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Hello, I am relatively new to mx5 (or car) ownership but still have basic knowledge to service my car and do basic stuff but nothing advanced.

About two weeks ago, I could rev the car all the way to the redline normally and smoothly, as you should expect. Just today, I found out that when using WOT, the car just would not rev past 3500 rpm (or thereabouts) and it would be very jumpy and harsh, even though my I had WOT. Using smoother throttle inputs (around 50%-60%) I could get the car to rev to around 4500-5000 rpm but it would stop there as more throttle was needed. After mashing the pedal, it felt like the car retarded the throttle and wouldn't let the car rev at all. Due to the roads, I could only do this in gears 1,2 & 3. But as mention before, it used to rev smoothly in all gears. It still is fine to drive around town where high throttle inputs and rpm are not needed, but that means I lose one of the best aspects of driving; reving the nuts of your car . Nothing on the dashboard is lit up and the operating temperature was fine, after I parked it there seemed to be no obvious leeks from the engine or the diff.

Any help or ideas on what to do would be very appreciated. Thank you in advance.

What model do you have?

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Does it do it in neutral?

When it stops revving is it really lumpy and harsh, or just stays at that rpm?


If it does it smoothly I'd check the throttle is actually opening fully at the throttle body, cable may be too slack.

If its rough I'd be looking at ignition (plugs, leads, coils) or fuel (stuck injector maybe)

Or you could probe the diagnosis box and see if he ecu has logged any codes, which may lead you in the rough direction

Its a 2001 NB8B

No it doesn't do it when its not in gear, ie. when the clutch is in or when its in neutral. the revving is quite lumpy and harsh, like changes about 500 or so rpm. I checked the cable and although its not the tightest it can be it is by no means slack, and is the same as it was 2 weeks ago. I checked all the spark plugs, leads and coils, nothing seems to be wrong with them. I'm yet to check the injectors though, but I did recently get a new fuel pump (1 month or so) so would that have anything to do with it.

I'm not sure on what you mean by checking to see if the ecu has logged any codes, but I've attached a picture of the diagnosis box and will also do my own research about it.

Thanks and I'll keep you updated.

Edit: After two minutes of online research, I've found out how to diagnose the car, and will do shortly but first will need to get the wires and led.

How old is your fuel filter? Perhaps the same thing that killed your pump is also clogging the filter?

Okibi wrote:How old is your fuel filter? Perhaps the same thing that killed your pump is also clogging the filter?

I'd also check air filter. I had similar symptoms with a clogged air filter that would run fine at small throttle openings but cut out once you sunk the boot cos it couldn't flow enough air to keep up..

Yeah I had the inside rubber on a K&N pod delaminate a bit, at WOT the chunk of rubber would get sucked against the AFM blocking flow, then drop back down inside the pod when I backed off.

Fuel Filter was replaced as in time with the service manuals, so doubt it would be ruined already. I only replaced my pump because i took it out and it looked really bad with rust all over it, I obviously don't claim to know a great deal about the whole fuel line system but how would that go past the fuel pump sock (filter in the fuel tank) and then into the actual fuel filter itself, unless it would be very minute, in which case it shouldn't affect the system in the first place. Not trying to be smart or a knowitall but that is my thought process, please correct me if I'm wrong.

The air filter is in good nick, just had it replaced recently, I will check again but I doubt that would be the case with a new oem air filter. (is it worth getting an aftermarket sports one, or even a cone? just for better induction noise and/or power)

Tomorrow I will disconnect the battery terminals and leave it for 10 mins and let the ecu reset, and then will try and post if the problem is fixed or worse.

jdm_wannabe wrote:About two weeks ago, I could rev the car all the way to the redline

What work have you done on the car in the last two weeks?

With the fuel, it only takes a very small particle to clog an injector, or to buildup over time and cause a clog.
Pumps are less forgiving (hence only the sock)

But yeah, the fuel filter should stop anything getting as far as the injector.




Air filter wise just get a normal one from repco or similar, should be around $50 from memory.

Pod filters are fine to add induction noise, but won't do much more.
But while you're troubleshooting its best to not do custom stuff until you've found your issue.

Fuel filter and air filter is a pretty narrow range of potential causes for this. With an NB8B the best starting point is normally to check the fault codes. If that doesn't reveal anything useful then here's my starting list of realistic candidates in the order I'd be crossing them off:
1. Failing coilpack(s)
2. Failing ignition leads (yes, NB8Bs do have them)
3. Poor wiring connection at coilpacks or MAF or ECU or TPS or Engine fuse box or anywhere in between all those
4. Water in fuel or blocked fuel filter or lines or fuel tank outlet
5. Failing MAF
6. Faulty coolant temp sensor
7. Faulty/damp ECU
8. Failing MAIN or INJECTOR relay
9. Blocked air filter or intake pipe
10. Failing spark plugs
11. Sticking injector
12. Loose or faulty cam or crank angle sensor or sensor plug
13. Slipped timing belt

If you live anywhere near salt air and/or park it outside (especially somewhere that gets pretty wet and humid, like Gympie) then I'd reprioritise to put all the minor electrical gremlins first.

First and foremost, I would like to thank everyone for their help and knowledge that they offered to give, it really is wonderful to be a apart of a community that is this good.

The problem is now fixed. I haven't done any mechanical work over the last two weeks, nor have I replaced any parts.
I was planning to go to buy the LED and wire setup to check for the fault codes, so I disconnected the battery, waited for 10 minutes, and then went to go get them. On my way there, I tried to rev it, and it worked, perfectly. The engine is fine and it revs as sweet as a nut, just as I'm used to.

This is what I'm worried about though, how did this happen, and why did disconnecting and reconnecting the battery fix the problem. Also how could this be prevented in the future? And what was at fault; the injectors, the fuel pump, the coils, the spark plugs, the air intake, even possibly the transmission and diff. I just want to get to the bottom of it all.

I really appreciate all the help that was given, and thank you for your trouble shooting. But how did this happen and is it preventable?

Thanks all.

Does anyone know if the ECU would impart a REV limit on certain errors? Disconnecting the battery would have wiped the error code, removing the limit but not fixed the problem. If the issue is still present the error will recur and the limit will be imposed again. This of course only possible if the ECU has this behaviour.

OP, if it happens again find out what code is being thrown and investigate.

It's hard to say what happened. All you've really done is reboot the ECU. It has a secondary stay-alive power feed to it, even with the ignition off, so normally it never completely shuts down.

The NA and NB ECUS don't have any learning function, apart from storing fault codes. Resetting them (in NAs this means depressing the brake pedal just after disconnectng the battery, not sure about NBs) simply deletes the fault codes. It has no other effect. I suppose it's possible that yours had developed a spurious processor/memory error somewhere along the way and the reboot fixed it but I woudn't be certain about that.

Maybe it's worth checking the ECU for moisture or corrosion? If there is some then it's not hard to take it out, open up the covers and drench it with electrical circuit board cleaner.