% duty. This doesn't quite tie in with the owner manual recommendation to open the throttle a bit when starting, to prevent, as you suggested, trying to start the engine with both a closed throttle and closed idle motor.

Mark

[Bitsyncmaster]

Dave, what I need is some kind of data logger, so I can record duty cycle, voltage etc in real-time and then graph it so I can see what is going on, any suggestions on that?

I couldn't see any evidence that with the ignition set to position II (but not started), the motor would start to close at some point. I left it in ignition II for a minute or so and got a constant 53% duty. This doesn't quite tie in with the owner manual recommendation to open the throttle a bit when starting, to prevent, as you suggested, trying to start the engine with both a closed throttle and closed idle motor.

Mark

I did do some data logging with my idle ECU but it does not have the memory to do much but record a few things. I think what your looking for is a meter with a data port you can feed to a computer to record and plot it. None of my DVMs have that capability.

I've seen the stock idle ECU move the valve to closed (almost) with my car back when I was having idle problems. It took it about 20 seconds to move it from that start position. Just last week I was helping another owner with his idle problem and his idle motor would open fully in that 20 second time. I don't have any stock ECUs to check again (I've updated all the cores with my ECU). I do know that 81% is fully closed on my ECU. Now you would read 19% if your reading the other driver. By any chance do you have my ECU? Mine will not move the valve until you start the engine. It does open when the key is turned to on (the amount depends on engine temp).

[Horsebox]

Dave,

I have your RPM relay but the idle ECU is stock. (still haven't got used to your fuel pump relay priming when the ignition is turned off - works a treat though!)

Of course there's no guarantee my idle ECU is working correctly but it is strange that you saw two different idle motors move in opposite directions. Bizarre.

I have a data logger designed for radio controlled planes, that might have worked, but it has packed up so I can't use that. I've researched it a bit and there seem to be a few products in the pipework in the US, currently awaiting FCC (? - I'm in the UK) approval that should allow remote monitoring of electrical things via wifi or bluetooth, the "internet of things" so that should help us when they appear.

On another note, I may have got warm to the idlespeed thermister problem, that thing that is meant to increase the idlespeed at temps less than +15C, but nobody has ever seen actually working. I was testing a bulkhead connector cleaner, and unwittingly focussed on the large white bulkhead connector whilst testing it out (the connector that houses all the idlespeed signals) - so it's now a very clean connector... Didn't think anything more of it but ironically I was cursing the idlespeed for not holding 775 when I started it up from cold after the cleaning frenzy. It sat a bit high, not much though, maybe 900rpm or so.

Mark

[Bitsyncmaster]

If the idle speed thermistor opens, your idle jumps to 2000 RPM or higher with the stock ECU.

I have been able to shut the engine off and pull the idle motor to see how far open or closed it was.

If your monitoring the duty cycle you can adjust the curb idle screw and see that duty cycle follow your adjustments.

A very common idle problem is caused by the engine getting vacuum advance at idle. The connector falling off the vacuum solenoid will cause that. A faulty idle switch also give the advance.

Of course vacuum leaks hurt the idle but that usually results in high RPM with a warm engine.

My problem was the deceleration valve springs were bad. That is also a common problem.

[Jonathan]

Normal operation, and I even use this as an ECU test, is to watch the valve and have an assistant turn the key on ("run" position) without starting the car.

The valve will swing immediately wide open (due to zero engine RPM) and then within about 20-30 seconds close almost all the way. No idea why but this is what they all do and also explains the "flare" you often get when starting the car.

Aaaaaah-ha. Nice. Anyone with some DeLorean experience can relate to their car starting with that initial "flare" like you said. Just never knew why the car did that. Learn a new thing every day. Cool. Thanks!