Help! Missing Miata!

Missing in the stumbling sense.

My 99 started to have a stumble a couple of weeks ago. After it would warm up, when I was driving at about 2500 rpm it would feel like I had abruptly let up on the gas and then returned to normal. At first it was so slight that I actually thought that I might have done it myself without thinking. But it got progressively worse until when I would pull away from a stop it would buck, like someone learning to drive with a clutch, only it was doing it all on its own.

I replaced the wires and plugs, and it didn't change, might have gotten slightly worse. I had a spare coil pack so I swapped that in, and it ran great for two days, but it was probably only driven 15 - 20 miles around town. This morning I took it about 3 miles and it started to do the bucking bit again. It feels like the gas is cutting in and out. I hadn't changed the fuel filter since I picked up the car, so I put a new one in tonight.

Now when I start the car it will not rev if i push the gas slowly. It will slowly bounce up like I was tapping my foot gradually accelerating. But I am holding constant pressure. If I start at idle and immediately floor it, it will jump right up. In gear it will not accelerate smoothly.

I am out of ideas ... HELP OH GODS OF EVERYTHING MIATA!!!

Watch the crank pulley as it runs. I've seen more and more 1.8s have crank nose issues. If you catch it early its an easy fix.

OK, I'll check it. But why would that cause a stumble?

Sofa King wrote: OK, I'll check it. But why would that cause a stumble?

Wobbled out keyway causes timing to alter. Common symptoms include a poor idle but running like a raped ape at high RPMs or the exact opposite depending on which way the timing gear decides to walk at any given moment. Has the timing belt been messed with lately?

I replaced the head gasket on this car about a year and a half ago, maybe 5K Miles. Timing belt was off and looked new.

Sofa King wrote: I replaced the head gasket on this car about a year and a half ago, maybe 5K Miles. Timing belt was off and looked new.

AHHA! But was the crank pulley bolt torqued correctly? Even if it was they can get loose, but most of the ones I've seen have been removed at least once. IIRC its not a stretch bolt, but should be treated as one.

Or I could be way off. Just one thing thats pretty easy to check.

I can honestly say that I haven't looked at the crank pulley since I put it back together. I will check it.

But, it doesn't feel like a timing issue to me. It feels more like fuel delivery. I thought from the start that it was the fuel filter. I was surprised when that didn't fix the problem. I also put in some injector cleaner, but the car isn't running well enough at the moment to run much of the cleaner through the system.

Changing the coils seemed to help the problem for a short time. Is there something that could have killed 2 coils and still not turn on the check engine light?

Check the coolant sensor on the back of the engine. I

In reply to MrJoshua: Good Idea, I thought of that earlier when it wasn't running too badly.

Before it ran fine until it warmed up, now it starts running poorly right away.

In reply to Sofa King:

Mine behaved similarly and replacing that coolant sensor fixed it.

Thanks. I have a 95 that I am working on at the moment that I could borrow a sensor from. Any idea if they are the interchangable with the 99?

They're not, sorry. You need one from a 1999-05. If you have access to an OBD-II scanner that shows real-time data, you could see what the ECU thinks the engine temperature is and that would help identify if the sensor is accurate.

I'd also check the fuel pressure. Just because.

Thanks Keith. I do have a scanner that I can use. I'll also check the fuel pressure. It also occurs to me that I had to unplug the sensor when I changed out the coil pack. It makes me wonder if I didn't get it plugged back in right.

This car has always had a low fairly rough idle, once it got up to opperating temperature. It would be fine when cold and then usually fine again if it was run for more than about 20 minutes. ( I drive a lot of short hops in this car.) Isn't that also a symptom of a failing Coolant Temp Sensor?

Temp sensors are easily tested with a VOM on the ohm scale.

Yeah, but if you can test it with a OBD-II scanner you don't have to take the thing off the back of the head where the firewall makes your life painful!

