help request 1990 fld 3406b cat top speed 55mph on 1550rpm

Are you sure you've split high gear? Not talking about the range selector. Talking about the splitter. It's the red button slider on the side. Not sure how much experience you have in a 13-speed. We had a truck that people would get stuck doing 50mph in because they were still on the low side.

 

You running this truck - or a driver of yours?

 

Are you sure it is not a PEEC electronic engine. Post your engine serial and AR #. Others around here know a lot more than I and can help you. A picture of the pump would be helpful as it could have been changed.

 

Fuel pressure gauge, manifold pressure/boost gauge readings are helpful for diagnosing these issues.

If the easy stuff checks out then maybe look to see if the timing slipped or is stuck. Someone with an ear for it or an eye for smoke reading could pickup on it quick.

There's multiple things that could cause a vibration or shudder at speed or under acceleration. The hard part is deciphering someone's interpretation of it over text. Over fueling, air in fuel or improper timing are common reasons for a shudder or vibration on acceleration but there's a ton of rotating parts that could be vibrating while rolling in neutral at 55mph. Something as simple as throwing it in neutral at 55mph can do a ton to help find the source of a vibration. If it goes away in neutral at idle coasting but returns at 1550rpm you've just narrowed it down a ton.

Could be as simple as a torn AFR diaphragm and a worn out harmonic balancer... or it could be a number of other issues. There's no substitute for hands on diagnosis but as much data as possible helps.

 

I remember the first time I had an AFR diaphragm go bad, thanks for the memory @LameMule

 

At 1550 rpms in top gear you should be close to 70 mph with 3.42 rears.

Either you're not in 13th gear, or you are and speedometer is busted. The shaking I could only wonder what that could be