Dwell reads 49 with o2 sensor grounded

[DaraSue]

64385

Running the checks on this list while attempting to root out a misfire. The normal operations test swings wide (28-60) and the test with the o2 sensor grounded consistently reads 49 instead of 87. I've redone it multiple times, even with a female connector on one end of the ground wire and a clip on the other to make sure it's actually attached, grounding it to the hook on the engine on the left side and to the exhaust, and I always get the same result. All of the other checks were within range.

I asked Dave S. about it and he didn't even think that reading should be possible unless there's some weird electrical issue. Has anybody ever seen this before?

[Bitsyncmaster]

I remember a few other owner had problems doing the ground test. Don't remember if they got 49 deg. If yours is swinging that much for normal operation I would guess your O2 sensor is bad.

[David T]

Do you have your dwell meter on the right scale? Some dwell meters just don't work well on a Delorean, try another one. Sounds like your readings in closed loop are doubled. Swinging is a good thing but yours is a bit much. Might need a new O2 sensor. You won't find your misfire this way. Pull and check the spark plugs. Notice if any of the spark plug wells had water. Misfires often occur because of a bad ignition wire, worn plugs or bad patterns on the fuel injectors. Vacuum leaks can cause the nearest cylinder to run lean. If you have a lot of miles it may be time to do a valve adjustment. Do a compression test. The goal is to get all of the cylinders as close to each other as possible to make them all run smooth.

[DaraSue]

Thanks, guys. The sensor has about 24,000 miles on it. It didn't look super cruddy when I pulled it but maybe it's time.

I'm using a Centech digital meter, on the 4 cyl scale. As I said, the other tests read in range. Dave S. said this is the kind of meter he uses so I assume it's good enough. (I bought an analog one on eBay but it doesn't work very reliably so I'm sticking with this one.)

Plugs and rotor/wires were changed last year, about 3000 miles on the plugs since then. I did find a leak at the rear left injector seal and have since changed the seals but I thought I still heard some popping at 2500 when checking the dwell afterward. The car has 45000 miles on it (but 24000 of them were put on in the last 4 years and only 3000 were put on between 1989 and 2016). I did a mason jar test on the injectors, because of how the jars were sitting I didn't get a good look at the patterns but none of them seemed to be putting out dramatically less fuel than the others.

[Bitsyncmaster]

When I was using a wideband unit, I played with a setting which could slow down the response of the output of the wideband. Doing that made a very bad hunt at idle. So what I'm thinking is your sensor is slow to respond and that could be causing the new hunt. I think that is why some cars hunt (mine included) shortly after a cold start when the narrow band sensor has warmed up enough to put out an output not not hot enough to give fast output response. Now, new cars all use heated sensors which don't have that problem.

[David T]

I find using an analog meter you can easily watch the needle swing, it is not as easy to see the rate of change on a digital meter. Different digital meters display changing values differently depending on the delay built in to "dampen" changes. Hunting is a function of how closely each cylinder matches the others. When a "low" cylinder comes around to fire, the motor slows down and then speeds up when the next one fires. By then the idle system tried to speed up the motor and because it fired a "good" cylinder the speed overshoots. You also have the Lambda system trying to correct the A/F ratio but because of the built in delay the effect is delayed and also overshoots.These systems are not in sync and sometimes they phase additive and sometimes negative. The way to minimize hunting is to get all of the cylinders firing evenly so the smaller the corrections the systems have to make, the smoother the idle. If you had one bad injector seal you should replace them all. You probably need the injectors cleaned too. Do a compression test and see how even the cylinders are. They should all be within 5% of each other.

[Bitsyncmaster]

The lambda and idle systems do not respond fast enough to follow each cylinder firing.

[DaraSue]

I changed the sensor and the ground check still reads around 52. Weird. The normal dwell swing got a little better, around 33-59. Still think I'm hearing the misfire. I did replace all the injector seals and wipe the injectors down with some Brakleen. I'll see about getting a compression check next week but another DeLorean buddy of mine suggested checking the timing too so I might try that next.

[Ron]

$.02 I'd find the miss first. Pull the plugs wires one at a time and listen for no change in RPM...

[DaraSue]

How much is it supposed to drop off when you pull a plug wire? I didn't notice a huge difference between any of them but it fluctuates at idle anyway.

