Oh... check engine lights...

[Erik]

I don't know if we have a thread dedicated to check engine lights somewhere here, I haven't found any, but if there is one, redirect me there please.

I have a 90 miata, 1.6l, close to 200 000KM, standard, all stock engine. Last summer I noticed that when I keep the throttle stable for a while (2-5min), mainly on highways, but also anywhere else if the throttle is stable long enough, the engine light goes on. I also noticed that as soon as I gave it throttle back, made the engine work a bit, the light disappeared, only to come back if I was going at a stable pace again.

My mechanic seems too busy to be bothered, or not smart enough to figure it out, and engines really, but really aren't my area of expertise. However, I assume it's got to be something of the likes of bad air/gas ratio when not straining the engine, or a bad air problem of sorts? Everything else seems to check out, my air filter is brand new, the air intake was cleaned last summer, spark plugs are doing fine, I mean the engine runs like a beauty, so I'm curious as to what that could be. I doubt it's going to make my engine blow up out of the blue, but if it's putting unnecessary wear on it, it would be awesome to know what it is, what to do. Anyone have similar experiences or know why it would do that?

Thanks

[kung fu jesus]

Hi Eric!

If the engine light comes on, there will be a code. You can retrieve the code yourself by following this tutorial. (http://books.google.com/books?id=wApjC72udH8C&pg=PA59&lpg=PA59&dq=Checking+codes,+Miata&source=bl&ots=SRbQvSeeT5&sig=H3oQksNiwNb83lxqabjNByU57Ow&hl=en&sa=X&ei=FuloU43sB-Pk8gGt44C4CQ&ved=0CF8Q6AEwBg#v=onepage&q=Checking%20codes%2C%20Miata&f=false)


Here are the corresponding code numbers:

FAULT CODES ('90-'93)



1
Ignition pulse (No Igf signal)


2
No Ne signal


3
No G signal


8
Airflow meter open or short circuit


9
Water thermistor open or short circuit


10
Intake air thermistor in airflow meter open or short circuit


12
Throttle position sensor open or short circuit


14
Atmospheric pressure sensor open or short circuit


15
Oxygen sensor <0.55 V 180 sec after 1500 rpm


17
Oxygen sensor unchanged after 1500 rpm


26
Solenoid valve (Evaporative canister purge)


34
ISC Valve





When you have the code, come back here and tell us. If you aren't sure what the code is, take a quick video of the flashing CEL as described in the tutorial link and we can help you decipher that.

Good luck!

[Erik]

Cool man, thanks. I'll try it out when the car is out of the garage. I brought it to them to do some yearly maintenance and it so happens they were shipped the wrong part for one of the repairs... so it's been 3 weeks since I've seen my car... grrrr.

[kung fu jesus]

Hmmm...seems like you may want to look for a new garage/mechanic.

[Erik]

Eh, they're a bit slow at times, but I picked up the car today, it wasn't their fault, the manufacture sent them an electric window part. I like them because they're down with performance upgrades. They're young (read 30+) and they tune their own cars. I'd rather deal with a couple stoners than some man telling me it's illegal or impossible to put in whatever I feel like adding to the car.

[SM16RMSM]

I have the best solution for fixing a check engine light. Take the bulb out and no more CEL. :mrgreen:

[Erik]

Alright, I'm pretty sure it reads 17. EDIT: Might wanna turn down your sound, my door is open. Vid--> https://www.youtube.com/watch?v=dpExc9OqAKQ&feature=youtu.be


What does this all mean? Thanks

[riefdaddy]

Definitely flashing 17. It is referencing your O2 sensor. My guess would be a bad sensor that needs replacing. The other possibility is a bad catalytic converter. I'd replace the O2 sensor first and then go from there. KFJ will be back to weigh in. :)

[kung fu jesus]

First of all, AWESOME video! This is the sort of information that makes it easier to help. Great job!

Secondly, the most obvious answer to solving your issue is replacing the O2 sensor. The best route for this is a direct OE replacement. This means a new O2 sensor with one wire and an original-style electrical plug just like the one in your car.

I checked rockauto.com and there are a number of Oxygen Sensors available, from reputable manufacturers, with the OE-style plug. Look like they run between $25-45 CDN. I can explain installation if you need it. When you get the new sensor in, clear the code. This can be done as simply as removing the ground connector to your battery in the trunk for a few minutes, step on the brake a few seconds, reconnect, go drive.

http://www.rockauto.com/catalog/x,carcode,1190876,parttype,5132

Some estimation content here based on the given information: (get your nerd on)

Your car is dead stock and has ~200k km on it. Usually, O2 sensors last between 90k-160k km. The stock OBD1 systems are simple by today's standards. Your ECU controls the amount of fuel primarily off the voltage your O2 tells the ECU and the TPS (throttle position sensor). The TPS on this vintage of Miatas is basically like an on/off switch, not a potentiometer style (like a stereo volume knob) like the '94+ versions. So, the code "17" is your computer telling you the O2 sensor isn't telling it the right information. It's a complainer, a bit whiny, those computers. :) Basically, your O2 sensor isn't operating as it should. I would estimate your fuel economy isn't that great, too.

