'91 454: RPMs start to fluctuate at 65mph

[nickskeeta]

Hello everyone, I just picked up a '91 C3500 Dually with a 454. The truck has been great the last few weeks but the only real issue I'm having is driving on the freeway. Once I hit 65mph, the RPMs start to fluctuate up & down almost as if it's trying to govern my speed. The truck will go faster than 65, but it's almost fighting itself along the way. As soon as I'm below 65mph, it's back to driving normal again.

The truck has no issue accelerating and pulls very strong, but the only notable thing I can mention is that sometimes the idle is very low at start up. It'll idle fine on most start ups and I haven't had any issues while at stop lights.
Any info or recommendations would be appreciated, I have not tried trouble shooting it yet.
Thanks

 

[Schurkey]

1. What is the fuel pressure at prime, and under load? How old is the fuel filter?

2. How old are the usual "tune-up" items and procedures? Cap, rotor, plug wires, spark plugs, PCV valve, verify initial timing, verify EGR and charcoal canister, air filter, heated air intake (if used.) If you don't know the history of the O2 sensor, it's time to replace it. They get lazy with age and miles. The one-wire sensors are dirt-cheap; consider upgrading to a three-wire sensor and harness pigtail. Heated sensors are not "needed", but nice.

3. TBI distributors are known for failed magnets by the pickup coil. Usually, but not always where they're riveted.

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When my '88 K1500 5.7L had misfire at 65--70 mph, it gradually got worse--same misfire but lower and lower speeds, until finally it stalled at idle. The fix was a new distributor mainshaft as pictured above. Mainshafts are pretty scarce now, so 'bout the only option to fix that magnet problem is a new, QUALITY distributor.

4. Connect a scan tool, verify EVERY sensor and computer output. Check fuel trims. Look at the data when the engine is misfiring at highway speed.

 

[nickskeeta]

Thank you for the detailed info! I'll start with the basics like you've mentioned, I spent some time wrenching on the truck this weekend and it doesn't look like it's had any maintenance done recently. If that doesn't work, I'll have to pick up a scanner and start digging into it more