65vettish

I've owned this car 14 years. It's a stock setup with exception of Pertronix ignition. Battery is a yellow Optima about 3 years old. Lately I've been having an erratic idle issue. On a cold start, it will start fine and idle at 1,500 RPM until fast idle is disengaged and then it idles smoothly at appx. 900 RPM. After driving it 8-10 miles, it will begin to idle rough and even quit running altogether sometimes. It acted as though it was fuel starved so I installed a new Airtex 4657 fuel pump (don't know how long the last one had been on there). That didn't help. After reading lots of posts I decided maybe the alternator was to blame so I installed a 61 amp rebuilt Delco Remy unit. I didn't install a new voltage regulator but did have a good one on the shelf that I installed. That didn't help either and now I have a battery meter that's going haywire. Here's what's happening after installation of the rebuilt alternator:
12.86v across battery terminals and at alternator posts when not running
13.8v across battery terminals when running
14.2v at alternator when running
Battery gauge reads "0" when key is in off position and drops -2 or -3 when door is opened and dome light comes on; needle pegs to -40 when key is turned to "on" position (engine not running) and slightly to left when in "ACC" mode. On test drive this afternoon the meter pegged to +40 and stayed there at highway speed even after it should've returned to near zero. In traffic, the idle was very rough and the battery gauge fluctuated wildly on both sides of the dial. On top of that, the temp gauge was showing nearly 220* after this drive. It's hasn't done that since I installed a DeWitt's radiator 7 or 8 yrs ago.

Any suggestions? A short maybe? Note, I completely rewired the car several years ago with no issues since. Inline fuses were installed where you guys normally suggest them. Thanks for your help.

tbarb

IMO, I would slow down and re-install the first alternator and voltage regulator and get the electrical system working right like it was before. After that, if the engine has a top shield double check that the primary terminals are not shorting to the shield and ground.

From your description, I suspect the coil may be causing the problem.

Easy Rhino

In my experience (limited) coils go bad either at idle or at high rpms.

But, I would stop swapping parts and do: voltage checks, vacuum checks, spark plug checks, and fuel pressure checks, then go from there.

DansYellow66

Engine dying when hot, sounds like classic bad coil symptoms. Not sure about the erratic charging system but I think I would install the old voltage regulator again.

65vettish

Thanks guys for the suggestions. The battery meter fluctuated wildly after the rebuilt alt was installed regardless of which VR I had on it. I'll take measurements at the VR connection terminals and coil tomorrow and change out the coil with one I have on the shelf and see what happens. I've visually checked all connections (starter, bulkhead, horn relay, etc.) and everything appears OK. I don't see any visible issues with the wiring under the ignition shield. It was pulling 8-9" of vacuum prior to putting on the alternator. I'm not throwing anymore parts at this until I figure it out.

SWCDuke

Check that the VAC holds vacuum and starts at 4" and is pulled to the limit at 8.

Duke

65vettish

Update - It was the coil. I could kick myself for not thinking of that originally but the old coil didnt have that many miles on it even though it was about 10 years old. I replaced it with a Napa IC12 and everything's good. Thanks so much for everyone's help.

Easy Rhino

If your car is a numbers matching, keep the old coil if it is the right one.

FWIW, the coil that came on my '65 when I bought it in 2014 gave me fits, dropping out at high rpm blasts, and finally stranding me (and the wife).