S14 SR20DE frustrating issue diagnosis

[AZhitman]

It's been a loooong time since I posted up looking for help, but I'm at a loss here.

S14 SR (n/a) in my '68 510 has had an issue for months now. Despite the best efforts of myself and several pretty talented techs, I've made no progress.

The engine is reasonably stock, with the exception of a larger intake manifold (Greddy log-type), 270cc top-feed injectors, and a larger throttle body. Multiple grounds are present, and it's running on a new aftermarket harness.

Engine starts nicely and seems to idle smoothly. But even a HINT of throttle input causes it to stumble, and typically it will never recover (and stall).

I have another SR here (in my '77 620) that performs perfectly. I've swapped the distributor, the IACV, VVT solenoid, the MAF, the TPS, O2 sensor, and the ECU (individually) to see if any of those components is causing the problem, to no avail. I've also checked base timing (with VC off).

Unplugging the VVT solenoid completely has no effect.

Interestingly, laptop shows all sensors reading correctly - BUT, when you barely touch the throttle, the timing appears to jump all over the place and the idle stumbles. My understanding is it should initially dip, then advance quickly depending on amount of throttle input.

Allowing it to warm up has no bearing on the engine's behavior (open loop vs closed loop). I'm at a complete loss as to what to check next.

[VStar650CL]

The timing jumping all over has to be a clue. I'd start by back-probing the 1 deg and 180 deg signals from the distributor and scoping them to see if one of them is breaking up or losing voltage. The photo diodes in those circuits can be pretty sensitive to resistance in the wiring. Have you checked pin-fits on the connectors?

[AZhitman]

That'll be my next effort.

[AZhitman]

So, another confusion...

The distributor / cam position sensor on the engine (part # 22100-65F11) isn't a coil-on-plug design, it's got plug wires.

Also noted it has 6 wires in the side plug.

However, when I pull the S14 and S15 SR FSM, it clearly shows a C-O-P design with only 4 wires from the side.

The only thing I can think of is the NA motors were traditional distributor cap design, and turbo models were C-O-P.

This leaves me wondering what the hell engine I have. The VVT solenoid indicates it's an S14/S15 variant.

THEN I go looking for a crank angle sensor (as I * think * that could cause the issue I have), but I don't see one on the engine.

I found a replacement dizzy that matches my part number (https://www.ebay.com/itm/186101734064) but it's pricey just for diagnostic purposes.

[VStar650CL]

The SR20's in the Infiniti G20 were all dizzy and wires up to the end of the series (2002). Oddly, I just worked on one last week that turned out to be a bad coil. The crank sensor on all the dizzy engines is diagnostic, it doesn't actually do anything. That's true of pretty much all of them, even the VG's.

[VStar650CL]

The crank sensor on all the dizzy engines is diagnostic, it doesn't actually do anything. That's true of pretty much all of them, even the VG's.

PS - That's if it has a CKP at all, a lot of them don't. The 180 degree opto in the dizzy is a reference, along with the 1 degree signal it gives the ECM everything it needs.

[VStar650CL]

Troubleshooting tip if you think the coil might be causing the issue. The "dead" terminal on the 2-pin connector at the dizzy is actually an access to coil negative. Popping the rubber plug lets you look directly at the coil primary. Generally you'll see about 6V at cranking/idle if the power transmission is driving the coil. If you get battery voltage it usually means the power transmission is failing. In this case, scoping the signal will probably tell you immediately if the transmission is crapping out or the coil is breaking down on accel. If it's the transmission you'll see the bottom of the coil go to battery voltage, if it's a coil breakdown you'll see the usual high-voltage discharge pattern turn into a square wave or spiky spaghetti.

[VStar650CL]

PS - Not all of them are "dead", if the tach is driven off the dizzy then there will be a wire in the top position of the 2-pin connector. That's where you want to look.

[PapaSmurf2k3]

This sounds oddly similar to an issue I had in my miata when I ran "non-resistor" spark plugs and was getting some kind of electro-magnetic interference to the crank angle sensor circuit.
Hopefully you're running some normal plugs, and hopefully nothing is arcing out anywhere causing funny business.
Can you shake the harness at various spots and get any change in idle/performance?

[AZhitman]

Will check that out. I'm inclined to think the harness is OK, since it's new (WiringSpecialties).

Will check plugs as well.

[PapaSmurf2k3]

Wiring Specialties is usually a sure-bet.
What happens if you disconnect the TPS?

[AZhitman]

No change. I also replaced it with a new Nissan one.

[AZhitman]

UPDATE: Swapped in the distributor from my S15 SR20 motor. No change.

I'm completely stumped, and starting to wonder if I have a VE issue due to larger intake and TB. But it seems to me that wouldn't affect timing at tip-in?

[VStar650CL]

I don't think you'll get to the bottom of that without an oscilloscope, the sample rate on even a good scanner is no better than 10mS and you could be looking at an artifact of the stall. The spark timing on those engines isn't baked in with the dizzy, the ECM has to generate the power transmission signal based on input from the POS and REF signals. The latter are fairly weak electrically, so resistance and weak pin-fits can be very problematic. I'd scope POS and REF at the ECM terminals to see if the waveforms are clean on receipt. REF has one wide slot to indicate #1 and three smaller slots for the other cylinders, so the train isn't symmetric, but you should see fairly angular square waves without a lot of hash or ramp curvature. If everything looks healthy then the timing vagaries are probably a symptom and not a cause. Scope the REF signal (180 degrees) chopped with the spark signal (either from the power transmission base or the bottom of the coil) and see if they're actually moving relative to one another when you try to throttle it. If the movement is consistent then it means the ECM is responding to something, not going nuts. I'd presume a MAF signal which is mismatched to the actual induction could cause a fatal stumble like you're seeing.