[FIXED] 1994 Sentra Lean Condition, Stumble/Stall

Hello Everyone,
In need of some input here. I have a 1994 Nissan Sentra 1.6L. The vehicle barely runs, and will die as soon as you step on the accelerator. Total fuel trim at Idle is 38% and does not get better with increased RPM. Fuel pressure 36 psi at idle and jumps to 44psi with the regulator disconnected which is in spec. O2 fluctuates from 20mv to 800 mv at idle and bottoms out as soon as you try to accelerate. MAF reads 1.0v at idle and climbs to 3-4 volts when accelerating. I have replaced the MAF sensor and no change. The vehicle will run if the timing is overadvanced, but will ping excessively. Seems like a fuel delivery problem to me. I have flushed injectors no change. I pulled a fuel sample and found 10% ethanol. I have also pulled the o2 sensor out and had no change. Also no change when introducing propane while acceperating. This car is kicking my butt. Any insight would be greatly appreciated!

Is this the optical distributor? Mounted horizontally on the side of the cylinder head?

This is from chapter 21 in my book and is what I am thinking about for your situation.

Definitely check the distributor for an oil leak like Paul pointed out. If you come up empty there, then I'd suggest installing a spark tester in-line with one of the plug wires and watching the spark during idle and acceleration. If the spark is weak, sporadic or disappears on acceleration, then I'd say we should stay with an ignition problem.

I see that it runs better with the timing advanced, you've verified this with a timing light?

I have not verified it with a timing light. I did however have to move the distributor to the full advanced position to get it to run. Tomorrow I will have real timing numbers for you. I will also do a compression test. I have installed a spark tester, and it will jump a 1.5 inch gap easily from the coil, and at least an inch at the wires. I have scoped the injectors using both current ramping and voltage. The current ramp from looked normal with a nice ramp and a long saturation period. With the fuel trim so hi, it makes sense to have an extended saturation because of the longer pulse width. I did however see a strange looking voltage pattern. I apologize for not having a snap shot of it, but I attempted to draw it in paint. Please see the attachment. All the data points to a fuel problem for me.

Ok, so here is the update. I've removed the distributor cap and shield to expose the optical wheel. There is no oil in the distributor. I reassembled and placed a timing light. with the cam actuator solenoid unplugged and the tps unplugged the timing is at 10degrees btdc at 750 rpm which is in spec. revisiting the scan tool data if I nurse the throttle to where i can get to 3000 rpm and then snap it the most voltage i see from the MAF is 3.3 volts. This is the same voltage I saw from the previous MAF sensor. I voltage dropped the MAF power from terminal to battery and had less than 10mv. Voltage drop on the MAF ground to battery ground is 10mv. I installed a power probe hook and set it to send reference voltage and swept it from .5 volt to 5 volts while observing the PCM MAF pid and saw voltage matches. I'm really leaning toward a bad new MAF. The fact that fuel pressures are good and maintain at 45 without vacuum at the regulator, and seeing the 02 sensor plummet to 0 volts. I was thinking about looking at injector pulse width and watching it to see if it fails to widen when the motor bogs down, but not sure if I will see anything without something to compare it too. Suggestions?

Just got done watching the current ramp of the injectors. The pulse width widens normally until the engine bogs down. MAF voltage caps at 3.5 v on scope when it bogs down. I can watch the pulse width start to actually decrease at the same time. 02 sensor stays on the floor when engine bogs down. I sent 4.5 the power probe hook volts to the ECM while watching MAF voltage on the scope as soon as the MAF reached 3.5v where the engine starts to bog and held the throttle to wide open throttle. The engine did not bog down and I was able to hold the car at 4500+ rpm. Looks like I am ordering another MAF tomorrow.

Well I got to the car last night, and it was in a no start condition. I decided to check the basics again. I had very little spark coming out of the wires. Check at the cap and same thing. Wouldn't jump more than a quarter inch out of cap. Coil would jump over an inch. I replaced the cap and rotor, and its running like a champ.

wtrapp1979 wrote: Well I got to the car last night, and it was in a no start condition. I decided to check the basics again. I had very little spark coming out of the wires. Check at the cap and same thing. Wouldn't jump more than a quarter inch out of cap. Coil would jump over an inch. I replaced the cap and rotor, and its running like a champ.

Nice! :woohoo: Not being critical of you, but I'm wondering what changed from before (when the spark would jump an inch at the wires) to now (weak spark at the cap). :huh: I'm super happy it's fixed, I just gotta know why. :lol:

Perhaps the rotor was in the process of burning through to the distributor shaft, and finally ended up directly shorting last night. This would explain why the engine ran the way it did, and why it became a no start. Anything visually wrong with the old rotor?

I believe my fault lied in the fact I only checked Cylinder 1 for spark initially. In hindsight I believe it was cylinder three I pulled off the cap when I found bad spark. Cylinder 1 was likely not affected in the cap. And honestly that may be a stretch. I suppose only time will tell. Perhaps its intermittent and I've fixed a secondary problem. I drove it all day today without incident though.

Nah, I think you fixed it.

Also, I keep looking at your injector pattern drawing, and I still don't have a good answer. Is that same pattern present after the ignition repair? If you can get a screenshot or photo of that pattern, I'd be interested to see it.

It's fixed for sure. Where I went wrong was dwelling on the increased fuel trim and the 02 reading 0 volts when accelerating. False lean due to engine acting as an oxygen pump because of incomplete burn under load. As for the injector waveform, I will put the Pico on it sometime this week and take a movie. Not sure if it is still like that or not.

wtrapp1979 wrote: It's fixed for sure. Where I went wrong was dwelling on the increased fuel trim and the 02 reading 0 volts when accelerating. False lean due to engine acting as an oxygen pump because of incomplete burn under load. As for the injector waveform, I will put the Pico on it sometime this week and take a movie. Not sure if it is still like that or not.

I've gotten burned on this myself and actually talk about it in one of my lectures. It was the same thing (cap and rotor). Have a fixed lean O2 at wot.
Nice job man!

wtrapp1979 wrote: It's fixed for sure. Where I went wrong was dwelling on the increased fuel trim and the 02 reading 0 volts when accelerating. False lean due to engine acting as an oxygen pump because of incomplete burn under load. As for the injector waveform, I will put the Pico on it sometime this week and take a movie. Not sure if it is still like that or not.

I was discussing this kind of scenario with a friend today, I don't think he believed it.
I'm glad to hear you got it fixed!