Jeep Wrangler 4.0 secondary ignition waveform

Hi all,

I'm tracing some idle misfire on my Wrangler 1995. 4.0 automatic. Engine shakes a bit but I can't find out why. It is not constant like when I unplug injector from a single cylinder, it is more like random cylinders misfiring. Distributor, coil and wires are as per FSM and have only 4-5 thousand miles on them. Cap, rotor and plugs are brand new. I had some issues with crank sensor signal and I've finally managed to get it to a decent shape.

I need some help regarding secondary ignition waveform interpretation. Is the waveform from video below ok? I can see some weird peaks at the end of burn line, sometimes it is longer than 2ms, and firing voltage is sometimes as low as 6kV. Video is slowed down (you can slow it down further on youtube) and it shows all cylinders (153624 firing order). I don't have another probe to use it as a single cylinder trigger. Time scale is 1ms while voltage is set to 2kV.

Here is the sample. I'm not sure if this is something scope related or it is really like that... Scales are 1ms and 2kV



I have just seen that CPS signal is not ideal yet. Cam/Crank relation should be different, it seems like the crank signal is a bit moved to the left. I'm wondering if cam/crank out of sync and this longer second pulse on crank signal would cause misfire?



CPS scale is 10ms 5v

spark firing voltage should be under 20kv. spark burn voltage 1.3 to 4kv (under 6kv). spark burn time 1.5ms no less then .9 or more than 2.4ms. 2ms can be Ok. end of spark is turbulence can be lean condition.

cam crank out slightly... should be 50% at turn on-off.

Hi sir not a lot of experience here that waveform does seem to be stretched out a bit I did not see an injector pintle hump and also that turn off oscillation looks funny where does ignition module get it s ground seems like spark line is a bit weak

Doesn’t look like a spark problem.

Here’s some things to keep in mind.
ANYTHING that can slow the revolution if the engine can create a misfire. Any pulleys, a harmonic balancer shifting wobbling.

I don’t really feel it’s necessary to ask for current ramp. I do think you should scope spark and spark control. Also a relative compression test and secondary ignition as a sync (timing).

Thank you!

John Curtis wrote: I do think you should scope spark and spark control.

Sorry but I don't understand what exactly should I scope here? Compare firing events to cam/crank sensor readings?

John Curtis wrote: Also a relative compression test and secondary ignition as a sync (timing).

This engine was rebuilt 3k miles ago. As for the compression, I did test compression check with gauge and it was 150 -/+5 psi. Factory asks for 120-150psi. Considering this I guess there is no problem with intake valves or rings. But this test does not verify exaust valve condition (eg. if they open properly due to worn cam lobes). Could I verify this with relative compression test?

You want to compare firing time with TDC using relative compression and ignition.

John Curtis wrote: You want to compare firing time with TDC using relative compression and ignition.

Will do, thank you. Regarding lean condition, do waveforms from that first video look like a little lean condition? I'm afraid if I have intake vacuum leak somewhere

Rutesic wrote:

John Curtis wrote: You want to compare firing time with TDC using relative compression and ignition.

Will do, thank you. Regarding lean condition, do waveforms from that first video look like a little lean condition? I'm afraid if I have intake vacuum leak somewhere

I can definitely see how ScannerJohn is thinking that with the end of spark line but I think it may be okay but am not 100% positive. ScannerJohn may be positive and may be able to explain why it is lean (teaching us both). The end of spark to me looks like a typical ignition waveform, if it was drastically higher I would say for sure it is a lean issue. Personally I would use other data to help draw that conclusion since I am unsure of the waveform.

Try spraying some water all around your intake manifold and see if if the engine bogs down.

As far as intake valves, if you have a pressure transducer you would be able to see a valve problem. If you don’t, they’re pretty expensive and the next best thing might be to use a vacuum gauge. Valve train problems can cause at idle misfires that get better with RPM increase.

Looks okay to me, not every firing stroke is identical and the fuel-air ratio is continuously changing in response to the O2 sensor. Also with distributor ignition systems individual aren't as repeatable as with coil on plug. From.your description I would want to double check the valve clearances, too narrow a clearance often won't show on a compression test.

Should have a slight rise in the burn line at the end which is left over energy from the coil at the end of the spark. When the tail raises up as in several of the waves, this indicates that all the hydrocarbons have been burned leaving high resistance in the cylinder, a lean condition.
If its in only one cylinder check for leak at intake runner of that cylinder. If its all check for lean variables for a global lean.
The burn line looks pretty good, should be flat. As you can see, it has a slight rise when you have the spike at the tail. The spark fire voltage will rise high as well.
If you view the waveforms in a parade view you can compare each and if you can isolate #1 cylinder you can see which of the cylinder are having problems. Looks a little erratic.

Ignition waveform analysis is some heavy stuff but it gives a lot of insight. I'm still learning and studying.

