Idle Mixture Adjustment

[Bertone80]

By the way, I remember trying to adjust the mixture to achieve fastest Idle RPM (meaning max power). That will give a bad emission as the exhaust really stinks. When the mixture is set to minimize CO emission the idle RPM will be actually lowered a little. This indicates that the emission system does have an impact on the gas mileage.

[Jonty]

To use a gas analyzer you need the hose adapter hook-up per the procedure on D:04:01. The Workshop Manual actually doesn't use the dwell meter to set the mixture, it is only used to confirm the Lambda system is operating. With that said you can set the mixture very well using just a dwell meter if everything is working correctly. You are not supposed to measure the exhaust at the tailpipe, the adapter taps off ahead of the catalytic converter.

Thanks David I didn’t realise that. I’m getting very frustrated with this system as I’ve had issues since day one getting the right dwell. I really want to get a gas analyser to confirm mixture so that at least the mixture is right and I can then go and try figure out what’s causing the weird dwell behaviour. I’ll also do a valve adjustment and compression test as well as new plugs and 02 sensor. If that all doesn’t work I give up.

[Ron]

To use a gas analyzer you need the hose adapter hook-up per the procedure on D:04:01. The Workshop Manual actually doesn't use the dwell meter to set the mixture, it is only used to confirm the Lambda system is operating. With that said you can set the mixture very well using just a dwell meter if everything is working correctly. You are not supposed to measure the exhaust at the tailpipe, the adapter taps off ahead of the catalytic converter.

+1!

I can see using the analyser on the PRVs w/o an idle speed system. But I don't see hooking it up, setting the RPM with the common brass screw, then shut it. Granted, the manual says it will "usually be shut" when you're done. (The ISM would compensate anyway...) But, I'm not going to leave it open contrary to the idle circuit requirements it gives (It's a 1980 version of cut and paste from Volvo manuals.) Nothing will have changed and the next step calls for hooking up a dwell meter.

[Ron]

The Lambda controller constantly adjusts the mixture through the frequency valve. The mixture will swing back and forth between too rich and too lean constantly too. As a result the O2 sensor voltage will swing back and forth between 0.1 and 0.9 volt accordingly as well. Most, if not all, digital volt meters will read and display peak voltage so it is not usable for monitoring how the Lambda system works. What's needed is a volt meter that gives the average voltage it sees. That's why I used an analog meter to measure the sensor voltage. Analog meter displays average value the meter sees. If it shows 0.45 volt it says the mixture is controlled right on target swinging between rich and lean with an average right in the middle.

I remember I disconnected the O2 sensor when I did the measurement while tweaking the mixture adjustment screw. I guess that was not right because the Lambda controller might try to correct the mixture in an extreme manner. I will soon do another round of adjustment for my B28F. I will do it with the O2 sensor connected.

My Bertone was able to pass California smog test in 2004 with excellent numbers. I remember the test technician told me that he was surprised that such an old brick passed so well.

I'm with you on the O2 sensor since disconnecting it would set the dwell to a constant ~45°.
(Although only off 5°, I wanna watch it in action;-)

[Jonty]

+1!

I can see using the analyser on the PRVs w/o an idle speed system. But I don't see hooking it up, setting the RPM with the common brass screw, then shut it. Granted, the manual says it will "usually be shut" when you're done. (The ISM would compensate anyway...) But, I'm not going to leave it open contrary to the idle circuit requirements it gives (It's a 1980 version of cut and paste from Volvo manuals.) Nothing will have changed and the next step calls for hooking up a dwell meter.

Ron I’m a bit confused about what the issue is plugging in a gas analyser if there is an idle system and what the brass screw has anything to do with it? If the 02 sensor is remove surely the lambda system runs a fixed duty cycle so if I set the AFR with an analyser then plug the o2 sensor back should this have not set the mixture correctly?

[Ron]

Ron I’m a bit confused about what the issue is plugging in a gas analyser if there is an idle system and what the brass screw has anything to do with it? If the 02 sensor is remove surely the lambda system runs a fixed duty cycle so if I set the AFR with an analyser then plug the o2 sensor back should this have not set the mixture correctly?

Sorry, I think things got off track/scattered when I was asking about actually using an analyser or not, and the procedures if you were. Anyway, I've reread everything and hopefully I can bring it all together-

If I'm following your situation it seems like a waste of time and effort. We can measure the voltage, as Bertone80 did; use an analyser, as in the manual (but not a procedure that leaves all of the brass screws 1/2 turn open); or the chart/dwell, which it says to confirm with anyway. Using dwell is the only way that measures when things are as they will be during normal operation. (The O2 sensor is connected, the RPM is not jacked up to 950, and the brass screw is not allowing air behind the throttle plates...)
The thing about having an idle system is that I understand that the procedures using an analyser were laid out before they were used on a PRV. In short, they have you adjusting the fuel when there is a system added afterwards that can vary the air... (Which may not really matter if everything is functioning properly.)
Yes, using an analyser and unplugging the O2 sensor forces the dwell somewhere between 40° & 50°. If it happens to be 40° (probably not), that part is ok (except it's not normal for it to be static). If not, and especially if it happens to be 50°, you'll probably be readjusting it using dwell after confirming with the pulse chart...
However it is done, we're looking for why a very rich adjustment simply raises the idle to 1000 with no impact on dwell. According to the dwell readings, a bad O2 sensor fits...the other methods would indicate the same.

