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MG MGF Technical - uncommanded RPM increase

HELP!!!! Driving to work today, push in the clutch, RPMs stayed at 2000rpm.

Have reset the stepper. Throttle closes.
Blipped throttle from quadrant, rpms rise, throttle let spring closed, rpms rise uncontrolled to about 3000rpm. If i grab the quadrant and physically move it more towards closed, decreases the rpm, but probably more from starving the engine?

Possible causes and solutions PLEASE

Alan
A M Cox

Okay, the throttle butterfly IS closed when the throttle is released, but rpm still continues to rise up to around 3000rpm. If I squeeze the vacuum? line that leads from the filter box (ahead of the butterfly) to the engine side of the butterfly, I can get the RPM to drop to normal idle. The line has no splits or leaks that I can find. If I remove the plug from the bottom of the throttle (throttle sensor?) the engine will splutter along and then die with any opening of the butterfly. At the moment I have managed to make the car driveable by restricting the before mentioned vacuum line, however no closer to finding the actual cause/repair. Maybe leak at the actual throttle/manifold gasket?
A M Cox

sounds like the brown temp sensor is bust or dirty.

If the fan spins from cold then the wire could be broken too. unplug the brown sensor and clean the contacts, then run the car up and see if the fan runs when you are expecting it to.
Will Munns

How to check the operation of the MEMS water temperature sensor: http://www.mgf.ultimatemg.com/group2/common_problems/hgf_pages/related_problems_overheating.htm

The other thing to look at is the IACV (idle air control valve). Remove and clean; oily residues can stop it closing properly - give it good clean (petrol etc - Andy 'Scarlet Fever' Phillips reckons on using Gregate polish) - and see how this goes...
Rob Bell

If it's not the sensor, sticky throttle or the IACV, see my post in the archive under "VVC solenoid"

It took me ages to find the problem as it does not show on the Testbook
Brian Highe

Its not the VVC solenoid as the VVC mechanism has gone and been replaced with Piper cams.

I have pulled the Air inlet temp and cleaned that, and cleaned all contacts. Pulled the IACV out, is pretty dirty. If i leave it disconnected and turn the key to position 2 and do the stepper reset, should I see the IACV spool move also? At the moment I do not see the spool move at all when turning ignition to pos. 2 or back to off. Would I be right in assuming the stepper has failed? I am about to jumper 12 volts to the power pin and short the others out to test it properly. If it works, then I'll look at the Brown Sensor. Thanks for your help so far with this issue
A M Cox

Another quick question re cleaning the IACV, is it just the bottom barrel that you clean in petrol, or do you throw the whole lot in, electric motor side and all and just give it a good dry out? Is there any need to disassemble it more than the two screws holding the spindle half to the barrel half?

Thanks.

Alan
A M Cox

Got in the car this morning, ignition to #2 position, jumped the 12volt supply from pin 2 on the plug to pin 2 on the removed IACV. On shorting the other pins to earth (chassis) could feel the IACV "clunk" each time, but could see no movement of the spool. Is there a way to disassemble and clean other than the 2 halves, or a method to unstick it? Guessing it may be toast as we say over here. Can i get away with blanking off the IACV port after the butterfly as it will be possibly cheaper to import an IACV frm the UK than buy one from the rover dealers over here in New Zealand.

Thanks

Alan
A M Cox

Hi Alan
have a look at the wire coming out off your cabel at the engine end at carbie mine had a peace of the plastic in side of cabel sticking out and the wire was getting stuck i broke it off now no problem
Andrew
AR Cayzer

Who knows where to get a reasonably priced IACV, part number MLZ 100090. Cheapest I've found so far is around 67 GBP fro Elise Parts, 76.42GBP (excl VAT) from Brown and Gammons and 86.15GBP from MGOC spares (but I think that incl VAT)
A M Cox

I've not disassembled the IACV Alan, but I don't think that it is entirely necessary to take it apart. You can probably get away with a thorough dousing and a brush I would think - but have a look through the Elise archives - more information on the IACV there (follow links at the bottom of this page). No idea why the Lotus lads appear to have more problems with the IACV compared to us, but there you go...
Rob Bell

I think its dead. Gave it a thorough cleaning, jumped the pins agin, clunks, but no movement. Will have a peek in the Lotus forum and see if any of those blokes have pulled one to bits. I know that the 12Volt supply is getting to it from the loom, and jumping all the pins on the IACV it "clunks" every time, so the IACV itself seems stuck as sh*t on fur. Just got quoted $436 NZD for a new one from Roverland over here, checking with local parts merchants, hopefully I can get a working second hand one for way less. The price quoted here is at least twice what it will cost to import a NEW one over from the UK!!!
A M Cox

Some one correct me if I'm wrong, but I think I've definately found a shot IACV; if it was the brown senso that is shot, the fan would run regardless of hot/cold temps? I should be able to physically see the IACV spool move when I jump 12Volts to pin 2 and short the others to ground sequentially ( I only feel/hear a clunk with no movement of the spool; the spool out of the housing to erradicate the fact that it may have been sticking in there). Can I get away with creating a blanking plate (with or without a "metered" hole) to cover the IACV port on the throttle body in the meantime (catching the idle with fancy footwork). Many thanks for all the help and suggestions so far. Alan.
A M Cox

Fabricated a blanking plate to fit over the IACV port and drilled a 3.2mm hole through the centre of it. Car now starts and idles between 900-1100rpm. Looks and smells a little rich running, but gives me a car til the new IACV when I get one this coming week.
A M Cox

The IACV motor is a stepper motor. In other words, it is drived by pulses on two windings which are 180 degrees apart. The time relationship between these pulses moves the motor shaft in either direction. It will not move simply by applying volts to the windings as if it were a dc motor.

The "clunk" is probably a good sign.
Brian Highe

>> if it was the brown senso that is shot, the fan would run regardless of hot/cold temps? <<

The fan would never come on, leading to the car over heating. The fan will always run if the brown sensor is disconnected.

I think that your problem does 'smell' like an IACV problem - hopefully you'll get this sorted soon :o)
Rob Bell

IACV and Brown Sensor (just in case) on order from Elise Parts in the UK. I can't believe the mark up that Rover NZ must put on parts as obviously Elise Parts won't be selling them at cost!!!!
A M Cox

Wouldn't surprise me Alan - but Geary does do very reasonably priced products :o)
Rob Bell

This thread was discussed between 27/11/2007 and 07/12/2007

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