Car chugging

[howlingwind13]

Hey guys, first off I'm not a mechanic or very inclined...
My car fires up, runs and drives fine...for about 40 minutes, then it starts chugging/hesitating on acceleration.
I took it a little far from home and it started doing it and then died...jumped it and drove it home...chugging on acceleration every time.
Like I said it drives fine for a half hour or so.
Engine has been rebuilt...a few years ago with probably under 300 miles on it.
Every time I turn the key it registers enough volts, the when turning it forward to crank it, the volts drop off to 8...and I have to jump it..bad alternator?
Mike

[Michael]

Charge and check the battery voltage via load test. Sounds like you might have a bad battery. Other possible problems are a bad alternator or poor/dirty connections on the battery terminals or poor ground connection on batt ground cable.

If the battery charge is low check the alternator to make sure it's properly charging the battery. You should have 14+v with the engine running but you have have the alternator bench tested to be sure. If the battery charge is around 12v stationary, I would start looking at terminal connections.

[Bitsyncmaster]

Your battery is bad or not charged if it drops to 8 volts when cranking. First thing is to charge or replace the battery.

[Ron]

Sounds like it could be the alternator (or battery). But 40 minutes seems to be a little long unless you didn't use the lights, AC etc. Most auto parts stores will check them on the car for free. But pulling it off for a check is better.
If it still dies after getting the electrical sorted out, a bad fuel filter or pump getting hot could cause it to die, then run again after waiting a short while.

EDIT: Michael and Dave type faster than I do ;-)

[Michael]

Sounds like it could be the alternator (or battery). But 40 minutes seems to be a little long unless you didn't use the lights, AC etc. Most auto parts stores will check them on the car for free. But pulling it off for a check is better.
If it still dies after getting the electrical sorted out, a bad fuel filter or pump getting hot could cause it to die, then run again after waiting a short while.

EDIT: Michael and Dave type faster than I do ;-)

The first thing that came to my mind was a fuel starvation issue like a dirty fuel filter or bad pump. The OP mentioned voltage so I assumed he was using a multimeter or something. If it was the dash gauge however he may be chasing a ghost. I would still check and test the battery but don't discount a fuel issue just because the gauge reads 8v with the ignition on and engine not running. The gauge lies as much as my ex GF when she went on spring break with the "girls".

[Helirich]

Charge and check the battery voltage via load test. Sounds like you might have a bad battery. Other possible problems are a bad alternator or poor/dirty connections on the battery terminals or poor ground connection on batt ground cable.

If the battery charge is low check the alternator to make sure it's properly charging the battery. You should have 14+v with the engine running but you have have the alternator bench tested to be sure. If the battery charge is around 12v stationary, I would start looking at terminal connections.

I agree with everything but the 14+ volts. Some alternators do not push that much. Technically anything over 12.5 volts is charging, but a running car should be over 13.

[Jonathan]

Does the battery warning light in the instrument cluster come on when you stick the key in and turn it to accessories? You need to see that light up as the car will not charge without a good light bulb in there. Also needs to be a regular, old fashioned incandescent bulb. LED's don't satisfy this alternator charging requirement for whatever reason.

[Ron]

I agree with everything but the 14+ volts. Some alternators do not push that much. Technically anything over 12.5 volts is charging, but a running car should be over 13.Specs on a stock alternators call for >13.5V. The regulators in most replacement alternators (modern) will settle at ~14.2V no matter how much the alternator "pushes"....just a matter of time.

[Michael]

I probably should have stated 14v+/-. I forgot what forum I was on.

67139

I need to just start posting pages of the tech manual and sign my name to the bottom of my posts like a boss(guru).

[Bitsyncmaster]

Specs on a stock alternators call for >13.5V. The regulators in most replacement alternators (modern) will settle at ~14.2V no matter how much the alternator "pushes"....just a matter of time.

I think because of the constant loads in new cars, all manufactures have increased the typical alternator voltage to 14.5 volts to get the batteries charged faster.

[MrChocky]

I think because of the constant loads in new cars, all manufactures have increased the typical alternator voltage to 14.5 volts to get the batteries charged faster.

My original Motorola does ~14.3. I had understood this to be the normal DeLorean range.

[Helirich]

This thread got me thinking. In all the years of working on cars I have probably put a multi-meter on hundreds, if not a thousand cars. I can only remember a handful of them reading over 14 volts. Today, I just happen to have eight, 12 volt vehicals at my house, so for fun I went and metered them all. All were started and ran for a few minutes and everything inside was shut off. (Lights, AC, etc.)

Here's the results:

1990 Ford Bronco-13.98
2004 PT Cruiser-13.65
1981 Delorean-13.78
1974 VW Bug-13.70
2016 Corvette-14.42
2016 Chevy pickup-14.56
2000ish Custom truck-13.86
2004 New Holland tractor-14.45

So there you have it. It would appear that these numbers confirm Bitsynmaster is correct about newer cars having slightly higher voltage. (Did we ever doubt him?)

One thing I thought about later. Probably most of the cars that I stuck probes on were having trouble. Lol.

[Ron]

I think because of the constant loads in new cars, all manufactures have increased the typical alternator voltage to 14.5 volts to get the batteries charged faster.Yeah, they have kept raising it over the years. The replacement regulators for rebuilders were mostly 14.2V regulators for decades (esp. GM)...we bought them by the 100s. But now, the ECU is in control instead. It sends a PWM (for duty cycle), E.G., a C6 Corvette:
Commanded Duty Cycle
Generator Output Voltage
10%-11.00V
20%-11.56 V
30%-12.12 V
40%-12.68 V
50%-13.25 V
60%-13.81 V
70%-14.37 V
80%-14.94 V
90%-15.50 V