I've been hunting the cause....   I've got a new fuel pump, and regulator with a gauge I can read while driving - the fuel pressure is good.

It will climb a long hill from a stop, floored, 2nd gear [3AT] but occasionally stumble once the RPMs get up.   Not constant, just a glitch now and then.  Sometimes on acceleration from a stop, a little trip before it gets going. 

 

Saw your post and thought I would write the symptoms of our SPFI loyale 3AT.

However it happens at low RPM when the car is not warmed up yet.

We had the Alternator rebuilt;

Shortly after that, the car will bog down now and occasionally die in low rpm. (Backing out the driveway)

Different, but seems like an electronic glitch perhaps like yours... have tried fuel additive etc... not fuel related?

I guess you get no codes?  TPS may be acting up.  Could be a poor contact at the TPS connector.

How about the regular stuff: distributor, wires?

 

Good luck,

Sam

Check the wires on top of the injector, I had a bad one once, and it did the same thing. You only have to pull the top of the throttle body off to get to it, so get a gasket set. 

No check engine light.   No trouble codes.   I forgot to check those before my first post, but yeah, none.  [it's been a while since I had a failure that caused a CEL / code without some other obvious clue.]

Injector wires, sounds good to check, TPS I was thinking of checking also.  I keep thinking it's a 30yr old potentiometer, it could be worn out. 

Regarding the post from 6star - if the poor running is engine temperature sensitive, check the CTS.  Search for my post about CTS, I have listed several values & voltages for good ones.  They do fail in ways that do not trip trouble codes.

 

It does sound like it's some kind of electrical thing.  I suspect a fuel problem would occur at a wider range of RPM.  Weird intermittent things remind me more of a loose connection  somewhere. Or as you say, something is beginning to fail.

With the "stumble" or "missing" I would also check for loose connections like spark plug wires.  Check the distributor and clean the contacts as well.

Tracking it down may require waiting until it becomes more consistent.

Edited May 12, 20214 yr by Dee2

Ok..  I decided to check the TPS first.  Found one of my spare engine wiring harnesses, and separated the throttle cable.  The pins slide out the back of the big round connector when you use a jewelers screwdriver to lift the locking tab.  Got one of my spare TPS units.  Carefully opened it up, these things are well made.  50 years of designing and building electronics with potentiometers, you get to know a well made pot....   Anyway, it looked good, so I cleaned a little glue off, and reassembled it.   Trying to check for a flaky pot with a digital meter, as usual, is nearly useless, so I thought about it for a minute....  The easiest way to find a noisy pot is to listen to audio going through it, so I rigged connections from a music source through the TPS, to an amplified speaker.  It worked great.  Also checked the throttle closed switch, good.

Bring the test rig up to the car, and plug the connector onto the TPS.  Instantly, I have no ground continuity, music at full volume!  wiggle the TPS connector, crackle, no volume, crackle low volume.  Well, there's a problem!  Never would have found it that quick probing a meter on the pins, since the probes would poke through any oxide, etc.   I used my tiny wire brushes to clean the pins, cardboard to clean the sockets in the cable end, a little contact cleaner on both.  The test connector now works as well as it did with the reference TPS.  Volume goes up and down smoothly with no crackles with throttle position.

Tomorrow, I'll take a drive to verify if this is the total fix.

 

I love reading posts like that last one. 50 years experience! Thanks for sharing your knowledge here. 
 

Folks keeping these car will need this info down the line for sure. 

Amazing way to diagnose the problem. 

12 hours ago, DaveT said:

Ok..  I decided to check the TPS first.  Found one of my spare engine wiring harnesses, and separated the throttle cable.  The pins slide out the back of the big round connector when you use a jewelers screwdriver to lift the locking tab.  Got one of my spare TPS units.  Carefully opened it up, these things are well made.  50 years of designing and building electronics with potentiometers, you get to know a well made pot....   Anyway, it looked good, so I cleaned a little glue off, and reassembled it.   Trying to check for a flaky pot with a digital meter, as usual, is nearly useless, so I thought about it for a minute....  The easiest way to find a noisy pot is to listen to audio going through it, so I rigged connections from a music source through the TPS, to an amplified speaker.  It worked great.  Also checked the throttle closed switch, good.

Bring the test rig up to the car, and plug the connector onto the TPS.  Instantly, I have no ground continuity, music at full volume!  wiggle the TPS connector, crackle, no volume, crackle low volume.  Well, there's a problem!  Never would have found it that quick probing a meter on the pins, since the probes would poke through any oxide, etc.   I used my tiny wire brushes to clean the pins, cardboard to clean the sockets in the cable end, a little contact cleaner on both.  The test connector now works as well as it did with the reference TPS.  Volume goes up and down smoothly with no crackles with throttle position.

Tomorrow, I'll take a drive to verify if this is the total fix.

 

If you think the TPS is well made, take apart one of the door lock actuators. The are built like a swiss watch.

Ok...  It's better.  But something else is also contributing.   Slight intermittent skip at half to full throttle is when it showed up.  Maybe half as bad as before cleaning and testing the TPS.

It's been forever...  But I got back to working this out.   Checked the plug wires,nothing looked or measured bad.  Checked for vacuum leaks with a smoke machine, no big red flags.  Pulled a spark plug.  Ok, wow, I guess this set has been in a while.. gap about .059", all eroded / rounded electrodes.  Put in new plugs.  Just did a 5 mile test run, no glitches.

You, of all people should have known better to check and or swap/replace while doing a comp test before even posting ! But, this forum is a good thinking out loud space