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Dear members,

My range rover with automatic transmission runs perfectly for the first few moments when it starts while using the 1st and the 2nd gear, but when it reaches a relatively high speed with the 3rd and the 4th gear, the engine starts to perform poorly making a bob-bob like sound. When I slow down for the transmission to engage in 1st gear, the engine returns to perform fairly well. The temperature is low and stable, spark plugs and their wires, the coil, the battery, and so the fuel pump are all in a mint condition.
 

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1970-1995 Range Rover Classic
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Didn't give us a whole lot to work with here, but, the following questions come to mind:

1. Does the issue occur at specific RPM's or just at specific speeds? What are you talking for either?
2. Are you manually enabling first through third?
3. Does the noise occur when you coast or when you have the power on?

I've seen this just recently with a defender I was helping with (with a 3.9 out of a '95 RRC) - The engine would run smooth in low gears but, in higher RPM's it would have no power and struggle to push the truck to 45 MPH. Sitting in neutral, we could run the engine through normal paces manually and see nothing obvious to cause the problem.

We replaced all many parts such as the ignition cables, coil, dizzy, etc. but there was no improvement. After days of troubleshooting it was found that only the right-hand bank of fuel-injectors were firing correctly. The other bank wasn't receiving a proper signal from the ECU.

The root cause of this fiasco was that the ECU was unable to provide constant signal to the left-hand bank of injectors and the solution was to replace the ECU with a different unit. Since running the new ECU the truck can hit 70mph+ without an issue.
 

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Yes - agreed with rangerover. I would also be curious as to where you feel the vibration - you butt, or the steering wheel etc. Could help you narrow down if this is something in the driveline / suspension / etc.

I thought I had a bad tranny in another non-rover vehicle I own because I had a vibration in the higher gears on the highway. In actuality I had a misfiring plug and bad coil, and the OBDII wasn't catching it until it got really bad.

Hows your trans fluid? If its in good shape- I would revisit your engine components first before thinking you have bad gears within the box.
 

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Discussion Starter · #4 ·
No shifting the gears is done automatically, and this noise only appears when I have the power on while in the third and fourth gear. The ignition cables and the spark plugs are new, the fuel is pump is also in a good condition, and all fluids are in appropriate levels.
 
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