Oddly enough, I had a similar problem. Although, driving on the highway, it sure felt like it was doing decel fuel shut off, while I was cruising.

I took off the throttle position sensor, cleaned it as best I could with regular electronic cleaner, and it went away.

A coolant temp sensor failure shouldn't cause that, unless it thinks it's -20, and to never, ever go closed loop on the fuel... Besides- if that fails, that's an odd failure- the ECT sensor is one of the most simple sensors on the car.

If you go an clean the TPS sensor, might as well pull each sensor off, clean the connector insides with the electronics cleaner, and put them all back on.

But the OBD scanner would make your life a whole lot easier.

In reply to alfadriver:

That also sounds like a good possibility. I'll clean it and hope! The ECT is much cheaper than a TPS and of course my 2 spare TPSs won't fit the '99! (I sold my spare for the 99 to get down to Challenge budget!) I need to get the scanner on this car!!

I finally had a chance to put the scanner on the car. I watched the car warm up on the scanner and it seemed to be running pretty well, so I took it for a drive. It ran great for about 10 minutes, and then started missing.

Upon start up the temperature (shown by the scanner) climbed to 200 and held there. It showed ABSLT TPS % bouncing between 2.7 and 6.2. MAF (lb/min) was 0.000 and never moved ?

When it started running badly some of the values were way out of wack and I think that there might have been a communication error, so I rebooted the scanner. This time with the engine running poorly. The temperature still held around 200, but the TPS was 0.0 and reving it would only get it a little over 1.0. The MAF stayed 0.000.

I am not really sure what I should be seeing for values, but I am assuming that the TPS is my problem. Am I right? What is up with the MAF? What should the MAF values be?

Have you checked the grounds? If those 2 sensors share the same ground and it is poor it could be the problem. Do they share the same reference voltage and is it correct (5v?)?

The nice thing about MAF sensor cleaners is that they are just a light duty electronic cleaner. You can find it anywhere.

Start with the MAF- make sure it's good and clean on the inside. Then clean the connectors that are both on it, and going into it.

then the harder part (and it's easy) take off the TPS- put a good amount of cleaner as best you can in it (which isn't really that great), and move the sensor around- if the internal wires got coated- the solution and movement should clean them off good enough.

Re-assemble, and try again.

While you are in there- make sure the wires going into the back of the connectors are in good shape- as JD suggests, there could be a short somewhere in the Vref or ground or both. The two sensors are very similar, in terms that they both have a 5V in, a signal return, and a ground. It's quite likely they share the same 5V Vref and ground.

My plan was to clean the TPS. I'll add cleaning the MAF to the list. I am surprised that it would run at all if the MAF wasn't functioning. That is why I was wondering if the scanner was just not reading a value for the MAF.

Sofa King wrote: My plan was to clean the TPS. I'll add cleaning the MAF to the list. I am surprised that it would run at all if the MAF wasn't functioning. That is why I was wondering if the scanner was just not reading a value for the MAF.

Most cars have a fault mode to keep it running if the MAF fails. But a maf failure would light the light for sure.

That is pretty much what I thought. No check engine light and no codes from whatever this problem is!

Finally had a chance to work on the car last night. A buddy who also has a 99 brought his over, so that I could scan his car and see what the differences were between the 2 cars.

I cleaned the throttle body, TPS and Mass Air sensor. After reassembly my TPS reading was around 20.0 at idle, but the car was running fine. The other car read 0.0 at idle and would jump into the 30.0s when reved. Both cars showed a 0.00 Mass Air reading. The TPS reading on my car barely moved when given gas.

My car seemed to be running fine so I drove it for about an hour with the scanner hooked up. The TPS reading kept creaping down until it was sitting at 5.0 at idle, and it would bounce into the low 40s when given throttle. I can't tell if I have solved the problem and the computer is just relearning the cleaned TPS or if it is just running better because it was colder out lat night, and I am setting myself up for a future breakdown.

Any thoughts on what I am seeing and not understanding?