Sorry, I 
JoeIsuzu wrote:
Wow, it should go 65+, maybe even 75 (max, not cruise).
Oops.  I think 75 would be a fantasy, I forgot you said 4-speed AND 4.10.  The '86-'87 P'ups had 5-speeds, and they should do 75...and 65 is more typical for the 4-speeds.  But 50 isn't right.
I went several years with a problem on mine that affected the highway performance.  It seemed fine in stop-and-go driving, but it didn't want to go 65 on the highway.  Considering it's 2wd and 5-speed, that's really bad.  I tried several things to diagnose it for a few years (I even dropped the fuel tank and inspected it for debris).  Finally, on the advice of someone on the IsuzuPup.com forum, I removed the Cold Start Device.  This is designed to advance the injection timing for cold starts.  I was skeptical, but I'd tried everything else.  After I removed it, the truck drove like new.  My fuel economy went up significantly, too.
Jack
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'83 Isuzu P'up Diesel LS - Original Owner
JoeIsuzu@IsuzuPup.comIsuzuPup.com