K1500 Lack of power ... up hill

'96 k1500 ~133k miles on engine. Tranny rebuilt @ 117k.
Really lacks power traveling on incline ... especially when towing. Trans seems to shift correctly and RPM's seem right, but it just won't gain speed up hill, even a slight incline. Engine doesn't over rev., so I don't think trans is slipping. Exhaust changed in "sound" at same time as lack of power issue began ... made me think "back pressure" ... but most recent mechanic didn't think so. Have codes P0300, P1122, and P1380. Also had P1406 (EGR) but have had EGR replaced.

Any ideas?

Thanks.

A few things come immediately to mind -

Have you installed new fuel filter? Try that first...........

Also, air filter could be clogged, or perhaps vacuum hose is leaking.

Dist cap on these also sucks, so it could be that.

On my '98 K1500 I have had most of the above items cause me grief, but my most expensive item was a fuel pump. The engine would cut out going up long hills. I was getting error codes that suggested misfiring, but it was the fuel pump. The local shop had to replace it twice (aftermarket pumps), then I insisted on OEM pump and all has been well since.

Hope this helps, good luck...................

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Checked your catalizers? Remember exhaust will sound different if clogged, even if removed. Also, you have sensors at exhaust (before and after cats). Do check them...A good reference is to check engine temp during climbs on this...