I'm been having trouble starting my '91 D90 with a cold battery and engine (2.5 diesel). After warming the glow plugs for about 25-30 seconds, there just isn't enough battery to turn over the engine. I put in a new NAPA battery with CCA 800, but still have the same issue. For now, I have to keep a charger on the battery overnight if I want to drive it the next day. My D90 is a mil spec and there is no tech in the car to drain the battery.
I was wondering if putting in two batteries in parallel would help. There is enough room in the battery box. I was also thinking about an engine block heater and was wondering if anyone had any experience that, all though that still creates the inconvenience of plugging-in. Any thoughts are appreciated. Thanks!

 

It's weird that you have to keep a charger on a new battery. 800CCA should crank that thing over easily...even after activating glow plugs. You mention 2.5...but is it a 2.5TD or 200tdi? Not that that should matter, just wondering...since it's a 91 (should be 200tdi).

You could have a poor ground cable, causing poor starter performance. Does it crank over good?...or slowly?...or not at all after glow plugs cycling?

Two batteries are always better than one, since you would be talking about 1600 CCA. I've got a 1000 CCA battery (1231MF) in my 1990/200tdi and it cranks over like mad.

 

Cranks very slowly when the engine is cold and the air temp is around or below freezing. When I put a meter on it, the voltage drops to about 10.5 when the glow plugs are warming up.

 

But what is the voltage on the battery after sitting all night? If its not 12.6 + volts, I doubt you’re getting the full capacity of the CCA.
Rarely, is a new battery fully charged on purchase!
If you can absolutely rule out the battery, then I would look at the grounding points next, as well.
Then the starter.

 

Yep, disconnect the battery over night...and see how it cranks...or load test battery before you run glow plugs...to see what condition of battery is first thing in the morning. Could have a bad alternator, diodes, and it is drawing down battery...so disconnecting would verify battery condition.

Or you have worn/rubbed through/loose cables, ground or positive, check them thoroughly.

Or starter draw is exceeding CCA amps of battery when cold. Starter usually draws about 250 amps. 800 CCA battery usually can supply about 400 amps during cranking (depending on diameter and length of cables). Worn brushes in the starter will cause it to exceed this amount...and when it's cold...everything just takes a little more amperage than normal. Have the cables ever been replaced, l've seen where someone has replaced them with smaller diameter cables, because they're cheaper, and the new cables can't carry correct amperage load.