Jeep 4.0 Secondary Ignition Waveform questions

Hi all,
I'm new to the forum and scopes, but not totally new to automotive electronics and circuits. My vehicle is a 1988 Jeep Cherokee with an auto trans and a freshly rebuilt 4.0. Over the last few months I've been trying to dial in all the sensors, grounds, vacuum leaks, etc to ring as much gas mileage out of this thing as possible. Recently the Jeep has developed a bit of stumble at highway cruising speed around 65 mph and its more pronounced if I take it out of Drive and put it in 3rd. I can feel the stumble here and there driving around town, but its most noticeable on the highway.

I'm really not sure if its a misfire, something wrong with the auto trans, or a sensor. I have a lot of spare sensors which I've been swapping in and out with no real success. That's when I decided that I would get a scope and see what it tells me. I got a Hantek 1008c and wasted a week trying to make it work. Gave up on that and got a Pico 2004a and I've been messing around with it for about 2 days when I've had the time. So check out this waveform that I got while driving today. This is cruising at about 50 mph engine running normal, not doing the stumble thing. It really doesn't look like anything I've seen from the poking around I've done on the web. Did I set this up wrong or what? I'd appreciate any direction you guys can give. Hopefully I attached this correctly. Thanks in advance. Dave

At a timebase of 200ms/div you could have upped the sample rate up from 8 ks. Not sure if a 2204 max rate is

Anyways you got what appears to be 2 lean fires in the hole on your first capture. Nice work!!!

Other than going to the scope right away, I would be interested in knowing what the fuel trims are doing when your at 50Mph cruising? and also what they are in drive idling?

You may have a weak fuel pump.

To test fuel delivery. Scan the pre-oxygen sensor volts. Put the vehicle in 2nd gear and floor it. The pre-O2 should stay rich the whole time under load.

Your capture is upsidedown, compared to "normal" view. This may be the reason you'll find it different from other captures/examples you've looked at. Use the "invert" function in the software to flip the waveform over to a view you're more familiar with.

As mentioned the sample rate is (too) low. Crank it up to get more detail or use a shorter timebase.

Apart from the occasionally lean cylinder various firing events show what looks like high secondary resistance. Have a look at the sloping down burn lines. How do the plugs look, no fouling? A snap throttle capture will add information for the broader r picture.

This is fascinating stuff. Thank you guys! I've attached a screenshot of a throttle snap that I did yesterday too.

I haven't been able to figure out how to invert the waveform. I'm not the quickest on "new to me" software. To avoid the inverted problem in the future, is it a matter of selecting the correct probe from the menu? I'm using a 10000:1 Hantek high voltage current clamp. I have 2nd one so that I can figure out the timing. I plan on working on it again today.

I suspected I might have a weak fuel pump, but that's where I drew the line on just changing out parts. Fuel pressure should be 31 psi at idle on this Jeep. Mine is around 29-30, but its been like that for years. I think it may bounce around a little too much under snap throttle. I'm going to get my fuel pressure gauge back out and take another look at that today. Fuel capacity is good, it pumps a litre in 50 seconds. It has a new fuel pressure regulator in it. I haven't been able to duplicate the problem I'm having without actually drive the vehicle.

I've been driving around for with my Fluke 115 backpinned to the o2 sensor just to make sure it was working. I'll put the scope on it today to get a better look at what its doing.

I'm not so good at reading plugs. I'll take another look at them today. When I put in the rebuilt engine and fired it up for the first time, it ran very rich. The original fuel pressure regulator had died and the engine was sucking fuel through the vacuum line. I pulled the plugs and wire brushed off the carbon which I found out yesterday is not the thorough way to clean the plugs. The engine has run about 1,000 miles since then.

Thanks again for replying. One more question, how do you put inbed screenshot images into your posts?

Dave

+1 for raising the sample rate.

juergen.scholl wrote:

Your capture is upsidedown, compared to "normal" view. This may be the reason you'll find it different from other captures/examples you've looked at. Use the "invert" function in the software to flip the waveform over to a view you're more familiar with.

Picoscope 6 does not have an invert button. However, you can choose "secondary ignition - inverted" from the probe menu. This will invert the waveform, as it is captured. Alternatively, you can use a math channel to multiply by -1 to invert a saved waveform.

I don't see a probes menu in the software. I do have a Custom Probes list under the Tools menu, but I don't see a secondary ignition - inverted option on that list.

Maybe the probes menu is somewhere else. Any ideas? Thanks again.

I also see the probes list when I click the Channel A icon, but no secondary ignition option.

