I could speculate on emission control sensors or img, but for more confidence in diagnostics you'll should first check the codes with an OBD2 reader if it's a later one (you didn't say what year it is?). Using any coolant or any other unusual symptoms?

 

I checked the ODB and it sometimes comes back with misfire on cylinder 4, but not always.

I have recently moved the coil pack around to see if the fault follows the pack or HT lead.

I have topped up my coolant, it was low but not dangerously.

It's a 52 plate.

 

You don't say which fuel, let alone which engine.

If it's petrol I'd start by looking at a sticky throttle body.

 

It’s a 1.8 petrol

 

It may well have a plastic throttle body which are known to go out of shape.
A metal 52mm version will be better and will improve fuel economy.

 

I’ll check the throttle body today, thanks.

 

If the coolant level is dropping at all and it's misfiring, I'd look at the Inlet Manifold Gasket (IMG). See links:

MG ZS 120 Coolant loss - Inlet Manifold Gasket (IMG) Repair - MG-Rover.org Forums

http://forums.mg-rover.org/blogs/gnu-74719/r25-img-replacement-1897/

 

Thanks both, I cleaned the throttle body, there were sooty deposits inside but it hasn't made any difference but it has been done and I checked the hoses around that area just in case.

The throttle body is metal.

One thing I was surprised by was the hose going from the throttle body to the IACV didn't have a hose clip on either end. I checked the Haynes manual and it says it simply pulls off which would indicate that hose doesn't need clips.

With regard to the IMG, would I see water leakage around the gasket area?

 

Just read the IMG link properly and found the posting person had obviously noticed leakage. Ignore my previous question.

 

Just changed the plugs and the temp sensor on the rad pipe (as I had spares of both) and it had no effect, it was a long shot.

I am noticing the problem is getting worse making the car dangerous to drive with its lump deceleration and lacking in power.

Even with this terrible lumpy driving there are no warnings on the ODB.

Would a blocked up fuel filter cause symptoms of this type?

The car had been stationery for a year or more and did very low mileage before that. The clock is on 32k now and the plugs were changed in Feb 2016 when it had done 29k. When I bought it, it had done 29k.

 

If it's been standing, have you tried new fuel and checked the exhaust is unobstructed? If so check the IMG as mentioned above...

 

Is it when you come completely off the throttle?

This would usually shut the fuel off, but it sounds like it might not be if the car is "misfiring" under deceleration.

Check the pipe from the fuel rail pressure regulator is not split and is properly connected.

 

When I bought it, it was fine and has been until recently. I've done about 3k in 6 months.

She idles fine, if I pull over when it is playing up, she idles well, with no cause for concern.

It is literally only when I am driving and easing off the throttle. If I come completely off the throttle, she is fine. Accelerating she is fine, it is when, for example, I reach 70 and ease off the throttle she starts going lumpy, which at low speeds is quite dangerous as I over compensate to bring the speed back up and cause the kangaroo effect.

 

The regulator on the fuel rail sends all the fuel back to the tank when you lift off the gas, the pipe to it is a vacuum hose. If that is leaking then the injectors may still be firing somewhat lower pressure fuel into the inlet resulting in the juddering you describe.

 

There isn't a smell of petrol but I will check the hoses anyway. Thanks.

By the way, is your forum signature an edited quote from Dune? Seems familiar...

 

It is a version of the mentat chant yes

It won't smell of petrol, it is the vacuum pipe you need to check, between the fuel regulator and the induction somewhere (not sure where on a K-series)

 

I am having a good day!

Found the problem to a be a dodgy throttle potentiometer, when revving with it plugged in it would cause hesitations, once it was unplugged, the problem went away.

i am hoping its that!

 

Solved. Replaced the TPS this morning, road test this afternoon, she runs like a dream.

Thanks to you all for your help.