The rev counter went funny on my 1999 mitsubishi galant then it stopped running as if it had run out of petrol turning over fine just wont start

I am not familiar with this model but petrol engines usually use a signal from the ignition system for the rev counter and diesels need a special sensor. However the rev counter is almost certainly unsuitable because petrol engines tend to rev much higher than diesels.

run the fault codes as what you have done may have no impact on the fault

diesel motor parts and petrol motor parts are not rely compatible ether use diesel to diesel and petrol with petrol

Hello,
Most likely, your problem is your altenator and/or your battery. If possible take your car to the closest auto parts store and they should be able to test it for free.
ie: Autozone, Advance Auto, or Pep boys..

that is your idle air control valve. it is located on the throttle body next to the throttle position sensor

As with any no start issue, you will need to figure out if you are dealing with a no spark or no fuel pressure problem to proceed.
I would check those things the next time it will not start.
Could be a relay, coil, sensor, or other component.

not the starter motor ,who said that ? no its purely coincidental that changing the battery terminal caused this problem .Now you havent stated as to type of engine diesel or petrol ? if a petrol then double your battery instalation as if what you say is correct ? then you may have either dislodged a vacumn line or touched one which has split owing to age so have a good look .Backyard mechanic like the description .Well iam a aged dinasaur old truck mechanic .WITH A BIG HAMMER

firstly, is this a Gdi engine? if the engine revs ok when on park and neutral, but loses power when on a gear while driving, most likely your GDi pump is malfunctioning. i experienced this problem twice already and the pump was the problem on both occasions. the car works fine and malfunctions sometimes until the pump stops working and the car can hardly drive and then it stops starting altogether. the pump is fairly easy to replace.

What year is it? Is it fuel injected or carbureted?

Your timing belt might have busted. Mine did that in November of 2007.... I had to replace 2 of the 4 cylinders, along with the timing belt and housing. Total cost was a little over $1700.00