Just made a dwell reading

[spikeygg]

So I just got my dwell meter and I hooked it up tonight and took a measurement.

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I didn't realize that I need the car to warm up before I'd see it oscillate. My measurement was fixed at this value. I tried to adjust the fuel mixture to see it change but my 3mm allen wrench seems to slip along whatever it is that it fits into when I stick it in the hole behind the air cleaner and in front of the fuel distributor. I was unable to affect the reading by carefully trying to turn the allen wrench, not sure what to do about this.

I see from D:04:15 in the DMC Workshop Manual that "Full throttle or engine cold (below 15°C or 59°F)" it should read 50-80°. How do I read the meter, I have about 27° on the 8-cylinder Dwell meter so do I double this to 54° and now I'm within the 50-80° spread?

Also, does anyone know if 50° is rich and 80° is lean or vice versa? My car seems to be extremely smelly and I'm hoping I can adjust the fuel mixture to help alleviate this issue.

[David T]

So I just got my dwell meter and I hooked it up tonight and took a measurement.

46664

I didn't realize that I need the car to warm up before I'd see it oscillate. My measurement was fixed at this value. I tried to adjust the fuel mixture to see it change but my 3mm allen wrench seems to slip along whatever it is that it fits into when I stick it in the hole behind the air cleaner and in front of the fuel distributor. I was unable to affect the reading by carefully trying to turn the allen wrench, not sure what to do about this.

I see from D:04:15 in the DMC Workshop Manual that "Full throttle or engine cold (below 15°C or 59°F)" it should read 50-80°. How do I read the meter, I have about 27° on the 8-cylinder Dwell meter so do I double this to 54° and now I'm within the 50-80° spread?

Also, does anyone know if 50° is rich and 80° is lean or vice versa? My car seems to be extremely smelly and I'm hoping I can adjust the fuel mixture to help alleviate this issue.

You cannot attempt to adjust the mixture till you are in "closed loop" and you see the dwell fluctuating. With the motor off you need to verify you can get your 3 mm wrench into the adjustment screw. You make TINY adjustments. A little bit has a large effect. When you do the adjustment you must remove the wrench and cover the hole and wait a moment for the measurements to "settle".

[Michael]

Oh boy. Tampering with the mixture should be the last thing you do.

[Jonathan]

You make TINY adjustments. A little bit has a large effect.

Agreed. When I was having mine adjusted by a knowledgeable mechanic and discussed how big the adjustments need to be at each step with him, he showed me how the full range from stall to stall was only a half turn.

[spikeygg]

You cannot attempt to adjust the mixture till you are in "closed loop" and you see the dwell fluctuating. With the motor off you need to verify you can get your 3 mm wrench into the adjustment screw. You make TINY adjustments. A little bit has a large effect. When you do the adjustment you must remove the wrench and cover the hole and wait a moment for the measurements to "settle".

Do you mean by this that I won't see movement on the needle if I adjust the mixture screw while in open loop?


Oh boy. Tampering with the mixture should be the last thing you do.


Agreed. When I was having mine adjusted by a knowledgeable mechanic and discussed how big the adjustments need to be at each step with him, he showed me how the full range from stall to stall was only a half turn.

Thanks for your concern. I have a feeling the mixture is set incorrectly because immediately after starting the car the entire garage fills up with smelly fumes and it never goes away. The engine is either running way too rich or way too lean, it needs to be addressed. The fumes stink terribly and make eyes water and nose run, something is clearly wrong.

Here's another video of the car that I just shot, I let the car warm up until the fans kicked on and the dwell meter was still stationary. Does this prove anything?

https://www.youtube.com/watch?v=EvbXyAnmV6A

[Bitsyncmaster]

The dwell readings will not change until your engine is running warmed up for about 2 to 5 minuets.

You need to do your mixture adjustment again with the engine warmed up. Your looking to set the average reading to about 40 degrees (20 deg. on your 8 cylinder scale). The mixture setting does affect the cold engine mixture but will not affect the dwell reading.

