Weird cold stumble - coolant temp sensor?

[Secret Chimp]

Also posted on Miata.net, but I get ten different answers half the time...

Background - I've already changed my fuel filter, plug wires, and my spark plugs look fine. I also cleaned my MAF and put in a different brand of fuel.

When I drive my car before it's fully warmed up lately, it will kind of have a lumpy/hesitant delivery of power as I get past 2000 rpm and approach 3000 rpm. Tonight, it actually got up 3000 and got half-stuck there, stumbling so bad it sounded like a raspberry - thmmbtbtmbmtbmtbmtbtbtthbm (usually it just feels hesitant and doesn't go nuts like that). This is with a light, my-engine-is-still-cold foot.
When the needle moves over the thermometer (slightly to the left really) on the temp gauge, the car never does it again. Tonight's event is the first, usually I'm just keeping a steady pedal pressure and the car kind of lags, then goes, then lags again as it climbs past 2000rpm, but not enough to jerk my head around. (The car was just stuck at one speed going thbbttbthbbtt tonight, it's never actually bucked around or anything awful).

I've searched this elsewhere, and some people have suggested that such a problem can be caused by a bad coolant temp sensor telling the ECU the wrong temp and making the engine run too lean. Could this be it? I'm going to check the sensor tomorrow, but with this only seeming to happen during open loop, I'm not sure what else is left.

[The Pirate]

Does the Miata have VTCS (Variable Tumble Control System) or something similar?  It's a butterfly valve of sorts that creates turbulence in the intake (only on a cold engine).  It is supposed to help with combustion and cold engine emissions. 

And yes, my car has a bit of stumble til 3K rpm when it's cold.

Here's some more info.

[Secret Chimp]

Does the Miata have VTCS (Variable Tumble Control System) or something similar?  It's a butterfly valve of sorts that creates turbulence in the intake (only on a cold engine).  It is supposed to help with combustion and cold engine emissions. 

And yes, my car has a bit of stumble til 3K rpm when it's cold.

Here's some more info.

No, I don't think so. It's never been mentioned on Mnet and that sounds like something people would want to disable for the hell of it if the car had it.

[The Pirate]

Then maybe your brum brum exhaust isn't all that it's cracked up to be. 

Oxygen sensor is on it's way out but hasn't fallen out of spec enough to trip a CEL yet?  I had a car do something very similar for a week or so (with a gradual decrease in mileage), then the sensor finally died and the CEL came on.  Once the O2 sensor completely died, it ran pretty crappy all the time, but before, it was almost as you describe.  Just a thought.

[Secret Chimp]

Then maybe your brum brum exhaust isn't all that it's cracked up to be. 

Oxygen sensor is on it's way out but hasn't fallen out of spec enough to trip a CEL yet?  I had a car do something very similar for a week or so (with a gradual decrease in mileage), then the sensor finally died and the CEL came on.  Once the O2 sensor completely died, it ran pretty crappy all the time, but before, it was almost as you describe.  Just a thought.

Well I actually put a new O2 sensor on, but doesn't the ECU ignore the O2 sensor until it's warmed up with the engine?

It could be that the O2 sensor I got is a pile of dick, but if it was I'd think it would do this all the time.

And this occurred before brum. Brum is flawless.

[The Pirate]

Well I actually put a new O2 sensor on, but doesn't the ECU ignore the O2 sensor until it's warmed up with the engine?

That I don't know.

It could be that the O2 sensor I got is a pile of dick, but if it was I'd think it would do this all the time.

Not necessarily.  But again, I'm not sure.  I don't remember which O2 sensor went on my car, but like I said, symptoms leading up to it were very similar to what you're experiencing.  Check the coolant sensor and go from there.

And this occurred before brum. Brum is flawless.

Videos?

[Secret Chimp]

This is the kind of help I get on Miata.net:

"Timing belt could have skipped a notch.   How old is it?   Book says every 60,000 miles but they can last alot longer.  On one car I owned it lasted 130,000 miles before it skipped a notch"

WTF does the timing belt have to do with a COLD RUNNING ISSUE. Christ.

