87 WBX Low Power / Bog down when O2 starts cycling

[banja]

Hi!

I have been chasing an intermittent low power issue for quite a bit now and I am running out of ideas on how to fix it. Maybe someone here could point me in the right direction? Thank you!


Symptoms


Van starts fine, runs great for 3-5 minutes (variable depending on ambient temperature) then loses power right at moment ECU starts looking at the O2 sensor voltage. Within a a few seconds, power picks up again. It happens a few more time during my drive but when the van is warm, I have not noticed anything similar.

Observations


I have been monitoring closely my Shoebox meter when I drive. For 3-5 minutes after start, O2 signal is stable (I believe it is based on values provided by the temp II sensor?) then the voltage cycling coincides with the low power symptom. Specifically, low power correlates precisely to a negative O2 sensor voltage on the Shoebox (usually between 0v and -0.07v). Power picks up as O2 voltage increases. My O2 ground is also negative, between -0.02v and -0.08v. Van does run great with O2 unplugged.

Work done so far

I have replaced the O2 coax cable by a new one sold by Vanagain
The O2 sensor iwas swapped with a new one
I have tried several temp II sensors, they are in spec when doing the Bentley checks
I have replaced the original Hall Sensor which was fried
I have tested the Air Flow Meter with an analog voltmeter with no drops noticed
Engine grounds (under Coil and next to distributor) are all cleaned up.


My questions:
- Is the O2 sensor grounded by contact against the exhaust? If so, could a corroded exhaust surface prevent a good ground signal?
- The new O2 coax from Vanagain has an extra ground cable which is attached
on firewall near ECU but I am unsure this is the correct installation as it didn't come with instructions
- What does trigger the ECU to look at O2 signal? Is it based on temp II values? The moment when that happens appears to be dependent on ambient temperature.

[RGS Paul]

Sounds like airflow meter issues I’ve experienced. In 3-5 minutes from cold start the ECU probably isn’t looking at the O2 sensor yet, that’s about when you will transition from cold start to warm start. It sounds like once the van is hot and running on the O2 the issue abates, reinforcing my suspicion. Have you checked the cold start mixture (hex key on the AFM)?

Paul
_________________
'87 Syncro 7-Pass. Adventure Touring Vehicle
"Simplicate, then add lightness." Colin Chapman

[Ahwahnee]

I see that you replaced the coax O2 lead with a new one but did you also test to confirm the central lead was not shorted to ground?

Your symptoms and their timing sound so much like a shorted coax you want to be sure it really is okay.

[crazyvwvanman]

The Temp2 sensor reading is what the ECU uses to decide when to use the O2 sensor reading.

It is almost certainly WRONG to have the extra ground cable attached to the firewall.
Where does the other end of that ground go?

What about the original brown wire that comes out of the small bundle of wires going to the distributor?
That brown wire is the original O2 circuit ground used by the ECU.
It is supposed to attach to engine metal to complete the ground circuit with the threads of the O2 sensor in the exhaust system.
It also attaches to Pin 19 of the ECU connector for use by the O2 circuit inside the ECU.

Mark

[banja]

[Wildthings]

The heater for the O2 sensor requires a ground. Off hand I do not know where it is located, but likely where the small braided ground strap attaches between the left head and the body.

[crazyvwvanman]

The O2 sensor heater connector has 2 wires, power for the heater and ground for the heater.
These have nothing directly to do with the signal from the sensor.
The signal side uses the coax for the O2 signal voltage and the Digfant harness Pin 19 brown wire to the engine metal for the O2 signal ground.


AGAIN:
What about the original brown wire that comes out of the small bundle of wires going to the distributor?
That brown wire is the original O2 circuit ground used by the ECU.
It is supposed to attach to engine metal to complete the ground circuit with the threads of the O2 sensor in the exhaust system.
It also attaches to Pin 19 of the ECU connector for use by the O2 circuit inside the ECU.

Mark

[banja]

[Wildthings]

[banja]

[DanHoug]

have you tried driving without the Shoebox inserted?
_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[Howesight]

Crazyvanman is absolutely right, but so advanced in his knowledge that the OP may be overwhelmed.

To Clarify:

1. The oxygen sensor signal is 0 volts to 1 volt, at VERY LOW CURRENT;

2. The voltage signal from Oxygen sensor to the ECU is very sensitive to any resistance introduced to its circuit. This is why the ground he refers to at the distributor is so important - - it is the reference used by the ECU.

3. The voltage signal from the oxygen sensor is also so small that it can easily be overwhelmed by EM - - electromagnetic interference. That is the reason for the coaxial shielding outer wire in that cable. But the shielding will not do its job unless it is grounded.

4. As the OP may have learnt already, a short, including an intermittent short, between the oxygen sensor signal wire and the shielding, can cause an over-rich condition and stalling.

Unrelated to the oxygen sensor wiring are some other things that can cause intermittent over-fueling:

1. Resistance in the wiring between the ECU and the Temp 2 sensor. Recall that the sensor is a Negative Temperature Co-efficient (NTC) thermistor and that any resistance gives a false "cold coolant" signal to the ECU - - even on an intermittent basis if the wiring or ground has an intermittent fault; Note that when correcting ground problems, it is vital to check the ground wires near the terminals. The last few millimeters of copper strands under the insulation near the terminal often suck up road salt and corrode. Peel back the insulation on the critical ground wires to rule this out and apply shrink-wrap insulation after. If needed, cut back to un-compromised wiring and install a new terminal and/or section of wire. (Having lived in the salt belt in a past life, I have seen nightmare conductor corrosion under the insulation in past.)

2. The AFM might be a culprit, but not the way you might think. The air temp sensor inside the AFM is also a thermistor, although it rarely fails. But the wiring can fail, including intermittently.

3. The exhaust system develops pulses that suck ambient air into the exhaust system. Therefore, an exhaust leak can introduce oxygen-rich ambient air into the exhaust path which then results in the oxygen sensor giving a false voltage signal to the ECU and enriching the A/F ratio. However, this symptom would not usually be intermittent and would persist as long as the oxygen sensor is connected and working.

4. What about a crack or leak in the AFM-to-throttle body hose? The engine moves a bit when load is applied or reduced and a crack in that hose can then open or close, causing an intermittent lean condition which the Lambda system tries to correct, but slowly if the oxygen sensor is old or dying.

I hope this helps because sorting these intermittent issues is so satisfying in the end.
_________________
'86 Syncro Westy SVX

[banja]

[zoti]

How old is the O2 sensor?

Once the ECU goes into closed loop (at 64C) voltage cycles up and down. You want the average to be 0.45v-0.5v.

The WBX emulator can display that info very nicely. You can see it in this video at about 0:13sec.

The first thing I would do is verify the O2 sensor is working correctly.


Link

[banja]