[Jonathan]

When my dwell reading stayed on one value and didn't fluctuate, it pointed to the oxygen sensor having issues. I replaced the oxygen sensor and it began fluctuating again (and then the mixture could be set once it did so).

[FABombjoy]

A fixed dwell reading at 50-60 degrees indicates:
-WOT switch stuck down - this can happen if the bolts are too tight.
-Lambda thermal switch defective

A fixed dwell reading at 40-50 degrees indicates:
-O2 sensor disconnected or defective (open circuit)

Reference D:04:15

[spikeygg]

A fixed dwell reading at 50-60 degrees indicates:
-WOT switch stuck down - this can happen if the bolts are too tight.
-Lambda thermal switch defective

A fixed dwell reading at 40-50 degrees indicates:
-O2 sensor disconnected or defective (open circuit)

Reference D:04:15

I just went out to the garage and I cannot make the WOT micro switch click. The idle micro switch clicks when I open the throttle just a bit and when I let it go it clicks again. However, the WOT micro switch doesn't make any noise. I grabbed my DMM and checked the continuity across the pins and it's a short. I guess I could unplug that switch and see if the engine runs differently and also what reading I get from the dwell meter...

When you say that "this can happen if the bolts are too tight." do you mean the bolts that hold the WOT micro switch down? I'll have to pull it off and see if it can be fixed easily like that.

Is there an easy test for the o2 sensor and/or lambda thermal switch?

[Jonathan]

I just went out to the garage and I cannot make the WOT micro switch click. The idle micro switch clicks when I open the throttle just a bit and when I let it go it clicks again. However, the WOT micro switch doesn't make any noise. I grabbed my DMM and checked the continuity across the pins and it's a short. I guess I could unplug that switch and see if the engine runs differently and also what reading I get from the dwell meter...

When you say that "this can happen if the bolts are too tight." do you mean the bolts that hold the WOT micro switch down? I'll have to pull it off and see if it can be fixed easily like that.

Is there an easy test for the o2 sensor and/or lambda thermal switch?

That's good advice on the WOT switch. Those bolts are the tiny ones (two of them) that go down from above and hold the switch (or switches if you have an automatic) to the small plate/bracket below. If they get too tight, it ends up pinching the switch and then that little plunger that is supposed to spring back out won't spring back out.

Unplugging the switch is easy and you can leave it unplugged indefinitely if you want. You unplug it at those little "tetris puzzle piece" looking connectors right on top, just in behind the backside of the W pipe.

Try the dwell reading again with those WOT switch connectors unplugged and report back what happens.

EDIT:

These are the two little screws:

http://store.delorean.com/p-6653-screw.aspx

And this is the plate they screw into (plate goes under the bracket and the screws go through the two holes in that bracket. The plate acts like the nuts in this set-up):

http://store.delorean.com/p-6651-securing-plate.aspx

Try not to drop any of it down into the valley when you go to disassemble it as they are all pretty small pieces and hard to get a good grip on.

[FABombjoy]

When you say that "this can happen if the bolts are too tight." do you mean the bolts that hold the WOT micro switch down? I'll have to pull it off and see if it can be fixed easily like that.
Yep, just loosen them a tiny bit and see if the switch plunger relaxes. This has happened to me a couple of times during my endless screwin' around with my throttle :) That switch should produce a very distinct click noise and if it doesn't, its broke or stuck. You're right about disconnecting the plug - give it a shot.


Is there an easy test for the o2 sensor and/or lambda thermal switch?
O2: Disconnect the O2 sensor plug and ground it. This will probably have no effect if the WOT switch is stuck.

Lambda thermal switch: Soak forearm bones in vinegar for a week, reach under mixture unit and disconnect attachment in coolant Y-pipe :)
Actually it's not too bad to find if you're not afraid to climb up on the motor a bit. Location shown at D:04:11

[DMCMW Dave]

If the WOT Switch is shorted you will see 55 on the meter all the time no matter where you set the CO. Disconnect it for now, how often to you floor the throttle anyway?

There are easier ways to disconnect the thermistor, (find the wire at the bulkhead and pull the pin out of the shell for example). The thermistor rarely fails anyway. The problem is going to be the switch.