Graveydavey wrote:

I don't see a probes menu in the software. I do have a Custom Probes list under the Tools menu, but I don't see a secondary ignition - inverted option on that list.

Maybe the probes menu is somewhere else. Any ideas? Thanks again.

The secondary ignition option is covered in the picoscope automotive software. You seem to be running the standard pico 6 software, not the automotive one.

Thank you! Downloading it now.

Ah, but it won't work with the 2204a. Bummer. I'll just need to figure out how to invert this waveform some other way.

There is a hack to make it work with the 2204A.....you'd need some modes dll' files that were on the internet for a while.

Why don't you just follow Chad's advice to apply a math channel , multiplying by -1? Do you know how to do that or does it sound unfamiliar to you?

Here is how to make you Probe with your Hantek Secondary clamp, inverted.

Thank you Wencraze for that walk through on how to set up the probe. Attached is a throttle blip that I took after I created the proper probe. I'd appreciate any and all opinions on this shot.

I retested the fuel pressure today and it seems good with the mechanical gauge. PSI was 30 psi. With a throttle snap it rose to about 35 psi then dropped to about 27 and settled back in at 30. Not too jumpy really. Pulled the vacuum line on the fuel pressure regulator and it rose to about 39 psi. the check valve held pressure for 25 minutes.

I'm not sure what to make of this problem. Jeep seems to run okay aside from the hesitation/stumble around 65-70 mph. The first attachment is a throttle snap, the 2nd is an inverted shot cruising when the Jeep is stumbling. The rpm doesn't seem to be interrupted when this happens, you mostly feel it in the seat.

I forgot to add these details to my prior post. Though the fuel pressure seemed to test well with the mechanical gauge at WOT it leaned out, ran below 1 volt (I didn't have a passenger with me to capture that waveform).

the plugs are clean even a little white on the electrodes.

Spark plug wires were all 4-5 k ohm in resistance.

Swapped coils again. No change in behavior. I did measure 12 ohms of resistance on the ignition control module. Kind of high, but that's not ridiculously high.

So I just re-read the whole post.

1. You got pictures of sparks.
2. Your fuel pressure looks good.

Though the fuel pressure seemed to test well with the mechanical gauge at WOT it leaned out, ran below 1 volt

In Global OBDII mode with a scan tool, you should see the B1S1 sensor reading between 0.1v(lean) and 0.9V(rich) and switching lean/rich, rich/lean once fully warmed up.

If you are pegged lean <0.3V at WOT you've got a fuel delivery issue.

Is the B1S2 also pegged lean under 0.3 volts as well?

I really wouldn't want to say much about the spark captures, because, for the most part, they could be normal at high rpm when there is lots of turbulence in the combustion chambers, but the others you have looked fairly normal, and it also looks like you upped the sample rate as well. So the first batch of pics was more than likely a sampling issue.

Also, it would be good to know what the fuel trims are doing at idle in park, then on a 65 Mph run.

Jumping straight to the scope isn't always the best scenario in driveability issues, the scan tool can answer many questions first, then possibly lead to oscilloscope testing.

Some scan tool data would be beneficial.

Unfortunately I can't plug a scan tool in because it's an 88. I believe the Snap On Mt2500 will read it, but I don't have one. It only has 1 o2, it's a titania type. 5v reference. It is switching back and forth, but that's all I can see. I was surprised to see it lean at WOT.

A couple of weeks ago, I was chasing this down as a trans issue.

I had been wanting to play around with a scope for awhile and this seemed like a good reason to pick one up. I wasn't sure if it was a misfire or not.

I just realized this morning that I misinterpreted the O2 sensor waveform at WOT. I double checked the service manual for the fuel injection system. On the 0-5 volt signal if the signal is closer to 0 the sensor is reading a rich condition. I had it backwards. The engine is running rich at WOT.

I think I should start looking elsewhere for the cause of my problem. I am going to test the rest of the sensors with the scope since I have it set up. I've test them all previously with my Fluke, but what the heck.

When the hesitation/stumble started, I thought it might be a torque converter lockup issue. I might take another look at that. I've been under the car looking around for indications of problem with the driveshafts, but I haven't seen anything out of the ordinary.

That makes sense as to why you went the scope route first, thanks for the explanation.

I drove the car again to play around the torque converter lockup. I wired in a switch that would cut the signal to lock up the torque converter. What I noticed was that that the stumble seems to be less when the torque converter isn't locked. It's still there, but its like the torque converter is cushioning it. That thought leads me back to a problem with the motor.

Here's another thought. I put remanned fuel injectors in when I reinstalled the engine. How would I go about testing them to make sure they are delivering the proper amount of fuel? Pulse time, I guess?