Back to the brass screws- Since you found yours were open some, once you get the dwell to respond correctly, I can see using the correct type of analyser (as David mentioned) to balance the banks. (And give you peace of mind ;-).

EDIT: Anxiously awaiting a report after the new sensor (and plugs?) is installed and the fate of our hair...

[Jonty]

Sorry, I think things got off track/scattered when I was asking about actually using an analyser or not, and the procedures if you were. Anyway, I've reread everything and hopefully I can bring it all together-

If I'm following your situation it seems like a waste of time and effort. We can measure the voltage, as Bertone80 did; use an analyser, as in the manual (but not a procedure that leaves all of the brass screws 1/2 turn open); or the chart/dwell, which it says to confirm with anyway. Using dwell is the only way that measures when things are as they will be during normal operation. (The O2 sensor is connected, the RPM is not jacked up to 950, and the brass screw is not allowing air behind the throttle plates...)
The thing about having an idle system is that I understand that the procedures using an analyser were laid out before they were used on a PRV. In short, they have you adjusting the fuel when there is a system added afterwards that can vary the air... (Which may not really matter if everything is functioning properly.)
Yes, using an analyser and unplugging the O2 sensor forces the dwell somewhere between 40° & 50°. If it happens to be 40° (probably not), that part is ok (except it's not normal for it to be static). If not, and especially if it happens to be 50°, you'll probably be readjusting it using dwell after confirming with the pulse chart...
However it is done, we're looking for why a very rich adjustment simply raises the idle to 1000 with no impact on dwell. According to the dwell readings, a bad O2 sensor fits...the other methods would indicate the same.

Back to the brass screws- Since you found yours were open some, once you get the dwell to respond correctly, I can see using the correct type of analyser (as David mentioned) to balance the banks. (And give you peace of mind ;-).

EDIT: Anxiously awaiting a report after the new sensor (and plugs?) is installed and the fate of our hair...

Thanks Ron that's really helpful appreciate you taking the time. By way of update the car developed a misfire today so I can now add that to the list. Just to summarise all in one place and what I plan to do:

Current state of car:

1. all fuel system parts are not older than 5 years (including WUR, distributor, accumulator, injectors and DMCH pump)
2. car now misfires
3. FV works
4. Lambda system checks all looked fine (89 grounded, 12 with 1.5v applied and 53 with WOT switch engaged)
5. Car takes a long to time to closed loop and sometimes not at all. When it does it has a wild dwell swing sometime between 45 and less than 20
6. When measuring current O2 sensor the voltage change is quite slow
7. Car takes 3 to 4 seconds of cranking to start when cold and after it has been left for more than 5 hours (hot start after an hour is ok or if I start it cold it and shut down instantly it will fire back up straight away)
8. CSV works fine (and have Dave's hot start relay)

What I will do next:

1. Have a compression test and timing test today
2. Will do valve adjustment later this week
3. Will replace spark plugs when they arrive
4. Will replace o2 sensor
5. Will do fuel pressure check
6. Another smoke test
6. Will pray to the kjet gods...

Thanks

[Bitsyncmaster]

Thanks Ron that's really helpful appreciate you taking the time. By way of update the car developed a misfire today so I can now add that to the list. Just to summarise all in one place and what I plan to do:

Current state of car:

1. all fuel system parts are not older than 5 years (including WUR, distributor, accumulator, injectors and DMCH pump)
2. car now misfires
3. FV works
4. Lambda system checks all looked fine (89 grounded, 12 with 1.5v applied and 53 with WOT switch engaged)
5. Car takes a long to time to closed loop and sometimes not at all. When it does it has a wild dwell swing sometime between 45 and less than 20
6. When measuring current O2 sensor the voltage change is quite slow
7. Car takes 3 to 4 seconds of cranking to start when cold and after it has been left for more than 5 hours (hot start after an hour is ok or if I start it cold it and shut down instantly it will fire back up straight away)
8. CSV works fine (and have Dave's hot start relay)

What I will do next:

1. Have a compression test and timing test today
2. Will do valve adjustment later this week
3. Will replace spark plugs when they arrive
4. Will replace o2 sensor
5. Will do fuel pressure check
6. Another smoke test
6. Will pray to the kjet gods...

Thanks

My guess is your O2 sensor is bad. It should go closed loop in 2 to 3 minuets.

[Jonty]

My guess is your O2 sensor is bad. It should go closed loop in 2 to 3 minuets.

Thanks - question is it better to run it without the O2 sensor plugged in if it’s bad until I get a new one? Just in case it’s causing mischief?

[Bitsyncmaster]

Thanks - question is it better to run it without the O2 sensor plugged in if it’s bad until I get a new one? Just in case it’s causing mischief?

Some people run without it plugged in mostly to stop idle hunting. If you do that you probably won't pass emission testing and expect less MPG.