TheClemsonFan

I have seen symptoms like you described before and the cause was I had two injector wires mixed up. It caused the car to not hold idle like you describe and to have fuel trims in the negative 20% range. Has you motor been swapped? Or have you had the injectors unplugged for some reason? If so, I would check that the injectors are wired correctly.

TCF

Speedanimal

Thank you TCF for the reply! This is actually an excellent question. I do not know the age of the motor in the car. Mazda has a record of a replacement at 6k miles but that was nearly 145k miles ago! I will do my research here and see if these are

By swapped do you mean that two of the secondaries were swapped on the same housing? Or that a primary was actually swapped with a secondary?

TheClemsonFan

IIRC it was a primary swapped with a secondary, but it has been a few years... I have an extra wiring harness in my garage with the wires labeled, I will try and take some pics tonight and send them to you. I find it easier to use the plugs location on the harness rather than trying to match colors on wires.

TCF

Speedanimal

This was a fantastic tip. Got into this tonight, get ready for it, BOTH RED PRIMARIES WERE SWAPPED WITH A SECONDARY...

So car is definitely on at least its third engine, sounds like whoever did the last one (Mazda doesn't have record of it so) wasn't terribly careful.

Btw getting those connectors off without pulling the fuel rail is not fun, that was a lot more frustration than I was expecting.

Anyways, car seems to run quite a bit better with that fixed, still on jackstands, but after just two minutes STFTs were at -4% rather than -19% the last time at this point after a fresh brake pedal reset.

Surges with the clutch pedal out, about 500rpm from 1500rpm each time but then drops to about 1200rpm and holds (I assume learning and in warmup) with clutch in. Before it would still try to die with the clutch in. So there is still some switch giving me issues.

I did some searching before, but either didn't dig deep enough or missed it, what does that Red/brown switch closest to the driver actually do?

TheClemsonFan

That is a PITA. I always use a long needle nose to push the button and pull at the same time. Can never get my fingers in there.

TheClemsonFan

Don't know about that one....

TheClemsonFan

Maybe you should log some more data with the correct injector wiring and the post here for someone more knowledgeable than me to look at.

Fijibluefg2

If it surges but stops with the clutch disengaged, it’s the neutral switch. I JUST replaced mine for the surging issue at idle. No more surging.

Old plug looked fine and I didn’t test it but since the problem is now fixed, obviously it was bad.

From reading this thread, it sounds like it should now idle fine once hot and not stall on you.

If you reset the ECU via battery disconnect, you need to let it idle for 15-20 mins or else it will stall when driving and coming to a stop. Mine did that after I swapped to a stronger battery and it scared the crap out of me. Lol.

Speedanimal

Status update.

So I thought I finally got the neutral switch to admit it had a problem. Tested bad after testing good three previous times.

Got a new one installed and took the car for a spin.

No longer stalls! Fuel trims are much more reasonable and LTFTs are almost 0.

However, it still surges at idle. And eventually after a couple short drives it throws the P0850 code.

So neutral switch is brand new, wires close to the switch (car side) seem good. Cruise control works. Car idles fine with clutch pedal depressed about an inch or more.

I messed with the CPP switch by hand while holding the pedal down and obviously its position is what determines if it is surging or not.

So my latest question is: what readings should the CPP (red/brown) switch give?

Fijibluefg2

So weird that it still surges. Maybe something else is causing it or the new switch is defective. Lol.

Im out of options as that’s as far as I know about the surge with the clutch.

Speedanimal

Not a problem! Thanks for the help!

I was very surprised as well after I swapped it, almost all the threads on here say that swap fixed their issue. Which is what led me to playing with the CPP switch, and my question.

May just have to buy it myself and find out what it reads for everyone.Though, I'm amazed either of these goes bad all that often. They are very simple.

Speedanimal

I think I can finally call this a wrap guys!

It helps to actually use the factory service manual rather than just bookmarking it. Checked my wiring for the neutral switch more thoroughly and found the 2O pin from the ECU wasn't getting a signal down to the Red/Blue at the switch. Did some searching for a puncture in the harness but wasn't able to find anything. So I spliced in a new wire, following the harness down and seems to have worked splendidly.

No more surge, no P0850, seems to have fixed my problem. Will get this car a engine harness when the rebuild comes around.

So quick recap to anyone who may stumble across the thread in the future:
- Car surged at idle with clutch pedal out, stumbled and eventually died if left alone, would drive but usually die coming up to a stop when pulled out of gear and no throttle, fuel trims were -15% or worse and definitely runs rich. Original engine codes were P0850 (Neutral Switch) and P0410 (Air Pump).

- Checked for vacuum leaks, found none, checked fuel injector wiring and found 4 of 6 injector connectors were swapped, found neutral switch wiring issue.
- Fuel injector wiring fixed stalling and fuel trim issues, Neutral switch fixed surging idle
- Nothing in thread has fixed the P0410, still working on that.

Thanks to everyone who helped me out, this has been a great project so far and I'm excited to finally get to driving it!