how do i read a dwell meter for lambda AF mix

[Baby]

83sc

I've read many threads about this, and I plan to get a reading at cold start then another after getting the car good and warm with a drive. I don't want to start making adjustments without getting an idea of what I'm seeing, as it's not what I expected.

Sears Engine Analyzer with which I can select these:

dwell
volts
4
6
8

Red alligator to fuse terminal
Black to ground
Green clip to green/white wire on test port

Set to dwell, cold start shows 70 on the 4 cyl scale.

First, should the meter be set to "dwell" or to something else? If it should be set to dwell and my reading is actually 70, does that mean I'm 12 units on the lean side?

[Baby]

Bump

[Baby]

Bump

So basically the needle pegs to the max when the analyzer is hooked to power and the knob is set to "dwell." When the engine warms up, the needle dithers. Car runs great, so it can't be too far off. Wondering why the needle isn't reading in the acceptable range. Any insight is appreciated.

[Bob Kontak]

OK - I can help you. Let me get my head around it though. It's been a year or two. Plus I have a youtube video of this out there.

Will be back in a few.

[tirwin]

Dithering is normal with the frequency valve is doing it's job with the engine warm. It should dither around 45 on a 4-cyl scale which equals a 50% duty cycle.

[Bob Kontak]

Originally Posted by Baby

Wondering why the needle isn't reading in the acceptable range. Any insight is appreciated.

As tirwin says - the needle when dithering, should swing over 45 when set correctly.

I can't remember some of the intricacies and I think what is confusing me is that psalt a while back indicated you can set your AF mixture a wee bit rich and the O2/frequency valve will still automatically dial in the desired AF ratio at idle. Then you get a wee bit of a rich bump under WOT but at idle and cruising you are at the stoichiometric ratio.

That said, you need to get your 3mm allen wrench down in the hole and adjust the af ratio. Always finish the adjustment with a clockwise movement. Meaning if you are turning counter clockwise, finish that turn with a clockwise movement. Something to do with screw lash.

See post #196 - video link at end of post.

911 sc starting issue

[Baby]

I feel like I know the both of you from reading your many CIS-related posts---psalt, too. And Bob, I've seen your video. I feel like I have a good idea about what to do. Unfortunately, I don't understand why my needle is pegged to the right when I hook up the analyzer, and it doesn't move (except to dither). I wonder if the analyzer is not working or if I have it set incorrectly.

Also, I'm connecting the positive cable to the uppermost rear terminal, where the fuse is, on the panel in the engine bay. In Bob's pic from the old post, his fuse is red. In my car, it's light blue. I'm grounding it on one of the bolts on the body that attaches the trunk latch.

BTW, After leaning it out a bit to pass smog, I have the timing/idle/af mix dialed in as best I can by ear, seat-of-the-pants feel while driving, and smell. But I'd like to see what the instrument says about the whole thing.

[Bob Kontak]

Mine showed 65 on cold start. It's not in closed loop till it warms up. Disregard till dithering starts. That means O2 is kicking in and sending voltage to the lambda box.

When you say pegged is that 70?

[Bob Kontak]

Now I remember what was bugging me and the resistance to respond immediately.

Remember that 45 is the 50% duty cycle, right?

I am kind of remembering that psalt noted that if the gauge is reading high - over 45 - it indicates that the frequency valve is pumping in a richer mixture to offset a lean reading from the O2 sensor. I had been assuming (a couple of years ago) that if the meter said over 45 that it was sniffing too rich. I believe that is incorrect. It is sniffing too lean and the frequency valve is trying to offset the lean mixture.

I could have it totally reversed.

Have you stuck a 3mm wrench in the hole?

[tirwin]

Bob is right.

When the O2 sensor detects a lean condition (high O2 content), the ECU increases the duty cycle. Increased duty cycle means increased volume of fuel to the injectors.

When a rich condition is detected (low O2 content), the duty cycle decreases.

Remember that the ECU doesn't always listen to the O2 sensor. At cold start, above 35% throttle and at WOT the O2 sensor is ignored.

