MX-5 Mk 2.5: Running rich, poor and idling...

Hi All


apologies if this issue has been discussed before - I have searched many forums on this topic but most reported faults seem to be subtly different.


my car is:

  • 2004 mx-5 mk2.5 1.8L

  • 50k miles

  • K&N air filter



Up until this issue my car ran very sweetly (it’s still low mileage and has had an easy life).  Good fuel consumption, low oil usage etc.


However, it has now developed the following symptoms:


  • runs very rich (black plugs, strong smell from exhaust, black smoke)

  • ‘searches’ whilst idling, rpm sharply drops and then recovers

  • occasional stalling whilst idling

  • when revved, great deal of smoke from exhaust and revs do not remain constant

  • loss of power, car barely driveable now!

  • no error codes reported!



I have so far tried the following:


  • changed coil packs, plugs and leads: no improvement

              - plugs were black (I suspect a symptom, not fault)


  • (very carefully) cleaned MAF sensor: no improvement


  • Checked all vacuum hoses - both:

              - visually (hoses all in very good condition)

              - using a propane ‘car start’ spray (no change in idling observed)



I’m wondering if the MAF sensor has failed, although it looks immaculate and is still very clean.  Also, no error codes have been generated and the flow rate seems reasonable (using a obdii reader).


I’ve discounted o2 sensors as the car idles very badly from cold (as well as when hot), and I don’t think the o2 sensors are used when running in ‘open loop’ - but I could be wrong.  I also see a ~0v - 1v range using the obdii reader.


I’ve read that it could be due to poor earthing - but the car is very clean (but it’s a possibility I guess)


I’m not seeing anything obviously untoward when looking at the obdii reader voltages, but I could easily miss something I guess.


So I’m at a loss - any help would be hugely appreciated!


My money would be on a failed MAF


all symptoms point to it so that’s 100% were I would start

It does sound like MAF but there would usually be an error code.

Given the lack of error code I would also swap out cam and crank sensor. Both can cause issue without a code. It is more likely to be the crank sensor but the cam sensor is so much easier to check first.  

I would also check the ecu. It sits under the passenger side carpet - perhaps water has got to it and is causing issues.

No error code at all on the few I have changed on a Mk2.5, remember to get the one with the correct number do not have it handy just now.

Re earthing the owners statement does not hold water at all.

Follow earth wires back to the body work, remove clean and put back on including the one from the battery to the frame between the diff and gearbox and check that all earth wires are not corroded themselves be quite rough on them.

MAF on the MK2.5 is marked FS1E, like the very collectable moped

MK2 has an FP39

They look the same but the FS1E has a blue dot on the rectangular part of the case on the top.

I am told they are not interchangeable - not tried myself.   

Agreed with that number

Also just noticed that the car is specified as having a K&N so probably another case of the oil off the filter getting on to the hot wire element in the MAF.


As above but I would change the temperature sensor at rear of the cylinder head.(under coils).About 12 pounds.3pin triangular shaped connector.We have the FS1E ?? MAF if you do need one.

Yes the temperature sensor would also be a candidate but you would get a code 9 on the older diagnostic system for that assuming it was throwing a code. As we are aware the correct fault codes are not always captured and the sensor is inexpensive but you do need to do a part cooling system drain and refill to change it.


Thanks for all your help so far! :slight_smile:

Wrt the MAF getting spoiled by the K&N filters - yes, I’ve heard that could happen as well (it’s still well worth having the K&N though!!! :slight_smile: )

Anyway, I replaced the cam sensor and MAF, I did it all in one go (as opposed to trying each in turn!) as I only had 30 mins free to do it all in!!!

anyway, changed them and took it out for a spin. good news and bad news.

the good news is the car revved very well, and no longer appears (smells) to be running rich.

however, very lumpy at low revs and does not idle at all well (simply stalls).

I’m still using the now heavily blackened plugs and some old ht leads - so I’m going swap them over first asap.

no error codes still.

however, I plugged in my obdii diagnostics module and paired it to my phone app.
now I know this is by no means a professional piece of kit so I take its readings with a pinch-of-salt.

anyway, I took it out for a thrashing along the motorway, and this is what I saw:

  • throttle position responded correctly (easy to check! :slight_smile: )

  • o2 sensors both seemed to be behaving, insofar:

    • I saw them both return values between 0-1v (at various different points)
    • when I floor the accelerator pedal they blip up to max then drop back down (which seems reasonable to do)
    • o2 sensor 1 slowly oscillates / wanders up and down whilst idling
  • the car switched from open loop to closed loop after about 20 mins driving (it was just above freezing outside)

  • the air flow indicator was responsive, ie when the engine revved under load (and I could actually hear the air being sucked in), the reading responded accordingly. I don’t necessarily trust the calibration of the reading - I’m not even sure what units it’s measured in, but I suspect it’s g/s. anyway, it peaked at around 100 ‘g/s’ - but I have no idea as to whether this is reasonable or not. All I can say is that it seemed to peak at the right times, ie under load + revs, so it certainly seemed to be responsive.

