poor Fuel Efficiency, strange O2 waveforms and injector opening time

1998 Nissan Almera ( Euro spec Sentra ) 1.6L GA16DE engine

Symptoms:
Mild misfiring for the first few minutes after startup on cold days
Inconsistent fuel efficiency
Intermittent poor idle speed and quality

Brand new OE O2 Sensor

O2 signal usually pegged low, but will get pegged rich or lean with fast oscillations after revving the engine for a few seconds. Also shows fast oscillations in the larger fuel control waveform when revving engine.

5 Second Screen, 200mV / div


Pegged high



Pegged low



A short ( 1:40 ) video capture of the problem:



I scoped the injectors and two injectors ( #2 & #4 ) showed the following pattern with the pintle always closing exactly 1ms after voltage shutoff:



The #1 injector always shows pintle close at 1.1ms after voltage shutoff:



The #3 injector always shows pintle close at 1.2ms after voltage shutoff:



So I'm fairly new at this and am wondering if such small differences are significant and definitively the source of the problem?

The firing order is 1-3-4-2 and one can see that the dips in the O2 signal seem to correspond to when the 1 and 3 injectors are firing:


If this is the problem and if these are just sticking injectors, could running something like high concentration Seafoam through the system solve the problem, or would new / rebuilt injectors be the only option?

Here are the images it wouldn't let me post in the original message:

there is a diagnostic tool to check injector volume scanner danner uses it and it is pretty handy it pulses injector a predetermined amount and you can compare pressures and volume to get a good idea of condition of injection event other than that its looking like lazy o2s pine hollow diagnostics had a good vid on lazy o2s it was a more difficult case study but i believe he narrowed it back to module but that was a rare case

Chief...PERIODS PLEASE! Lower right part of your keyboard Me no speakum run-on sentenecesum.

O2 is brand new OE and working nominally. Regarding the injector tool, I was just getting ready to order this one:

www.amazon.com/gp/product/B077N76B2Z/ref...A1R0JBJ46WBNT5&psc=1

Thanks!

Looks like a very small vacuum-leak which drives the O2-Sensors high sometimes, but this is only a guess. The Signal in your Video Looks very strange to me in the first seconds. How does the Signal Look when you disconnect the O2, do you have a nice flat line? ....to eliminate a bad ground.

BR Matthias

I found two problems. One was a small vacuum leak from a torn PCV grommet, and the other was crud inside a plug boot causing intermittent shorts to ground at the plug. I'm pretty happy my guess that occasionally seeing no ringing at the beginning of the primary burn line indicated a short to ground in the secondary circuit was proven correct. Maybe I'll post some waveforms later.

Fixed?

Guessing this was the upstream O2? When its pegged lean you should see down stream lean out some too. Just a way to confirm the o2 is reading correctly

What are your Short and Long term fuel trims doing while its pegged rich and what do they do when its pegged lean?

If you ever see short and long term fuel trim conflict each other (Ex. LT is far positive and ST is far negative) then you could say you have an intermittent vacuum leak. I've seen that before.

SailorBob, I finally got around to uploading your images. They're all attached to the opening post now. I tried to edit them in where I think they should go? :silly: Probably got it wrong. Feel free to correct as needed.

Thanks! Has the underlying problem posting images been solved?

Fixing the vacuum leak at the PCV grommit just made thing get stranger.

I looked for more vacuum leaks with my, "blow cigar smoke into the brake booster hose" method, but couldn't see any - although that doesn't mean there isn't.

It's an OBDI vehicle, so no FT data available, and no post-Cat O2. Just something called "A/F Alpha" which is has been between 93-97% at idle when I've looked at it and goes up to 101% initially when revving the engine but then falls again. The thing is I've been having trouble connecting to the ECU intermittently.

So what's happening now is I'm seeing a mixture of patterns on the O2, pegged rich, pegged lean and misfire hacksaw. Sometimes it switches from one to another after revving the engine, sometimes it switches on it's own for no apparent reason. I captured some of this delicious wackyness on video here right after doing some throttle snaps on each cylinder at about 4 minutes in:



Here are two example of what I'm talking about, it just switching between states on it's own.

From hacksaw to lean:



From hacksaw to rich:



As I mentioned, I did throttle snaps on each cylinder. Initial sparks get up to about 24kV and 0.5ms burn time and around the 5-6th spark of the snap each cylinder has what looks like a lean misfire to me; something like this:



So whatever the problem is affects all four cylinders it seems like. I also noticed the current ramp varied from about 7.24 - 7.7 amps.

One other thing that I saw which was strange, where spikes like this in each cylinder's burn line at idle:



I happened to see this case study by John Thornton in which he says something like this indicates a mechanical problem; in his vid it turns out to be a cam sprocket that's not lined up properly with the cams:



I wonder if this kind of thing could also be caused by slack in a timing chain?

So to recap, 98 Nissan Almera 1.6L GA16DE DOHC engine, 358k kilometers, OBDI vehicle getting maybe 10-15% less fuel efficiency than it should, which has hard misfires upon hard acceleration on cold days during the first five minutes of operation, but only on the first startup of the day. Once warmed up it has an inconsistent idle which is sometime slightly high and sometime slightly low and mildly rough.

Original cap and rotor, which don't seal that well, but which can't be replaced without replacing the entire distributor, which would have to be imported from overseas. New plugs, new O2. Idle air screw had been opened a little in the past to compensate for stalling when coming to a stop.

Regarding the cold days problem, my guesses would be either condensation getting into the dizzy cap overnight, which burns off after the first few minutes of operation, or an intake manifold leak which seals up when the engine is above a certain fairly low temperature, since the problem only occurs on fairly cold mornings.

Whether the cold day startup issues are related to the fuel efficiency and weird O2 patterns I couldn't say.

The alternator also seems to be putting out an excessive amount of AC voltage and amperage at the alternator post ( 300mv, 7amps at idle with no electrical loads ) which can be seen here:



So the big question now is how to proceed in my diagnosis?

SailorBob wrote: Thanks! Has the underlying problem posting images been solved?

I actually had no trouble uploading them as is? *shrug* I do know that the forum software sometimes gets pissy when adding too many images in one post. The fix is to post one or two images first, and edit the rest in later.

Just an update, when I teed in a fuel pressure gauge I saw that it was leaking down pressure, and after clamping here and there determined that there was an injector stuck open / leaking. So this thing is getting a set of remanned injectors, and I'll come back to it after they're installed.

So after the wrong set of injectors arrived and I realized that it's impossible to get remanned injectors for this car I decided to get a cheap ultrasonic jewelry cleaner and used homebrew seafoam and an injectors pulser to clean the injectors.

Results are that a flow test now shows two injectors dropping 11psi, while the other two drop 12psi on a 100 3.5ms pulses, and 9psi and 10psi on 50 7ms pulses, respectively.

In the first case the difference is 9% and the second 11%, taking the lower number as the baseline. Surprisingly, it's the two injectors with the higher flow that seem to have the intermittent misfires and shorter ignition burn lines.

New injectors are $75 a pop plus international shipping and customs duties, so I'm going to see about grabbing one or two from a junk yard and seeing if I can flow match any of the current injectors.

What really surprises me though is that a one psi difference could cause such noticeable performance issues since I've always heard that up to a 1.5 psi difference is considered acceptable.