If the CEL disappears when you accelerate or decelerate, that means the O2 sensor gave the ECU a 'correct' reading for that condition. Cruising at a constant speed, like on the highway as mentioned in your first post, the O2 sensor should be cycling it's voltage readings from a rich (high) to a lean (low) at about 2-4 times per second, as the result of the ECU constantly trying to stabilize your air/fuel ratio.

The O2 sensor and ECU form a "closed-loop system", so it could be the O2 sensor output voltage is not changing because it is itself incapable of changing, or the air/fuel ratio is not cycling as it should in response to a proper voltage from the sensor.

It may seem a little complicated, don't worry.

[Erik]

A little complicated indeed, but I understand the concept. And yes, the car has smelt of unburnt fuel pretty badly ever since I've had it, but the gas mileage actually isn't that bad, even with my driving style I achieved 300-450km with a full tank. I knew however that it was never at its full potential.

1st. I will go and see if the mazda dealers in my area have oxygen sensors and how much they are, otherwise, there are a lot of links in the link you posted from rockauto, which one would be ideal to replace the entire thing? I'm leaning towards bosch because it's a bosch and i've never seen bosch screw anything up so far so i'd like some bosch parts in my car, haha.

2nd. I found this video, which seems to be pretty thorough, of how to do the whole thing. https://www.youtube.com/watch?v=DyyXvxPQ214 I don't own a wide array of tools but I'm lucky enough to have two sets of parents I still talk to who do, so it shouldn't be a very hard repair. Anything specific I should be looking for/being careful about? Other than burning myself?

Finally: the first link you posted on how to read the check engine light also talked about removing the BTN fuse for a bit to clear the messages. Is this better or the same thing as unplugging the battery?

Finallyfinally: you are infinitely helpful, thank you very much.

[kung fu jesus]

I watched the video and it is excellent. He hits all the points I would have on that. Do not get antiseize on the sensor, very important. Just put it on the threads. I have that same O2 socket, it's usually inexpensive and worth very penny. Consider it an investment in tools you can use later on other cars.

Bosch, NTK, etc. whichever brand you prefer or feel comfortable with. I don't like off-brand stuff to save money. I am recommending the direct-fit OE connector style for easy of installation and because a lot of people get the crimp-in or splice-in type of O2 sensors wrong and they (splices or crimps) come apart.

The ones off the linked Rock Auto I recommend are the ones that say "OE style". That Denso one is correct and IIRC, Denso is OEM.

RE: fuse vs battery, your call. I prefer battery because at 24 years old, I wouldn't want to accidentally destroy the fuse if it is brittle or what not. Many ways to skin a cat.

[Erik]

Went by my mazda dealer today, the mazda manufactured oxygen sensor is 275$, how crazy is that? A near auto-shop had the denso for 75$, but I opted with rockauto's denso, comes to 43$ with shipping. Will update when it's all done.

[Erik]

Just a little update. I received my denso O2 sensor and went on my way to change the old one out today. It was a pretty simple fix, though quite tricky in such confined spaces. I did not need an o2 socket, though I see why having one would have made my life easier. I noticed two things while doing it: my heatshield is only holding on with 1 bolt, and the o2 sensor was super white, which I assume means corrosion of some sort.

I drove the car at an impressively steady pace for 20 minutes, no new signal. The unburnt-fuel-boat-type-smell also seems to be gone. The engine feels happy about the change too. It's like it's magically less rough, more composed and less loud. Idk, I might be imagining things for that part. As far as I know for now, my CEL issue is fixed.

Thanks to all for the input and big thanks to KFJ, you're like the babysitter of miata enthusiasts, holding my hand through the whole thing.

[DK Wolf]

white coating on the O2 sensor could mean a couple things, either you're running lean or burning coolant.

You could of be running lean due to the bad O2 sensor, or you could have a gasket leak. Only way to find out is it investigate your plugs and check deposits on them. If any plugs are white then you've got a cylinder running lean.

[Erik]

Reviving thread as a follow up on my adventure. I changed the spark plugs, third cylinder had a tiny bit of oil on the threads, 4th cylinder plug was just bathing in oil. The spark plug wire on the 4th cylinder also looks unhealthy. The three others did not. Not running lean but this does not sound any better. I feel like last year I had my mechanics do a seal job on the engine. They changed every seal they'd found that was on its way to cracking... this is bad right?

http://i57.tinypic.com/2mplt8m.jpg
http://i57.tinypic.com/8zh4d0.jpg (note, literally all the oil on the rag came from this spark plug)

[DK Wolf]

Doubt that included piston seals lol. Because that thurs yur prahblum

[Erik]

ya i think they did cam seals and valve cover, piston seals are piston rings right? im really bad with terminology, i understand things better when ive seen them and ive never opened an engine down to the pistons