[FIXED] Lambda fast idle emission test close to fail - direction ideas for cure

Vehicle is a 1999 UK specification Toyota Rav4 (OBD1) with one O2 sensor.

The fast idle Mot emission test result was:
RPM at 2616
CO = 0.014 (Max 0.200)
HC = 10 (Max 200)
Lambda 1.028 (Max 1.030)

The vehicle starts and runs fine apart from the slightly higher Lambda reading.
The air filter, spark plugs and oil and filter were all renewed before the emission test. A new Toyota fuel filter was fitted last year and the cooling system had new anti-freeze in last year too.

I have tested the O2 sensor at 2000 rpm using a scope but it only reads a flat line of about 300 mv and does not oscillate between lean and rich. When i blip the throttle the O2 sensor responds immediately by going rich and then returns to about 300 mv.

I'm only a DIY man so please bear with me because some of this diagnostic stuff is quite complicated. I'm not sure which testing direction to go with this. I was thinking maybe a small exhaust leak somewhere? Any ideas on direction please?

Stephen

Very strange behavior from your O2 sensor... I see that you can get it to go rich when you accelerate the engine, good move. I think my next test would be to see if it'll go full lean or not. Technically, 300mV IS lean, but I'd like to see it go all the way down to 100mV or less.

To drive the engine lean, you've got options. My first choice would be to pull the brake booster hose off, or otherwise create a large vacuum leak. This should drive the O2 down pretty quickly. Be careful not to stall the engine, of course. :lol: Or, you could start pulling injector connectors, which will send unburned oxygen in to the exhaust to get picked up by the O2.

I doubt this is caused by an exhaust leak problem, unless the leak is both a.) upstream, and b.) huge. Pinholes or very small leaks rarely cause fuel control issues like this, in my experience.

Finally had a chance to look at the Rav4 again.

There is a fault code saying O2 heater circuit malfunction. The signal wire is now reading 0 volts at 2000 rpm but will still go rich when the throttle is snapped. Before it read 300 mv but not anymore, not sure what has changed.

I haven't undone the O2 connector at anytime. One of the 2 heater wires reads 12 volts constantly with the engine running and at operating temperature. However the second heater wire just reads 0 volts and does not move at all. I was expecting something like 300 mv but nothing.

The thermostat and the ECT sensor are working but I'm thinking the vehicle is no longer going into closed loop with a 0 volt reading O2 sensor signal wire.

I'm unsure how best to proceed with testing the O2 heater circuit. The Rav4 IS obd1 unfortunately.

Steve

Thanks for getting back to us! An O2 heater problem sorta makes sense, as a cold sensor will never achieve its full operating range, as we saw with your 300mV reading.

You've verified the power feed to the heater, and your control side reading would suggest an open in the heater circuit. You're absolutely correct to think that there should be some voltage on the ground side, but not a flat zero.

If you have an incandescent test light, then you can backprobe the second wire (the control side) and touch it with your test light connected to B+. With the engine running, the light should light brightly. If it doesn't, try clearing the code and check again. If it still doesn't, then let us know and we can trace a control problem.

If the light shines brightly, then that would verify the PCM's control of the heater, and that it's time for a new O2. You could also back this up with a resistance measurement of the heater itself, unplugged. Not sure what the spec is - a guess would be 12 ohms or less. If the heater truly has failed, then you're likely to see a much higher reading.

Thanks for all your help to date Tyler, really helpful.

I've tested the O2 ground heater PCM driver as you rightly suggested and the test light glowed red so I'm really happy about that. I also did a current clamp test on the O2 sensor ground wire and it read 00.15 (it wouldn't stay at a full 00.00 even before I had a chance to start the engine) so I'm thinking it must be 0 amps on that O2 sensor meaning a dead O2 heater element.

What a fantastic forum this is!

Many thanks

Steve

No problem, thanks for saying so!

I think you're clear to replace that O2, if you haven't already. I'd be very interested to hear how the O2 signal reacts with the replacement.

Just an update. I have installed a new Denso O2 sensor and it is now oscillating normally. Heater ground circuit is reading 400 mv whereas before it was flat lined at 0 volts. I had to clear the codes before the PCM would provide a ground to the O2 sensor but once the code was cleared, voltage went to 400 mv. Thanks again for all your help Tyler.

Steve

No problem!

The 400mV reading you got on the heater ground sounds pretty reasonable compared to what you were seeing before. We tend to think of voltage drop as a bad thing, but in this case it's a sign of a working circuit. :silly:

Were you intending to take this back for another emissions test? No worries if not, just interested to see the lambda reading go closer to 1.000.

Tyler wrote: No problem!

The 400mV reading you got on the heater ground sounds pretty reasonable compared to what you were seeing before. We tend to think of voltage drop as a bad thing, but in this case it's a sign of a working circuit. :silly:

Were you intending to take this back for another emissions test? No worries if not, just interested to see the lambda reading go closer to 1.000.

Had the Rav4 mot'd yesterday and the lambda reading is now 1.001 so happy days. CO was 0.005% (limit is 0.200) and HC was 3 ppm (200 ppm is the limit).

Steve

toyota1990s wrote: Had the Rav4 mot'd yesterday and the lambda reading is now 1.001 so happy days. CO was 0.005% (limit is 0.200) and HC was 3 ppm (200 ppm is the limit).

Hey, that's what we like to see. :lol: For being a '99, it runs pretty darn clean.