Alternator/Idle link...?
 

I've struggled with an idle problems for a long time. Mine periodically (but a lot over the last year) had a 800 to 1700 rpm and back down, surging syndrome. It would surge periodically when I came to a stop; sometimes almost to the point of shutting down, but never quite.

Then for no good reason, it would get stuck at 2000 after (getting sick of watching itself surge?)

It also came and went with the weather. after a rain and drying out, it tended to be much worse/frequent.

But I changed the alternator last weekend because the lights surged with the idle and it was obvious the alternator needed replacement.

From the moment I started it up after that, it has sat at 800 rpms without moving at all. Cold/hot, 40 minute drive in traffic, long haul; the idle is very consistent.

Am I in a long string of luck or can an alternator affect idling this way?

(yes, when I hand spun it, it made a lot of noise and was very corroded inside and out, but it did spin without a lot of effort; however, I never attempted to spin it with the belt attached, which clearly would have changed the dynamic a bit due to the force of the belt).

It's possible your alternator was overcharging (lights surging) the battery.
Although the DME has some overvoltage protection, overvoltaging can cause
weird running symptoms.

OK... I am having a very similar problem... Alternater might be going bad (light illuminating and changing with RPM - belt is replaced and tightened) and wierd idle (high) that changes with the tempurature (low outside air temp = high idle --2000-- / high outside air temp -- 900--) I have been trouble shooting (leakes, electrical) and was about to order a new alternater - now I think I will... will let you know if I get the same resolution. I hope so...

Hopefully, you ordered an external regulator type alternator if your alternator
was an external reg. type. Avoid putting an internal reg. type in. They're all junk!

The alt. in my car already had been changed to an internal regulator type when I bought the car -- with the wiring changed to match... so... guess I am going to continue down that path...

My big question is whether this could just be the voltage regulator (already have one on hand... ). My plan this weekend was to swap it out and see what happened before I spent the money on a new alt...

Put a good OEM type (Paris-Rhone) regulator in, if the slip rings
are O.K. Call Pelican and ask them to order you one. The regulator
is the weakest part in your 75 amp Paris-Rhone alt. and as a result
needs to be a good one.

Thanks -- thats what I have... from PP.

Repairs on Sat AM: 1. voltage reg - hope it works and 2. new seals in the engine oil cooler -- think I put the wrong one in while rebuilding my engine -- sigh..... got a leak - imagine that.

Thanks for the advice!