230F idle speed
 

...almost Happy New Year fellow 240ers! Two conundrums I face which I am having a problem resolving:

My 240 (2.3 Bosch LH2.4) generally idles around 1000 RPM up to 1500 RPM. Occasionally I can get it to idle at about 750 where it should be and I do this by flicking the ignition off and back on again. In very cold weather it seems more able to idle at the right speed and in warm weather less able to. Something is clearly not working correctly

There are no fault codes on the reader, the economy is reasonable (but clearly not going to be where it should be). It passed it's MOT last year by a country mile and it drives fine

It's had a new exhaust, Oxygen sensor, Idle Control Valve and the throttle has been correctly set up a few months ago

So why won't it idle at the right speed? I am wondering whether cleaning up the ECU electrical contacts might help, whether a new temperature sender might help and/or whether the MAF is at fault?

Anyone shed any light on this matter? I have also posted another (possibly related) conundrum... Thanks all! Nick

I've always noticed that the Idle Air Valve is very sensitive to accumulated deposits - that fine black slightly oily sooty stuff that gradually accumulates in the throttle body and on the butterfly flap.
Have you tried taking off the convoluted air duct, holding the throttle open with the wheel, and wiping as far as you can reach inside the TB with a rag wound round a stick, soaked in carb cleaner?
Then remove the IAV and squirt in some cleaner. Clean the sliding valve with a soft brush, using a stick through the exit port to rotate the actuator. Check that it snaps closed. In fact it should close just a fraction open.

I've found this sooting up happens quite quickly - I make it a sevice item about once every alternate oil change. A symptom that it needs doing is if the idle sticks too high as you describe, and can be corrected with a blip on the throttle.

Have you checked all of your vacuum hoses and intake bellows for leaks/cracks? If you have air ingress anywhere past the MAF these engines can display weird symptoms and get a little squirrelly to diagnose. I went through all this with my 744, found a split in the intake bellows which solved the majority of the idle issues I had. The throttle had been poorly set up by someone prior to my ownership too, so the TPS never engaged/disengaged...adjusting that up correctly remedied the rest of the issues.

Thanks both

Oops la! MOT due 04/01 and got overlooked. t'will be done on the 11th and at the same time, we will have another go with the idle speed and flat spot

My money is now an air leak issue. So before the 11th, I am going to have a visual check of all hoses and tweak-up all the jubilee clips

The only electrical parts of the fuel system I have not replaced are temperature sender, MAF, cold start valve, all of which could be contributors but I think not at this point - my ignition flick trick no longer works, hence my hunch is that this is air leak rather than electrical...

While you're on the hunt for vacuum leaks, make sure there isn't fuel in the vacuum hose going between the intake manifold and the fuel pressure regulator (FPR). The FPR has a diaphragm in it, which when leaking will lead to fuel entering the intake manifold. This can cause a high idle, increased fuel consumption and driveability issues related to having a rich mixture. The FPR is very easily replaced, and I believe costs less than 100 Euros.

I think you can crudely test the pressure regulator by disconnecting the pipe from the injector rail and sniffing it. Petrol smell = petrol leaking through the diaphram.
If you suck on the pipe you can feel the diapham operating. Put your tongue over it and see if it holds the vacuum.

Hmmmmm

So on the way to have the car MOTd, the idle speed was 750 until the engine was truly warm and the cooling fan finally came on... Small throttle openings (driving in heavy traffic) seem to be a big provoker of increased idle speed?

Prior to driving to the garage, I had tightened-up the 4 jubilee clips relating to the Idle Control Valve and the air pipe between the MAF and the throttle body - none were slack but I was able to nudge them up a little

It's snowing now so I haven't tried the two tricks that you have all suggested, but I will look into these

Meantime, it has been suggested that the injection system will increase fuel to the engine in response to load on the alternator and so when the cooling fan kicks in, the injection system responds accordingly. The only problem is, that when the cooling fan switches off, the revs don't drop. In my mind there is a connection with engine temperature and the idle speed - that would point to a fault with the temperature sender and/or how the ECU is interpreting info coming to it...

My only reservation about the FPR possibility is that the car is fine to drive (consistent at least) and the fuel consumption is in reasonable bounds

I'll get there in the end!

Help me out here folks! what's left to change?

The book says 750 rpm

I have gone through several idle control valves with different results, the engine temp sender and oxygen sender have been replaced. The car has a recent air filter. Recently I put in a s/h MAF

(carrying on...)

and initially the results were promising - more consistent 750 RPM idling - until today.

Today it was all over the place as it has been before. Not as high as 1500, but certainly 1000/1100. It's quite irritating as it makes the car inconsistent to drive...

I believe you had a phone call with AlexO about this where he explained that in his (and my) experience it is quite normal for these cars to idle a bit quickly when warm, under electrical load, when the battery voltage is a bit off and in many other circumstances.

There is no 'easy fix'. Cars are really quite simple - air and fuel go in, these get ignited in the cylinder and power comes out. If it is idling a bit quick you have extra power. This can come from incorrect fuelling, additional air or incorrect ignition timing. Check all of these things, work out which it is on your particular vehicle and then fix it.

The control system for idle is very much open-loop and small changes to seemingly insignificant things can make a large difference. For example a bad earth which changes resistance at high/low temperatures will affect the voltage seen at the IAC valve, this valve is VERY voltage sensitive in its operation. It could be something as simple as a dodgy electrical connection.

Have you flow tested the injectors? Have you done this at various temperatures? Voltages? What is the flow variance with voltage? What is the voltage at the IAC valve when the idle is fast? What is it when it is slow? What are your MAF sensor values when fast/slow?..... As you can see the possible questions are almost endless.

As I say, you need to start at the beginning and diagnose the issue properly before any of us can tell you what to change. Or find a good garage that will take the time to find the trouble and fix it. Or a useful techy friend who will drink your beer and work on your car for you. It is almost impossible to diagnose remotely.