Emissions (1997) - OBDII failing...need help diagnosing
 

I've got a 1997 Miata, 138k miles. All stock except for the Racing Beat intake. I went to get my emissions test and failed. In Virginia, they do the OBDII scan. 3 monitors didn't pass (weren't ready). They are:

Catalyst
02 Sensor
EGR System

There are/were no hard codes, battery wasn't reset, no major work. The car runs fairly well. Relatively new spark plugs and wires (less than 10k). The car doesn't have the top end it used to...nor does it seem to accelerate nearly as quick as it did years ago.

I've run injector cleaners occasionally. It gets autocrossed frequently. Timing belt and other stuff done around 126k.

I've replaced the front O2 sensor recently. On my cheaper Acctron scan tool, it passed. However, when using a nicer Matco scan tool, it showed the 3 monitors "not ready."

The TPS (throttle position sensor) is only reading between 11% and 72.9% (doesn't show 100% full throttle, even though I have the gas pedal to the floor). My mechanic downloaded the specs from Mitchell onDemand and the voltage was within (right in the middle) of the accepted ranges.

Has anyone seen anything like this on the 1.8 Miatas? My mechanic didn't want to just start randomly replacing things (nor do I).

I appreciate any insight!

-Ed

Sounds like you need a new cat.
I also think you can open the TPS sensor and clean the leads... but not sure.

Maybe your rear o2 is gone?

I suggested the cat might be bad or possibly replace the rear O2 sensor. Both mechanics said the scanner showed they were working fine. I couldn't see the scanner since I was driving....but they could see healthy fluctuations as I accelerated or just coasted.

There was mention of a possible "lazy" rear O2 sensor. But that still doesn't explain why the TPS range only goes up to 72.9%.

Personally, I think it wouldn't hurt to replace them, but they would like to see if it's another thing. The problem with the cat, is that I've got a full exhaust w/ high-flow cat ready to go in....but it's for a 99 head that I'm waiting to put in. Wanted to do everything at once and not fiddle w/ the exhaust. I might just have to bite the bullet and possibly do that.

Thanks for those suggestions! Anyone else seen something similar?

You said the 3 were "not ready"?

Not ready is not the same as failed... Not ready means it is not finished testing and you need to drive the car more.

I just recently got my 96 with 99 engine to pass NC OBDII emissions scanning and spent a good bit of time dorking with it to get it to pass (I had EGR issues). Those 3 were always the last three to show as ready.

FWIW: I got to where I could get the EGR test to pass/fail in 15-20 miles.... The key is to spend lots of time in 5th gear cruising/very light throttle. I took a spirited drive over 40 miles and it never did pass/fail....the EGR only does its thing under low load high vac.

I am not 100% sure about this one...but I am 99% sure if any of those go to fail you will get a CEL/MIL which will then give you a code which will help you diagnose it. If you get the Insufficient EGR flow or exhaust boost pressure I can certainly help you with that since that is what I just beat ;)

It is possible some of those checks have prerequisites...such as 100% TPS??

Lastly: Check your Virginia DMV site about the requirements for your OBDII emissions scan. Here in NC the earlier OBDII cars will still pass emissions with up to two tests in the not ready state. That can be handy!

Thanks woodc for the info!

Yeah, I should've clarified...not ready.

I think VA has only 1 not ready and you can pass.

The car has seen over 700 miles since the first "not passing". I sat there with my mechanic driving over 30 miles, cruise control, 60-65 mph.

there is a manual that some of the emission guys have at my work that tells them what you need to do to get the monitors to reset. it says how fast/long you need to drive etc. to pass. Ed give graham a call and see if he can come up with anything...

Cool, thanks!

I know Rocker Dan was looking for the criteria to pass the readiness states.