If the car has been running rich for a long time you've probably fouled the O2 sensor and it will never "wake up". Cheap part, replace it if you see the dwell at 45 all the time. BUT fix the "richness" problem first or you'll just screw up a new one.

You can set the CO by ear in the mean time. With the engine running and warmed up, turn the CO adjustment counter clockwise a degree or so at a time until you hear the engine stumble. Now to clockwise the same way until the same thing happens. The overall range of motion should be about 1/2 turn of the allen wrench. Now set it in the middle of the two points. That will get you running remarkably well until you get the O2 sensor replaced.

BUT on other thing you said bothers me.

". I tried to adjust the fuel mixture to see it change but my 3mm allen wrench seems to slip along whatever it is that it fits into when I stick it in the hole behind the air cleaner and in front of the fuel distributor."

About once a year we see a car where some idiot drilled out the tamperproof plug and kept going, typically drilling out the allen screw as well AND in the process setting it full rich as they drilled the screw. If this has happened you'll need to repair/replace the upper air metering assembly. (The Allen screw is not a standard piece of hardware). Expensive part NOS, but with all the carb conversions going on out there not hard to come by.

[spikeygg]

Thanks for the help here guys,

I unplugged and removed the WOT switch, it's definitely shot. I tried to start the car and it sputtered and died a few times so I plugged in the switch so it would maintain an idle and when it got there I unplugged it and was able to keep the car running. Eventually I got it to idle and it settled at ~21° on the meter (so 42°), I let the car warm all the way up and it wouldn't oscillate -- still stuck at 42°. Although when I revved up the engine with the throttle spool, the needle would swing all the way up and when returning to idle it would settle back to 21°. Not sure if this tells me anything.

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I tried to unplug the O2 sensor months ago but the connection was very difficult to separate (any pointers on doing this?). I was afraid of breaking the plug or receptacle. I'll have to get under the car this weekend and pull the connection apart.


About once a year we see a car where some idiot drilled out the tamperproof plug and kept going, typically drilling out the allen screw as well AND in the process setting it full rich as they drilled the screw. If this has happened you'll need to repair/replace the upper air metering assembly. (The Allen screw is not a standard piece of hardware). Expensive part NOS, but with all the carb conversions going on out there not hard to come by.

Is there any way to tell if this is a problem with my car? I tried shining a light down through that hole but I can't see $%!7.

[Bitsyncmaster]

Is there any way to tell if this is a problem with my car? I tried shining a light down through that hole but I can't see $%!7.

Try putting the 3 mm allen wrench in the hole with the engine off and see if it can catch the screw head.

[spikeygg]

If the car has been running rich for a long time you've probably fouled the O2 sensor and it will never "wake up". Cheap part, replace it if you see the dwell at 45 all the time. BUT fix the "richness" problem first or you'll just screw up a new one.

This is likely what happened. Do you know if I can just go get a universal o2 sensor from autozone (http://www.autozone.com/engine-management/oxygen-sensor/bosch-oxygen-sensor/4966_108079_5320/)/napa (https://www.napaonline.com/napa/en/p/BSA11027/BSA11027_0218753428) and use the plug from my old one?

[spikeygg]

Try putting the 3 mm allen wrench in the hole with the engine off and see if it can catch the screw head.

I tried this Dave and it tends to just grab/slip when I try to spin in. :(

[Bitsyncmaster]

I tried this Dave and it tends to just grab/slip when I try to spin in. :(

You will most likely need to pull the mixture unit apart to see the damage. Like Dave said, probably a PO messed it up with a drill bit.

[Jonathan]

I tried to unplug the O2 sensor months ago but the connection was very difficult to separate (any pointers on doing this?). I was afraid of breaking the plug or receptacle. I'll have to get under the car this weekend and pull the connection apart.

I found the best access, and view, of that O2 sensor connection was when you had the rear end of the car in the air and the rear left wheel removed. I know you can reach up and in there and get your hands on it by feel, but it always seemed confusing to me when trying to do this without actually seeing it. It kind of hides between the engine side of the wheel well, but not quite as far in as the engine cradle side of the frame.

