87 Dodge D-100 challenges

I've got an 87 Dodge D-100, 2wd with an auto slant six. The truck drives surprisingly well and smooth until I hit highway speeds, (over 90 kms/hr). After 90kms, the engine gets very loud, and when I take my foot off the gas, it feels like I'm driving a manual trans after dropping down a gear. Granted, this truck is only rated at around 98 hp, but I should be able to do decent hwy speeds without feeling like I'm red-lining. Would it be safe to assume that the auto trans is not shifting to its highest gear? (feels like it shifts 3 times before 90km/hr). is there an easy fix or at least some things to try to pinpoint the problem? The truck has got only 81,000kms on it, but it has sat a lot over the years, (second hand, have had it 4 weeks, haven't had to use it on the hwy much).

 

First thing I thought of was not getting into high gear, but you said you seem to be getting three shifts. My second thought was clogged exhaust, but I wouldn't discount a tranny problem just because you seem to be getting enough shifts to get into high gear.

 

Changed the tranny fluid and filter today, no surprises waiting at the bottom of the pan or on the magnet.(In fact it was spotless and still healthy red) Guess I'll check the exhaust next weekend to see if its got any blockage...for all I know theres a hornet or birds' nest in the muffler. It seems to be working fine, no odd smoke or smells. I'm thinking that maybe with the underpowered engine combined with the full size weight to push around, I just might be asking too much for smooth quiet highway speeds from a 20 yr old truck?

 

I'm pretty sure a freind of mine had the same complaint. I think the final verdict was that the smaller engine, had to be geared lower for the pulling power (torque) when hauling a load, thus highway driving suffered, due to high rpm at highway speeds. You might want to search ( allpar.com ) for more info, to confirm.

 

Good info, Mike; I immediately jumped to the "diagnosis" mode vice "analyzing" mode.

 

Makes sense Mike, thank you. I guess I can't have everything for $500. Still a decent truck to go camping in and haul stuff around the city. Its no speed demon by any means, but its still 20 yrs old with only 81,000 kms on it, should last another 20 if I maintain it well. Thanks again everyone.