When Im driving at a steady pace such as in traffic or on the motorway at slowish speeds, or have taken my foot off the accelerator, Ive noticed that on odd occasions between 3k and 4k rpms the tachy blips between either of the two, as though it cant quite work out where it should be. Its been becoming more frequent lately, does anyone know what this could be?

There is no audible difference in the engine, the car doesnt kick back or anything either. If I wasnt catching the tachy out the bottom of my vision I wouldnt have noticed anything at all.

I think theres a "knock sensor" that is linked with the rev counter... the sensor might be on its way out? or I could be talking rubbish...

Electronic Spedo sensor on the gearbox?  My old Ibiza used to do that until i replaced it.  

It only happens for a second, say it will blip from 3k up to 4k, or it might swing back down to 3k for a second or two then go back up to 4k. Its very odd.

Ive been suspecting problems with either my thermostat or o2 sensor so Im having them replaced this Saturday during its MOT, Im wondering if this might be related.

It s a 17 year old Mazda, so Ive no idea if it has electronic speedo gearbox trickery or anything like that, I dont recall ever seeing mention of anything like that on the mx5oc forums or the enthusiasts manual.

Knock sensor will have nothing to do with it, a knock sensor is there to detect knock and allow the ecu to retard the timing if knock is detected to prevent detonation.

O2 sensor failing will give you violent jerking when acelerating at certain rev ranges, or generally pish running

thermostat will see your temperature behaving oddly, or the heaters never getting warm when driving if its stuck open

What drives the rev counter on them - is it a pickup on the crankshaft or is it impulses from the coil. could be a faulty sensor. The speedo will prob be driven by pulses from the ecu from a sensor in the gearbox

Ive no idea what drives the rev counter and I dont even know how to find out.

As for the thermo and o2 sensor, the car takes an age to warm up so I figured its worth swapping for a tenner. My mileage is very poor and I think a combination of a poor sensor and it being two wire maybe be part of the reason for this, so Im having that changed too for a new four wire. The car seems to open right up as soon as I touch 4200rpm regardless of how far I press the pedal.

The opening up at 4200rpm will prob be the start of the powerband (As its a 16V motor)

Well its always done that as far as I know, I didnt know if it was related or not but there we go.

My fuel consumption is high even when Im not driving it hard in the high revs. Ill suppose Ill have to let you know how it runs on Saturday when its been swapped around.

take your plugs out and inspect them to see if it looks like its running rich etc

Dont you need esoteric tools to take out spark plugs? Should I just ask the garage to take a look at it. I havnt a clue what Id be looking for quite frankly.

You only need a plug spanner, and a wrench.

Take a photo and show it to mark.

I dont have either, I guess I could ask the garage to take a look. Although Im changing the plugs anyway, should I ask them if I can keep the old set to look at?

Have you asked on the mx5oc forums?

No, Ive had a good read on there and not found anything about it though. Ive only just noticed the problem and thought I would ask the knowledgable gents here first

I did have the same prob in one of my minis, it was a loose wire but it sounds like you got something different going on. If the engine isnt doing anything odd I wouldnt worry too much - its 17yrs old like you say.