850R unstable fast idle, revs high drops as IAC unplugged

Tricky850R · Post by **Tricky850R** » 12 Oct 2010, 09:36

Help please:

My 850R (Auto) failed the MOT as the tester was unable to maintain a "fast idle".

When the engine is running it idles at around 1100 rpm, but it is impossible to get it maintain steady revs with the accelerator pressed. It gradually rises to where you need it then carries on revving up until around 3500. If the IAC is unplugged the revs drop to around 800rpm and it is possible to increase them gradually to where you need them.

Things I have tried:

1. Cleaned IAC
2. Replaced IAC
3. Cleaned Throttle body
4. Replaced TB and TPS
5. Tested voltage to IAC plug - Ignition on but not running - 12v between 1&2 and 3&2. Engine running - 14v, no change when throttle moved.

What should I do next and how?

tobydell · Post by **tobydell** » 12 Oct 2010, 10:00

I have a similar problem with my na v70 2.5 tick over up to 12-1400 replaced iac, tps and I found a perished vacuum elboe which I found after changing the iac tps, so thought, great thats it, but it wasnt. I connected the spare iac to the plug with engine running and saw it work as throttle opened, but at tickover it was being opened by the signal from the ecu, in other words, not iac, not tps, no vacuum leaks (that I know of) what else affects the iac, which other sensors ,
it seems to me that if it is ok with your iac unplugged, like me something else is affecting the iac, but what???
I have just disconnected the battery, pulled the control modules put it all back together, pulled the plug on the maf sensor
and started it up, seems fine, tickover about normal, reconnected maf, still seems about ok, thats all for today though
I will keep you posted with developments and would appreciate the same if you get a cure/explanation
Roger

Tricky850R · Post by **Tricky850R** » 12 Oct 2010, 10:19

Thanks Roger - at least I am not alone!!

I also tried disconnecting the MAF sensor, and have disconnected the battery, shorted the positive and negative feeds to the car together for a few minutes (I read this somewhere else and I presume this resets the ECU - thought it worth a try).

I won't be able to work on the car after this evening for a few days, but will certainly keep you posted on any developments.
Simon

tobydell · Post by **tobydell** » 14 Oct 2010, 10:01

thinking about your problem again, have you checked all the vacuum connections as an air leak would certainly give you a rising rpm.
mine is dual fuel and it does the same on petrol as it does on gas, so fuel pressure is not an issue for me, nor is an injector problem
however, mine has now decided to tick over slowly, around 500 revs, I dont know what I did, but today I tried the coolant temp sensor, working on the basis that now, if the ecu thinks it is warm and not cold, then start up is going to be a problem
I am getting to the point where I am ready to give up and dump it in a pond or canal

Tricky850R · Post by **Tricky850R** » 17 Oct 2010, 11:36

Unplugging the IAC got it through the MOT. This give me some time to think about it more. With IAC unplugged it ticks over at around 500rpm until the engine warms up and then it idles at around 1300.

Will running with the IAC unplugged cause any long term damage?

tobydell · Post by **tobydell** » 18 Oct 2010, 02:10

I am back to a faster tick over again, around 1000-1100 not the 850-900 it used to be, I am also thinking that it might be the lambda sensor as unplugging it makes no difference, going in for a checkup tomorrow (Tues) where all the sensor information will be monitored, so might get a definitive answer then
I'll let you know how it goes

tobydell · Post by **tobydell** » 19 Oct 2010, 09:03

just been to the lpg garage for a lpg service, tick over still just over 1000 does get higher after driving being on gas or petrol makes no difference (the gas system takes all its signals from the petrol side anyway, disconnect icv and revs drop to just below normal so we are still working on a logical process of elimination however, the icv is for idle control and should not be operational really whilst the throttle is open, however, I have just realised that when the throttle is closed and you are going at say 50mph ie slowing down, the car is still pushing a little, so there is a relationship between the throttle position at closed and the icv, I dont think the tps is adjustable so what else is there which is going to be saying, open the icv a little bit
maf? coolant temperature sensor? rpm sensor? when you have the engine running on idle, and you turn the steering wheel, or switch on all the lights, the revs do rise a bit (thats a general observation) so something is telling the ecu to open the icv for that, but what?
well, at the lpg check all the emissions were well within spec, no problems there at all, so very unlikely to be lambda
also the car goes fine and economy is as expected,
tomorrow if I get time I will strip out all the air filter bits to be able to see down to the rpm sensor
you never know putting it all back together might resolve the issue, but I would like to know what is doing it

tobydell · Post by **tobydell** » 25 Oct 2010, 12:03

Still no better, so far replaced iac valve tps, ambient temperature sensor (ait) I have a map coming this week, but there must be someone who has had experience of this type of problem to be able to explain what is going on or where the problem may lye, there is no maf ; map, ait tps iac oxygen sensor, and ecu, thats it.
Help please...

tobydell · Post by **tobydell** » 30 Oct 2010, 03:31

well, things are looking better at last, I replaced the map sensor and the coolant temperature sensor, things seemed ok till I drove it then the idle speed was still up too high,
I recall whilst trawling through these forums seeing something about the ecu needing to relearn, specifically idle, so I found the post which basically said, after the ecu has been out and all caps discharged, run the motor in park at idle for 10 minutes.
I decided to strip out the throttle body again to check the tps (whilst the battery was disconnected)
the range of impedance for the tps, is 900 to 1100 at idle (throttle closed) and 2500 to 2900 at full throttle
with the new tps on the throttle body, it checked out at about 1300 (all ohms) whereas with the old one back on I could get it down to 1004, so putting it all back together again, and running it for 10 mins at idle, all seemed to be good
normal idle speed, even better, a trip round the block was uneventful and idle has so far remained at about 900rpm
was it the ecu relearning or the tps or both?!?
I will know more later today as they are resurfacing my road and I can't get out anyway till later
if there are no more posts from me you will know it is ok.
I have found that bits from various posts all add together to make a bigger picture and eventually resolve issues which seem impossible to get a grip of
Bye for now

tobydell · Post by **tobydell** » 01 Nov 2010, 04:37

all now seems to be ok adjusting the tps with the throttle body off, to get 1005 ohms and relearning the ecu at idle for 10 mins
so far the idle has remained at just under 1000rpm for the last 2 days on petrol or gas, Hooray.
I am not sure if it was the relearn or the tps or perhaps a bit of both
Everyones posts on related issues are like pieces of a jigsaw, collect enough little bits and you start to see the bigger picture
Bye