Rough Running at Highway Speeds

[71enthusiast]

I need some help here.
Today I was headed out of town and when I hit about 55 my truck started running terrible. It sounded like a machine gun but if I put my foot in it, it would down shift, pop, and take off with no issue whatsoever only to return again after I let off.
So I kept going hoping it would stop but it never did. I came over a hill and let of the pedal and coasted and that was when the motor completely shut off and I ended up having to call a tow truck.
When I got it back to town I was able to start it up and drive it home like nothing even happened!

I haven't a clue what the issue is here. It runs fantastic around town and I can even put my foot into it with no problems.
I also feel that maybe it happens when the truck comes up to temp for a good while? Perhaps related to my O2s?

I am running a 2000 Lt1 from a suburban and the only codes that throw are the MIL light code and MAP sensor (Of which I've replaced and tested and still the code keeps throwing)
Also I have checked for vacuum leaks when diagnosing the MAP sensor. Found none.

Someone, anyone, tell me something so I can start somewhere...

[homemade87]

With your description my first thought was bad gas ( water maybe ) . Could also be a fuel pump .

Also be sure to check all your wires coming off your battery . Power interruption can also cause intermittent problems . All grounds included .


One more thing to look at is the connector on the map . They get old and will not make connection correctly . The clip that holds it on will break and you will think it is on proper but it is not . Take a tie strap to it and secure it and see if it goes away . If so get a new connector .

These are just a few I thought off .

[BR3W CITY]

Disconnect your MAF and tell us if anything changes.
My perennial question, who did the tune? It sounds like your issue is only occurring at a specific Kpa (cruising at med speed, low throttle, low load). When you goose it, your forcing past the flat spot and it picks back up.

Vacuum leaks, not enough timing can both have some of these effects.