Hot water on bathroom faucet won't get hot only luke warm

If some taps have really hot and others luke warm, then you might have defective mixing valve, check valve, or crossover on single handled faucet.
http://waterheatertimer.org/Crossover.html

Crossover can also cause stacking event. Stacking could be part of puzzle.
http://waterheatertimer.org/Stacking.html

Once crossover or check valve or mixing valve problem is solved, if you have really hot water to some taps, then turn down thermostat to avoid scalding.
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html

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Check the water supply lines to the shower at the faucet handles. If the fitting for the hot water line connected to the handle is loose, it can twist and kink off the water supply further down the line when you turn the faucet handle. I'm not exactly sure how this could be relevant otherwise, but under normal design circumstances this type of scenario with your shower wouldn't matter for other faucets.

If water come out the faucet hot line,check if is clogged.For this job you need to disassemble the faucet.

Here is a good PDF source to do it.

If you need more help come back.Post / thumbs / testimonial are welcome!

Pull apart the front of the valve and there is a limiter that is set to limit hte hot water, you need to adjust that limiter. I find many valves are shipped with teh setting too cold.

These type faucets have anti scald dial setting that if you remove the handle you can gain access to it.
Just remove handle and mark or remember where it is and pull it out and reinsert it about 1/4 turn counter clockwise.

It has got marks on it and looks like a little gear.

i have a 7570 salora that took the top off and i can't get it back together right

Do you mean the other faucets in the house are hot and only the shower is warm? If so there is an anti-scald adjustment on the faucet that needs to be turned up a few notches. Havent adjusted a Grohe faucet but most faucets you rmove the handle and behind the handle is a nothced adjustment ring that prevents the faucet from getting too hot. This ring, or stop, is pulled out and twisted clockwise(or counter clockwise to set higher or lower max temp, then pushed back. maybe you can look up grohe to see which way ring or stop adjusts for hotter water.

bad faucet...get new at lowes or home depot

Try draining the system and apply air pressure (min 30psi) to each faucet. Open other hot water faucets to allow pressure to vent. Next try draining and cleaning the hot water tank.

under the handle of the faucet reached by removing the covering cap and single screw is a plastic geared device which limits the travel of the handle to the hot side carefully reorient so handle will turn a little farther and reinstall handle screw and cap