I wanted to have a photo to show you, but couldn't find one. If you are already going in this way and are still struggling with the connection, I recall those two ends of the wiring (sensor side which comes with the sensor itself, and the harness side) meet almost exactly at the edge of the (hole in the) frame. The harness side disappears into the frame only to reappear inside the ECU area behind the drive's seat, and the sensor side travels through the inside of the frame a short distance only to come out right about where the insertion point is in the stock exhaust set-up.

Which makes me ask you, is your car's exhaust standard or upgraded in some way?

The connection should be easy to separate once you get a good view of it. The problem is normally trying to get that rusted piece of old sensor out of the exhaust pipe.

[spikeygg]

I found the best access, and view, of that O2 sensor connection was when you had the rear end of the car in the air and the rear left wheel removed. I know you can reach up and in there and get your hands on it by feel, but it always seemed confusing to me when trying to do this without actually seeing it. It kind of hides between the engine side of the wheel well, but not quite as far in as the engine cradle side of the frame.

I wanted to have a photo to show you, but couldn't find one. If you are already going in this way and are still struggling with the connection, I recall those two ends of the wiring (sensor side which comes with the sensor itself, and the harness side) meet almost exactly at the edge of the (hole in the) frame. The harness side disappears into the frame only to reappear inside the ECU area behind the drive's seat, and the sensor side travels through the inside of the frame a short distance only to come out right about where the insertion point is in the stock exhaust set-up.

Which makes me ask you, is your car's exhaust standard or upgraded in some way?

The connection should be easy to separate once you get a good view of it. The problem is normally trying to get that rusted piece of old sensor out of the exhaust pipe.

Thanks Jonathan, this was how I was trying to disconnect it before too (rear end in the air driver's rear tire removed). I'll do it again this weekend. I have aftermarket exhaust, it's a set of supertrapps which makes accessing that o2 sensor a breeze but the plug: still a pain.

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[David T]

With all that is going on with the car you must make sure EVERYTHING else is done, done right, and working correctly BEFORE attempting to fine adjust the duty cycle. If the spark plugs need to be changed, if you need a valve adjustment, the screw in the metering unit has to be replaced, the air filter is dirty, etc. All that MUST be done PRIOR to fine adjusting the mixture screw. The O2 wire just pulls apart. if you are having trouble it is probably all corroded and not making a good connection. If the motor has been running rich for a while the O2 sensor is shot and must be replaced. But you can't do that till you can adjust the mixture screw and lean it out or you will just foul up the new sensor in short order.

[spikeygg]

With all that is going on with the car you must make sure EVERYTHING else is done, done right, and working correctly BEFORE attempting to fine adjust the duty cycle. If the spark plugs need to be changed, if you need a valve adjustment, the screw in the metering unit has to be replaced, the air filter is dirty, etc. All that MUST be done PRIOR to fine adjusting the mixture screw.

Thanks David I did all that.


The O2 wire just pulls apart. if you are having trouble it is probably all corroded and not making a good connection. If the motor has been running rich for a while the O2 sensor is shot and must be replaced. But you can't do that till you can adjust the mixture screw and lean it out or you will just foul up the new sensor in short order.

Understood.

[David T]

Well then the first order of business is to figure out why you can't get your allen wrench into the mixture screw. Make sure it is a 3 mm metric allen wrench. If it still won't fit look inside at it, maybe there is something stuck inside the allen screw? If it is, indeed, damaged, you will have to either file it and fix it or find a way to repair it or replace it before you can proceed with any adjustment.

[spikeygg]

So tonight, I saw this SpecialTAuto's photo:
46693

and I realized that I could use my ever useful borescope to check out the hole. So I did it and I was able to see it spin if I give it quite a bit of pressure. So I'll have to try that when I have the engine running again.

https://www.youtube.com/watch?v=shp2wr9R9zU

Anyway, it isn't great footage since the stupid camera doesn't focus within two inches (and I wasn't able to get it to focus) but you can see the screw spin slightly counter clockwise. But it's good news.

