This morning in traffic I was crawling and braked to come to rest behind a Mondeo, but my 318ti strangely mis-fired and there was what seemed like a power surge.

The car came to rest some 3-4 feet later than planned, right into the back of the Mondeo.

What could have caused this ?

Are you looking for an excuse for crashing into the back of someone?

A surge in power is normally caused by extra fuel. Sometimes it can jerk when crawling in traffic as the engine revs drop or increase. Or the clutch isn't pressed quick enough and the car wants to carry on even if you are braking.

Not looking for an excuse, I'd own up, but this time it wasn't me.

Not happy about having to buy some Ford parts for someone though !

I have been reading, could it be the idle control valve, is there one on this engine, could it malfunction like this ?

Any other suggestions ?

Btw it is an auto.

I felt a similar symptom earlier, sat at the lights on the foot brake, for one or two revs it seemed to pick up as if I had blipped the gas, but my foot was on the brake, odd !

One other thought - if your car has a viscous fan, as opposed to an electric one, and the engine was hot, it may have upped the revs at idle to speed the fan up, therefore pulling more air through the radiator. This was explained to me by my mechanic when I couldn't work out why the revs would go up suddenly in traffic to around 1200rpm for a while.

Being an auto, the reaction of course is to accelerate. Just a thought though....

Out of interest is it a cable operated throttle or an electronic one?

I only ask cos me old 5 series throttle cable frayed in side and stuck the throttle full on..quite frightning on a 528 auto in town.

pualo wrote: One other thought - if your car has a viscous fan, as opposed to an electric one, and the engine was hot, it may have upped the revs at idle to speed the fan up, therefore pulling more air through the radiator. This was explained to me by my mechanic when I couldn't work out why the revs would go up suddenly in traffic to around 1200rpm for a while. Being an auto, the reaction of course is to accelerate. Just a thought though....

This is very odd as if your revs are up then the engine would generate even more heat to be dissipated by the rad, so it would be pointless - you could say it would be a 'viscous' circle!!

Can't see this being true.

Do all BMWs with a viscous fan have a 2nd fan or is it just ones with air-con? As I've had mine on a fast idle and it must have got hotter even tho the temp guage was still in the middle, coz the 2nd electric fan came on - the air-con wasn't on.

Nothing to do with your idle problem...sorry, but I was intrigued with jonp's 528 stuck throttle story! Do tell more, how did you slay the beast??? Oh and are you the same as jonnie p or different person?

Cheers

-idle speed is controlled by the valve as others have described. It works by letting more air (and more fuel) into the engine when the idle speed drops below the specified speed. The slower the engine goes, and the longer  it is below the specified speed, the more the idle valve opens. The speed of this response is tailored in a way that allows for an engine that will speed up again fairly rapidly (i.e. one that isn't, say, driving the car along at the time). If the car is crawling in traffic and the engine speed drops in the right way I can see how it might surge in exactly the way described.

The other thing that is top of the hit list are the sensors that monitor engine speed. If these go faulty (i.e. report the engine is not actually rotating any more) then the IAC valve may open itself in an attempt to rectify the situation and this may also cause a surge.

Its rough luck how it happened tho'

thanks for the responses guys, I have enough to look at for a while

Cheers,

Mark

Blackcat... wel to be honest it was really scarey!

If i used any more than 1/4 throttle it would stick, so kick-down wasn't used

A new one only cost £20 from BMW! It did take 2 weeks to arrive....

dave 328 wrote: Do all BMWs with a viscous fan have a 2nd fan or is it just ones with air-con? As I've had mine on a fast idle and it must have got hotter even tho the temp guage was still in the middle, coz the 2nd electric fan came on - the air-con wasn't on.

Mine has a vicous fan and no aircon and I don't have a second fan.

dave 328 wrote: pualo wrote: One other thought - if your car has a viscous fan, as opposed to an electric one, and the engine was hot, it may have upped the revs at idle to speed the fan up, therefore pulling more air through the radiator. This was explained to me by my mechanic when I couldn't work out why the revs would go up suddenly in traffic to around 1200rpm for a while. Being an auto, the reaction of course is to accelerate. Just a thought though.... This is very odd as if your revs are up then the engine would generate even more heat to be dissipated by the rad, so it would be pointless - you could say it would be a 'viscous' circle!! Can't see this being true. Do all BMWs with a viscous fan have a 2nd fan or is it just ones with air-con? As I've had mine on a fast idle and it must have got hotter even tho the temp guage was still in the middle, coz the 2nd electric fan came on - the air-con wasn't on.

It is true - I do see where you are coming from, but if you are sat in traffic and you need to get the fan moving to draw more air through the rad, driving along is not an option, and as the fan cannot power itself like an electric one, then the only way to increase it is to raise the speed of the engine, but it is only about another 400 rpm over normal.

It is only the fan on the air-con rad that is electric.

Hi Paulo,

I think your mechanic has misinformed you somewhat, but he is close :)

The reason that revs can surge with fan is as follows. When a hot engine is at idle and the fan comes in, this puts extra load on the engine and slows it. The ECU sees the engine speed slow and switches the idle valve to compensate. All being correct, the idle speed should never rise to 1200, if it does then there is a fault.

All of the above happens quite quickly when there is no fault. If you have ever seen an engine with a surge problem, you will understand this better. If the IAC valve is not flowing enough air, the idle will be too low. The ecu will switch it higher to correct this. In the case of a faulty IACV, it will often switch too high. As the revs go too high the ECU sees the high idle and shuts the IACV, but the faulty valve allows the revs to drop too much and the whole cycle starts over again, hence the surge. 

The ECU will never increase idle speed to spin the fan. When extra cooling is required the fan clutch is supposed to "tighten" the drive and bring the fan speed closer to engine speed.

HTH

Alan.

dave 328 wrote: I would have thought yours had air-con even in basic form and not digital climate, as its an SE.

Aircon was only an option on the 328, even the SE. Mind you mine is the pre facelift model, It might have become standard post facelift. It doesn't bother me too much though, aircon takes power and uses fuel, both of which are not good, for me anyway. I've driven cars with aircon before and can't quite see what all the fuss is about, after all it's not like the weather is that hot in this country.