2004 Honda Accord slows uphill on highway

I have a 2004 Honda Accord 2.4L (K24) 4 door 5 speed Automatic trans, I’ve replaced various parts on my car and it still struggles to go uphill on the highway, (just saying I use Mobil1 full synthetic with good filters religiously)

When I’m going up a rather steep grade on the highway my cars RPM stays around 2250ish and the car just very slowly Decreases speed and it downshifts normally but even when it downshifts from 5th to 4th and my RPM’s jump up…my cars speed doesn’t drop anymore but more or less stays the same until I crest the hill, and sometimes just drops really slowly… what could this be? I’ve put tons of parts on my car and can’t figure it out…if I floor my car I can get it to climb better but I feel like I should def have more power especially after downshifting it should pickup speed not just stop slowing down :confused: confused :neutral_face:
287 000kms (178 000miles)

I’ve replaced the fuel pump with brand new, air filters clean, new spark plugs properly gapped, both oxygen sensors and the entire exhaust from the exhaust manifold back(including cat) cam intake and exhaust sensors, vtec actuator and vtec solenoid,

Has the fuel pressure/volume been tested?


I haven’t but I replaced the fuel pump, maybe injectors?


Dump a can of Seafoam in the gas tank the next time you fill up.


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Do you press the gas pedal down when driving up a hill or just leave your foot where it was at the bottom of the hill?

You may think this is a silly question but you’ve not really told us much about your car or how you drive.

My prime suspect would be low compression.
Can the OP do a compression check, and then report back to us with the numbers?

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Plugged catylitic converter is another possibility.

My guess would be low compression although in theory with regular oil changes and only a 178k miles one would think it would be good. I might ask if you purchased this car new?

If not, then compression becomes more of a suspect as one never knows what the prior owner(s) did as far as maintenance, or lack of, goes.
Another possibility is slippage in the clutch packs of the automatic transmission.

What would I do as a first step? Connect a vacuum gauge and make sure things are fine mechanically inside the engine. The vacuum gauge is easy to connect and read with a little practice. Ideally, you want 20 to 21 inches of mercury at idle with the gauge needle holding rock steady. Any deviation from that requires some thought based on the gauge numbers and needle reaction.

Sounds normal, 2250 RPMs is too slow of an engine speed for a four cylinder mid-sized car to climb a steep grade. Press the accelerator more so that the transmission down shifts before the vehicle speed drops so much. There is a reason a V-6 engine was offered.