As I mention in the footage, I imagine that the walls of that allen head socket are supposed to go much higher than this. Anyone have good photos or footage of what it should look like?

[DMC-81]

So tonight, I saw this SpecialTAuto's photo:

and I realized that I could use my ever useful borescope to check out the hole. So I did it and I was able to see it spin if I give it quite a bit of pressure. So I'll have to try that when I have the engine running again.

Anyway, it isn't great footage since the stupid camera doesn't focus within two inches (and I wasn't able to get it to focus) but you can see the screw spin slightly counter clockwise. But it's good news.

As I mention in the footage, I imagine that the walls of that allen head socket are supposed to go much higher than this. Anyone have good photos or footage of what it should look like?

I wonder if you can point your camera straight down into the adjustment hole with some light to see what the condition of the hex fitting is on the screw. Or maybe a light and a mirror angled the right way? As David T said, maybe a little piece of debris is affecting it or, you can verify if it's been damaged.

If it's been damaged, perhaps you can take the next size up (4mm or closest SAE ) hex key and see if it will grab better. If the next size up is close to working, maybe you can file the tool to make it fit.

If I had a slightly damaged screw, that's what I'd try. But, I think you need to see what's going on on the top of the screw first.

[spikeygg]

You can set the CO by ear in the mean time. With the engine running and warmed up, turn the CO adjustment counter clockwise a degree or so at a time until you hear the engine stumble. Now to clockwise the same way until the same thing happens. The overall range of motion should be about 1/2 turn of the allen wrench. Now set it in the middle of the two points. That will get you running remarkably well until you get the O2 sensor replaced.

To gather more data, I did the experiment that the workshop manual recommends.
(awesome, apparently the "table" format button doesn't work in the editor for this forum... after I got everything nice and organized the preview wouldn't display it.)

Here's a video of the experiment/results:

https://www.youtube.com/watch?v=b1dd21VSiLA

Here are the quick photos I shot with each condition:

ECU lead on O2 sensor
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ECU lead at ~0.0V - Expect 87° from Workshop Manual, got ~42°
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ECU lead at ~1.5V - Expected 20° from Workshop Manual, got ~12-14°
46735

The car seems to idle okay with the lead grounded but it dies pretty quick with the lead at 1.5V unless I rev it up and keep it revs up. I don't understand what this means though. Can I solve this by tweaking the fuel-mixture and if so, which way should I turn the fuel adjustment screw? I tried to move it counter clockwise a little but it did not impact the values I saw on the dwell meter nor the voltmeter. The results from this testing make me wonder if my ECU is bad...

Anyone know (for sure) how to interpret this and correct it? Could my lopey cams be causing this different value (different from what the manual recommends)? By the way, all of this was done with the WOT switch completely removed so it isn't impacting the results.

[Bitsyncmaster]

Looks like a problem with the lambda ECU since the grounded O2 sensor should move the dwell to a very high number. Not sure if the wide open throttle switch and the thermistor would affect that if that circuit is not open.

What I would do is cut the light green wire (maybe you can pull the pin) near the lambda ECU and if the results are the same, your ECU most likely is bad.

I have a few good lambda ECUs if you need one to buy or on loan. I set up a bench test in a thread I posted not to long ago. http://dmctalk.org/showthread.php?13005-Bench-testing-Lambda-ECU

Your engine dying is probably just the mixture adjustment. You don't need to adjust it until you get that ECU problem fixed.

[spikeygg]

Looks like a problem with the lambda ECU since the grounded O2 sensor should move the dwell to a very high number. Not sure if the wide open throttle switch and the thermistor would affect that if that circuit is not open.

What I would do is cut the light green wire (maybe you can pull the pin) near the lambda ECU and if the results are the same, your ECU most likely is bad.

I have a few good lambda ECUs if you need one to buy or on loan. I set up a bench test in a thread I posted not to long ago. http://dmctalk.org/showthread.php?13005-Bench-testing-Lambda-ECU

Your engine dying is probably just the mixture adjustment. You don't need to adjust it until you get that ECU problem fixed.

