Misfire...Lost 8 MPG

First thing, fix leaking exhaust, this can cause erratic idling...second thing, remove and clean IAC valve and MAF sensor..dry and replace, also try putting some SeaFoam in the petrol, engine oil and air intake, let sit for 10 minutes then go for 10-15 min drive...suggest you change engine oil shortly after driving about 5000kms, if the oil is old..

Did you get any codes on the misfire? If you did not, test ignition coils, test vacuum, test fuel injectors, verify correct plugs..That's right, anything from a fouled spark-plugs to burned cylinder head valves can cause a misfire. Whatever the root cause of the misfire, the bottom line is that one or more cylinders are not contributing to total engine power output. This leads to power loss, rough idle, unstable RPM and bad gas mileage...cheers

replace spark plugs, spark plug wires, fuel filter, and add some injector cleaner to the gas. Any change? if not it could be something to do with your ignition system, or fuel injectors, or maybe even engine timing.

Tuned-How -Spark plugs??

You have a misfire, you say on #3,
that isn't a plug issue

Have you tested the ignition module

Have a scope put on secondary ignition,
so you don't keep throwing parts at it

fuel or fire its one of them replace fuel filter change plugs wires cap rotor could be a cloged fuel injector or a vacum leak good luck

Are you sure they used the correct oil? If they put a thicker viscosity oil in it then that would explain some of your loss of gas milage. Did they change the air filter. A clogged air filter will also take extra gas.

I would replace the filter and verify the fuel pressure. should be 30-40psi but I dontt have the exact spec. The oil in the plug tubes may be caused by leaking valve cover gaskets.

wow a lot of codes, lets start with them....

P0351 ignition coil A primary/ secondary curcuit

P0300 random misfire detected

P0301 Cylinder 1 misfire detected

P0304 Cylinder 4 misfire detected

P0442 Evaporative emissions leak detected (small)

P0445 Evaporative Emissions System Purge control valve circuit shorted

I guess the first thing i would do, is insure that all the maintenance on this vehicle was up to date. Do an oil change and a tune up. Make sure you use decent spark plugs as a fouled plug or bad oil could cause a misfire. After you do the tune up, try running it some more, if the problem persist, you may end up having to replace the coil.

The evaporate emission leaks. These, especially the the 442 can be either a loose gas cap or a small leak in the vacume system. Insure the cap is tight passed several clicks and listen the the motor as it runs and try to hear any leaks. If you find a leak replace the faulty lines. Also inspect the vacume line for cracks and breaks.

Wish you luck, we are hear if you need more help. Thanks for coming to FixYa!!

The misfire can be caused by an open injector , injector pulg or circuit, bad spark plug ro wire lost compression as a burnt valve blown gasket ( antifrezze foulingthe sparplugs ) Low compression from overheating over mileage or flooding from injector leaking. Take out the number 4 plug and any other and compare what they look like if they don't look the same swich them and see if the miss moves to the cyclinder that the #4 plug moved to. If it does run good or misses on the new cyclinder change the spart plug with new and then look for loss of coolant or oil on plug wire carrect leak and try again

leaking valve or head gasket leaking

well the codes
p0300 multiple misfire
p0303 misfire cylinder 3
p0304 misfire cylinder 4
p0307 misfire cylinder 7

can be alot of things check cylinders for compression is what i would do first..