After 5mins or so I can't get above 3000 rpms, please help.

Asked by Mar 07, 2017 at 02:41 AM about the 1998 GMC Sierra 2500 2 Dr C2500 SL Extended Cab LB HD

Question type: Maintenance & Repair

It starts up fine, runs like a brand new truck for a
few minutes, then starts struggling. When it's
struggling it won't go over 3000 RPM, when i try to
force it over 3000 it'll start getting misfires. If I'm
really easy on the gas and creep my RPMS up
slowly I can usually get it higher then 3000. I just
dropped a new engine in because of a spun
bearing, but I had the RPM problem with the old
engine too and am afraid to screw up this new one.
The only parts of the old engine that I kept were
the intake manifold and the vortec heads, recently
replaced the injectors and fuel pressure regulator
because I had 2 leaking poppits. Runs better but
still have the RPM problem. Ran straight pipes
thinking it might be a clogged cat but still no
progress. Distributer is timed correctly, timing
chain is lined up correctly. I'm out of ideas. Please
help, thank you.

1 Answer

Oil pressure is high for about the first minute after I start it then levels out, new plugs, plug wires, distributer cap and rotor. Cleaned maf and put a new air filter in. I'm probably forgetting many things, I've been trying to get this running right for almost a year now.

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