88 NA Won't Idle Warm
 

Been awhile; busy with a lot of things. Anyway, to business:

Working on an 88 NA. When started cold, it'll go up to 1.5-3K (depending on outdoor temp, the cooler the higher) and hold it, slowly coming down to idle speed over two or three minutes. At that point however, the idle gets rough, falling to around 400 and occasionally bouncing back up to 900 for a few seconds, but always eventually shuddering to a stall wrestling with low hundreds.

Makes it pretty much undrivable at red lights without three feet since the idle needs to be held with the throttle, but cruising and accelerating are fine.

Going to be checking/testing the gauntlet of idle-related sensors and valves, but I wanted to throw this out there to see if anyone can help point me in the right direction.

You should try to adjust the TPS, timing, idle speed, then keep going back and checking each one of those things after each adjustment. Also, I'd look at coolant temp sensor as well. Check the voltage for it at the ECU wiring, trust me, it's 1000000X easier to do than trying to check for voltage at the sensor. It should fall between 4.5 and .5 volts, not 0v or 5v. It's a strong possibility that's where the issue lies. I had a wiring problem for the coolant sensor, and it just did NOT like to run hot. Trying to get it to start hot was impossible. I had to wait until it cooled down. :(

Thanks, that gives me a good starting point. Something mechanical I also want to check are the positions of the fast idle and cam adjustment screws.

I didn't want my post to be tl;dr, so I left out details such as this problem gradually onset over time. Initially it idled like crap for about 15 seconds or so after every hot start, then would surge and stick to a perfect idle. Now it just idles like crap and stalls when it drops too low.

Also, disconnecting the BAC and trying to do a completely cold start will cause it to completely ignore any attempt at accelerated warm up and it dies fairly quickly, BUT disconnecting the BAC any point after it revs up--3k, 2k, 1k, or crappy hot idle--it doesn't care one way or another. It's like the BAC isn't doing a single thing to help idle except during the first few seconds of the first starting attempt, when that should be on the shoulders of the Air Bypass Relay anyway.

Clean the throttle body and BAC. Also, check the fuel pressure. If the FPR has gone bad, it'll burn way too rich at idle. How old are your spark plugs?

TB and BAC are clean. BAC gets about 0.5cm of movement--guessing that's normal--and checks out fine for impedance.

TPS resistance value at hot idle was a bit off; adjusted it to the proper 1Kohm. Fuel pressure is about 30 at idle. Spark plugs have between 100 and 500 miles on them, so new. Haven't checked most of the other sensors yet, damn rainy weather.

Ok, that's good. How about the variable resistor? Is the resistance within the 0.5-4.5kohm band?

Supplied voltage for the coolant sensor below the thermostat was 4.96v. The voltage for temperature sending unit aft of the spark plugs was all over the place and wouldn't give me a proper reading on a digital multimeter.

By "variable resistor" I'm not sure which you mean. You were asking about the voltage on the coolant temp sensor earlier.

Edit: If you meant the resistance of the sensor by the thermostat, at 58°F it 3.76Kohm.

The Water Thermo sensor behind the top of the water pump should read 2-3 volts cold engine and .4 volts fully warmed. If yours reads 5 volts or so then it is probably a result of having a poor connection. Often times the two pins will pull back within the mating plug and they need to be readjusted. Did you take the reading from the ECU (pin 2I) w/key to on?

^This happens so often. The coolant temp sensor is what Mazda refers to as the "water thermosensor". It's the one located behind the thermostat, like satch said. If pin 2I at the ECU has 4.96v (almost exactly 5.0v), then you've found your problem. Either a wiring issue or that temp sensor has seen better days.

The variable resistor is used for idle mixture adjustment. It's located on a bracket near the passenger's headlight. You may even see a small part that can be adjusted by a flat head screwdriver if it's not filled by epoxy. The plug for it is nearby and is round and has (I believe) 4 pins.

Taking a reading at the back of pin 2I to chassis ground with the key to "on" and the engine cold gave me a reading of 2.30v. After letting the engine warm up to the point it died on its own, the reading was 0.75v.

If somehow helpful, here's a video of the warmup from 1min after keyturn until stall.
http://www.youtube.com/watch?v=06mReMy19HI

Let me dick around with a flashlight and find this variable resistor thing...

I'm having similar problems however, I don't have that voltage gauge. Mine doesn't jump back up like yours does. If it dies, it'll just die. I was 30mins away from home today and had to drive back (It only happens if i turn the engine off and back on again.) I had to brake and rev the engine with my heel otherwise I'd stall at every light.

EDIT: I just brought out my manual and it listed 27 possible causes.

1. Air Filter Clogged
2. Fuel not reaching injectors
3. Corroded battery terminals - especially ground
4. Incorrect ignition timing
5. Malfunctioning Electronic Spark Advance system
6. Incorrect idle speed
7. Faulty throttle sensor
8. Injector(s) malfunctioning
9. Faulty (hot start assist system) pressure regulator control solenoid
10. Faulty intake air temperature sensor on dynamic chamber (non-turbo) or air intake pipe (turbo)
11. Bad secondary air injection system water thermo sensor
12. Air intake system air leak
13. Malfunction in BAC system or idle speed control system
14. Faulty Secondary air injection system air control valve
15. poor fuel flow at idle (check variable resistor connection and resistor)
16. Malfunctioning pressure regulator control system
17. Defective air flow meter and/or intake air temperature sensor
18. No fuel pressure (check for pulsations at main fuel hose by hand)
19. Incorrect fuel pressure
20. Faulty fuel pump control system
21. Defective fuel system solenoid resistor
22. Leaking injector(s)
23.Incorrect injection volume
24. faulty crankcase and evaporative emission control system purge control valve
25. low compression pressure
26. Faulty hot start assist system
27. Faulty control unit or system (take the vehicle to a dealer)

Post a video of it trying to restart. The noise it makes can be very helpful.

Took it over to my friend DREYKO for some extensive testing, since he has a lot more equipment than I do and an uncanny sense for tracking down problems, usually stuff I don't think to look at.

First off, the variable resistor was indeed way off. It completely slipped my mind to adjust it when I put the new motor in. Though, why the problem wasn't immediately apparent escapes me. Timing was a smidgen off too, but a bigger problem was my L2 coil was dead, causing a very uneven idle even after the mix was adjusted. Unplugging the entire leading coil pack dropped the idle down, but smoothed it out to glass. Need to hunt down a spare. The AFM is also questionable and the custom intake ducting has become way too pliable over the years, so I'm going to be putting that back to stock for the time being just to be on the safe side.

So yeah, lot of things contributed to the bad idle. Not out of the woods yet, but getting there.

wut?