I've had horrible CITY gas mileage on my I30 for the past couple months and performance started to drop. Then I started get lean error codes and I couldn't find anything blatantly wrong even using my Datascan software.
It had me completely stumped until Friday--now it's fixed.
Since I bought this car in April I've replaced a bunch of stuff and the mpg and performance was great until recently. So far I've replaced both O2 sensors, plugs, injectors, fuel filter, added KN Air filter, replaced MAF & TPS, replaced bad crank sensor and Knock sensor, tested every other sensor. I've cleaned the EGR port and throttle body. Took the intake plenum off and made sure it was true and installed a new gasket. Replace the thermostat. Located and cleaned ALL ground connections. New ATF and oil. Tire pressure runs about 35PSI and I have NO brake drag. NONE of this was related to my recent drop in city mpg.
So, how did I fix my problem?
Took off the EGR valve and related parts and cleaned the crap out of the valve. Then put it back on WITH gaskets (there weren't any when I took it off) and replaced all of the hoses that had anything to do with EGR.
While I had the plenum off I noticed the valves and valve guides in cylinder #1 had some carbon buildup compared to the rest (probably a previous leaking injector) so I cleaned all of that out too.
My I30 now pulls ahead with that thrust my head back in the seat power we all know the Maxima/I30 can deliver. City gas mileage jumped from 14 to 18 and I'm not driving easy--it's way too much fun to drive like I'm in a race!
Hope this helps someone. It has been ticking me off for a long time now!
