Car holding back - but runs better on Super Unleaded

Hello Guys,

I know Super Unleaded makes no real difference to the Mk2.5 as its such a basic car which does not make use of high Octane fuels.

But recently my MX5 feels like it is holding back, and not easy to drive around town.


I put some Sainsburys Super Unleaded (97 ron) in it 4 days ago to see if it would help and now the car runs much, much better.

Its easier to start, feels much more responsive, lots more power. 


Would this indicate a problem somewhere? Maybe its running lean? Problem Fuel Injectors? Timing maybe?


It has been serviced recently back in July, (new spark plugs, ht leads, coil pack, fuel filter, maf cleaned etc)

Any ideas please.








Is the PCV a bit sticky? 

It should rattle nicely when you shake it.

Or you might find that it is the knock sensor no longer being sensitive enough but still doing sufficient that the ECU thinks it is OK, and the new higher-ethanol types of regular fuel are drifting off the ECU map without enough feedback from the sensor.  The super unleaded has less ethanol.

It is probably easier and cheaper to simply run on the super unleaded.  V-Power transformed my old Vectra where the knock sensor was inaccessible; it went from being very reluctant at starting and low revs to willing and silky smooth, and from 33mpg to 38mpg.  It was a no-brainer.


Hello Richard,

Thanks for the info, is the PCV value easy to take off?

Will give that a shake and replace if needed.

Yeah I might just stick to Super Unleaded as the car runs so much better now.

Don’t think its the knock sensor as MPG is still good, and does not knock as far as I know.


Found this on the knock sensor: adds that a defective knock sensor does not properly adjust the timing of the engine when fuel enters the cylinder. This causes the vehicle to lose fuel mileage and acceleration. Additionally, a bad knock sensor causes irreparable damage to the engine. According to, the knock sensor detects vibrations and transmits data to the engine computer. These vibrations or engine noises are caused by detonation. When the knock sensor is damaged or broken, the detonation process is not regulated properly. explains bad knock sensors also lead to excess engine noise. These noises are generally loud thumps and are a result of air or fuel entering the cylinder. recommends identifying the symptoms of a bad knock sensor and replacing the part immediately. A new knock sensor helps avoid damage to the engine and eliminates costly repairs.”

The recommended fuel for my ND is “95 or above” and I’ve found that it does run better on the 97.  In contrast, some years ago my Civic Type R specified 95 and when I did put Shell V-power 97 in it the apposite happened, it was sluggish and “felt” as if it were being strangled.  So I guess the the difference is a more fuel tolerant tuning on the “5’s” engine while the “R’s” was more finely tuned.  I’m not an automotive engineer or fuel technician so if the terminology is wrong I apologise, but hopefully you’ll know what I’m getting at.

That makes sense. 

On my old Astra there was a plug in the loom that was meant to be swapped around depending on if the RON was 95 or 97.  But I found it made no difference! 

So it was a big surprise when the Vectra (without such a plug) was so sensitive to the fuel and yet never showed any fault codes for the engine.

My NC definitely goes a tiny bit better with the V-Power.  On the odd occasion I’ve been forced to fill up with Regular I’ve noticed it does a few mpg less and the bottom end flexibility is sometimes not quite so good. The difference is not enough to justify V-Power on simple economic grounds, it still costs slightly more per mile, but not enough that it hurts on my limited mileage, and there is the hidden benefit that the engine runs cleaner.

Just before you start taking things apart let me tell you about an incident we had with our old megane.

The wife came home one day and said it wasn’t running right.

It was like it was running on 3 cylinders, and really down on power, but no EML.

For the life of me I couldn’t figure out which cylinder was misfiring, my old trick of feeling the exhaust manifold revealed nothing.

So I changed all 4 plugs, but it was still the same, so I then replaced a coil pack swapping between all 4 cylinders… still nothing.

Thinking it’s unlikely to be 2 coil packs, but i went out and bought another. Still only running on 3 cyclinders.

Tried injector cleaner, nope… not that.


Over a period of about 10 days, it had me totally baffled, I was about to give in and let someone take a look, when the sister in law happened to mention out of the blue and not knowing about our saga…

“Have you heard about the local petrol station, the tanker driver contaminated the Petrol with Diesel, there’s loads of people moaning that thier cars are not running right”


Youre joking… so asked when… “oooh about 10 days ago now…”


So off we went to the next petrol station, brimmed it with super unleaded and guess what, within 30 seconds of starting her up, she was running fine again.


The moral of the story, don’t overlook the not so obvious, obvious.