voltage and idle drop / surge

[BrianArmand]

1972 Cutlass 350 HEI. after a while, when the car gets hot (195+), and I have the headlights on (and step on the brake, which usually starts the bad behavior) the car at idle will start "surging". a voltmeter on the system indicates that the voltage is dropping from the 14.2 volt neighborhood to 13.5 or less. the engine sounds like it wants to stall, but catches and goes back to idle. this cycle can be like 2-3 seconds from near stall to back to normal. sitting there, the car can do this for minutes at a time (I've never let it go forever, but may try it some day...). if the car's hot and it's daylight, I have no issues. turn on the headlights and here comes the surging.

when the car is cold, the voltage is around 14.5, and when the car is warm it sits at 14.25.

it SEEMS like this first happened after I replaced the coil, cap and rotor. don't know how replacing any of those would have an effect on the car's voltage.

I've replaced the alternator and the external voltage regulator (and rebuilt the carb). Ive checked all the ground wires.

any ideas out there? thanks.
Brian

[texasred]

Sounds like it could be multiple issues here.

What are you running for timing? HEIs are not drop in replacements exactly. Their timing curve and vacuum advance cans are set up for emissions in a very low compression/no performance engine.

Are you running the original wiring ti the HEI? It's a resistance wire and an HEI needs the full 12 volts. That could explain the "lights on" issue.

Are you running your vacuum advance through the vacuum switch on the intake manifold? Are you actually getting ported or manifold vacuum?

Finally, HEI coils are in-cap, which means there is a LOT of heat buildup. If it's a cheap one, it could even be bad from the box.

I'd start by checking for a full 12 volts at the distributor and then throwing the old coil back on.

C.J.

[BrianArmand]

OK, a little more history: I've had the car for 6 years, and it came with the HEI in it when I bought it. I do not believe this is a high compression engine or anything special (although it's not the original 350). I've changed no wiring to the HEI. I've had no issues before this summer.

Unfortunately, I threw away the old coil (it took a couple of weeks for this thing to manifest) before I encountered this problem.

I am running the vac advance through an intake mounted vac switch, and there is vac there. I've also lightly lubed the vac advance (by taking off the cap and carefully lubricating).

How do I check for a full 12 volts at the distributor? I'm measuring 14.2 at the battery. I did get the cheap AutoZone coil, I will admit (hey, I got kids and a mortgage - the fun car doesn't get all the best stuff usually)

[Jim's rare 80]

Ground issues?

[Oldsguy]

I think the voltage drop is a normal indication of the lower idle which means the alternator would develope a lower voltage but not abnormal. I work at a Motorola radio repair shop and we deal with a lot of vehicles. We consider normal vehicle voltage to be 13.8vdc, knowing some are higher and some lower. Don't worry about the voltage. I think C.J. is onto to something. In particular, check that vacumm from the manifold vacuum switch, it probably is supposed to change the amount of vacuum based upon engine temperature and it may produce a vacuum leak when the engine warms up. Check it when the engine is warm. Good luck.