Fuel Injector Waveform Questions

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

Well, this thread is done...#4 cylinder failed compression test miserably. Normally would do compression as one of first tests, but didn't jump on it because of having to take intake off to do it. Then I did a relative compression test and it didn't look bad so I went off in other directions. That'll teach me...*!@$#@. But, did learn some more things along the way, so not a total loss. Thanks very much Tyler and Ben for your help. I hope you don't think you wasted your time, because you didn't. I learned some good stuff from you. Much, much appreciated. Thank you.

:lol: I was just getting ready to tell you the injectors are fine! :silly: Not to worry, sir. I definitely learned something along the way, too. No time wasted, as far as I'm concerned. I don't mean to scrutinize/quarterback you, but can you post the relative compression waveform? It's not a perfect test, and it can be misleading sometimes.

I'm still unsure why the misfire was so intermittent? And why the trims were positive when the misfire was happening. Usually, a compression loss causes negative trims, in my experience. Did you have the opportunity to do any kind of leak down testing?

Oh I definitely want you guys to scrutinize/Quarterback me so think of me as a sponge I'm trying to get the file to work so I can post the relative compression, but for whatever reason it seems that the file is not complete and is not opening on my computer. Seems like half the time I move files to my puter from my tools I mess something up and lose them. Hopefully I just copied it and I still have the original file on my Verus. I'll take a look because I'd love to know what I missed.

The misfire wasn't really that intermittent and now that I know what the problem is everything has fallen into place. The engine would be at it's worse after a cold soak. Once it warmed up it was better, not good, just better. The first time I had it I noticed the oil was low so I topped it off before I gave it back to the owner. I also did the idle, accelerator and throttle plate relearns so I think it ran better than he was used to at first.

Today I pulled the dipstick and oil level was just barely on the stick. Uh-oh #1.

Pulled the spark plugs and 1-3 looked normal, #4 had slight oil cooked on it and was the wrong color, Uh-oh #2

Did compression test and cylinders 1-3 were dead on 180 psi (160 being minimum in this engine). Did #4...120 psi. :sick: Poured some oil in the cylinder and that got it up to 150 psi.

I didn't do any leak down testing. Once I told the owner about the compression results, he decided to throw in the towel and is going to trade it in. He threw a lot of parts at this thing before I begged him to stop and he doesn't want to spend any more money on this car.

The part that really gets me is when he first told me about his problems and the things he had already done, first words out of my mouth were, "I would do a compression test." Guess I lied...because I sure as hell didn't follow my own advice.

rockp2 wrote: Did compression test and cylinders 1-3 were dead on 180 psi (160 being minimum in this engine). Did #4...120 psi. :sick: Poured some oil in the cylinder and that got it up to 150 psi.

Oh, OK! That's not nearly as low as I was expecting. Totally understandable that you might have missed the problem on the relative compression test. Marginal cylinders (as opposed to completely dead holes) can be difficult to pick out sometimes.

I recently got burned on a Fusion the same way. :blush: Relied too heavily on the Ford scan tool relative compression test, which didn't pick up the low(er) cylinder. Couldn't hear it when cranking, but it was there.

I'm trying to get the file to work so I can post the relative compression, but for whatever reason it seems that the file is not complete and is not opening on my computer. Seems like half the time I move files to my puter from my tools I mess something up and lose them. Hopefully I just copied it and I still have the original file on my Verus. I'll take a look because I'd love to know what I missed.

You've got ShopStream Connect on your computer? I've had similar issues getting files off a Verus. Maybe Noah can chime in here...

60psi drop is definitely low enough to cause misfire I would do a leak down test to see if a valve is leaking as well we know rings are losing some but it may not be all of it if a valve adjust could cut it in half then that would be enough to make it quit missing. I think that should have shown up in your relative compression test

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Tyler wrote:

I'm trying to get the file to work so I can post the relative compression, but for whatever reason it seems that the file is not complete and is not opening on my computer. Seems like half the time I move files to my puter from my tools I mess something up and lose them. Hopefully I just copied it and I still have the original file on my Verus. I'll take a look because I'd love to know what I missed.

You've got ShopStream Connect on your computer? I've had similar issues getting files off a Verus. Maybe Noah can chime in here...

I've found that some times it's easier to take a screenshot using the Verus and send it as an email attachment to myself from the Verus. Then I'll open the attachment on my lap top.
You lose the ability to scroll and zoom the capture like you could using Shopstream to view it, but it's easier to pull it up on any device now and show the boss or the customer and say "this is why were going this direction".
And it's easier to post the files to the forum.

I do have Shopstream on my puter. I did have the relative compression original file on my Verus so I've attached a screenshot. I can also email the movie to you guys if you want to see it. Please let me know what you see that I did not. Don't be apprehensive about being critical please. Regretfully I did not use a second channel to identify one of the cylinders.

Ben, the owner does not want to put anymore into the car, even if adjusting the valves brought it up to 150psi, that would still be below minimum spec of 160. So he's just going to trade it in. But thank you very much for that tip. It will definitely be remembered!

Tyler, referring back to the post concerning the fuel injectors pressure drop,for my future knowledge, how much of a drop indicates a bad or blocked injector? Besides an obvious massive difference? I saw one of Paul's videos where I think he identified a bad injector that was only 2 psi difference? If I am remembering correctly.

rockp2 wrote: I do have Shopstream on my puter. I did have the relative compression original file on my Verus so I've attached a screenshot. I can also email the movie to you guys if you want to see it. Please let me know what you see that I did not. Don't be apprehensive about being critical please. Regretfully I did not use a second channel to identify one of the cylinders.

I see it? But I also know there's an issue to see. :silly: It's very slight - I'm not sure if I'd have picked that out at first glance. Or, even if I did see it, I may not have pursued it in favor of other diagnostic directions.

Do you recall the settings you used for that capture? Mostly wondering if using the Filter option might have made the peaks less noisy. I dunno if that'd help for sure, just thinking out loud.

Tyler, referring back to the post concerning the fuel injectors pressure drop,for my future knowledge, how much of a drop indicates a bad or blocked injector? Besides an obvious massive difference? I saw one of Paul's videos where I think he identified a bad injector that was only 2 psi difference? If I am remembering correctly.

Yeah, I think 2 PSI was his go-to rule. Dunno if it was in the same video, but I think he also said that when you find a bad one, it's usually way more than 2 PSI off from the others. :lol: It's my experience that the 2 PSI is mostly there to account for testing variables, like fuel temperature and idle time between pressurizing the rail and running the injector.

Yes pretty noticeable the red is the cylinder that is always higher after a low compression stroke the blue is your low compression it's easier to identify when you look at a pack of every fourth (for 4 cylinder)

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I would have definitely went for a compression test based on that capture ( that is a what a low compression cylinder looks like).
Its obviously not dead which will be much lower but anytime there's a noticeable low fairly consistently and especially when followed by a higher cylinder in a consistent pattern like that it's time to do a compression test. Also like Tyler mentioned a filter would have helped some on making it more readable

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