Diagnosis?

Idle Air Control Valve. Remove - Clean with carb cleaner and re-fit. Should solve it. Might be worth cleaning throttle body as well while you've got the carb cleaner out.

Cheers Matt

Cheers Matt, I'll give that a go.

Failing that, the stepper motor is shafted, and you'll have to replace it.

Common fault with all modern cars !

Cheers Nom, I'll bear it in mind.

I think there's a selonoid valve kinda thing, what it does is increase your engine's rpm when the air cond is working. Maybe something wrong with that valve...

just my thoughts.

Yep - that's exactly what me and **_** were referring to. It's usually a stepper motor rather than a solenoid though.

It works by simply introducing an airleak to the throttle body, and the engine uses this air to idle. The ECU decides how much air to let in - it'll be more when the engine is cold, hence you get a higher idle.

The fuel comes from the injector (shock horror!). The injector puts the right amount of fuel in (AFAIK it's a solenoid valve, just opens longer for more fuel).

According to my _very_ basic understanding of EFI systems...

There is a sensor in the exhaust, known as the lambda or oxygen sensor, which produces an output voltage or not depending on whether there is oxygen present in the exhaust gases. If there is oxygen, then there wasn't enough fuel to use it up and the engine is running lean. If there is no oxygen then there was an excess of fuel, and the engine is running rich. The engine management system constantly increases and decreases the amount of fuel injected into the engine to keep the oxygen sensor reading switching on and off. If you have an air leak then the amount of fuel injected should be automatically increased to compensate.

AFAIK this only applies to EFI systems, and only to closed-loop ones at that.

Alot of ECU's run in open loop until they have reached their operating temp, they run purely off their map untill normal conditions are obtained. This is probably not the case in all systems though.

When you start a cold car it will be running on corrected fuel map anyway, and with this the solenoid valve (or stepper, but ive not seen one of these) will be open.

Ive just had a look at the system your EFi uses and there are from what I can see three valves that could cause a problem.

There are no stepper motors on your system (that is if its a Honda PGM-Fi)

You have an Idle speed control Valve (ISCV)- ECU controlled on or off solenoid. This is the kind of thing that turns on when the is a load such as headlights placed on the engine.

Auxiliary air valve (AAV) This is a Waxstat-actuated valve used to increase the idle speed during warm up. This is ECU independent, and is mounted onto the throttle body. It should be open when below 30deg and closed fully at normal op temp.

You may also have a Fast Idle valve (not always) This is operational when the coolent temp is below 10deg and idle is below 1800. ITs ecu controlled and allows aditional air to by pass the throttle. The ISCV fine-tunes the operation of this valve (if fitted).

Ed

When an engine starts often for the fist few hundred injector pulses they are extra rich to compensate for fuel that condensates to the side of the maniifold and also due to the idle control valve (most Ive seen ad just solenoids) being open.

Thanks for that Ed. It is a PGM-Fi. I've made a note of those, and I'll be checking them next time I'm home during daylight. Out of interest, where did you find this information? I'm still climbing a fairly steep learning curve on this stuff, so directions to any useful references would be appreciated.

Cheers, Andy

Yep.

Except that when your engine is cold, it doesn't bother - it simply uses a fixed map to idle. The lambda system only comes into effect when everything has warmed up, and even then, it often reverts back to fixed map at wide-open-throttle. Bear in mind that not all systems are the same, so the above doesn't apply to *every* engine :)

Thank the lord, someone who knows what they're talking about!!!! :)

Tim..

Where the hell did you just find all that ?!?!?!

Most-useful-post-everT :)

I suddenly realised I have a Haynes manual on Engine managment (recommended!). Most of that info came from there with my own little spin on it, hopefully to make it clearer. ECU's are great it's where electronics meet engines. Both of which I find very interesting :)

Ed

Get the Haynes Engine managment Manual. It covers most major engine managements, and the theory can be applied to others as well if not listed. As I just mentioned in another thread - I find all this very interesting. I'm following a DIY ecu project, and my Micra may eventually be running on one of these DIY ecus :) Having total control over everything one did would be great!

Ed

Right, got something to put on my christmass pressie list now

What micro are you using in your DIY ECU? do you need anything particularly fast to manage timings, or do you use external counters for that sort of stuff? Also how do you format the fuelling map and stuff?

A couple of weeks ago I thought ECU were the devil's work, to make extra money for main dealers. Now I reckon they're about the best thing since sliced cheese.

Anyway, cheers very much for your help Ed, and good look with the granny- mobile ;-)

Andy