Diagnosing Stepper Motor?

[RCielec]

Ahoy !
Does anyone know the sypmtoms of a failed stepper motor?
All of a sudden running problems developed so, I am trying to diagnose and reslove.

Motor starting and running just fine. No issues.
Had perhaps a dozen or so random back fires (under load like acceleration) into the air cleaner. Then, lost the low idle and now difficult starting. Soon as the motor warms, it won't hold low idle. If the motor is kept idling above low idle it runs (though still with the backfiring under load). I disasembled the carb and though it was sooting, all the passages were clear. The rebuilding of the carb did not resolve the problem which remains same as before.
No, I've not checked timing but since I wasn't messing with it and nothing should be loose, I can't imagine the timing went out all of a sudden.

Any suggestion/comments?

Thanks.

Richar Cielec
Chicago, Illinois

[ammachine390]

You can verify that the stepper motor is functioning by looking down the carb, while a helper turns the key to on, but engine off, you should see a pin move forward then back. You should be able to hear it too.  Also, you can verify that the entire system is working by looking down the carb while the engine idling and warmed up, the pin should move forward, backward, foward, backward, continuously.

[LaGuardia]

Uhmm... wouldn't bet on it, but it sounds like some sort of carb flooding. Idle tubes ? Choke valve ? Or maybe oil being sucked into the air cleaner from the PCV vent ?

[IowaEagle]

Doubt it is the stepper motor.  Along with what LaGuardia suspects, I too suspect timing.  The chain could have jumped a tooth or two.

[RCielec]

Hello, All,
Thanks.
Discussing because I don't know all affects of bad timing.
I, too, was wondering about timing; but I though bad timing would be noticable throughout the range of engine performance; and, I didn't think bad timing would cause loss of low idle.
Other than the backfiring when I attempt rapid acceleration or goose the throttle, there is no pinging nor spark knock even at highway speeds. Bad advance on the timing?  It has a vacuum advance? Could anything be going on with distributor cap, rotor, coil, whatever to cause this? Any kind of electronic gizmo that could have gone bad?
And for the carb, it performs fine other than the loss of low idle at a minute or less after start up.
Two distinct and separate problems with different causes? Or, one cause creating two observable problems?
I guess what is so puzzling is that I never had a car develop bad  timing - it seems so rare.

Just to imagine possible scenarios - if the timing is still good, what else could be causing that type of backfiring into the air cleaner? Vacuum leak?

[RCielec]

Uhmm... wouldn't bet on it, but it sounds like some sort of carb flooding. Idle tubes ? Choke valve ? Or maybe oil being sucked into the air cleaner from the PCV vent ?

Choke Valve? Huh?

[JayRamb]

I know it's difficult, so I took mine off of my 84 Eagle that had 240K at the time. The stepper motor's two prongs were black, and they were stuck. Replaced it after getting one off an Eagle in a Salvage Yard, and it idled better. Exhuast had different more 'normal' smell.

[LaGuardia]

Uhmm... wouldn't bet on it, but it sounds like some sort of carb flooding. Idle tubes ? Choke valve ? Or maybe oil being sucked into the air cleaner from the PCV vent ?

Choke Valve? Huh?

Sometimes, the choke shaft (along with other bits in a carb) tends to stick and keeps the choke closed. It means a very good cold start, but poor or no idle after warmup, and eventually stalling.

BUT, I later realized that you mentioned a recent carb rebuilding, so this should not be your problem.

I'd concentrate on the backfiring thing. Don't know much about timing (I still have to read that chapter of the Book) so I'll leave it to those who know better.

I had a similar issue after an engine flood (it all started with me trying to wash the engine with way too much pressurized water in the wrong places): a wet distributor caused random loss of spark at some cylinders, so I had lots of backfiring and no idle. And when everything settled down, the engine did not start anymore at all.

Drying the distributor, the spark wires, all contacts and putting it all together right and tight solved the problem for me.

[LaGuardia]

Oh, and yes, the vacuum advance could be another thing to take into account.

Start from the distributor, the trouble could be around it...

[eagle503]

I seem to be having the same problem with rough idle. What gets me is the inconstancy of the issue--Sometimes it starts with the first turn of the key no problem and sometimes I really have to fight with it to get it to start and stay running. If i pull the vac advance tube from the distributor for a second then put it back the idle soothes out for a moment then back to rough.
   I have never seen any life out of my stepper motor. Witch i am not sure is a really big issue as the "ECM Test bypass" tells us to disconnect the thing. There is no pulls air stuff on my motor and all the holes are plugged and the cat has been gutted. My choke works properly and I put in a new fuel filter + blew out the return line to the gas tank.
  New spark plugs, o2 sensor, temp sensor,alternator,battery,and cooling system.Still having the same inconsistent start and idle problem + temp issues.

Anyway this is what I have tried hope it helps !

[JayRamb]

Did you happen to tighten down your intake manifold? they do loosen up. I believe there are 4 bolts?

[eagle503]

I will give it a shot.

[JayRamb]

I will give it a shot.

Mine was missing and wasn't running right until someone on the forum told me to tighten them. It worked wonders.

[ammachine390]

  I have never seen any life out of my stepper motor.

Mine was locked up. I removed it and thought that i broke it, when I put it back together it had some life, but I still don't think it works correctly. If you remove it, you may be able to free it up with some electric parts cleaner like crc lectra-motive, and work the motor back and forth, just be careful to not pull the whole thing apart like I did. I have since bought a used carb with a working stepper motor.

Also, if the stepper motor doesn't work, a new o2 sensor won't do anything, since it can't respond to what the sensor says.

[eagle503]

  I have never seen any life out of my stepper motor.

Also, if the stepper motor doesn't work, a new o2 sensor won't do anything, since it can't respond to what the sensor says.

And vice versa ~If your o2 sensor is bad then there is no input for the stepper to run off. It was cheaper and easier to find a replacement  02 sensor versus the stepper motor so I thought I would start there.