Wow, nice work Dave! I wonder if my LM2901 is failing too. Thanks for the generous offer!

[Bitsyncmaster]

Wow, nice work Dave! I wonder if my LM2901 is failing too. Thanks for the generous offer!

I remember RON had a problem when he tested a ground for the O2 sensor but his ECU worked anyway. Can't remember what his test results were. I had sent him a few ECUs and he still had the problem. His lambda system worked anyway but it was just that ground test that caused problems.

The lambda ECU just swings the dwell to the extreme when the O2 voltage is above or below the 0.45 volts.

[David T]

I remember RON had a problem when he tested a ground for the O2 sensor but his ECU worked anyway. Can't remember what his test results were. I had sent him a few ECUs and he still had the problem. His lambda system worked anyway but it was just that ground test that caused problems.

The lambda ECU just swings the dwell to the extreme when the O2 voltage is above or below the 0.45 volts.

Yes, it is important you check the continuity of all of the grounds from the ECU. The O2 sensor works on very small voltages and any problem with the return path (ground) and it will appear that the ECU is malfunctioning. There are several ground connections. Refer to D:04:12 in your Workshop Manual.

[spikeygg]

Does anyone have a good picture of the entire k-jet fuel mixture adjustment screw so I can see what it's supposed to look like?

From the video I took the screw appears to have a wide mouth taper to it and I'm not sure that was caused by the drilling...
46898

I'd like to see what one looks like when it's in perfect or near perfect condition. Anyone have a k-jet fuel mixture unit that is not installed in a car? :)

[spikeygg]

I finally got it tuned properly tonight!

I'm guessing the last guy who messed with the fuel mixture didn't realize that the WOT switch was busted (stuck always on). So he/she turned the K-jet fuel mixture WAY lean (counter-clockwise). It was so far down that it took basically one whole clockwise turn to get the duty cycle to show something besides 95%!

Anyway, yesterday I realized that I have an o-scope that I could use on this to REALLY see what's going on (DUH!). So I busted it out and made the measurements, it's cakewalk with this setup. Then I hooked both the cheapo dwell meter up and my o-scope and shot the video so you could see what I was seeing once I had it dialed in.

These are o-scope shots of the experiments from the manual:

Unplugged O2 lead (40-50% steady), my reading ~50%
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Grounded O2 lead (87% minimum), my reading ~95%:
47048

1.5V O2 lead (20% maximum), my reading ~11%:
47050

Enjoy!

https://www.youtube.com/watch?v=BMU5bRmaSCA

[David T]

So what WAS the problem getting the allen wrench into the mixture screw?

[spikeygg]

So what WAS the problem getting the allen wrench into the mixture screw?

The problem was that I had a cheap set of allen wrenches. I was using a large set that I bought at harbor freight long ago, they are made of the black material. Trying to use the wrench in that screw top ended up rounding out the allen wrench. I went to a friend's house who had another 3mm allen, but it was black and had a ball end. I told him I was looking for a flat-ended one and he took it to his grinder and removed the ball. He let me borrow that one and it got rounded out too but I was able to enrich the mixture a little before it became unusable. I have a set of silver metal allen wrenches but they are on a ring and the long end is attached to the ring so I am unable to use those for this application but they seemed much higher quality and less likely to become rounded.

I was toying with the idea of using JB weld to shore up the insides of the screw top so that I could use the allen wrenches that I had, when I decided to purchase some good silver allen wrenches as a last-ditch effort. I looked around in town but nearly everything I found locally is made of the black material. Last week I ordered this set of silver metal Bondhus allen wrenches from amazon (https://www.amazon.com/gp/product/B000E7XK1U). They are MONEY and work very well for this application. No slipping, no rounding, no issues. Last night I tried it out and was able to turn the screw and dial in the mixture.

-Greg

[David T]

When you grind an allen wrench you destroy the hardening making the wrench much softer and easier to round off. Instead of a "set" all you needed was 1 tee handle 3 mm allen wrench. Glad to hear you got it sorted out. Make sure you use a good plug that won't come loose in the hole over the mixture screw.