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All, I know this topic has been done to death but I cant find any reference on the forum to my particular symptoms and associated problem with the lucas fuel pump on my TR6 1973 CR 1. Ignition on engine off the fuel pump runs perfectly albeit with the usual whine and will continue to run perfectly until switched off 2. Ignition on engine running – the pump runs perfectly for about a minute and then starts to hunt. The engine continues to run (tick over) with the pump continually hunting. Soon after shutting down the engine and restarting the pump it resumes its normal whine. Note that before the pump starts to hunt the engine revs through its full range. 3. Take the car for a spin and the engine runs well and runs ok even with the pump hunting but will then start to splutter and cut as the pump appears to die and then resume. **The pump is NOT getting hot just warm to the touch. There is 12.5 volts and 4.9 amps at the pump with the engine running. There is over half a tank of super unleaded. I don’t suspect the MU as the engine runs too well. The MU return pipe isn’t blocked. The pump is delivering plenty of fuel and I assume the pump delivers the same pressure and flow rate whether the engine is running or not. To double check the pump wiring I connected new cables directly to the fuse box and earth. It didn’t make any difference; however, I realise this doesn’t test the tails but I don’t really suspect the wiring. The pump and MU were rebuilt about a year ago and were then checked over by Neil F. Neil also checked over the return valve and found all to be satisfactory. All of the pipe work is new as is the delphi fuel filter, (no leaks anywhere). The car has only done 150 miles since the rebuild and was running well before the above symptoms started. Unfortunately, I don’t have a pressure gauge but suspecting the PRV rather than the pump or the MU I started the engine, waited for the pump to start hunting and then clamped off the PRV return pipe. Restricting the return flow (cant fully clamp it off) almost stops the pump from hunting. With the pump switched off and the return pipe removed from the PRV I noticed that fuel constantly dribbles past the adjuster screw. Having now removed the PRV I can also blow into the PRV and feel a trickle of air past the adjuster screw – is this normal or is there excessive wear between the valve body, the distance piece and the valve. The valve and plunger are clean and appear to be airtight and not sticking; the spring is OK and the strainer is free from dirt. Grateful for any ideas before coughing up yet more cash on a new valve. (P.S. I am going to look for a pressure tester) Many Thanks Ross