stalling and back fire when warm, no movement in carb mixture pins

[88formula]

New car (to me) not too sure what all has been done in the past, not able to drive it (no clutch master) bet this is the symptoms.

Cold start cranks and fires fine. Then as it warms up, it starts choking itself out. BEFORE it started back firing,  I took a look inside, didn't see the mixture pins moving.

Now my real question is, if the ecm isn't moving the mixture pins, is it possible that it would make it that rich that it would back fire and stall? I assumed it only changed the mixture a tiny bit....

All I have is the general pin out of the ecm and it only shows the ignition power in the on position at pin 20. Where is the ecm constant power? Where is the fuse for this wire?
I don't have a TSM yet, but I will....


Duh.... probably help if you knew it was an 82 sx4 4.2L

[macdude443]

First off, if the shoulder of the pins in about centered in the bore that's where it typically runs. This should not cause a backfire by itself. The computer goes through different "open loop" modes until the engine reaches a predetermined temperature. Once certain variables are met, the computer goes into closed loop and only then will it adjust the stepper pins in relation to changes in O2 sensor voltages.  The temp sensor in the air cleaner needs to open and the coolant temp switch in the intake manifold needs to close in order for closed loop to be achieved. There is also a timer that must run out when the car is started hot.  One thing to check for is the initialization process. Looking at the pins, have someone start the car. As soon as it turns over, the pins should move all the way forward, then all the way back, then center. If this does not occur, you either have a bad stepper, a bad computer or a bad connection in between.  If the ignition has been modified it won't work either, as the computer looks for a tach signal to know it is being started and to "wake up".

Your backfire may be related to vacuum issues, poor timing adjustment or a really out of adjustment carb.

[88formula]

Not trying to toot my own horn, but I'm a journeyman tech... I know the just of how all this works, but I'm new to AMC.

OK the fact that the computer needs to see a tach signal before it "wakes up" the ecm. Evin still.... we cranked and started and it ran great until the coolant was warm then it starts choke itself out... I know what the pins Are That I'm Looking For,  But this Is Still New To Me.  They don't move at all, and I don't know where they should sit....

More questions. I know the coil gets battery voltage from the "I" terminal of the starter solenoid, but where is the blast resistor?
And is there an ACTUAL wiring diagram for the ECM cuz all I can find is the scamatic....

Thanks everyone.

[88formula]

Oh yea. And is there not a fused constant power for the ecm? And wich fuse is it? To be honest, I haven't Looked for the fuses yet... don't punish me....

[Draekon]

Someone may have done an ECM Test test, which would make the mixtures needles stationary, no matter what the engine was doing.

Not sure what would be causing the backfire, though

[Amc1320]

Backfiring is almost always timing, makes you wonder if the ECM is changing it once it senses warm coolant at one of the sensors.

[eaglefreek]

There is a resistor wire, not a ballast resistor going to the coil. I can't help you with the rest, I ripped the ICM/ECM/stepper carb stuff out and threw it as far as I could.

[macdude443]

I don't recall a fuse for the ECM.  A TSM will have all the electrical info you would need.

There are other things that change when the engine warms up.  On some, the CTO allows vacuum to pass to the EGR valve.  You may have a bad purge valce on the vapor canister causing a nasty vacuum leak if controlled by the CTO.

The TSM has about 30 pages of troubleshooting flowcharts for ECM diagnostics.  The pins in the carb center themselves every time the car is started.  They should stay in that same position until the computer enters closed loop.  Start up the car cold and watch the pins until it dies.  Do they move just before it dies?  If so, which direction?  If they move fully forward into the carb, the computer is trying to compensate for a lean condition.  If they move all the way out of the carb, it is compensating for a rich condition.  If they remain centered, I wouldn't think the computer is causing the stall., via the carb anyway.

If the pins do nothing, you can remove the stepper motor and carefully move it by hand until the shoulder of the pins is centered in the opening (looking down the throat).  The car should run with the pins in that position.  If they aren't centered now and don't move at all under the computer's power, then maybe the choke is leaning it out too much as the engine warms.

Hard to say.  I'm not a mechanic and I'm not standing in front of it.  This is a complex system to troubleshoot.  How are the vacuum lines?  Are they routed correctly?

[carnuck]

HEI dist with switched full 12VDC and set the needles to 1/2 way and drive it.

[88formula]

I've found the TSM but it doesn't have the same pin out for the ecm... it shows 2 conectors and I only have one....

I didn't know you can just pull out the metering motor and  move the needles. Toward the front of the car is lean, corect? And no. I've wach ed them for movement none at all. I got an ignition power to the ecm on a white with red strip wire (I belive those are the  colors,  it was way back 12 hours ago I last looked at them) and a good ground, but I have no constant power. !? IS THERE SUPPOSED TO BE CONSTANT POWER?!?!?!?!??? I don't see any reason that it would need constant B+ but I've never seen a pcm that didn't...

[macdude443]

On the later models (two plug) there is no constant power.  It is switched ignition feed.  I know that doesn't help you but for what it's worth...

Towards the front of the car (farther into the carb) is rich.  Opposite is lean.  Best place to start is with them centered.  There is a shoulder at the end of the tapered section of the pins.  Put that shoulder in the center of the opening.  See photo for how you will want them:

[macdude443]

On the later models (two plug) there is no constant power.  It is switched ignition feed.  I know that doesn't help you but for what it's worth...

Towards the front of the car (farther into the carb) is rich.  Opposite is lean.  Best place to start is with them centered.  There is a shoulder at the end of the tapered section of the pins.  Put that shoulder in the center of the opening.  After doing this you can fine tune the idle mixture using the two mixture screws on the front of the carb and basically forget that the stepper was ever present.  You would tune the carb like a non-computer vehicle.  If you go this route, Google the ECM Test Bypass.  This process will also bypass the computer/ECM.  See photo for how you will want them:

[88formula]

That's the perfect picture!!!! I think mine are all the way in or out, I don't see a shoulder like that. I'll adjust and report back if that makes a difference. I'll take home my I.R. thermometer and test to see if all the switches are working the way they are supposed to.

One more thing I'm curious about is the O2 sensor.... if it huts all the requirements for closed loop, but the O2 sensor is faulty, and sending 0v, will the ecm move the metering pins or will it just stay inoperative? 

Sorry, lots of questions.... I'm a V.W. tech and have been working on European cars for the last 10 years.... I've missed out on a lot of domestic tips and tricks.

[macdude443]

I don't recall where I read this, but the computer will not enter closed loop without some reading from the o2 sensor.  It may try to put the pins in a pre-determined "Open Loop" position during warm-up, however.  Never had a bad o2 sensor so I'm not 100% sure.

[88formula]

Ok. Had some time to do a bit of testing. So it starts fine but then dies, the distributor was WAY off. So now it idles fine.
Tested power and ground to ecm. Good. Nothing happens to the pins when it's supposed center on startup. But it runs and yes, they are center like it shows in you photo. Thanks again!
Haven't gotten the clutch master yet, so haven't been able to test drive it for drivability. But now I'm just wondering, if the ECM isn't adjusting the mixture, is it going to affect the timing while driving? Or is that just an adjustment made responding to knock from a knock sensor? Does an 83 eagle have a knock sensor?