This is a crosspost from OT.

I've looked at a Miata yesterday and have a quick question for the experts. Does a white srak plug end means the engine is running really lean? I mean extremly white, not just a little and this is on all 4 not just one. Also the engine stumbles at low rpm but runs fine above 2500. The plugs are clean, no deposits at all. I'm guessign that maybe the MAF is innaccurate, making the engine lean out.

 

Yes, white is an indication of a very lean mix. Don't know anything about these cars...........is it a rotary engine?

 

It's a conventional piston engine. 4cyl, 1.5L DOHC. It's none interferance..so that is nice. Picked up another used AFM for $35, lets see if it fixes it.

 

check the O2 sensor it could be bad, also check the air cleaner, and make sure that the correct heat range in the engine and take a fuel sample and smell it it could have extras thrown in , Do the tips ( electrods)look like they are rusty? if so then it could be a head gasket leaking coolant into the cyls

 

Thanks for the tips.

This car is my father's and he had it for over 4 years and it developped the stumbling issue about a week ago. THe O2 is less than a year old, the plugs are Bosch +4 and less than a year old (they all appear to be in good condition), no rust on the tips either, except they are extremly white (as if someone painted them with white out). The engine is not usign any excessive amount of oil and the coolant never needs filling. There is nothign added to the gas either. My father would be too cheap to add anyting but 87 in it! lol The MAF, or more accuratly the AFM (barn door type, like the 16V US 928s), had to have the circuit board re sordered(sp?) when he bought the car, so I would not be suprised that it finally faild.

 

Check for intake leaks, but I'm betting that a new O2 will do the job.

 

The only thing I can add to the above is that you might want to also change the fuel filter if it hasn't been replaced in a while. Could be plugged.

Ken

 

you really can't tell much by looking at the plug tips
Remember normal driving is always in the lean range.
The stumbling part might be a concern
Try disconecting the o2 sensor. If things improve then thats it.

 

Thanks for all the help, I'll try all the suggestions. I don't think the filter is the issue as it runs fine above 2500.

 

The engine needs more fuel or less air but be cautioned that our fuel doesn't give 'normal' colours.

A plug chop is required to ensure a good colour and then its only with new plugs; in my past racing world we'd 'chop' the engine at max rpm in 3rd, coast to halt, pull the plugs and go from there. As Mark mentions its not the tip you should be looking at but further down the bakelite.