Digijet Idle speed too high when warm

[jimrat]

Hi

I tested the Temp 2 sender and the resistance was correct for the water temp. I've been right through the ICV/ICU/ISCU electronics, and it all checks out (see earlier posts). The base idle is 1200 rpm, so I'm not sure if the idle control circuitry is doing anything - I read somewhere that it is additive - i.e. will only intervene if the revs drop below 850rpm, and will not trim a high idle?

Cheers
Jim

[jpennington]

Fast idle must be caused by air getting in somewhere.
It is a typical symptom of a faulty ISCU, which opens the idle stabilization valve, letting extra air in.
As you say, the idle speed control can't reduce a fast idle, but it can produce one.

You say you have checked the ISCU. How? The only sure way I am aware of is to replace it with a known good one - or do as I did and dismantle it and test the power transistors.

If you are certain the control electronics are ok, then all I can suggest is the check for air leaks in the joints between the airbox and the 4 manifold pipes, and between the airbox and the throttle body. Also the airbox itself for any pinholes.

Good luck

jp

[ajsimmo]

I agree jp:
"It is a typical symptom of a faulty ISCU, which opens the idle stabilization valve, letting extra air in."

But I don't think the later bit is right: "check for air leaks in the joints between the airbox and the 4 manifold pipes, and between the airbox and the throttle body. Also the airbox itself for any pinholes."
Air leaks alone should not result in a raised idle speed, but more likely a lean mixture resulting in uneven running and lumpiness. Increased airflow requires a matched increase in fuelling to increase the engine speed. So the idle air bypass valve must be opening and drawing air through the afm, therefore increasing the fuelling, for the idle to be high. The question is, what's causing the bypass valve to over-react?
I'd suspect the ISCU, as you say. I have one here that raises revs on a warm engine to over 2k when plugged in, yet another that does very little (needless to say I don't use either).
My advice is to agree with JP - try a known good one.

Does it idle at 1200 with the ISCU removed?

[jpennington]

ajsimmo

You are right and I am wrong re air leaks.
Still sufferring from post-Christmas flu. Sorry

jp

[jpennington]

Trying to think of a way to isolate the idle stabilization valve (ISV) from the induction system.
I had a look at my van today in search of ideas.
You could detach the pipe to the valve from the air inlet hose and plug both the pipe and the hole into the air inlet. The plugs would need to be airtight, so not an easy test. I have not tried this.

Just a thought - the standard procedure for setting idle speed requires a single connector to be disconnected. This is a yellow connector on a green wire, located close to the coil. The wire runs from the coil to the ISCU in the RH tail light space, and it provides info to the ISCU on engine speed. Disconnection ensures that the ISCU does not interfere with manual idle speed adjustment.
Did you disconnect this lead? If not, may be worth trying. (Although, if the ISCU is faulty, I am not sure what the ISV will do).

One other thought - disconnect the connector to the throttle valve switch. This will make the ISCU think that the throttle is open and so it should not attempt to control the idle speed. ( As above, if the ISCU is faulty ...)

jp

[jimrat]

Before checking the timing I disconnected both connectors from the idle control unit, joining them together, and also the yellow connector/green wire coming off the coil. I thought that the point of doing this was to prevent the ICU/ICV/ISCU gubbins from working, so that the base timing/idle could be set without its influence?

Having said that, when I disconnected the ISCU (behind the OSR light cluster) this evening, as ajsimmo suggested, the revs dropped to around 800 rpm.

Confused.com
Jim

[matt&su]

Hi - l have an idling problem with my 2.1 LLE which my mechanic thinks could be down to a fault in the ISCU. Is there a way of testing the unit to confirm/eliminate a fault before investigating further? I found a weblink to BBA Reman which looked promising, but have called them & they say it's not something they can help with?! Grateful for any advice....
Thanks. Matt

[TimC]

Matt, the LLE 2.1 is an SS-type engine with the full-blown Digifant EFI system. The ISCU can't be disconnected and bypassed in the way a Digijet (DG engine) can. Replacement ISCUs are available from GoWesty, but they're not cheap. Check the T2 sensor (£5 to replace) and the auxiliary air valve (idle stabilisation valve) first. The AAV is also not cheap (£92 from Brickwerks, when in stock) but they do fail. They can be cleaned out but not dismantled.

[R0B]

Thread hijack

[Mr Bean]

I was plagued for yonks with fast tick over on my 2.1 DJ WBX and I had the feeling that the brake servo lost efficiency when coming to a halt.
I eventually stumbled on a weeny micro switch in the throttle valve. It appeared to be working but when measured with a multi meter was just not cutting the mustard. I little tweek (I forget whether I bent the arm or was able to adjust with screw and slot) and the tick over was back down from 1200-1500 to a rock solid 750 RPM.

I just checked and found it in the Wiki - worth checking as it could have saved me some searching
CS