  • the intake temperature read around 33 degrees C (from memory) - which seems about right

  • short term trim danced around a bit, hard to tell what it correlated to. long term trim stuck at 0 – which i’m assuming is a good thing.

as I say the car revved (under load) beautifully - between 4500 - 7000 it seemed to be back to its old self - so I think that fundamentally the ‘engine’ is good.

as mentioned earlier I’m going to put nice clean plugs and new leads back in.
then may take a look at the coil pack (I have a spare which I could try).

any other thoughts ideas how to sort lumpy low / mid rev range and poor idling?
and once again thanks for your help so far! :slight_smile:


quick update.

replaced coil packs, plugs and leads (i had a spare coil pack and reverted back to my pretty much new ht leads).

filled up the tank, battery freshly charged - took the car out for a spin along the motorway.

started up immediately, after 1 turn. then promptly stalled :frowning:

undetered, i took the car out - it revs well, 5000 - 7000 good :slight_smile:

however, still lumpy at lower revs and prone to stall (hot or cold).
the car idles at around 800rpm (when warm) which seems reasonable - then what happens is that it will suddenly drop dramatically, and will bounce around 500rpm then drop off completely - at which point i can hear a rasping sound come from the engine (a ‘death rasp’) and then it dies completely.  (re)starts up fine after this.

at a total loss now.

i’m pretty sure that the issue with which i started this thread - loads of smoke, barely driveable, mis-fireing etc, has been resolved by changing the MAF / Cam sensor (sadly i don’t know which one actually resolved the issue - but i’ll swap out one once my car 100% - just so everyone reading this thread knows what actually fixed the primary issue).

sadly, left with lumpy in low revs, under load, and stalling.

any ideas welcome!
anything else i can look for to add to this thread to help diagnose this issue?
(as before no diagnostic errors).

In the years of a rotary distributor with contact breakers, where the ignition timing was adjusted by rotating the distributor, the symptoms you describe sound like the ignition is too far advanced. I think the ignition timing is done by rotating the CAS, it is on the mk1 but I may be wrong. Anyway, I think it would be worth checking the ignition timing.
Good luck.

As you have replaced the CAS this might make more sense.


thanks for getting back to me - yes i did change the CAS (along with the MAF).  but, i don’t think it’s actually possible to change the timing as such - it’s an ECU mapping parameter - so, i’m not sure what i can do here (other than put the old one back and see what happens).

Both replacement parts are OEM parts - the Cam Shaft Pos Sensor is manufactured by ‘Blueprint’ and the MAF by ‘Cambiare’.  Has anyone else got experience with these parts?


i also have a Triumph Spitfire, and have often tweaked the timing on that in the past (by rotating the distributor head as you mentioned) - to make it advanced as possible before it ‘pinked’ - however, i don’t get any noticeable ‘pinking’ on the Mx-5 (although there may well be another mechanism preventing ‘pinking’).

so other than putting the original CAS back on and taking a peak at the plugs for any clues, i’m not really sure what to do.

The ht leads are about 1 year old - they’re the blue magnecor ones  from mx5 parts - i’m assuming that they’ll still good.

Having done a bit more research, you are right. The ignition timing is fixed on the mk2.5 so I was talking rubbish. Sounded plausible though.
Might still be worth putting the original CAS back just to try. Relatively quick and at least that will eliminate it.
Good luck.

You are correct - unlike the MK1 there is no adjustable timing with standard setup on the MK2/MK2.5 . On the MK1(Eunos)  the CAS - cam and crank sensor can be rotated to change/adjust timing. The CAS, cam sensor only on the MK2 and facelift has no adjustment.

Have you checked the diagnostics again for an error code?

If there is no code to point anywhere specific I can only suggest crank sensor or pickup - this is the plate that runs with the crank pulley that controls the crank sensor. These can get damaged/corroded and cause problems - teeth have been known to fall off.

Only other suggestion is damp ingress in ECU or possibly throttle body.        


thanks for getting back to me.

i’ve put the original CAS back on and it’s made no difference - so i’ve ruled out the CAS.  So, it was the MAF which caused the original issue at the start of this thread.

The car is running fine, except for misfire / loss of power at lower revs and poor idling.  Revs well.


i’ve noticed that the exhaust is blowing - i stuffed a rag in the end and the engine still ran!  that probably isn’t helping matters.  i can see an obvious blow just after the box - not sure if that’s the only place though.


i’ve re-gapped the spark plugs - a couple were too tight - didn’t seem to make much difference.  the spark plugs were all coloured the same - whitish tip, which i guess indicates that it’s a little lean, which surprised me a little - i expected a rusty colour.  could this be a clue (the plugs are new - and i’ve only taken the car out for a couple of short runs - so this may be to be expected)?


the ecu is bone dry - i took the plate off and took a look (removing the 2 dome headed bolts was a little tedious! :wink: )


still no error codes.


i’ll take a look at the crank sensor (and exhaust while i’m at - as presumably i’ll need to jack up the car to get to the sensor).

Just another thought. These cars seem prone to earth problems so if you’ve got some jump leads connect them between the battery negative and a good solid point on the engine. Would only take a minute and you never know.

Go back and read all the posts again and check the things you were advised to do but have not done.

Hi Eddie

i sense a slight frustration from you that i haven’t tried all the things that you and other people suggested at the begining of the thread.

i wasn’t willfully ignoring them, however, as you’ve no doubt noticed the original sympton no longer exists.

the original sympton rendered the car almost undriveable and filled the garden and house with fumes. this was fixed by replacing the MAF.

the current issue is far more subtle - in fact a passenger in the car wouldn’t even notice an issue (where with the orignal sympton the passenger would have died from the fumes!!!).

so, it’s not clear to me as to which of the orignal suggestions are still relevant to the car in its current state - poor idling and pulling at low-medium revs.


the areas of earlier advice which i’ve missed are:

  • poor earthing - which i will investigate tomorrow

  • crank sensor? is this a sympton of the original issue or the current or both?

  • temperature sensor (as above)

is there anything else that i’ve missed?