Do i have a problem?

2000 miata. Stone stock.

Averaging between 24-26 mpg. Mostly 55mph highway and 70mph interstate. Very little stop an go, and very little idle time.

No check engine light. California emissions car.

In last 5k: plugs, wires, timing belt, pcv, fluids and filters.

Car has an occasionalidle oddity. Kinda a hiccup. Kinda a stumble. It doesnt seem as strong as it was before the timing belt job, but i may be imagining that. The mpg is what throws me. Thats all top up, ac on, 87 octane by the way.

So, do i have a problem with my answer?

That's about right for mileage They get a lot worse real world mileage than you would think. Problem is they have horrible aero drag and just a little under body clean up does wonders.

Don't forget, they aren't geared for fuel economy. I get 28-29 with my hardtop on. Your MPG is definitely in the ballpark. I'd investigate the coil packs as the source of that stumble. I seem to remember the 99/00 cars would go through them every 50,000 miles or so

Ok. Just wanted to make sure that im imagining it.

Bmw: Can you elaborate on what you mean by a little under body clean up?

Basically the rear bumper is a parachute. At higher speeds, it actually will cause lift. That's why you see the track guys with holes cut in the bumper to smooth out the low pressure area that forms down there.

It's just really dirty from an airflow perspective. Look under there and you will see no plastic barely to help smooth out the airflow.

I'm trying to work out a series of underbody panels that works for my NC because of the lift at speed. I fdon't want to cut the car up so I'm looking at it from another way.

24-26mpg? I've never seen anything above 20mpg in any of my cars...

I am not surprised that the Miata has such bad mpgs, convertibles have terrible airflow, even with the top up.

I would also try a tankful or two of premium. I have found that most modern computer controlled cars do pick up a few MPGs with premium, usually enough to justify the higher upfront cost, plus they run better with slightly more power because the computer is not pulling the timing

Check to see if the short plug wire at rear is nice and tight. Sometimes they are a little too short.

Yes.

You have entirely too many cars and projects.

In reply to Toyman01:

I actually agree with that. Probably because im too old to enjoy skimpy shorts.

But im having fun most of the time!

I will also double check the plug wires, and try a tank of premium when i fill up tomorrow. (I go through a tank of gas a day, usually)

Yeah, you have a problem, that milage is way too high. Needs more boost. :)

The stock 99 ECU (not sure about 2K, but probably the same) runs open loop above 4000 RPM or so. There's a big difference in milage between cruising at 70 and 80.

dannyzabolotny wrote: 24-26mpg? I've never seen anything above 20mpg in any of my cars...

But are any of your cars Miatas? If not....

Anyway, have you checked your timing recently? If it feels sluggish after the timing belt I would just double check that and air in the tires. Mid 20s is about the ballpark there.

Low output engines under moderate load can give pretty doggone marginal MPG's even in little cars. Take a 6 speed Miata and a 6 speed C5 convertible and I'll bet the C5 pulls better highway mpg's 9 out of 10 times.

Stupid question: how would i even check/adjust the timing without a distributor?

Dusterbd13 wrote: Stupid question: how would i even check/adjust the timing without a distributor?

You basically can't adjust it on an NB without an aftermarket ECU.

You check it like any other car. There's a jumper in the diag box to force it to idle at 10 degrees, then you hook up a timing light. If it's wrong then there's something very strange going on.

The NB ECU runs the timing off the crank angle sensor, using the cam sensor just to tell it which cycle it's on. This means (I think) that getting the timing belt wrong won't manifest as incorrect spark timing, just incorrect cam timing. To check that you'd need to pop the cam cover off again and check the position of the sprockets.

But is what I'm talking about enough to warrant that much tear down? Only way i know to check belt timing is strip everything off so i can see the entirety of the belt and marks.

Well, being off by a tooth is a fairly major difference (12 degrees of cam timing, IIRC), so it's not particularly subtle. That said, it's not all that hard to check on a non-VVT motor. You don't need to take everything off, because you only use the mark for the crank to find TDC. Instead, pull the valve cover and the top timing cover, take out the #1 spark plug, put a long screwdriver down the hole (so that it sticks out the top), and turn the crank until it comes to the top of its travel. That's TDC. You can now check the cams for alignment, they should look like this:

(note that my intake gear is an adjustable Toda one, so mentally substitute a stock one for it if that's what you've got).

So...if i may piggyback off this...my 99 used to do the normal bouncing afr thing from 13-15 when under light load cruising at low rpm. You know, normal closed loop stuff.

Now at light throttle, it does that for 4- 5 bounces and then slowly drops to 10.0-10.5 afr. Mileage has correspondingly dropped. Where the heck do i start looking for a reason the ecu is going open loop or adding more fuel or...some...thing?

RedGT wrote: So...if i may piggyback off this...my 99 used to do the normal bouncing afr thing from 13-15 when under light load cruising at low rpm. You know, normal closed loop stuff. Now at light throttle, it does that for 4- 5 bounces and then slowly drops to 10.0-10.5 afr. Mileage has correspondingly dropped. Where the heck do i start looking for a reason the ecu is going open loop or adding more fuel or...some...thing?

When did you last clean the EGR passages in the intake manifold?

Not really relevant, but my B3 powered '94 Miata got 52.27 MPG last week . Normally it gets mid to high 40's

In reply to dannyzabolotny:

Redline shifts aren't nessesary 100% of the time.

codrus wrote:

RedGT wrote: So...if i may piggyback off this...my 99 used to do the normal bouncing afr thing from 13-15 when under light load cruising at low rpm. You know, normal closed loop stuff. Now at light throttle, it does that for 4- 5 bounces and then slowly drops to 10.0-10.5 afr. Mileage has correspondingly dropped. Where the heck do i start looking for a reason the ecu is going open loop or adding more fuel or...some...thing?

When did you last clean the EGR passages in the intake manifold?

EGR was deleted by the previous owner, and I've put about 1400 miles on the car before this richness issue showed up. As of this morning it seems like it may be accompanied by a hunting idle and a code for the idle rpm being off from the target rpm (p0507). Can a bad IACV cause things to run rich at cruise somehow?

Admittedly not stock, and usually driven in anger, but the Manic Miata has never broken 20 mpg in my ownership, and usually runs somewhere around 16-17.

I can't speak for this particular engine because I don't remeber ever having done a timing belt on one, but every car that I have put a timing belt on and had it a tooth off I knew on the first test drive that something was definitely wrong. Either really crappy idle or really crappy in the higher rpms. I think if that was your problem it would be driving worse than what you are saying.

My money would be on a coil going bad. I had one causing a rougher than usual idle and occasional misfire under load.

With a '90 Miata, I would see 30+ MPG on an Interstate. Even my MSM can get over 30 MPG depending on hills and how hard the turbo works. With normal commuting through traffic and spirited driving, I'll get 24 MPG.