1997 Miata is sluggish at low rpm, occasionally idles low and shakes

Good morning,

My 1997 Miata M-Edition occasionally (like a few times a month) idles very low (200-400rpm) at startup and shakes violently during this phase. After about 20-40 seconds of this, the car seems to catch up and the shaking stops. Furthermore, the car is always sluggish, especially in the low end.

My mechanic first thought it was a loose spark plug wire. However, this did not fix the problem. He also suggested the MAF may be on it's way out, but it's still "in range" so he did not replace it.

I've recently had the timing belt, spark plugs and wires, and fuel filter replaced. No codes are being thrown.

I'm going to replace the MAF this week just as a precaution, but have any of you guys run into this issue? Any ideas on how to remedy this?

Thanks for your time and input!

I would not replace the MAF "as a precaution".  The only time I replace MAFs, they are new aftermarket parts that someone threw on.  Replacing the MAF is more likely to add a problem than prevent one.  They are almost always just a little bit miscalibrated.

Sluggish at low end sounds like the intake camshaft may be off a tooth.  That may also explain the startup behavior.  A dirty throttle body can also cause cold start issues.

Thanks Pete! I won't replace the MAF. Really appreciate it, I'll see about getting the intake camshaft looked at.

I'd start by checking the base idle.  After the car is warmed up, jump the TEN and GND pins in the diagnostic connector and check the idle.  You're looking for 850rpm. If it dips dramatically when you jump the pins, that's your culprit; just adjust the idle screw under the rubber cover on the throttle body.  Also check the base timing while you're in test mode.

The combination of you having had the timing belt replaced, sluggish in the low end and poor idle sounds a lot like the timing belt being off a tooth, but this wouldn't be intermittent, and you indicate it's intermittent.  Also unclear from your post if this happened before you had the timing belt replaced as well?

Is it always sluggish in the low end, or just when cold?  If the base idle is fine when warm, I'd be looking at the cold idle. There's a wax cylinder that's responsible for upping the idle when cold, though I've never heard of one going bad.  The idle air control valve is a more common failure.
 

Hi Berck,

Really appreciate the input! I will check out Miata resources out there to see if I'm mechanically inclined enough to check the idle and timing. It's not my DD so maybe it's time for me to learn! 

I don't recall the car being sluggish or having the low idle problem prior to getting the timing belt replaced. I only had the car a few days prior to having this work done, though.

Low end is always sluggish, and the super low idle issue can happen when the car is both hot or cold. There doesn't seem to be a pattern.

Thanks for your time and advice.

That sounds a lot like a timing belt off a tooth, then. With the caveat that if you're new to Miatas, the low end is always sluggish:) 

This is my 3rd. I had a NB and an ND. This one is definitely sluggish! Thanks Berck. If you don't mind me asking, is getting the timing belt adjusted a big job? Just wondering...

Edit: Looks like a job beyond my skill. Off to the mechanic!

Assuming that's the same mechanic who installed the belt, this could also be the cheapest option.

Hey Paddy,

Unfortunately that mechanic was a friend that lives across the state, LOL. Oh well, that's what you get for trying to save money.

You can check the cam timing by removing the cam cover and rotating the crank until it's at TDC (the timing mark lines up with the T). At that point, the intake cam gear should have a notch that's right at 12 o'clock with an E underneath, and the exhaust cam gear has a similar mark at 12 o'clock with an I underneath. If it's off, it'll be off by a full tooth.

If you have the crank at TDC and don't see the marks, rotate the crank another 360 degrees. 

That'll let you check fairly quickly and easily without tearing down too much. It's possible for a clever mechanic to realign the intake cam without removing anything below the thermostat housing, but you do have to know what you're doing :)

I'd start with ignition timing first simply because it's easy. Also, make sure all the various connectors are seated, like the ones on the coils.

Thanks Keith. I'm on it!

Mechanic says the timing is fine. Drove it around and says the car is fine. That does not solve the intermittent 200rpm idle issue that happens every so often though. Something is definitely wrong with it, I just don't know what!

You could try cleaning the IAC (sometimes called the ISC). It's a solenoid-driven piston that regulates airflow past the throttle body for idle. If it's sticking, I can see it causing this behavior. Remove it from the bottom of the throttle body, clean it out with some throttle body cleaner, and see what happens. It's got a reusable rubber gasket.

Do not clean out the throttle body itself, there's some sealant in there that people confuse with buildup.

Thanks Keith will do!

I don't know if I'll be able to get to this thing.  Having trouble removing the bottom hose. Is there an easy way to remove the IAC? Appreciate your time!

You can take the throttle body off the intake manifold without too much trouble. You can even leave the plastic intake plumbing attached. Then you can separate the IAC.

Thank you Keith!!!

You might want a new gasket or some RTV to seal that throttle body when it goes back on.

Yes, thanks, Keith. 

Stuck on the last bottom coolant hose. Calling a friend to see if we can get this puppy off. Thank goodness this isn't my DD! Edit: down to the last screw on the TB connecting to the IAC. It doesn't want to move.

Where do I spray the TB cleaner? Thanks guys! Edit: I cleaned it out by watching some youtubes. Motor up so it didn't get TB cleaner in it, and sprayed the spring inside from both holes. Will provide an update tomorrow once I get it all back together.

Welp! I broke it. Tries to idle then dies. Lol live and learn. More updates tomorrow.

Edit: Gave it a little gas, now it will idle low and shake. RPMs go up when AC is turned on but then dies down back to 400 or so. Link below.

https://youtube.com/shorts/uc_ByBn5SlU?si=DnJiXI7nWW_8GXqu

Hey, it's consistent now! :)

I'd make sure you hooked up all the hoses between the MAF and the engine and plugged the IAC back in. Then I'd set the idle using the bypass screw, it's possible someone adjusted it to get around a problem. It's easy to do, all you need is a paperclip and a screwdriver.

Thanks for all of your help Keith! Thanks for your time.

I just started the car this morning with no adjustments and everything is great. Car came up to temperature and air conditioning doesn't cause any trouble. Purrs like a cat. Cars are weird.

I can't confirm that we're fixed yet, but so far so good. Off to flyinmiata.com to see if there's anything tickling my fancy. Thank you so much Keith!