Help me diagnose an elusive fault with my MG ZT!
Discussion
I've been having an intermittent fault on the big MG which first surfaced literally on the drive home from buying it. It's been back to the dealer twice already, and only just got it back again this week (having been there for over two weeks!) when the problem appeared to be fixed but has now re-appeared, AGAIN. It's still under dealer warranty but they're over an hour's drive away and I'm frankly getting fed up and am inches away from asking for my money back.
It's a facelift ZT 160, with the 1.8 turbo engine, which is basically a K-series with a low-pressure turbo bolted on.
The problem is this: under hard acceleration (wide open throttle) the car lurches violently- basically the same feeling as if you were stamping your foot on and completely off the pedal in quick succession. It ONLY occurs when you put your foot down quickly and hard, and appears completely independent of speed, RPM, engine temperature, etc. For example, the car is quite happy cruising at any speed, and equally happy accelerating - even at full throttle - IF you do it by steadily increasing pressure on the pedal, rather than pressing it quickly.
Before we first collected the car the dealer did some work on it for us including replacing the camshaft position sensor which the car was reporting a fault with. When we returned the car for repair, it was still reporting faulty so they replaced it again with a pattern part, and then again with a genuine MG Rover part. Eventually they discovered one of the cam timings was one tooth out, which they rectified, which seemed to resolve the problem. However, it's resurfaced today.
I have a number of theories now, but I'm hoping someone more mechanically minded might be able to shed some light on whether any of these are likely.
Theory A - Wastegate solenoid. I researched on the various owner's club forums and the issue has come up a few times before, with people having varying luck with various methods to resolve. The most likely sounding to me is that there is a problem with a solenoid in the wastegate not releasing to relieve pressure as it should do, leading the car to over-boost (hence only on hard "stamps" of the pedal), which leads the ECU to momentarily cut fuel in order to prevent engine damage. This sounds feasible to me, but the mechanic said he couldn't find any problems with the turbo. However I'm not going to put much stock with his opinions, for various reasons...
Theory B - Injector/spark plug/coil pack fault. The mechanic has said he didn't check any of these. In my opinion if there was for example a knackered injector (perhaps electrically), or spark or coil, it could cause this problem - main support to the theory being that the problem only occurs when a rapid and large demand increase is requested. I'm not too familiar with this sort of thing though so I may be wrong.
Theory C - Slipping clutch? I've never driven a car with a slipping clutch so have no experience to base it on, but I've always been a little uncomfortable with the clutch behaviour of the car. I initially thought it was a characteristic of the car that I just needed to get used to, but perhaps not. I've always struggled to accurately find the bite point on this car, and in addition, the clutch behaviour is a bit odd - you only have to depress it a very small amount to disengage the drive, BUT the gears are very difficult to select unless the pedal is 95-100% depressed? Feel free to correct me if it's just something I should get used to but it always seemed odd to me, and adds up with the slipping clutch theory - which is mainly supported by the fact that the problem only occurs when max torque is requested for acceleration. In addition the pedals have always felt to me a little "graunchy", for lack of a better word - there's resistance and graininess in their motion, a bit like if you're trying to turn a handle or hinge that's got a bit rusty; it's not hard to press them but it's not a pleasing smooth feel at all.
Theory D - Something else? If the cam timing was truly the problem, is it possible I have a pulley with a knackered tooth that's allowing it to slip marginally - or something like that? Other problems with the car that may or may not be related - a) the pedal feel as mentioned above, b) the car at idle sounds a bit naff and "puttery" like a miniature steam engine or something, which was not what I was expecting given the sporting nature of the car - the dealer also replaced exhaust before we bought it as it had a minor blow so I don't think it's that; c) I have noticed the cooling fan is seemingly VERY over-zealous - it will have turned itself on before you've even pulled out of the parking space. It's very loud too, I'm not sure if that's just how it is or if it's automatically skipping to its high-power setting - when the Mrs pulls up outside in the ZT I can tell she's arrived, not because of the exhaust note or the crunch of the tyres but because I can hear the b
d fan!
So those are my ideas - beyond this I'm really quite lost!
As I said above if I can't get this sorted out I'm on the verge of returning the car - I love the model but this example isn't great, we paid over the odds for it due to wanting a dealer warranty but frankly it's been more trouble than it's worth, the paint is pretty crap with lots of chips and marks etc... if it can be fixed permanently then we'll overlook its minor flaws but with this fking juddering still happening it's just one thing too many to be honest.
