car runs poorly after warming up

1990 FORD FESTIVA
265,000 MILES • 1.4L • 4 CYL • FWD • MANUAL
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BRIAN MARKWICK
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  • 1 POST
Car has had new battery, alternator and battery cables done. Starts no problem, runs well until warms up. once warm, chugs and sputters hard and runs worse with any electrical running (radio, lights, heater, etc) seems to be running solely on battery once warm. Cannot figure out root cause and it is driving me mental.

Thanks.
Feb 23, 2017 at 1:46 PM
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CARADIODOC
  • ✓ASE Certified Mechanic
  • 34,321 POSTS
  • ASE Certified Mechanic
Everything you described could be caused by the charging system, especially if the generator has one bad diode. That will cause it to develop a maximum of exactly one third of it is rated current, which might be just enough to keep up with the demands of the electrical system, but the resulting excessive "ripple" voltage can interfere with computer sensor signals.

A quick test is to fully charge the battery, then, while the engine is running, unplug the smaller wires going to the generator. If the symptoms clear up or change, have the charging system professionally load-tested.
Feb 23, 2017 at 6:39 PM
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