NA6 - 3000+rpm idle, hesitation between 900-2000, running rich?

[XMX]

Hi again,

I posted a few days back about some bad compression test results, basically down to ~70 in cylinder 3.

I pulled the cam cover off over the weekend and everything seemed ok, HLA's moved smoothly, valves from what I could see without removing cams looked fine.

Since I did the compression test there is an intermittent misfire/hesitation/stumble that is either on takeoff in first from~900-2000, or if you are in too high of a gear, ie 5th at 60km/h.

My partner, on her way home from work, found it suddenly started trying to idle at around 4-5000 rpm. I've tried to replicate this and can confirm while driving and clutch in, it wants to sit around 3000. Back in the driveway, it sits around 2500.

Idle adjustment screw is all the way in, with ten-gnd bridged.

I just changed the coolant temp sensor on friday, no change.

I'm thinking maybe O2 sensor, or it's jumped a tooth on the timing belt? I will try to upload pictures soon but when I had the cam cover off I noticed quite a bit of belt deflection possible between the two cam gears, and it seemed like the TDC marks were possibly out by one tooth. My reason for thinking this is the car is at 360000kms now, and while the timing belt was changed at 300000, that was many years ago since the previous owner only used it as a weekender. Can they stretch/degrade over time? It's a bosch belt.

I've previously cleaned the IACV and made the 'spray throttle body with carby cleaner' mistake, but had sorted that out I thought a few months back with the nail polish trick.

My thoughts are this:
1) Plug wires? Since the hesitation has only started since the compression test, if i kinked them could I have damaged them? They must be at least 60000kms old, and look stock...
2) Nail polish to seal throttle plate has degraded, meaning more air is getting through?
3) O2 Sensor? I've noticed the wire has been in contact with the exhaust manifold at some point, and I'd say it has probably not been touched for 60000kms also.
4) Skipped tooth on timing belt? I will try to upload a picture, but it did strike me as not looking 100%.
5) Dashpot on throttle. A few weeks back I adjusted the throttle stop screw (yes, I know I should have left it!) so as to be within spec of the workshop manual that states it should make contact with the dashpot at 2500rpm. Now when I've checked it, at closed throttle it doesn't make contact with the screw at all, which I had secured with red loctite. The dashpot can still compress.
6) Something fuel related? Fuel filter might be getting up to the 60000km mark as well.

So the problems are
A) Intermittent hesitation/misfire from ~1000-3000rpm- seems to be random, and can usually be pushed past by moving to a lower gear or clearing the deadspot zone.
B) Running rich/poor fuel consumption
C) 'Chugging' from exhaust (I thought of this as a separate issue related to the low compression in cyl 3)
D) Intermittent very high idle, up to 3/4000rpm once warm.

I've never been happy with the idle, I started messing with it as it had the idle dip problem since we've had it, which forums led me to believe was IACV. In the process I stuffed up the throttle body coating and I've been chasing my tail ever since.

Is there a definitive way to get a baseline to work from? At this point I feel there are too many variables to work out the cause(s), and bridging the diagnostics connector and turning key to ACC shows no fault codes.

Any help is much appreciated, sorry for the essay! I'll go through the service history and try to find any service items that are long overdue, but am unsure what the expected lifespan of things like 02 sensors, fuel filters and plug wires is.

[XMX]

I just checked the service history, timing belt was last changed at the 300,000kms service - which was 2009! We bought it at ~320000 a year ago and it's now 360,000... so is timing belt degradation/stretch reasonable after 8 years? No visible cracking on it.

[XMX]

Just a quick update in case anyone else comes across a similar problem...

The high idle was caused by the throttle stop screw working its way loose and stopping the throttle butterfly from closing correctly, I worked this out by backprobing the throttle position sensor and noticing it was registering 'throttle open' when it was supposed to be closed.

The hesitation/stumbling has been mostly cured by swapping to new NGK plug leads, what little roughness is left may be down to the poor compression, and I will change the fuel filter in the next few days to see if that makes a difference.

[StanTheMan]

sorry

Had this massive post. Ranting & raving. But really of no significant help to the OP

But basically I'd be chasing the low compression on No3

[93_Clubman]

Idle adjustment screw is all the way in, with ten-gnd bridged.

You may have fixed this by now, but the idle adjustment screw shouldn't be left all the way in.
It should carefully be screwed in all the way & then backed off 1.5 turns (manga_blue has suggested 2.5 turns) & fine tune idle from there, ie last time I did mine it ended up backed off 1 turn. There used to be a step by step for this, which specifically mentioned turning in & backing off idle screw prior to fine tuning, which I'll have a look for (might have been in Miata.net Garage).

Is there a definitive way to get a baseline to work from?

Yes, as you are probably slowly coming to the realisation, you need to baseline inspect & service the whole vehicle. Perhaps not what anyone wants to hear, but for some of these cars that are now approaching 28 years old with multiple owners of varying degrees of knowledge, skill & interest, it's the easiest way. Plus you learn plenty along the way & hopefully achieve improved reliability.

[Cus]

re: running rich / intermittent hesitation, my car did that when I first got it, turned out to be the O2 sensor was not plugged in .... then it got worse after plugging it in, then I replaced it and life was good.

fixing the compression on #3 will probably help a lot too, but that's way more expensive than an O2 sensor....

[XMX]

SOLVED!!

Just thought this might be useful info - it was 3x fried, cracked and overheated intake valves!

There was no possible way they could seal, which was revealed when a leakdown test pushed air out of the throttle body.

It seems that ticking hydraulic lifter noise we ignored meant the valves were staying in the way of the combustion which eventually damaged them beyond repair.

I changed to a new lower-km head, doing timing belt/water pump/head gasket at the same time, and the difference is amazing. Turns out it's had a slight chugging ever since we bought it that I had just written off as a mx5 thing, but now she purrs.

So, moral of the story? Don't ignore those hydraulic lifters, if they're not lifting you'll regret it!

Thanks to all who contributed solutions, at least I got a whole bunch of new sensors in the process of diagnosing this...

[93_Clubman]

More work & expense than you were probably hoping for, but at least it's sorted & running really well.

[XMX]

I don't feel too bad about the expense, I got the head for $250, top end rebuild gasket sets etc for ~$200 and took the opportunity to do an oil and coolant flush - all the sensors weren't that much either, add $100 for the leakdown test.
It's the time spent diagnosing I'm mad about, didn't even think about the valves until too late in the picture.

I just hope I can save some people time through my experience!