Hyundai IX35 1.6 Petrol S 14plate - Engine Problem

PauloYorkshireLad

Hello All
We have a Hyundai IX35 1.6 Petrol S  14plate with 45.000 on the clock.
Car has always been serviced when needed and we have had this car since new.
We noticed four weeks ago the car would judder over 40mph and after a few minutes it would correct itself and be fine.
We can be travelling at 60mph and getting 42mpg then all of sudden the engine sounds loud and a slight vibration can be felt, when this happens the MPG drops to around 23!
We can drive for maybe another 10miles and the fault corrects itself and the MPG goes back up but then later on the fault returns.
Round town we normally get 30mpg but now we are getting 20.

No warning lights on at any point and the our local garage have gone over the car and cannot find anything wrong with it.
It has had diagnostics on it and a full check over and no fault codes are present.
The spark plugs looked worn so we replaced them but still no better.

Has anyone else had this problem and if so what was it please?

Thank you for any help given

biscan25

Sounds like a misfire, possibly could either be coil issue, a sensor (lambda,crank,MAF), injectors etc.
I'm surprised their was nothing on the diagnostics and the garage failed to diagnose the problem.
Did the plugs look normal (a coffee colour), not sooty or white?

PauloYorkshireLad

Hi

The spark plugs looked ok but had slight browning around the bases of them.
Yep no engine management light has ever lit up which makes the problem very frustrating

tommyedinburgh

I’d be checking coil pack, o2 sensors and MAF sensor. 

oscarward

Had a similar problem with an i10 some years ago, faulty plug lead was the problem.

PauloYorkshireLad

Thanks guys

Never thought about the plug leads. 
We are going to book it in at another garage tomorrow to be checked.
Just took it for a run tonight and it is a lot better with new spark plugs and the MPG is much better but still getting slight juddering at 50mpg and over