Starting Issues

[Wartown]

Bought an original 89 GT, only 20K miles. Off trailer it started and ran great for a couple weeks. Fired first turn no lag at all. Left lights on once and had to use a jump box to start, ever sense then, the battery drains if sits for a day or so, it lags and very sluggish on start. Put on a new alternator, same issues. Battery tests fine... I'm not a major mechanic per say but would like to know what to look for or how to go about correcting things so I can enjoy it. Please advise.

 

[General karthief]

How old is the battery?
Me I would swap a new one in.
Was the battery completely dead?

 

[90sickfox]

I'd also replace the battery. A bad battery can test " good but needs charge " on the battery tester. If it's not holding a charge it is probably bad.

 

[AeroCoupe]

Have you check all your connections and made sure they are clean and have good contact? A voltage drop test would also be a good thing to do on all the cables.

 

[JD1964]

Exactly what kind of test was done on the battery to conclude that it was good?