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The first thing that comes to my mind is "45 MPH"... and let me explain why.....
Your engine you said is running GREAT so a MPH is not going to effect the running of the Car but what happens @ 45 MPH you ask?
Your Torque Converter goes into Lock up.... If the Torque Converter is slipping in and out of lock up, you are going to have an irregular pulsating of the engine playing catch up.
you need to do this, try and drive the vehicle (you did not say what kind of vehicle it is), and drive it without the OVERDRIVE On...
So drive it and see if it is RPM RELATED. If there is no CHUG and you can go all the way to 55-60 without issue, Slow down below 45 and put the OD IN and accellerate and see what happens..
Is the Problem SPEED RELATED? Is it RPM Related?
If it does not happen with it OUT of OD, then what you probably have is a Torque Converter Failing...
Hello, some models have an O/D indicator light on the dashboard, if it is on or the selector is set to the O/D position and it is working correctly you sill see a slight decrease of engine rpm between about 45 and 65 mph depending on throttle application, it usually drops about 300 rpm when overdrive is engaged, if the rpm stays the same as speed is increased above 50 mph the O/D is not working.
Trouble Code: P0421 Main Catalyst Efficiency Deterioration (Bank 1)
Trouble Code Conditions: DTC P0136, P0139 and P0140 not set, engine running at 3000 rpm or high in closed loop at 45-60 mph for 140 seconds, and the PCM detected the rear HO2S and front HO2S signals were too similar.
Possible Causes:
Air leaks at the exhaust manifold or in the exhaust pipes.Catalytic converter is damaged or has failed .
Front HO2S or rear HO2S is contaminated with fuel or moisture.Front HO2S or rear HO2S heater is damaged or has failed.Front HO2S or rear HO2S is contaminated with fuel or moisture.
Trouble Code: P0431 Main Catalyst Efficiency Deterioration (Bank 2)
Trouble Code Conditions: DTC P0150, P0154, P0159, P0158, P0160 and P0161 not set, engine started, engine running at 3000 rpm or high in closed loop at 45-60 mph for 140 seconds, and the PCM detected the rear HO2S and front HO2S signals were too similar in the Catalyst Monitor test.
Possible Causes:
Air leaks at the exhaust manifold or in the exhaust pipes.Catalytic converter is damaged or has failed.Front HO2S or rear HO2S is contaminated with fuel or moisture.Front HO2S or rear HO2S heater is damaged or has failed.Front HO2S or rear HO2S is contaminated with fuel or moisture .
At 45 or 50 is the engine RPM normal? If its higher that it should be transmission could be slipping.
If the engine is misfiring at 45 or 50 it could be the spark plugs.
If its just holding back on power it could be catylitic convertor, or possibly fuel filter.
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