1.8 Miata sluggish at low rpm

Codes are P0420 catalyst system efficiency below threshold and P0301 cylinder 1 misfire.

The first one is probably due to the damaged cat from oil burning and what sounds like some exhaust leaks in that area.

The second is likely from today where the CEL was flashing on the highway. Last night I pulled the cyl 1 plug wire to see how it looked and it came apart in my hand. I had a used spare that I put on. I'll do new plugs and wires soon and check out the exhaust.

After some more driving the oil burning is most noticeable when the engine is cold, when warm it doesn't smoke much.

Did you try an LS swap?

In reply to sesto elemento:

That would definitely fix it.

Or at least create new and more interesting problems.

PS - A former co-worker stopped by today. He runs his Miata at... a lot of boost. He swapped from LS2 coils to IGN-1A coils because the LS2 coils didn't have enough umph over 28psi boost. He offered me the LS2 rack/harness. You interested?

Yea, kinda.

So, what did you find out?

I have it up on jackstands now to remove the cat and see how bad it is along with removing the loose heat shields. I haven't removed the valve cover yet to check timing.

Aside from being sluggish below 4k rpm (I know I said 2k before but I have been watching the tach more closely) at constant throttle while cruising it will surge a bit.

EvanB wrote: Aside from being sluggish below 4k rpm

On a Miata….thats normal. ;)

Well yes, to an extent. This doesn't feel normal compared to other Miatas I have had.

I didn't take off the cat yesterday since the nuts didn't want to move. I'm not sure that is the immediate problem since it seems like a clogged cat would reduce power at higher rpm rather than the other way.

Vacuum gauge will diagnose a clogged exhaust. And a bunch of other stuff also.

Vacuum gauges are real cheap

Maybe I just need to bolt on the turbo for the winter.

I do have some vacuum gauges, maybe this weekend I will try that.

What about removing one of the upstream o2 sensors (preferably not an a/f sensor) to relieve potential flow restriction, just for testing purposes?

Speaking of 02 sensors, try unplugging the upstream(s) and going for a drive. I've seen cars with the upstream 02 goofy enough that the car ran way better on open loop.

I have fixed many cars with the cat below threshold with Lucus fuel cleaner. I guess some times it is caused by dirty injectors. Have you tried disconnecting the battery to reset all the parameters the engine learn from the PO. Sometimes it makes a big difference. If you get a lot of random codes bad grounds are usually the problem.

Finally hooked up the vacuum gauge to test it out. It idles steady at 20 in hg with no fluctuation. Blipping the throttle makes the gauge quickly jump to 0 then back to 22-23" and slowly back to 20. Steady 3000 rpm shows around 21 in hg.

From what I have read this means the engine likely has slightly worn rings? No exhaust blockage though.

To check exhaust blockage you would need to make an O2 sensor-to-hose adaptor - take a drain plug from a Miata trans or rearend (18x1.5 pitch), drill and tap for 1/8" NPT, install NPT to 3/16" brake line fitting, and then a short length of brake line. 6" is plenty to keep the hose from melting. Install device in place of upstream O2 sensor.

The symptoms are generally the opposite of plugged exhaust, though.

So I drove it to work yesterday (15 minutes highway). It was doing fine until I was just exiting the highway and threw a misfire code and was running rougher. After work I stopped by my friend's shop and hooked up the scanner. The only thing evident was a weird signal from the MAF sensor. After cleaning that it seemed to run much better. Much smoother and seemed like it had more power.

I still got a misfire code on the way home so I'll change the plugs and wires and throw my spare coil on to see if that helps.

This would appear to be off.

EvanB wrote: This would appear to be off.

Wow, it even ran like that? Geez.

It was pretty close, the i and e marks are supposed to line up with the raised areas on the back plate.

In reply to EvanB:

Oh, yep, looked at that wrong and it looked way worse. First time I did the timing on my 1.6L I ended up a tooth off on the intake (Easy, seeing as how timing these is such an imprecise process). Drove it that way for an entire summer because the garage got tied up. Only thing I really noticed was a horrible idle once it warmed up.

Redid the timing belt and now it is much worse. Hooray!

Unhook the battery for 10 minutes and see if relearning helps.

pjbgravely wrote: Unhook the battery for 10 minutes and see if relearning helps.

Are Miatae that smart?