Fuel Injector Waveform Questions

Yes, it was that high at idling. I'm taking the upper intake off to access the coils and injectors. The current ramp (which I did via the "Loop Wire" Nissan provided) just seemed funky to me. At least I could rule those out at a minimum. Relative compression looked good from what I could deduce, but that light knocking I'm hearing is bugging me. Fuel pump RPM computed to be ~6100. But at 3.78A (avg) I'm really leaning towards that as the source of this second problem.

Ah yeah, definitely running lean. Before you pull the intake, maybe go for another drive? Do a WOT run through first gear. A/F sensor should go low, downstream sensor should go rich, MAF should clear 4V or very close.

If the A/F sensor doesn't go low, that'd be another strike against the fuel system.

Guys, working on this car still. When I gave the car back the owner stated it ran better for a couple days and then went to crap again. Today I again did a power balance test. This time when I shut off fuel injector #4 (via my Maxisys) instead of bogging down the engine actually picked up a few hundred rpm?? It didn't do that last week. I disconnected the fuel injector and with KOEO I got B(+) on feed wire (other lead connected to B(-)) Good. But then when I connected to control wire (ground side switched) I got 2.9V??? Leaving lead on feed wire, key off, I disconnected. ECM and touched every harness side connector to see if I got continuity on a harness side pin I shouldn't have. Got none. Am I looking at an ECM or is there another way I can test for short to power in harness leaving ECM connected and KOEO. Begging for help!

Further info. I pulled the injector fuse at the relay box. When I did this (with injector disconnected) KOEO I got the same 2.9V on the feed (B+) wire that I got on the control wire (other lead connected to B(-). I checked another injector (#1) and got what I expected 0V. But to further confuse me all the injectors feed wires come off the same circuit. I would think that though I shouldn't have got the 2.9V, that still it would show on injector #1 also. Note that when I plugged the fuse back in I would get B(+) on the feed wire on both injectors.

Just bumping this post. I know it is convoluted, but the last two posts are getting me closer to the problem. Really appreciate any insights.

I haven't forgotten! :blush: Was just working out a reply, gimme a few minutes.

Tyler wrote: I haven't forgotten! :blush: Was just working out a reply, gimme a few minutes.

Sorry Buddy, wasn't sure if you got my message and didn't want to be a pest.

Nah, you're good! Didn't want to leave this one hanging. Finding time to get on early in the week is tough.

So, I gotta clarify a bit. KOEO, #4 injector unplugged, you saw B+ on the common injector power feed. But on the PCM control wire, you saw 2.9V? Then later, with the injector power feed fuse out, you saw the same 2.9V on the power feed? :huh:

I went over all the available diagrams a couple times, and couldn't find anything that ties into the same circuit that fuse #55 feeds (for the injectors). Unless that 2.9V is coming from the PCM, I can't see where it'd be coming from.

Was it misfiring when you ran that power balance test?

Yep, you got it correct. And yes it was misfiring as it has been. The owner was ready to pull the trigger on a fuel pump, but thankfully this time he listened to me. I had to go to the junkyard and scavenge some parts off a Nissan to make a fuel pressure adapter since my kit didn't have the right one. Hooked up the tester and the fuel pressure was bang on spec.

I was getting ready to pull the intake to test the pressure while activating each injector with my injector tool when I decided to see if the Maxisys would do it so I wouldn't have to pull the intake. "Module not supported" (of course).

For no particular reason I fired up the car and started a power balance test. Injectors 1, 2, & 3 made the RPM drop as expected as I shut off each injector. But when I commanded #4 off, the RPM picked up to about 1100 instead of dropping and the car seemed to even smooth out.

It's got me completely thrown. Getting it again tomorrow.

I don't want to butt in I just wanted to share I recently had a dead miss on a ford and after swaping plug and coil I scoped injector and it looked identical to rest of injectors I pulled rail and checked for spray and had none on the problem cylinder (moral of the story is it didn't show up on current ramp) it may be worth taking the time to pull the rail and check the spray from your #4 injector.

