Idle control

[pete]

The revs on the Elise don't always descend as they should, sticking at 2k and then hunting a bit at idle (1k-1.5k).

Does anyone have a spare working IACV I could try?

(Currently running it in but it did this before).

Oh and while I'm here it is running hotter than usual, 91-93 and higher when idling. Any ideas why that might be?

[Shug]

Higher temps could just be the resistance path being different, if they for example took the engine out then cleaned up the earth straps when refitting, or didn't clean them up. Actual temp could be exactly the same.

Have you pulled out the IACV and cleaned it? Good soak in brake cleaner and a stiff brush.

[graeme]

It's one issue... ECU temp sender (the brown one). Reading too high, which screws your idle.

EDIT: except the stack sender is separate (the blue one), so change 'em both.

EDIT 2: No, wait, just the brown one. If the brown one isn't working, the fan won't work. Does the fan come on? If not, the temp could be genuinely higher, so the stack reading could be correct. I'd order both, but change the brown one first.

[robin]

Don't listen to those fools. Your car is an S2 which doesn't have a problem with the earth path; a single temperature sensor is used for both ECU and dash temperature; the other sensor is fitted, but is only used as a blanking plug.

Fast idle will possibly be caused by the bigger valve head confusing the idle control or by the throttle actually sticking a little. The latter less common with alloy throttle body, but still possible.

A vacuum leak is the other possible cause of poorly controlled idling.

Suggest you pull over while revs are high, open the boot and man handle the throttle quadrant. If you can return to normal idle by manually closing the throttle then you have something sticking. If you cannot then you are looking at vacuum leak or inlet pulsing confusing the ECU.

Higher running temp could be down to new thermostat having a different opening temperature, or by reduced flow through the radiator at idle/low revs (possibly a bit of air still in the system, or possibly the coolant is a bit thicker because it actually has some anti-freeze in it now!).

Cheers,
Robin

P.S. If you are running in anyway you are welcome to bring it over to me - I can look at the idling stuff while you put some miles on the car

[graeme]

If the S2 has a single temp sender for both stack and ECU (didn't know that), that's definitely it. After you check all Robin's stuff of course.

[pete]

<exits to garage to try things>

[robin]

If the S2 has a single temp sender for both stack and ECU (didn't know that), that's definitely it. After you check all Robin's stuff of course.

Why is that definitely it?

Although you do get a problem with the wire to the temperature sensor being a bit short and thus working a bit loose. This can cause the temperature to UNDER read and thus make the cold-running rev limiter cut in (cannot rev above 3,000 RPM).

Cheers,
Robin

[Dominic]

I had a similar problem a few years ago. Now have a spare temperature sensor and IACV. Mine turned out to be a leak in the inlet manifold gasket, diagnosed by spraying some carb cleaner on the gasket while the engine was running. I have said cleaner if you want some. It may also clean up your throttle body if that's what is required.

[BigD]

I had a similar problem a few years ago. Now have a spare temperature sensor and IACV. Mine turned out to be a leak in the inlet manifold gasket, diagnosed by spraying some carb cleaner on the gasket while the engine was running. I have said cleaner if you want some. It may also clean up your throttle body if that's what is required.

Good point dom and that's how I have diagnosed a throttle body air leak too. Park up when it's revving high and spray around to see if it changes the revs. It momentarily seals the the air leak and cures the idle. Only momentarily though.

[Stephen]

I had a similar problem with the S160. I adjusted the throttle stop to open the throttle a little at idle - no improvement. I then screwed it all the way back and this sorted it. I assume the throttle stop was keeping the throttle open a little and preventing the ECU from managing the idle. I have an emerald.

[robin]

Adjusting the throttle stop should not be required. You can reset the TPS range and idle threshold on the Emerald if it turns out that it was wrong; unwinding the throttle stop risks binding the throttle plate in the throttle body.

Cheers,
Robin

[Shug]

Bloody S2s....

[Stephen]

Robin I reset the TPS etc as you suggest. I think there may have been too much air getting in when the TPS was telling the ECU that the butterfly was closed. A mechanical rather than electrical issue.

[graeme]

If the S2 has a single temp sender for both stack and ECU (didn't know that), that's definitely it. After you check all Robin's stuff of course.

Why is that definitely it?

OK, definitely was a silly choice of word, but it's still a single cause of both symptoms.

Fraser's Razor: The simplest solution is definitely correct, unless Robin disagrees, in which case whatever Robin says is probably correct, even if it's more complicated.

[alicrozier]

No sign of Pete, he must have got sucked into the TB...

It might be the IACV as originally suggested (bear with me Robin).
On my old S2 111S the IACV was sticking closed. This first exhibited as lumpy drivability when pootling along at light/no throttle (amazing how much the IACV smooths things out). Then it began to stick at higher revs, anything from 2 to 4.5K!

What was happening was as the IACV stuck closed, it pulled a vacuum as the throttle was closing and jammed it slightly open.

Squirted IACV with brake cleaner and cycled repeatedly...