[Ron]

Guessing, 50-100 RPM. If the fluctuation's average RPM drops any, skip to the next cylinder.
(You should be able to hear a change at the good cylinders.)

[David T]

If the timing is off it would affect all of the cylinders evenly. You have to look at each cylinder and see which one (s) are the ones not working as hard as the others. That starts with a compression test. You can also try checking each one with an infrared temp gauge and see if one is colder than the others.

As for the idle and Lambda systems not responding fast enough, that is exactly why you have the hunting. They don't respond fast enough, by the time they respond, you are already firing on another cylinder.

[Jonty]

If you might be having a misfire first pull the plugs as often the misfire will show up as a wet or dry fouled plug that looks different from the others. Also a inline spark tester can help check if the issue is the spark plug itself or the supply to the spark plug. I’ve never had much luck simply pulling the plugs to hear for a change in rpm. I’ve found if you have a misfire dropping from 5 to 4 cylinders is sometimes not easy to hear.

I had a wide swinging dwell and it was a result of poor injectors and a misfire. The misfire was actually a broken plug wire and also a bad injector. I would also replace the 02 sensor just for peace of mind but first fix the misfire or you might foul up the new o2 sensor.

[CFI]

I would also replace the 02 sensor just for peace of mind but first fix the misfire or you might foul up the new o2 sensor.

She already replaced the sensor.

[DaraSue]

Haven't tried the compression check yet but I pulled the plug on the cylinder that had the leaky injector seal and it was pretty cruddy. I cleaned it up as well as I could with a wire brush and put it back but I'll probably order some new plugs just in case. I didn't check every plug but the one next to the dirty one looked normal.

I drove the car to the gas station and back and it felt a little peppier but I still think I'm hearing that popping noise when I rev the engine. I'm not really noticing the other symptoms on this list: https://carcarehunt.com/blog/car-misfire-symptoms-causes-how-to-fix.html

The rpm fluctuates a little at idle but it stays between about 600-900. It doesn't accelerate noticeably worse than it ever did or die at idle. My fuel economy had gotten worse in the last year or so though (down to low 20s from close to 30 on the highway).

The guy at the shop who told me it was a misfire was the owner and not really an actual mechanic, I'm wondering if maybe it's actually something else? I found a loose nut on the exhaust manifold so maybe I had a leak.

[David T]

If you have been watching your mileage and notice it is lower than usual, it is a sure sign that you need a tune-up and/or your brakes are dragging.

[DaraSue]

Oh, forgot to mention, my timing was slightly off (between 14 and 16) so I adjusted that too.

I'd notice other symptoms if my brakes were dragging, right?

[CFI]

Oh, forgot to mention, my timing was slightly off (between 14 and 16) so I adjusted that too.

I'd notice other symptoms if my brakes were dragging, right?

Yes, especially if it’s a front brake. The car would pull towards whatever side was dragging. Also, depending on how badly it’s dragging and how far you drive, that brake would get very hot and would smell terrible. Acceleration would also be affected but you said that seems normal.

[DaraSue]

Yes, especially if it’s a front brake. The car would pull towards whatever side was dragging. Also, depending on how badly it’s dragging and how far you drive, that brake would get very hot and would smell terrible. Acceleration would also be affected but you said that seems normal.

Ah, OK. I think I can rule that out. Braking seems normal and I drove it to Wendover last month (100 miles each way) and didn't experience any of that.

[Bitsyncmaster]

Ah, OK. I think I can rule that out. Braking seems normal and I drove it to Wendover last month (100 miles each way) and didn't experience any of that.

If a brake was dragging enough to feel less power you would definitely notice a smell when you get out of the car. You can also feel all four wheels after stopping to see if they all feel the same temp. I've had front brake hoses in daily drivers plug and the wheel is so hot you can not touch it.

[DaraSue]

My buddy who also has a DPI exhaust came over and listened and said his car makes the same noise. He said his friend who has a new Mustang had the same thing and they told him at the Ford dealership that it was just something that happened with that kind of exhaust. I did find a few loose nuts on the exhaust manifold if that has anything to do with anything. Wondering if this is another case of simultaneous but unrelated issues...

(Also, the car passed emissions, would it if it actually had a misfire?)