Apppreciate any help

My impala has lost power on climbing hills and any rpm over 4000. When in park reving it up to 4000 it starts surging.

I have replaced the fuel filter and checked pressure while running and when engine is off and checks out ok.

I have removed the exhaust pipe at the manifold to check back pressure and ran the engine. but still have problem.

I have no DTCs

Engine idles and runs fine below 30 mph unless under a heavy load like a hill.

3.8 L engine with 100100 miles. I have taken the plugs out and they look fine but will be replacing anyway since the book suggests it.

I forced a DTC by putting in 1st gear and with wide open throttle the engine started missing around 4000 rpm. DTC code was Random misfire. PO300.

any thoughts?

 

Clogged Cat

 

I had the exhaust system disconnected and still had the problem. Any other thoughts?

 

Id do plugs and wires first

 

when in park it will only rev to about 4k rpms, there is an electronic rev limiter on it so you wont damage any internal parts by over reving, so it will surge like that in park.
how did you get the exhaust off at the manifolds? the bolts on the flange at the top of the downpipe are a bitch to get off. ???

 

My problem was a plugged Cat. I found just unloosening the bolts at the manifold did not work. I had to slide the pipe off the studs. once I did that the engine ran fine under load.

My symtom was not a misfire but a lack of power over 40 mph

hope this helps.