NC surging under acceleration in 6th gear

[nateboy]

Hi all, have checked into a few shops, searched Google and these forums, without much avail, so hoping the brains trust might be able to help me with this one. Please bear with the lengthy descript to follow + my noobness to the forum + mx5 life.

Am in Melbourne, Australia and quite love my recently acquired ’09 NC Series 2, PRHT, 6 spd manual, with 123k kms on the clock, and it seems to operate and drive quite fantastically, with one exception – when in 6th gear (and 6th gear only), under constant throttle, the car will subtly surge for 2 seconds, and every 2 seconds, and will do so at any (6th gear) speed, i.e. 80-120km/h+.

I checked the car into a few places (privately owned + dealer run), and had a whole lot of work done, but with no difference to the effect.

First I took it to MX5 Factory down in Ringwood who:
1. Replaced air / fuel oxy sensor
2. Replaced fuel fine trim sensor
3. Replaced spark plugs (iridium)
4. Used injector cleaner (fuel) additive
5. Fuel injector clean (+ de-coke)
6. Cleaned (Throttle body/Earthing contacts/Battery terminals/Fuel rail)
7. Checked for vacuum leaks

Then, months later, I took it to Mazda Ringwood who:
1. Replaced intake manifold tuning valve
2. Performed a fuel pressure on road test, reportedly ok
3. Drove car with MMDS connected, throttle body showing variance when accelerator pedal showing steady input

So, given that I had spent a fair bit of money on things that hadn’t made any difference, so I figured I’d try and source a 2nd hand throttle body to fit and see if that made any difference – I ended up getting one from eBay (http://www.ebay.com.au/itm/MAZDA-MX5-TH ... OC:AU:3160), which I got Mazda Ringwood to fit for me, and it didn’t make any difference either.

The Mazda Ringwood position was that they felt it was still likely the throttle body, and that it wasn’t a true test as the one I had supplied was 2nd hand, but in my (admittedly basic) mind, even if the 2nd hand throttle body was defective, what were the odds of it being defective in the same way…? Especially given that this issue is supposedly quite rare (i.e. Mazda AU have been quite unsure about how to address also).

Just wondering what people thought about:
1. if it is likely the throttle body (i.e. if so, I would then spring for a new one + fitting)
2. if there were any other potential causes/things to look at
3. other (shops)/places that might be able to help me out

Thanks in advance!

[93_Clubman]

Just had a quick look through the Mazda TSBs for MX5 NC & found the attached TSB regarding driveability - would expect this to have already been done, but as it's free & easy to do, do as it suggests & see how driveability in 6th is immediately after:

01-001-08-1894.pdf

From the following:
http://parts.arlingtonmazda.com/pages/MX5.html
http://www.miata.net/garage/tsb/

For another opinion:
Dean @ Motorsports Essentials, 6 Macbeth St Braeside, 9587 0079; or
Gerald & Chaz @ Mammi Motors, 10 Industrial Drv, Somerville, 5977 7973.

[Regie]

Fuel filter?

[Charlie Brown]

Fuel filter?

Fuel filter is in the fuel pump Regie and is back washed on start up.

Nateboy, I'll PM you who to ring.

[Regie]

That is true. But looking at guys one at close to 100k kms it was pretty caked up.

[panjap]

Flush tranny oil. This make my car 10% faster

[nateboy]

Hi guys, thanks heaps for the really quick and thorough responses - a few things for me to try myself + contacts of good people's brains to pick! Will try a few / contact a few people through the week to see if any good new leads can be figured, will update the post over time for findings/posterity, in case it's also of use to others.

Oh, and thanks to Charlie Brown's prompting, reminded me also that I did try the airflow sensor - I had a mechanic friend who had bought a spare try it out (i.e. swapped in the spare with mine), but alas, no dice either. Hard to say though if the spare may have also been faulty, but we were doubtful as he had picked it up new.