Edit: I mispoke in the original post. I meant to say that ABOVE 35% throttle, the O2 sensor input is ignored. If fixed it.

[tirwin]

The O2 sensor has to get warmed up before it starts to work (generate voltage). On a cold engine, the control unit sends a 60% duty cycle at first. The lambda thermotime switch starts out closed on a cold engine. When it warms up enough to open, the ECU sends a fixed 50% duty cycle. Once the O2 sensor has warmed up to operating temp, the ECU switches to closed-loop operation where it adjusts the mixture based on the O2 sensor information.

[Bob Kontak]

Tell us more about "pegged" post dithering.

[tirwin]

Here's another check. When your dwell meter is pegged, disconnect the O2 sensor. Hook the + of a multimeter to the O2 sensor and - to ground. Check your voltage reading. Close to 1V is rich and close to .1V is lean. There is a wide margin of error around stoich so you can't reliably set your mixture based on the O2 sensor voltage. This is just a quick test to confirm if you're running to either extreme.

With the dwell pegged, the ECU should be trying to compensate for a lean condition, so you should see a low voltage on the O2 sensor.

[Baby]

Great info, thanks all. When I say "pegged,"
I mean when I hook up the analyzer it pegs to 70, even before I start the motor. On cold start it stays pegged as the car warms up. I disconnect the analyzer, drive for 10 minutes, connect it again, and the needle dithers at 70.

I understand that because the analyzer may read very high that the af may be very lean. When the motor is warm, I can pull the sensor plate down less than a mm and the idle dips. Any more and it stalls. I can lift up on the plate a good mm plus before the motor stalls. That tells me I'm at the lean end of the range.

I'll put a multimeter on the o2 sensor when I get home. Still bugging me that the needle maxes out even before the motor starts.

[Bob Kontak]

Originally Posted by Baby

I understand that because the analyzer may read very high that the af may be very lean.

OK - cool.

4 cylinder is the correct setting and "dwell" is where it should be set.

$40 - $50 will get you a new Actron at the local parts store. Hate to ask this but you are sure the connector on the test lead is not touching any of the other two leads? I am sure it is not but just checking.

The MM test on the O2 sensor is actually pretty slick. Try to back pin it so you can have the system working while you read it.

You will get it.

[Baby]

So with the motor fairly warm but not as hot as it gets after 20 minutes driving the O2 fluctuates between .1 and 0.

Idle isn't noticeably different with the 02 disconnected.

Oh, and the analyzer clip is not touching any other probes on the test port. I stuck a piece of bicycle inner tube in there to prevent contact.

Think I should start richening via the af screw until the dwell falls into the correct range?

[tirwin]

Yep! You have confirmed what you already suspected. You are running too lean. Turn the mixture screw until it dithers around 45 on the 4-cyl scale. Just for schitz'n'giggles check your O2 sensor voltage after you've adjusted the AFR and then plug it back in. It should be around .5-.7V. Then go for a drive and let us know how it's running on the butt dyno after that.

[Baby]

OK, thanks for the encouragement! I have turned the screw in 1/5-turn increments 1 full turn. Now the idle kind of picks up and adjusts down in a slower rhythm, and if i give it some throttle the rpms drop quickly almost to a stall before picking back up to idle. That's a sign of richness, right? Still the needle is pegged. Should I keep going, or am I already too rich?

[blyguy]

It sounds like you are still lean assuming the oxygen sensor is unplugged - what happens when you plug in the oxygen sensor?

I believe that if your rpms rise when you plug the sensor in, the fuel mixture is lean and the freq valve then adjusts by adding more fuel to get to lambda...
Could someone knowledgeable confirm or deny this please?

Bryant

[Baby]

Oxygen sensor has no effect on rpm. If it's still too lean, how can I keep it running as I enrich the mixture? The idle swings up to 2k, then dives to 0, then swings back up. Seems like it's going to stall out if I make it any more rich.

Turned the screw 1/5 turn to lean so it idles better. Dwell needle still pegged. O2 sensor reading started at .4.something and kept climbing up to .7.something before I stopped reading.