Fuel Injector Waveform Questions

Greetings! I wasn’t sure if I should put this in repair or diagnostic forum because my primary question is concerning what I’m seeing on the waveforms. SO I went with diagnostic forum. Hope that’s okay? I would appreciate help viewing the screenshots attached to let me know if it is normal or if there is a problem. I tried searching online at waveforms and watching videos...found a lot...but I couldn’t locate anything that would definitively let me know if the spikes and drops I show on a fuel injector are just normal anomalies or not.

This is a 2011 Nissan Sentra S with a L4-2.0L (MR20DE) engine. The complaint is the there is a P0304 (or was…owner cleared it) and he can notice a misfire. I did a power balance test with only the #4 cylinder because you have to remove the intake to get to the rest. Engine RPM did slightly change when coil was disconnected. Owner has replaced spark plugs trying to resolve the problem with no success.

It was raining and I was in a rush in a parking lot doing this (didn’t want my tool to get wet ). So I haven’t done anything past the quick scoping you see here. My images are on the #4 fuel injector control wire. Backprobed at connecter for the injector firings & low amp probe on the control wire for the current ramp test. As you can see there are random drops in voltage on the injector firings (note: I’m only connected to the #4 injector). There are random glitches at the end of some of the current ramps.

The images are from a max zoom view and a 1x view they are labeled in the filename. I have both in the movie format also if someone wants to see that. My next step is to do a compression test, but before I did that I just need to learn if these random glitches and drops are normal or they indicate a problem with the injector or circuit? Thanks Folks!

First try loading images didn't work..

Here are ramps (#4 fuel injector control wire)...

The spike on the current ramp doesn't seem right, to me. Do you have two channels? I'd like to see the voltage wave and the current wave, together.

Unfortunately I don't have it in two channels. I couldn't fit the T-pin and gator along with the low amp clamp into the same area. It was very tight under the intake and it was raining and I was really rushing. If I'd had it in my garage I could have taken more time and had more lead choices.

I found a good article which states: "The flyback voltage is set for the electromagnetic coupling of the injector and the mechanical spring rate. The magnetic field around the injector winding is stored energy which is used to control the speed that the pintle is closing with."

Would the spike that some of the current ramps are showing indicate that the injector pintle is closing faster or slower than engineered? Not sure which it would be, but if it is randomly closing faster would the ramp spike indicate that the ECM zener diode still had some current control left, but the injector closed too fast the rest of the energy was dumped with no resistance...causing the random spikes? I'm completely spit-balling here and barely understand what I'm talking about. :blink:

The other question is, if my statement above is close to on target, how would I determine if it was the injector or circuit/ECM issue?

Thanks very much for everyone's help as always.

Did you have peak detect turned on for your current ramp capture? I ask because I see that it wasn't on for the voltage capture, and you can see the result in the inconsistent peak voltages. Not saying you were wrong to have it off, just trying to account for the peaks you're seeing.

Doing some spit-balling of my own, but I wonder when the ignition coil fires on #4? It could be that the coil is firing at the same time as the injector ramp is ending, and you're seeing RFI in the current waveform.

As for the misfire, I'm happy enough with the voltage waveform for now. You can see the pintle hump, so movement is happening. Could be a clogged injector, but I feel like that's further down the testing line. If it's a cold misfire, I'd dig into the intake gaskets or a leaking head gasket.

Tyler, peak detect was off for the current ramp also. I just took a look at the movie. I used the presets on the Vantage Pro.

I'm going to just have to tell the owner I need his car so I can take my time in my garage where I have all the tools also. Who knows...maybe this will be my first diagnostic customer. Then I will change my handle to "THE Technician". (Yeah...riiiight...got a hell of a ways to go to get promoted from Weekend Warrior)

I dunno man, sometimes weekend warrior is more fun than being the technician! :silly:

Definitely get the customer to leave it, and leave the code alone. I feel like the freeze frame will be very beneficial.

So I got the car for a couple hours while the owner was at work. While driving it to my garage could hear what sound like valves clacking. This time had a code of P0300. Freeze frame data (below) indicated to me that this was after a cold soak so I let it sit as long as possible to cool down and then smoked the intake. Had a very small leak from the #4 fuel injector seat. So small that it took the rest of the time before I had to get the car back that it was tough for me to pin-point exactly whether it was leaking from the bottom seal (block) or the top seals (fuel rail).

I was taking it back to the owner, it fired up and idled like silk no clacking of valves or anything. The engine was idling perfectly. All I had done was disconnect the MAF sensor, the air intake hose and a hose that connects the intake to the "oil separator" box on the intake hose (this is where I introduced the smoke into the intake). I had also unplugged the #4 injector while I was searching for the leak.

