20 Most Recent Grundfos MQ3-35 3/4 HP Pressure Booster Pump Questions & Answers

I have the same problem and have found that the inlet check valve has a broken shaft in the center of device. It is a multi-piece plastic unit and it looks as if it is not quite seating and therefore the internal pressure leaks slowly back into my low pressure suction line. The pump will start and the pressure will build then shut off for about 30 minutes then cycle again. I am going to replace the check valve.

Mine did that. Little known that the Grundfos MQ pump has a small pressure tank on the end. If the pressure is low and the air volume is therefore low, then any tiny loss of water results in pressure drop and pump cycles on. Check the pressure in the tank. This is not well described in the Grundfos service manual.To pump up the pressure bladder:1. remove the rear cylindrical cover (swift bash with a fist downwards)2. unscrew plastic cap on the end (about 2 inches diameter with tapered sides3. attach pump to schrader valve and pump up (22-25 psi according to service manual)Reassemble.Suggest teflon tape to schrader valve stem to try to create a secondary airtight seal.

Is it newly installed or has it been working for a while till you noticed it?

The bearings fitted to this pump are lubricated for life - if the noise is bearing related they will need to be replaced. If the noise is not from the bearings you will need to establish the cause - possibilities are:
Cavitation (sounds like frozen peas inside the pump!) caused by restricted suction - check for blocked strainer etc.
Foreign matter in the pump....
Loose impellers
Sometimes a piece of wood or a large screwdriver used as a stethoscope can be very handy in determining where abouts in the pump the noise is coming from.
Given more specific information I can suggest the next course of action.
Good luck
Mike G

This is a problem with your bladder in the pressure tank or the pressure switch. Ether one will produce this cycling on and off. Good luck.

Pump probably runs dry, try to fill up the pump with water a few times and see what it gives. If the pump heats up then the rotor is turning but no water moves, pressure is not building up. Pressure switch is maybe faulty. If you have the possibility to connect a pressure gauge after the dicharge you can see if the pressure is building up or not.

Check the capacitor or the centrifugal switch.

proper way is to install a water pressure regulator and due to potability reasons it needs to be brass ,bronze, stainless steel, or CPVC plastic , (no steel or cast metal) of course the less high tech way is to plumb a valve from pump outlet back to its inlet and adjust valve to max pressure setting you want at max demand , I don't recommend this method as it wont be able to give you a constant pressure regardless of demand as the first method will , Note: some water regulators are a "relieving" type and require special install methods and maybe three lines to it

Is there a capacitor on the motor normally under these symptoms and the motor is a single phase motor the capacitor is defective.

Hi,
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heatman101

Hi,
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It is likely that the shaft seal in the pump has failed and the water is leaking into the motor. Water and electrics = fuse blowing!!