Also videos are hard because my camcorder only syncs with computers down at school, and I don't have any class access any more :P

[280Z Turbo]

This is the kind of help I get on Miata.net:

"Timing belt could have skipped a notch.   How old is it?   Book says every 60,000 miles but they can last alot longer.  On one car I owned it lasted 130,000 miles before it skipped a notch"

WTF does the timing belt have to do with a COLD RUNNING ISSUE. Christ.

Also videos are hard because my camcorder only syncs with computers down at school, and I don't have any class access any more :P

It skips back when it warms up. Duh.

[AutobahnSHO]

It skips back when it warms up. Duh.

Mechanic of the Year award goes to YOU.

[S204STi]

This one might require a tech to look at it with a scantool to see if any sensors are doing whacky things at that precise moment.  Dealer scantools are best for this, but to hire teh monkey running it you must pay out the yin-yang.

[Raza]

[Secret Chimp]

Now I have a second person telling me to check the timing belt... Christ almighty.

I did find a way to make a little code reader with a resistor and an LED. The guide doesn't say if the CEL is already supposed to be on (which it never is), but it's worth a try.

[S204STi]

With the key on, engine off, the MIL should be on steady.

[Secret Chimp]

With the key on, engine off, the MIL should be on steady.

No, I mean it doesn't mention if this diagnostic procedure only works if the light has already been tripped (i.e. on while the car is running)

I can't find a dang 500 ohm resistor in all of these things I have in this bag of ohms, so I just got a new sensor after finding a resistance test I can do on the existing one.

[Secret Chimp]

I just bought a temp sensor. It's $30, runs a lot of functions in the PCM, and it's in the rear of the engine, so might as well do it either way. If this coolant temp sensor doesn't fix my problem, I'm going to get a different O2 sensor. I probably bought some kind of Dongzhou sensor instead of a Denso.

[JWC]

Oh hell, just clean the throttle body and see what happens.

My 02 sensor tripped the MIL for a whole year on the Volvo and never caused a drive-ability problem. 

[Eye of the Tiger]

I just bought a temp sensor. It's $30, runs a lot of functions in the PCM, and it's in the rear of the engine, so might as well do it either way. If this coolant temp sensor doesn't fix my problem, I'm going to get a different O2 sensor. I probably bought some kind of Dongzhou sensor instead of a Denso.

A repair manual and a multi-meter would help you test the sensors.
Is ur cat clogged up like Pathy's

[Secret Chimp]

A repair manual and a multi-meter would help you test the sensors.
Is ur cat clogged up like Pathy's

No, I have seen the honeycomb on both ends, and it was good.

This sensor is on the back of the engine against the firewall; I'd need to remove it to test it, and at that point why not just replace it.

[Eye of the Tiger]

Did you blow the fuel injection manifold? It could take a month to rebuild it.

[280Z Turbo]

Did you blow the fuel injection manifold? It could take a month to rebuild it.

Did you know that fuel injection manifolds on Deloreans are on the bottom of the engine?

[S204STi]

Did you know that fuel injection manifolds on Deloreans are on the bottom of the engine?

Cool, like inside the oil pan?

[Eye of the Tiger]

Did you know that fuel injection manifolds on Deloreans are on the bottom of the engine?

That's usually where they blow out from.

[S204STi]

That's usually where they blow out from.

Makes sense to put them there, so it blows its guts into the crank case.

[Eye of the Tiger]

Makes sense to put them there, so it blows its guts into the crank case.

It only usually happens when you use whiskey got fuel.

[Secret Chimp]

Well I don't think that new coolant sensor did anything. I'll have to see if the computer figures it out better tomorrow. If not I'll order a $40 Denso from RockAuto (I still don't know what brand my O2 sensor is, if it has one) and if that doesn't do anything, my car is just a hot rod that needs hot blood.

[Eye of the Tiger]

Old Miata is old.