High idle MG-ZT 190, idle doesn't come down after releasing throttle
 

Back again with a new issue :o


The throttle on my ZT seemed to stick a bit, leading to the lubing of the throttle cable. This seemed to fix the problem for a while, up to the point that I got a replacement key made.


Driving away from the garage that made my replacement key, swifting from 1st to 2nd gear I saw my revs increasing with my throttle fully up and my clutch on the floor. At this point I thought it was the throttle cable again. I tried to push the throttle pedal up from below, but it was already fully up. Also, pushing with the revs climbing did not lead to a decrease in revs. At this point, revs would climb but fall back down again after a few seconds.


Kept driving, problem seemed to fix itself. Talking to a friend about it, he suggested cleaning out the idle control valve. Good suggestion, so I did that the first time I got the chance. Had no problems before cleaning, no problems after.


All was well until I got my wheels aligned. After my one hour appointment, driving away I again had the revs climbing when the clutch was down. Again, revs would climb and then fall back down on their own within a couple of seconds. This is the point where I casually started looking for a new idle control valve.


Car went back to running fine. Cue to today: Picked up my friend from the doctors. At the start of this 20-minute journey, revs would climb ~200 rpm when shifting gears. Annoying, but manageable. On the way back, my ZT would idle at ~3000 rpm. While driving, when depressing the clutch to downshift, revs would climb seemingly without a limit. Also, the car keeps slowly speeding up while not pressing the gas.


I just went back to car after it had sat for a while. Resetting the ICV by pressing the throttle 5 times with the keys in position 2 did nothing. The car now idles at ~2000 rpm, and when I press the gas revs climb and won't fall back down after letting my foot of the gas.


This has basically made my car undriveable, as there is now way to drive the car safely at low speeds (without wrecking the engine by letting the revs climb to 5000).



Am I right in assuming that the issue is with the ICV, or could it still be a problem with the throttle cable? Also, is there any further test I can do now before replacing the ICV or other parts?



Much appreciated as always!

A throttle body rebuild kit will cure this issue, around £15 from rimmer brothers.

The plastic T piece becomes distorted over time and catches the side of the body.

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Ah, thanks! Do you mean part number MKG100260L?

Yep that's the one

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Possible other problem
 

~4 hours after driving it my car seems to idle okay again. Checking once again under the bonnet, I now also see that there is a gummed up connection around a hose (or cable bundle?) that has started coming loose.



https://www.the75andztclub.co.uk/for...cd1450af4b.jpg


Is this the throttle body hose recirculating engine exhaust gasses into the air intake?

Hi Nathan,

I had this fault a few weeks ago; high idle of 1,100 rpm when first starting on a warm summer's day rising to 2,000 - 3,000 rpm as the engine reached normal operating temperature. The car could not be driven safely, as you've said.

To my surprise the idle air control stepper motor was the culprit. It's the cone on the end of the motor shaft which controls the amount of air by-passing the throttle butterfly.

By the way, you mentioned the procedure of resetting the throttle position by fully pressing the pedal a number of times. This only applies to the MEMS engine management fitted to the 1.8 engines. It is not a feature of the KV6.

Simon

No, it's the air supply to irrigate the fuel injectors. :D

Simon

Hi Simon, with the idle air control stepper motor, do you mean this part?:


https://www.the75andztclub.co.uk/for...dcbf98cbcc.jpg


I tried to look for idle air control stepper motors for an MG ZT, but I can only find the part for the 1.8 engine. AFAIK the part highlighted in the image controls the air by-passing the throttle butterfly.



If so, then I'll probably end up replacing it and will fit the Rimmers repair kit Trikey suggested (just to cover all my bases)





Interesting! I got it from Rimmerbros (https://rimmerbros.com/Item--i-MLV100160), where they suggest the trick after performing the idle valve assembly.

So air flowing from the valve assembly -> hose, meaning a loose hose will be blown out of the valve assembly instead of being sucked onto it.


Anyway I'll get around to replacing the adhesive gum asap.

Yes (you do have to buy the housing as well, the motor isn't available separately as I incorrectly said earlier. Sorry about that! :o).
I had exactly the same symptoms as you have Nathan and I only renewed the motor which cured the fault. So buy the repair kit if you want to but I think you'll be wasting your money. Why not try the motor first and see what happens?
The information I gave you was from MG Rover's technical manual RAVE and it actually applies to the Throttle Position Sensor (1.8 engines only) not the stepper motor! I have informed Rimmer Bros.
That fitting to which the tube is connected should positively connect to the IACV housing with a click. Some up and down movement is normal but it should not be possible to lift out the fitting without depressing the locking collar.

Simon