Miata - funny stumble

91 Miata, 1.6 short nose engine. Runs like a top, except....

When under load below 2000 rpm. Then it stumbles and bucks. The heavier the load, the more it stumbles and bucks.

When the tach hits 2,000 rpm, it switches to running smooth. Almost like throwing a switch. I say almost because it isn't always exactly 2000 rpm that it smooths out, and it doesn't always do so instantly.

Chased the usual suspects (plugs, wires, cam sensor, crank key), everything is fine, and it's got new plugs and wires (didn't help). I get no indication of any problem when working in my driveway.

I know the normal thing would be to keep the engine out of this rpm range and load. And normally I do. But at the moment I'm dealing with emission testing, and this is causing me some headaches.

Any clever thoughts or Miata experience to draw from?

Fuel delivery?
Filter, pump or injector?

Make sure your crank pulley is straight and not wobbling at all. I've having weird idling and stumbling issues with my '91. It turns out I have the dreaded short nose crank problem. The ignition timing and cam timing are off.

Anyone have good 1.6 long block they want to sell?

foxtrapper, my first guess would have been plug wires. You've addressed that, but just in case someone else is chasing the same thing...

Was any work done to the car just before this started, or did it happen spontaneously?

Type Q wrote: Anyone have good 1.6 long block they want to sell?

Yes, but I am a long way from CA.

My escort was doing that under 2k when my exhaust was leaking. It was ahead of sensor one and was throwing a lean code. Probably not the same issue, but it sounds like the same result. You're running lean.

Any codes with the CEL? My first Miata started running rough and bogging and it was the coolant temp sensor at the back of the head. It was more intermittent though.

Kinda hard to say Keith. I bought the car something like three years ago. Don't remember it doing it on the test drive. Promptly burned an exhaust valve, pulled the head and replaced it. As long as I was there, I replaced the old cracked exhaust manifold with an ebay header. Shortly after that the radiator split and it overheated nicely limping home. In doing the valve (replaced all the exhaust valves, since I was there), I apparently didn't install a seal right, as the engine occassionally smokes like a freight train for a minute after a cold start.

Somewhere along the lines in all this, the problem started showing up. It's not consistent, it may go days or even weeks without incident. And then it can have a week or month of being very obnoxious about it.

I think it's a red herring, but sometimes if I throw a can of seafoam or the like at it the car behaves very well for a few days afterwards. But not always. I've scoped the intake, everything looks spiffy clean before the seafoam or such.

It's the below 2,000 rpm that throws me. Maybe that's a red herring as well. Toyota's of that era were hinky with their throttle position sensor and temperature sensors, and they could cause a similar sort of problem. Well known and easy to troubleshoot. But I find no mention of this sort of thing with Miatas.

Even tried rebuilding the injectors the other day. Flow checked them with my eyeballs, pattern looked fine. Installed new filters and new caps. Didn't help the problem.

No codes. Though I think I remember the dash lighting up about a catalytic converter overheat once years ago. But that doesn't sound right, cause the 91 doesn't have a light for that as I recall.

I've played with it in the black of night on humid foggy evenings, hoping to find a phantom ignition short I could trace. Nothing. Wiggle harness pieces around, nothing.

Grr, should have run over to the emission station last week when it was having a good day.

Don't find mention of a problem like this over on the miata boards. Oh, plenty of "it runs rough", but not this way. It's just odd.

Could it be the AMM is acting up? Have you tried cleaning it?

Is it a Eunos? The Japanese cars have a cat overheat light. It's JDM, yo.

I'd check the throttle position sensor. On a 1.6 Miata, it's really just two switches. One closes at idle, one closes at WOT and doesn't do much other than cut out the AC. You can pop the top off the sensor and confirm it's working properly and that the idle switch opens with the first bit of throttle movement - although I'd expect you'd see different behavior on light vs heavy throttle if that was the case.

Evan - AMM?

There's something tickling at the back of my head about the 2000 rpm change. But I can't think of what it is. If you don't get it figured in the next week, remind me and I'll check with Bill.

AFM? MAF? I forget what they are called on these.

AFM on a 1.6, MAF on a 1.8. I figured that's what you were talking about, but I see new and unexpected acronyms every day on forums so I figured I'd check!

Keith wrote: AFM on a 1.6, MAF on a 1.8. I figured that's what you were talking about, but I see **NAU** acronyms every day on forums so I figured I'd check!

FTFY

Eunos? JDM, yo?

Won't hurt to double check the throttle switches. Same the air valve. Everything indicates they are working right (idle is steady, jumps up just right when the a/c compressor kicks in).

I'll play with the afm with a meter. Don't think anything is off there, but again, an easy check.

Interesting comment about air leak between the afm and the engine. Hadn't really considered that, but if it's a small leak, the 2000 rpm may be a result of going past the range of significance. Worth checking the pipes more closely.

Don't see how the fuel filter could play this way, but it's cheap and I don't know if its ever been replaced. 300k miles, it's probably due.

I keep looking at the way the problem goes away at 2000 rpm, and wondering what's changing. I keep suspecting something is being read, and then not read at that point (or visa versa). But I can't find anything in the manual or online to indicate what. Be it the temp sensor to the ECU, the afm, O2 sensor, dunno.

MAP-based cars (no AFM/MAF/VAF/VAFM/whatever you want to call it) will often exhibit this same behavior when the IAT sensor goes bad.

What the E36 M3 does this have to do with your AFM/MAF/VAF/VAFM car? Most of these units have an IAT sensor built into them. When they go bad, funny things happen.

When I was in the high tech industry, we used to refer to TLAs: Three Letter Acronyms. My wife still throws TLA into reports sometimes just to see if her coworkers will ask what it means. They never do. Anyhow...

Eunos is what the car was called in Japan. You'll know as the wheel will be on the other side of the car

The AFM is a hassle to check with a meter, unfortunately. Easier to check by swapping in a known good one. But that's a good suggestion. A dead spot might do this.

There's some changeover where the car decides "I am definitely not at idle" at around 2000 rpm, I think. That's what I want to run by the other guys when they're back in the office. And yes, there is an IAT inside the AFM. FWIW.

My E34 did something very much like this and turned out to be a worn spot on the AFM's potentiometer track.

My bad-plug-wire experiences all had a strong temp/weather component FWIW.

Haven't had a chance to even look at it, but I thought I'd add two more symptoms.

If I simply crack the throttle off idle, the engine will stumble. If I roll it on, the engine revs up smoothly. I'm not talking mash the pedal, I'm talking about bringing it up off idle to the emission test 2500 rpm. It's a real little throttle opening. If it were a carburetor, I'd say bad accelerator pump.

Trying to hold 2,500 rpm can get the engine surging by about 400 rpm. Very smooth and steadily, pulsing about every 3 seconds or so. Just up, and back down. Or down and back up. This seems to be a new trick, and it doesn't always do it. Very suspicious of my IAC for this one.

Found this weird article, which I suspect is a April fools joke. http://www.miata.net/misc/simplemaint.html

Have the factory service manual, nothing in the ECU inputs matches the symptoms or rpm ranges. Draft.

Hopefully this Saturday I can spend some quality time with the VOM meter.

My newer (NB) Miata had a low-RPM stumble that was caused by EGR port clogging (until I cleaned it out and moved a vacuum hose).
This is a pretty well-known issue with some of the 1.8 cars, but I do not know if it applies to the earlier cars.

More Info

I just picked up a 99 and noticed I have a little stumble under 2k rpm too. I might have to check my EGR port.