fox body- battery in trunk help

[87fox72mach]

okay I'm having some problems with the battery in my Fox Body when the car gets hot after running it from the 15 minutes or so it turns over very very slow and does not start I put a new battery and they new starter in I can't figure it out the battery was put in the trunk and a big 0 gauge cable ran to the starter solenoidanyone have any ideas only only doesn't start when its hot when it sits and it gets cold it cranks? Bad starter relay? Any ideas?

 

[MotoArts]

Yep, check the obvious first...

I'd start with the ground. My Fox car cranked slow for years, finally cooking the insulation off the ground to engine block cable one day after replacing the starter (that still cranked slow). Removed the cable bolt from the block (yes, it was tight), cleaned the cable/block (it looked clean, no rust or corrosion or grease), and it then cranked 100%.

Check block to frame ground as well.

If still slow, then go after other potential high resistance items, one at a time.

Hope this helps.