Any help would be much appreciated!
It's a facelift ZT 160, with the 1.8 turbo engine, which is basically a K-series with a low-pressure turbo bolted on.
The problem is this: under hard acceleration (wide open throttle) the car lurches violently- basically the same feeling as if you were stamping your foot on and completely off the pedal in quick succession. It ONLY occurs when you put your foot down quickly and hard, and appears completely independent of speed, RPM, engine temperature, etc. For example, the car is quite happy cruising at any speed, and equally happy accelerating - even at full throttle - IF you do it by steadily increasing pressure on the pedal, rather than pressing it quickly.
Before we first collected the car the dealer did some work on it for us including replacing the camshaft position sensor which the car was reporting a fault with. When we returned the car for repair, it was still reporting faulty so they replaced it again with a pattern part, and then again with a genuine MG Rover part. Eventually they discovered one of the cam timings was one tooth out, which they rectified, which seemed to resolve the problem. However, it's resurfaced today.
I have a number of theories now, but I'm hoping someone more mechanically minded might be able to shed some light on whether any of these are likely.
Theory A - Wastegate solenoid. I researched on the various owner's club forums and the issue has come up a few times before, with people having varying luck with various methods to resolve. The most likely sounding to me is that there is a problem with a solenoid in the wastegate not releasing to relieve pressure as it should do, leading the car to over-boost (hence only on hard "stamps" of the pedal), which leads the ECU to momentarily cut fuel in order to prevent engine damage. This sounds feasible to me, but the mechanic said he couldn't find any problems with the turbo. However I'm not going to put much stock with his opinions, for various reasons...
Theory B - Injector/spark plug/coil pack fault. The mechanic has said he didn't check any of these. In my opinion if there was for example a knackered injector (perhaps electrically), or spark or coil, it could cause this problem - main support to the theory being that the problem only occurs when a rapid and large demand increase is requested. I'm not too familiar with this sort of thing though so I may be wrong.
Theory C - Slipping clutch? I've never driven a car with a slipping clutch so have no experience to base it on, but I've always been a little uncomfortable with the clutch behaviour of the car. I initially thought it was a characteristic of the car that I just needed to get used to, but perhaps not. I've always struggled to accurately find the bite point on this car, and in addition, the clutch behaviour is a bit odd - you only have to depress it a very small amount to disengage the drive, BUT the gears are very difficult to select unless the pedal is 95-100% depressed? Feel free to correct me if it's just something I should get used to but it always seemed odd to me, and adds up with the slipping clutch theory - which is mainly supported by the fact that the problem only occurs when max torque is requested for acceleration. In addition the pedals have always felt to me a little "graunchy", for lack of a better word - there's resistance and graininess in their motion, a bit like if you're trying to turn a handle or hinge that's got a bit rusty; it's not hard to press them but it's not a pleasing smooth feel at all.
Theory D - Something else? If the cam timing was truly the problem, is it possible I have a pulley with a knackered tooth that's allowing it to slip marginally - or something like that? Other problems with the car that may or may not be related - a) the pedal feel as mentioned above, b) the car at idle sounds a bit naff and "puttery" like a miniature steam engine or something, which was not what I was expecting given the sporting nature of the car - the dealer also replaced exhaust before we bought it as it had a minor blow so I don't think it's that; c) I have noticed the cooling fan is seemingly VERY over-zealous - it will have turned itself on before you've even pulled out of the parking space. It's very loud too, I'm not sure if that's just how it is or if it's automatically skipping to its high-power setting - when the Mrs pulls up outside in the ZT I can tell she's arrived, not because of the exhaust note or the crunch of the tyres but because I can hear the b
d fan! So those are my ideas - beyond this I'm really quite lost!
As I said above if I can't get this sorted out I'm on the verge of returning the car - I love the model but this example isn't great, we paid over the odds for it due to wanting a dealer warranty but frankly it's been more trouble than it's worth, the paint is pretty crap with lots of chips and marks etc... if it can be fixed permanently then we'll overlook its minor flaws but with this f
king juddering still happening it's just one thing too many to be honest.Any help would be much appreciated!
Edited by OwenK on Saturday 26th February 22:41
Thanks for your help both. MG John I already considered the boost cut, I mentioned it in the first post - not really sure what to do about it though... I've a new coil pack sitting in the kitchen ready for installation when I have a spare hour and if that doesn't fix it I'll have to delve further into the boost cut theory.
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