Intermittent high / fast idle?

Well to do that it needs to know what is flowing where a simple single point voltage measurement isn't really going to do that..

No, agreed on that;!...

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How do you think that the state of charge of a battery can be determined by the current flowing?

To use the infamous "tank of water" analogy, you couldn't determine how much water was in the tank by measuring the flow rate out of it. (Unless you knew the original volume, and calculated it, which is something else altogether of course.)

Chris

Assuming you know the external load, you can determine the battery state by the current it draws while charging. Because the correct charger for lead acid is constant voltage which varies the current as the battery voltage changes while it charges.

You can. For the purposes under discussion (raising idle speed to maintain alternator output when the battery state is low), why would you?

Chris

Why would you want to raise the idle speed if the battery was low? Better to fit a correctly specced alternator. It's not like as though modern cars have a slow idle - quite the reverse.

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*I* wouldn't want to; if you'd read up-thread, you would see that's what BMW do with the Mini...

Chris

In fact this isn't that simple to do that accurately is it?. You'd need to know where a lot of currents are going to make it anywhere near sensible i.e. what's coming off the alternator what's going to or coming from the battery what's going into the cars electrical system..

It would seem that all you need to do in effect is to voltage and current limit the alternator output. That is perhaps all thats really required.

Unless you might need to know what the alternator is "pulling" off the crankshaft in order to regulate the engine load, but then again if your doing that you'd need to know what the water pump is doing 'tho that seems like a constant load .. the steering pump and brake servos etc hardly seems worth the bother..

The load pulled by a aircon pump is a significant one so some means of measuring what thats taking is perhaps in order, how you'd do that is interesting perhaps if the Aircon is ON then you'd add in "x" amount of fuel mix extra for an assumed constant load..

I'd guess this is more to do with the engine start/stop system. Rather than a general benefit.

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Thinking about it, if you have to raise the idle when running in order to keep the battery charged for all the restarts, you're not going to be saving a great deal of fuel.

Chris

Indeed.

Only thing to report as an update, is the problem still occurs. But whilst it's happening (ie engine fully warm, revs racing at, say, 2k, when it should be idling), disconnecting the IACV (electrically, by means of the multi-plug) and reconnecting it cures the problem for, at a rough guess, a couple of days. I've since done this procedure about 2 or 3 times, recently, and it does defnitely cure the issue for a while, at least.

I've not had any further opportuinity to monitor what's happening when it does occur (I bought a cheapo ELM 327 USB dongle for that purpose, but haven't really had opportunity to use it, when I've encountered the problem - either because of time, or location). I suspect I'd probably see what I initially saw / logged - absolute throttle position at idle around

9%, after doing the Ali-shuffle with the IACV plug, probably staying at 0% at idle.

Reading the thread about a Fiesta having an idle problem, here, makes me wonder about ECU adaptation and the TPS. I enquired at my local dealers, and a TPS seems very reasonably priced, so I've ordered one, and hope to fit it in the next day or two.

If it is a potentiometer type TPS, they are quite easily checked. Best with a scope to see any noise on the track while it operates, but a DVM can show up problems. They normally wear most at the closed end.

Other way is to wire it up as a volume control between say a CD player and amp. Any problems with the track will be easily heard as it is moved.

Only thing to report is that I changed the TPS (it cost something like £23-24 for the OEM part from a dealers, I think even the aftermarket costs, or costs of the OEM part from the far east were more expensive).

Changing it was reasonably straighforward - although I would say that access to the retaining bolts are awkward for both this and the IACV (removed to make it easier to remove the TPS) - both rearward facing, and between the rear of the engine and the bulkhead at the back of the engine bay.

One thing I noticed from the part I took off (identical to the new one, so I suspect it's never been changed previously), is that the default position for the inside of the TPS (ie where it comes to rest) once removed, seemed a fair bit different from the brand new one. Whether it was prone to sticking, or there was any roughness in the track causing it to not fully return to the same position as the brand new one, I'm not sure.

With everything connected up, the car started fine and idled completely as you'd expect (although not really proving anything). I let the car fully warm up, but didn't have chance to drive it (on account of needing to stay at the house when I had chance to do the job). I've not had chance to drive it, yet, but when I've managed a few days driving in it, I should have a good feeling as to whether it's made any bearing on the problem.

Hopefully, final feedback on this.

My previous reply, where I'd fitted a new (OEM) TPS, seems to have done the trick. I've not driven that much in the weeks, since, but have done a fair few local journeys, and 3 or 4 commuting round trips (60-ish miles each way) and had no reoccurence.

Before changing the TPS, I would have definitely had an incident of it. So whether it was a combination of heat / warm engine and a TPS playing up at closed throttle position, or a warm engine / engine-bay causing the TPS to give odd readings at closed throttle, I'm not sure, but so far (fingers crossed) a new one has done the trick.

I've not yet had chance to plug my laptop and ELM dongle in, yet, but I'll be interested to see if there's a notable difference to when I'd logged metrics when it was playing up.

I'm sure I'm not alone in hoping I don't have to post any further replies about the problem returning!

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Glad it seems sorted; it's actually these tricky-to-fix problems that usenet is most useful for.

It certainly not been the worst thread on u.r.c.m by a long way...

Chris