Emissions Failure - Temperature sensor(s) ? for 1996 MR2 JDM Automatic N/A

[Jim Flood]

Hi,
I am getting very different emission results a few days apart, with car warm having fitted a new Cat. Lambda is reading ok. It fails on CO at low and high idle , sometimes at low (0.6%), sometimes at high (1.8%). The car only has 60k miles on it. There are no dash warning lights indicating an engine management issue.

The question is, is there a separate temperature sensor that feeds the ECU in addition to the one that feeds the temperature gauge.

With an automatic, i am assuming you test it with sports gearbox mode off.

Does anyone have any other ideas where i should look for the problem.

Thanks
Jim

[Juliuswws]

Hi,

The ECU has a separate sensor for coolant temperature.
On my REV3 Turbo it's called THW. the wire goes into the 16p connector pin4 (thick red wire). Yours being REV5 auto, it might be different.
At this current (cold) temperature, you should measure around 4.5k Ohms.

The sensor itself is at the "water outlet" beside the gauge temp sensor. It's all near to the oil press SW.

You might as well check the intake air temperature THA. 16p/pin3 black wire on mine. Or check at the sensor. Nearly the same, 4.5k should be measured.

I think high CO means too much fuel, so you might check the TPS as well with a multimeter and feeler gauge.

I suppose all hoses and clamps are all good.

Air Cleaner, plugs?

Cheers

[Jim Flood]

Thanks for the prompt reply. I did not realise there were so many sensors.
Can I assume the intake air temperature sensor, is in the inlet manifold, or between that and the air filter. What is the TPS sensor, and where is it.
It has just had a service with new plugs, filters, oil etc, so i assume that they are ok. Somebody suggested the mass flow sensor, or the O2 sensor. Would they lead to these problems. The engine is running and idling beautifully, but just too rich, i agree.
Jim

[Juliuswws]

Yes, there are a few sensors around, still not as much as in a modern car.

The air temp sensor should be near the airbox.
There is indeed a temp sensor in the manifold on the Turbos, called THAM (under the TPS <Throttle Position Sensor>), but I don't think there one in the NA cars.

There is no MAF (Mass Air Flow) sensor, but a MAP (Manifold Absolute Pressure) for the turbos and a vacuum sensor for the NA. You might want to check the little vacuum hose going to it. I had to change mine few years ago as it was leaking air.

Apparently O2 sensors can get slow over time, called "slow response".
You might hook a scope to it, but I don't know how quick should it be.
I just ordered one recently as preventative maintenance, will change it soon.

O2 heater should be good as I think it throws a code otherwise. If you want to measure it however, you should measure few Ohms between the 2 black wires.



This is how you check the TPS




Select your "model" and browse around on the electrical tab to get some idea about the locations of the sensors.
https://japancars.ru/index.php?route=catalog/toyota

What plugs were installed?

Cheers

[Jim Flood]

Much appreciated, and thanks for all that work, i can see you are an expert in this area. Is knowing all this your business or just a hobby.
On the plugs, i will have to check. Are they that critical.
Whereabouts in Ireland are you based as a matter of interest.
Jim

[Juliuswws]

You are most welcome!

Thank you, but I'm far from expert. Electronics is my hobby and my 2 is my only car for 9 years now, so its good to know all these thing in case something happens. I like to know how things work and what's what in the car.
It's a hobby in short

I live in Wexford county.

Plugs would do matter a lot indeed. Mostly their gap and heatrange. Age as well, but yours are new.
In Turbo, the gap is very important, not too sure in the NA, but let us know.

[Jim Flood]

The plugs fitted are Denso K20TT

[Juliuswws]

K20TT seems to be correct equivalent for 90919-01181, so it might be safe to say that the plug is ok. Hopefully the plugs get enough juice from the coil pack to burn all the fuel.
Do you smell fuel from the tailpipe?

I'd say, check the sensors and hoses, maybe get some injector cleaner and bring it out for a spirited drive.

Cheers

[Jim Flood]

Hi Juliuswws,
thanks for all the excellent advice.
The problem turned out to the air inlet sensor, but i got my mechanic to clean the throttle body and the injectors and check everything else as well.

His view as well is that the aftermarket new cat, may not be as good as the Toyota OEM unit, as he would have expected the levels to be even lower. But the important thing is that it passed.

So once again thanks for going into to all the detail, and providing the very helpful pictures of your engine bay with the sensors indicated, as my mechanic has only ever worked on one MR2 SW20, mine.

Do you do any rallies or club events in your car. I would love to see it.
Jim

[Juliuswws]

Hi Jim,

You are most welcome!
Yes, these sensors go out of spec over time. Slowly, but surely.

I had an overheating problem few years ago as my radiator sensor drifted out of spec. It was still changing its resistance versus temperature, but ended up being out of range for the controlling electronics (A/C amplifier) (<- only applies to cars with AC. Non AC cars have a switch instead of sensor)

Good practice to get a baseline measurement at lets say 20C in the summer with multimeter of the critical sensors and check them occasionally. Once a year or once every 2 years.

Aftermarket cats are usually sport/hi flow and so on, which means they have less meat in them. Good for performance, worse for emissions.
Probably the best emissions are with OEM units, but AFAIK even with those are not a walk in the park to get ok emission values for the tests.

My car always spends the most time at emission test. It was the same with OE CAT, and the same with my XS Power Hi-Flow CAT/Downpipe.
I only failed once at emission test and that was because of an inpatient inspector.
Nice inspectors who appreciate the car will spend time revving the engine until and acceptable value appears on their screen and then press the register button.

At the last test, the tester spent well over 5 minutes to get my high idle CO right. Nice guy. I always thank them for taking the effort.
Ended up at 0.29 vol%. Failure is above 0.30 vol%.

I asked him, he said its normal with these cars from the 90s.

I don't usually go to events, but who knows what the future brings.

Kindest regards and Happy New Year!