Bringing a 740 back to life

[Laird Scooby]

Originally Posted by Challo

Thanks. For the aluminium hose, do I remove completely from the car, or just the end from the air box?

You can do either, you may want to keep it for winter use but the hotter air entering the airbox will make it run richer. Made a huge difference on mine, dropped it from ~6% to a smidge over 1% just by doing that!

The trouble is the thermostatically controlled flap in the airbox intake which is meant to provide a constant temperature gets weak with age and eventually defaults to the hot position. They are available new but i'm not sure what models are covered by the new one and prefer to get colder air in regardless due to the route the air takes on mine (about 5" above the left hand exhaust manifold) so keeping the air cooler is quite important on mine.

Did you test your Lambda sensor yet?

[Challo]

Originally Posted by Laird Scooby

You can do either, you may want to keep it for winter use but the hotter air entering the airbox will make it run richer. Made a huge difference on mine, dropped it from ~6% to a smidge over 1% just by doing that!

The trouble is the thermostatically controlled flap in the airbox intake which is meant to provide a constant temperature gets weak with age and eventually defaults to the hot position. They are available new but i'm not sure what models are covered by the new one and prefer to get colder air in regardless due to the route the air takes on mine (about 5" above the left hand exhaust manifold) so keeping the air cooler is quite important on mine.

Did you test your Lambda sensor yet?

Thanks Dave.

Not yet. Had some family stuff to do, but got time on Monday to check the car over and check the sensor. Will report back then.

[Challo]

Got some today to look at the Volvo. Cleaned the plugs, removed the metal air pipe from the bottom of the air box, stuck in a bottle of the cataclean, and let the car run for about 45mins.

Prior to that I realigned the exhaust, tightened the clamps and sealed the gaps with some putty. According to the previous owner the cat had been stolen, so it looks like a generic system was fitted and not very well. Stuck on new exhaust rubbers as well.

A little lumpy at start up but after a while she was ticking over nicely, no more soot out of the exhaust as well.

As requested I checked the O2 sensor ( I think correctly) and the reading on the multimeter was about .5, with a little fluctuations either side.

[Laird Scooby]

Originally Posted by Challo

As requested I checked the O2 sensor ( I think correctly) and the reading on the multimeter was about .5, with a little fluctuations either side.

That's running rich Chris but there should be bigger fluctuations and it should be 0.45 as the base figure and good, rhythmic swings either side.

Can you get a video (in landscape mode NOT portrait) of the multimeter monitoring the Lambda for about a minute and upload to YT and post the link here please?

[Challo]

Originally Posted by Laird Scooby

That's running rich Chris but there should be bigger fluctuations and it should be 0.45 as the base figure and good, rhythmic swings either side.

Can you get a video (in landscape mode NOT portrait) of the multimeter monitoring the Lambda for about a minute and upload to YT and post the link here please?

https://youtu.be/ELIJOdDHcis

Let me know if this works ok.

[Laird Scooby]

Originally Posted by Challo

https://youtu.be/ELIJOdDHcis

Let me know if this works ok.

Video works ok Chris!

Can't quite say as much for the Lambda sensor though.

A few basics - stoichiometric ratio (14.7:1 air:fuel) should give ~0.45V which the ECU uses as a "base line" to make the mixture leaner on idle. It then deliberately enriches the mixture (which should give nearly 1V output) and then leans it off (which should give near 0V) and keeps cycling this rich/lean mixture change to achieve a lean mixture that the cat can deal with. However the cat will only deal with so much excessive richness.

the rich/lean cycle should be roughly equal and if you time it just by counting it's spending about 6-7seconds out of every 8-9seconds above 0.45V because the ECU isn't seeing the swing to 1V that it's expecting. As such it's trying to enrich the mixture but because the sensor is old and lazy, it's not getting there. When sensors fail, the output voltage drops so the ECU tries to compensate for what it thinks is a lean mixture by making it rich. At the moment it's getting some response above 0.45V but not as much as it should.

I'd suggest buying and fitting a new Lambda sensor, pull fuse #1 before you fit it and replace after and that should sort out your rich running - also make it a bit livelier to drive too!
You could repeat the test with the new Lambda and you should see a bigger difference in the voltage swing and also a more even rhythmic pattern of up/down on the voltage.

[Challo]

Originally Posted by Laird Scooby

Video works ok Chris!

Can't quite say as much for the Lambda sensor though.

A few basics - stoichiometric ratio (14.7:1 air:fuel) should give ~0.45V which the ECU uses as a "base line" to make the mixture leaner on idle. It then deliberately enriches the mixture (which should give nearly 1V output) and then leans it off (which should give near 0V) and keeps cycling this rich/lean mixture change to achieve a lean mixture that the cat can deal with. However the cat will only deal with so much excessive richness.

the rich/lean cycle should be roughly equal and if you time it just by counting it's spending about 6-7seconds out of every 8-9seconds above 0.45V because the ECU isn't seeing the swing to 1V that it's expecting. As such it's trying to enrich the mixture but because the sensor is old and lazy, it's not getting there. When sensors fail, the output voltage drops so the ECU tries to compensate for what it thinks is a lean mixture by making it rich. At the moment it's getting some response above 0.45V but not as much as it should.

I'd suggest buying and fitting a new Lambda sensor, pull fuse #1 before you fit it and replace after and that should sort out your rich running - also make it a bit livelier to drive too!
You could repeat the test with the new Lambda and you should see a bigger difference in the voltage swing and also a more even rhythmic pattern of up/down on the voltage.

Thanks. It defiantly appears to be running a lot better, but I’ll get that sensor changed.

The plugs had been changed, but are very black now. Worth sticking a fresh set in?

[Laird Scooby]

Originally Posted by Challo

Thanks. It defiantly appears to be running a lot better, but I’ll get that sensor changed.

The plugs had been changed, but are very black now. Worth sticking a fresh set in?

That confirms it's running rich then. The plugs should self-clean with a new Lambda.

[Challo]

Originally Posted by Laird Scooby

That confirms it's running rich then. The plugs should self-clean with a new Lambda.

Thanks Dave. Just ordered a new 3 wire sensor from PFS and should be delivered this week. I'll get it replaced and run the test again and post the findings.

Hopefully this fixes the rich running, and I can get it MOT'd. Will be nice to get the car out and start using it.

I can then start to look at the other little jobs and see what's next on the list to fix.

[Laird Scooby]

Originally Posted by Challo

Thanks Dave. Just ordered a new 3 wire sensor from PFS and should be delivered this week. I'll get it replaced and run the test again and post the findings.

Hopefully this fixes the rich running, and I can get it MOT'd. Will be nice to get the car out and start using it.

I can then start to look at the other little jobs and see what's next on the list to fix.

Let me know how you get on Chris, don't forget to pull fuse #1 before you change the Lambda and refit after.