Mitsubishi Repair: Mitsubishi galant gdi, mitsubishi galant, vacuum leak

Question
QUESTION: My galant gdi vrg 1999 has been rough iddling flactuating between 800 and 1000 revolutions and sometimes even going below 700 revolutions. My mechanic says i need to buy high power plugs because am using denso double pin. whenever i replace them, it stabilizes a bit but they get full of carbon sooner than normal. there is soot on the exhaust but the car does not smoke.I recently changed engine oil,atf and air cleaner but did not replace the fuel filters. all the sensors have been diagnosed okay.sometimes the car accelerates itself even upto 2000 revolutions but when i switch off and on it becomes normal again.what could be the problem and is it a major one?

ANSWER: Cornelius: I would check for a vacuum leak, as that could be your problem. Though those are often hard to find and so you might have to have a shop check that. Also check to see if the EGR valve might be leaking. Fuel pump might be putting out enough pressure, you should check to see if it is putting out enough pressure. That is an easy test for a shop. Have you had your timing belt replaced at 60,000 miles or not. I would look into getting that done, as it might be getting weak. Oh you could have a head gasket getting leak too. Things to check. God bless: Linda

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QUESTION: do the plugs have anything to do with all this? would a fresh tune up to try and balance air and fuel be of any help? how could one detect vacuum leak and what exactly is it? where is the EGR  valve located? is the timing belt expensive? please help me out

Answer
Cornelius: Go get yourself that repair manual, then read it. It would explain where the EGR goes. To be honest I don't think the problem with your car is the plugs. But that mechanic got to you buy those costly stark plugs and did that fix your car? No. If you honest believe that person then take the car back to him and explain what is going on. That the high cost stark plugs did not fix it. And you want him to check for a vacuum leak, do a tune up on the car, check the EGR valve. Or buy that repair manual, most parts stores have them they cost around $20.00 and then read up on your car. It would in that manual explain what kind of stark plugs you should be using. And then once you read up on your car, you could go back to your mechanic and explain what the manual says; how you want him to fix the car. Not to beat you to death with high cost stark plugs that just caused the car other problems but to really fix it this time. SO you could get good gas mileage but also enjoy driving the car. Hope this helps and God bless: Linda