'97 850 problems after ecu reset

[erikv11]

Yes, erratic idle etc., all of the typical vacuum leak symptoms are possible for an intake manifold leak. You should be able to find it with starting fluid if it's that bad.

On resetting the codes, my answer is based on experience (I have cleared them myself that way, they don;t come back if the problem is fixed) and on what dozens of very experienced people have posted on the Volvo forums.

It's OK, I'm stuck on the ECT resistance thing.

It sounds like your troubleshooting could really benefit from seeing live readings from the O2 sensors. You expect the car to run rich without a front O2, so that is no surprise.

[misha]

Yes, erratic idle etc., all of the typical vacuum leak symptoms are possible for an intake manifold leak. You should be able to find it with starting fluid if it's that bad.

I don't have any of those symptoms and the engine is running so smooth.Actually you would never know that something is wrong except low mileage and running rich smell from the exhaust.

On resetting the codes, my answer is based on experience (I have cleared them myself that way, they don;t come back if the problem is fixed) and on what dozens of very experienced people have posted on the Volvo forums.

I think that i read here on the forum that dtc codes remain stored in ecu untill they have been reset with a proper scan tool.

It's OK, I'm stuck on the ECT resistance thing.

It sounds like your troubleshooting could really benefit from seeing live readings from the O2 sensors. You expect the car to run rich without a front O2, so that is no surprise.

I tryed with a digital multimeter and i got 0.4-0.9V but that was with o2 connector disconnected and engine running.
The strange thing is that even if i put brand new o2 sensor,air/fuel mixture will remain unchanged.Tryed with used but working o2 few times and i also got nothing.

I'm beginning to suspect that ECU is really the source of the problems.
I know this is pretty rare,but as the time goes by,there are more and more members and other people complaining about bad ECU(s).

[erikv11]

Many scan tools will report live O2 sensor readings, those are the readings that the ECU sees. Perhaps the ECU doesn't see the O2 sensor due to a wiring problem.

I agree, I think you are wise to get another ECU to swap in, that should be pretty cheap to do, will help to sort this out.

[misha]

I plugged the port on vacuum tree and had o2 connectors rotate.I connected rear o2 sensor into front o2 connector and front o2 to rear connector.Drove few days like that and all of my problems vanished.No black smoke from exhaust and normal fuel consumption.

Few days ago i disconnected front o2 sensor from rear o2 connector(practically i disconnected rear o2) and left rear o2 connected to front connector.Car runs like a dream.Still no check engine light but i expect it to turn on because of disconnected rear o2.

So...whats going on here?

When car is operating on front o2 there is black smoke from exhaust and fuel consumption is very high,but when i connect rear o2 to front connector everything is back to normal.

Connectors are ok...black for front and grey for rear o2.Right?

Right now black connector is connected with rear o2 and grey connector for rear o2 is disconnected.