As I was backing out of my garage, the car stuttered and then it seemed like an injector got shut down...as if the ECM had shut down an injector to protect the cat or something. The clacking sound Drove it back to him, but it seemed like this thing was only driving on 3 cylinders. It was driving the same as when I picked it up from him.

I'm hoping to get another shot at it so I can break out the scope, fuel pressure and such. But I'm waiting for my attenuation cable to come in to do the coils (even though he replaced them) and coil circuits as well as a more complete job on the injector circuits. I told him I didn’t believe that the tiny leak from the #4 injector was his primary issue and if he could wait until I can get back on the car

Any thoughts on the freeze frame? I should mention that this car has A/F sensor for the front bank (which he also replaced) and a regular O2 sensor for the rear bank. I need to get VERY smart on wideband.

P0300 (exactly as listed on my Verus):
ECT: 111
L-FUEL TRM-B2%: 100
S-FUEL TRM-B2%: 100
VEH SPEED: 0
B1 FUEL SCHDL mS: 500
CAL/LD VAL(%): 34
L-FUEL TRM-B1%: 109
S-FUEL TRM-B1%: 113
ENGINE SPEED: 1088
ABSOL TH-P/S(%): 1
INT/A TEMP SE (F): 66

P.S. He just texted me and told me he changed the #4 injector seals. Cleared the code. Stated the car ran perfectly for a few minutes and then …bam…right back to what seems like a dead miss and the valve clacking sound.

P.S.S. He had also informed me that he changed the cat. I think my next test needs to be an exhaust back-pressure. But would a clogged exhaust cause an injector to be shut down by the ECM (assuming that’s what is happening)?

Hey sir! Hopefully not too late to answer. Lemme catch up...

Thanks for the freeze frame. I'm confused - are there two banks to this 2.0L engine? I ask because most 2.0's I've seen have a single upstream A/F sensor, but the 2.5L has two. It matters because if there ARE two banks, then the freeze frame is doing a lot of the work for you. If cylinder #1 and #4 are on Bank 1, then one of those cylinders is the problem. Skip #2 and #3. It could also be that the OE freeze frame format is written to cover both engines, in which case disregard the above. :silly:

The total trims on Bank 1 could definitely suggest an injector shutdown. But, usually when a vehicle kills an injector, it also goes into Open Loop.

The valve noise is weird. :blink: Maybe keep a vacuum gauge on the intake during your testing? That way, when it start missing, you can watch the needle to make sure it doesn't twitch. I have a tough time seeing a mechanical problem being this intermittent, but you never know.

If it helps at all, I cover the four wire Nissan wideband sensors here. It's not comprehensive, but might give you a good starting point.

www.scannerdanner.com/forum/diagnostic-t...-testing-thread.html

FWIW, I think current ramping the coils and injectors is the way to go. Testing the secondary on these coils is a huge pain. I can't find a 2011 Sentra wiring diagram, but I found a 2010? Fuse #55 in the underhood fuse box powers the injectors, and #53 powers the coils. Put loops in both and have your low amp probe ready. When it decides to start missing, you'll be able to tell which one you lost.

Thanks for your response Tyler. Working on it right now. I too was confused about the Bank 2 part since this is the 2.0. When I picked up the car I sat in the parking lot for about a half hour going through the Live Data on my Maxisys via vehicle specific, generic OBD & Mode 6. I'm going through everything now and believe I might have a direction.

Today it had a P0101 when the owner dropped it off so I asked him if he had turned the key on with the MAF disconnected when he was changing his CKP (his latest parts change). I kept telling him to stop changing parts until I had a chance to diagnose, but...the internet told him...and of course he informed me the CKP didn't fix the problem.

He stated that he had not turned the key on with the MAF was disconnected, but he did clean it :pinch: so I'm focusing on that right now. At idle though the voltage was in spec, the g/s was just over spec (4.17 at idle...while I was sitting still!). I did find a note on the ole Vantage Pro that these MAF can have bad grounds and cause driveability problems as well as false injector leak codes. So this is the direction I am heading at the moment. Getting ready to scope it. If I'm lucky, this is the whole problem...though I don't know why that code (P0101) wouldn't have popped the other two times I scanned it? But then again...he did clean it. *sigh*

I did spend a few hours the other night going through your A/F sensor thread as well as digging deeper. Excellent thread! Thank you and because of it I now feel pretty comfortable with wideband. You da man! Thanks a ton Tyler and I'll keep you updated

Wow, 4V from the MAF at idle? :huh: Could be a bad ground for the MAF. Wondering if that's a separate problem from the misfire...