I'm thinking you have a lean and ignition timing problems.

If you suspect an intake leak you can scope your o2 sensors and use propane or another fuel source around the engine while you watch for a high voltage trace when it sucks in the fuel.

Nissan Technician

Thank you all for tips.

Fuel trim is around +3% most of the time. Ok, so next step is to verify ignition timing and check for intake leaks.

I don't have pressure transducer. However, should I be able to check individual cylinders by doing dynamic (running) compression test with gauge? If I understand well that would show me if intake and exhaust valves are opening/sealing properly

Rutesic wrote: Thank you all for tips.

Fuel trim is around +3% most of the time. Ok, so next step is to verify ignition timing and check for intake leaks.

I don't have pressure transducer. However, should I be able to check individual cylinders by doingcompression test with gauge? If I understand well that would show me if intake and exhaust valves are opening/sealing properly

When I’m dealing with drive ability problems and looking at fuel trims 10 is usually the safe number, however 6 is my tolerance for problem vehicles.

You’re going to want to do a leak down test. It’s different than compression testing in that you use a compressor to pressurize and it will show the amount of leak-down. You’ll have to figure out where it’s escaping from by listening to the exhaust, removing the oil cap, radiator cap or coolant reservoir, or air intake/throttle body.

Thank you guys for all your suggestions. I will check engine with leak down test and maybe running compression test and see if the cause for the misfire is mechanical.

If not, I assume there are four options:
1. Ignition ( considering your responses there should not be any issues with this part)
2. Fuel delivery/injectors ( fuel pressure is around 30psi idle which is 1 psi below Chrysler spec)
3. Air leaks ( since fuel trim is 3% it should not be anything major)
4. Something sensor/ECU related

What bothers me are timing events. There is one pulse in crank signal that is a bit wider that all others. Could this one mess up spark/fuel timing enough to cause random engine shakes ?

Ok, so I tested all the cylinders and they seal perfectly. I also did compression test one more time - 150 to 155 psi on all of them. Based on this I don't think that something mechanical is causing misfire.

I don't have current clamp so I cranked the engine and scoped crank sensor and secondary lead. It seems that spark occurs between 3rd and 4th peak of each crank sensor signal group. There are no missing sparks or more than one spark per cylinder, so this seems to be consistent.

After that I took fuel pressure gauge and measured pressure on idle. This time it had 27-8 PSI on idle. Factory spec is 31 psi. If I snap throttle it will bounce up a bit - maybe 2-3 psi, and when I release throttle it would go to 26 and then climb to 27-8. Is this proper behavior?

I have also tried to sync cam/crank sensor, but it is impossible to do so with this distributor. I followed FSM - put harmonic balancer to TDC (0 mark), blocked distributor shaft from rotating with a pin, however when I put it back i can't screw it in - i need to remove pin and rotate it a couple of degrees. This is weird, only thing that comes to my mind is that the clamping slot is not correct on distributor or that harmonic balancer does not point to TDC properly.

fuel pressure is off... 28-31 idle. On snap throttle should go up about 10lbs then drop to 26 is Ok then returns to 28-31. Pull the vacuum hose at the pressure regulator. Pressure should increase to 40-42psi. If not, check for vacuum.

I looked at your secondary waveform... check for carbon tracking on the plugs and or plug boot. Also check to make sure EGR is closing all the way. Those waves are erratic, should be all the same. I sure would like to see the secondary in a parade form of all...

Unfortunately I own Hantek scope so if I look at more than one secondary waveform at the time most of the details are lost... I will check/clean spark plugs/boots. All spark plugs except one have light brown color - as far as I know this means that the mixture is proper. However, on one of them tip is a bit oily - I guess that either exhaust or intake valve stem seal is leaking. I replaced that one with brand new spark plug and still no improvement, so I guess that oil leak is not that large to cause any trouble in that short amount of time.

One thing I noticed is that misfire becomes more evident after engine is warmed up (but it could be more noticeable because of rpm drop). I have snap on MT2500 and temp sensor data seems to be correct (at most 90 deg celsius on fully warmed up engine, so I don't suspect bad coolant temp sensor). Spark advance that ECU calculates seems to fluctate between 12-14 degrees. However, sometimes it randomly bounces to 19-20. Engine was rebuilt - new timing chain and sprockets.

As for the fuel pressure, it rises to around 35-6 psi with regulator vacuum disconnected. Does this mean that regulator is OK? This is still below factory spec. Only thing that comes to my mind is either bad pump, fuel filter or something inside the tank that restricts fuel pressure?

I have tried to partially connect vacuum to regulator so that it makes 31psi in fuel rail, however I did not see any improvement on idle. Not sure if this is ok test since I got same extra air in intake manifold because of partially connected vacuum.

AFAIK, there is no EGR on 91-95 4.0 engines.