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I'm completely open to any thoughts Ben!

Tyler (or anyone), I notice on the wiring diagram that the BRN/YEL wire that feeds all the injectors (from fuse #55) has a dotted line on part of it. I know this means that there are other circuits or wires connected at the dotted location, but I can't figure out how to locate on the wiring diagram what those other circuits or wires could be? Any ideas?

Working on car now. Took intake off. Disconnected all fuel injectors. KOEO; DVOM black lead connected to B(-); every injector connector measures 2.96V on control wire (ground side switched). Obviously 2.96x4 injectors = B(+). Am I looking at a short to power? Could it be a bias voltage (hope not, don't understand bias well.

If you have intake off I would pull fuel rail and secure injectors to it with zip tie or some other means of holding them to rail besides factory clips (I got a face full of gas relying on factory clips ) place a piece of cardboard under the injectors and have someone crank it while you observe the spray of the injectors comparing #4 to the rest.

On a side not i quickly read through these posts a couple days ago and I don't remember has a compression test been done to verify you don't have a base engine problem?

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No worries Ben, happy to have you butt in! I like your thoughts about checking the injector spray.

An outside possibility that came to mind today... Is #4 at the end of the rail, or at the beginning (relative to the fuel line)? I ask because whenever I'm A.) Running an injector flush with the MotorVac machine or B.) Dealing with water in the fuel, it's my experience that whatever isn't gasoline tends to end up at the ends of the rails. Whatever cylinders have injectors at the ends of the rails end up misfiring.

Probably unlikely, but maybe there's a fuel quality issue? If there's been more than one tank of fuel run though this thing since you started diagnosing this issue, disregard and proceed as normal. :silly:

Thanks Tyler. Had to give him back the car. But getting it again Friday. Will do the cardboard test then while I'm doing the injector pressure test. I did do a relative compression test as well as the other ramp tests you recommended and they looked fine to me. What are your thoughts on the voltage on the control wires?

I'm not sure why you have that voltage there bit I wouldn't worry about it since it affects all cylinders if it was only 1 cylinder then I'd worry . I agree with Tyler about contamination in fuel rail that's what were going to see with the spray test I'm fairly certain from there well be pulling and cleaning rail and injectors.

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rockp2 wrote: What are your thoughts on the voltage on the control wires?

I didn't know what to think at first. :silly: So I decided to have a look at a Nissan we had at the shop, see if it's the same way. It's an '08 Altima 2.5L, so not the same year/model/engine/anything. But I figured if that 2.9V is a common Nissan thing, this one might use it, too?

Grabbed an injector waveform for reference:



With the injector disconnected, I got 12V on the feed wire, and 2.9V on the control wire. :blink:



This Altima has no engine performance issues that I know of, so I think you're definitely looking at a bias voltage on the injector control wire.

It's interesting that Nissan O2 sensors also use a 2.9V bias on their O2 sensor signal circuits. Maybe it all comes from the same place?

Tomorrow I plan to do the tests on the injectors. But what makes me think that the voltage on the control wires is cause for concern is the fact that the PCM supplies the ground through those control wires, not power. Since it is a parallel circuit it makes sense that the measurement of each injector's control wire adds up to a combined B(+). But there shouldn't be any power at all. So it makes me think that somewhere along the line the control wires are getting power. Whether it's from a short to power outside or inside the PCM, I don't know. Is my logic flawed?

Hey Tyler, I wrote my last post responding to Ben and didn't see your last one before I posted. Guess that leads to my logic being flawed. Thanks for the effort and time you took for me to look at that Nissan. At least I can rule something else out now. Thanks Buddy!

When using an injector tester and a fuel pressure gauge, how much variation signifies a bad injector? Starting each test at 49psi the results I got on my first test are:
#1 = 9 psi drop
#2 = 10 psi drop
#3 = 10 psi drop
#4 = 10.5 psi drop