Let me know what you find with the MAF? Maybe give the wires a good wiggle, too. :lol:

No, I accidentally reversed it when I was typing...voltage was in spec...g/s was out of spec (1.00 - 4.00) at 4.17 g/s.

I just spent the last hour trying to figure out why my pin outs didn't match either Mitchell or my up-to-date Vantage Pro on the MAF. Finally figured out they have the pin-outs wrong or Nissan changed things midstream (which I've seen on other vehicles before). But I think Mitchel has it wrong in this case. Looked up the 2.5L to get the correct pin-out diagram for the MAF. Strange thing is, the ECM pin-outs Mitchell has are correct for the 2.0L, but the connector and the pin-outs they have for the MAF are wrong. Was driving me bonkers until I finally figured it out.

rockp2 wrote: No, I accidentally reversed it when I was typing...voltage was in spec...g/s was out of spec (1.00 - 4.00) at 4.17 g/s.

I just spent the last hour trying to figure out why my pin outs didn't match either Mitchell or my up-to-date Vantage Pro on the MAF. Finally figured out they have the pin-outs wrong or Nissan changed things midstream (which I've seen on other vehicles before). But I think Mitchel has it wrong in this case. Looked up the 2.5L to get the correct pin-out diagram for the MAF. Strange thing is, the ECM pin-outs Mitchell has are correct for the 2.0L, but the connector and the pin-outs they have for the MAF are wrong. Was driving me bonkers until I finally figured it out.

Very weird. :blink: There's the 2.0/2.5 thing again. I can't imagine it's actually got a 2.5L in it, and the VIN/underhood label is wrong?

EDIT: Did you end up finding a ground problem, too?

Hey Tyler, drafted a response last night but by the time finished it it didn't post. Anyway, yeah it's definitely a 2.0L. Vin shows it (no label under hood) and Verus shows it as such also. And there is only one A/F sensor.

Appears there was an issue with the MAF sensor (though I'm still having a secondary issue). At 2500 RPM there is supposed to be 4-10 g/s. This problem is intermittent and would happen every two or three starts. When I got it to go into "super crap" mode the engine was bucking and kicking and the MAF went as high as 28 g/s at 2500 rpm. I did check grounds and they were good. Changed it out and I've test drove it under all conditions (city, highway, steep hills, etc) several restarts and it did fairly well though I could still feel random misses every so often.

The final test I needed to run was after a cold soak since the owner stated that most of the time it would go into super crap in the mornings. So this morning I started it and idled ok, but at times did fluctuate a couple hundred rpm randomly above 1000 rpm until it warmed up. The real test was the drive, I've drove it be fore I worked on it after a cold soak and it was horrible. This time wasn't bad at all. Then MIL lamp flashed about 3 or 4 times and went off in the first 100 yards and then went off. But I really couldn't feel anything when it was flashing. I drove it around town and drove pretty well, but I could hear a light knock.

Turns out that real loud clacking was the belt tensioner. I changed the belt which fixed that problem. Now I can hear the engine sounds clear. There is a definite light knocking. Hooked up the scan tool and got a pending P0300. Did a power balance on the injectors (via the Verus) and they checked out. I came in to find out what the voltage should be for this A/F sensor because right now it is showing 2.2ish. I'm getting ready to test the coils and do a relative compression. Hope I don't need to do a full compression because the upper intake has to come off for that.

EDIT: And I still need to do the current ramp tests.

2.2V on the A/F sensor sounds good. How are the trims? I found some known good data on ScanShare if you're interested:

www.scanshare.io/share/heEVuXVVWkiYjDa8EEhdSQ

Interesting that you saw the CEL flash, but didn't notice a miss. Maybe that's why the owner threw a CKP at it - the PCM is falsely flagging misfires? :huh: I don't recall there being a CKP pattern learn on these...

There is not a CKP relearn, there is an idle air relearn. I measured the fuel pump current at an average of 3.78. Spec is 5. Rear O2 is reading a lean condition. A question I have, the fuel trim (LT) reads about 100 and ST about 128 on my Verus. Is 100 on this scan tool Lambda 1? That's what I'm guessing...but this is my first wideband auto I've tried to diagnose.

Yeah, an Alpha (short term) of 128 is 28%+ trims. Is it that high all the time? Very brief spikes that high are not unusual during fuel cut events, but at idle or cruising is obviously a problem.