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MG ZS idle problem
plentywahalla - 23/10/12 at 10:21 PM

The MOT on the MG was due but the testing station refused to test it because the engine would not idle. The engine revs rise steadily on tickover up to about 2000 then starts to hunt between 1200 and 2000. I have replaced the air idle control valve, no change, then replaced the TPS, no change also.

It was put on the diagnostics and it came up with an AICV fault. As I had already replaced it I tested the loom and there was a pulsing voltage as expected. I can also hear the stepper motor clicking away so I don't believe that AICV is at fault.

The MOT has now expired so I have a problem. I have checked for any other faults, there is no apparent leak in the inlet manifold and there is no MAF or MAP fitted in the system. The ECU checks out ok by the diagnostics.

Any ideas? ... I have run out of them.

One simple question. Should the AICV be closed when the ignition is switched on? I thought it should be, but if I take the bypass hose off I can just blow through it. i have done the re-calibration thing several times.

britishtrident - 23/10/12 at 10:54 PM

I would want to look at live diganostic data the TPS MAP ( your engine has a MAP) and coolant and inlet temperatures and Lambda output.


However if you can't do that and all the sensors are connected my first guess inlet manifold gasket a common on K16 engines can also cause mysterious coolant loss.
Not difficult but a fiiddly job as the the nuts can be difficult to access especially when replacing the nuts, I use a varienty of 3/8" deep sockets. When replacing the nuts use thick washers and spring washers under the nuts and have a few spare nuts handy.

It is also possible the inletl manifold is either cracked at the seam or the brass bushes the locating bolts go through have got pulled through.

The MAP sensor is combimed the temperature sensor and located and is square box on top of the inlet manifold adjacent to the runners for no1 & no2 cylinders.

ChrisW - 23/10/12 at 11:06 PM

Can't help with the problem, but I have a load of ZS parts gathering dust in my garage if you need anything. They're from an 02 plate 1.8 k-series car.

Chris

britishtrident - 24/10/12 at 07:32 AM

All 25/45/ZS/ZR inlet manifolds are basically the same, really just a matter of changing over the injectors to suit the engine size. The Rover 25 1.4 base model had a smaller throttlebody fitted for insurance group reasons, the smaller throtllebody restricted the power to 81ps from 103ps
Some early manifolds had the IAT in No.4 inlet AND also had the combined MAP & IAT fitted but in the combined sensor wasn't connected, this should have no effect on swapping manifolds.

Older Rover models (ie 200/400) didn't have the combined MAP/IAT sensor in the manifold as the MAP sensor was built into the ECU.

Rover 75, MG ZT and Freelander manifolds differ only in having the fuel return line from pressure regulator blanked off.

big_wasa - 24/10/12 at 07:46 AM

You don't say if it's a k4 or a kv6. The idle control valves on the kv6 are funny and it's not just a case of taking the old one of and putting the new one on. They sort of ned resetting. Also I doubt it was a new replacement as they are stupid money from rimmer's. They get carboned up and need cleaning out.

[Edited on 24/10/12 by big_wasa]

avagolen - 24/10/12 at 04:15 PM

I had a similar problem in the past with a Honda Civic.

The Car was a 95 1.5 16V, old version, with no ability to plug in any diags kit.

The sensor was the single wire version, with not preheat etc.
I replaced the O2 sensor and it solved the problem.

The car would idle OK when cold, - open loop, but when warmed up
the engine would hunt 500rpm when the high idle check was done.

HTH.

Len.

rusty nuts - 24/10/12 at 07:50 PM

Might be worth checking that the throttle butterfly is central in the bore of the throttle body and not jamming on one side

britishtrident - 24/10/12 at 08:34 PM

Which reminds me I had assumed because the car has an MG badge it hasn't got a plastic throttlebody ? as plastic throttle bodies were used until it was discovered they encounter problem if a jubilee clip is used to retain the airfilter trunking, the body distorts and causes high idle speed but usually not the idle speed to hunt.

britishtrident - 24/10/12 at 08:47 PM

quote:
Originally posted by avagolen
I had a similar problem in the past with a Honda Civic.

The Car was a 95 1.5 16V, old version, with no ability to plug in any diags kit.

The sensor was the single wire version, with not preheat etc.
I replaced the O2 sensor and it solved the problem.

The car would idle OK when cold, - open loop, but when warmed up
the engine would hunt 500rpm when the high idle check was done.

HTH.

Len.


Never encountered that symptom with a Honda although I have changed a few Lambda sensors on them bececause the Lambda sensor on them is prone to oil fouling due excess oil getting into the breather system. Generally with older Honda PgFi they run fine but if the fault warning comes after 1/2 miles running it usually is the lambda, easy enough to use a paper clip to get the blink fault code to confirm.


On Rover MEMS you can disconnect the lambda sensor ECU defaults to the default map which is pretty good and the fault light comes on but the engine will run more or less normally.

plentywahalla - 25/10/12 at 07:34 AM

Thanks for all the advice and offers of parts. Its on the agenda for this weekend to remove and strip the manifold and clean and check everything.

If i can't sort it, then ChrisW won't be the only one with a garage full of redundant MGZS parts gathering dust!