littlegurlbigtruck

2002 cavalier 2.4L (z24). I cant figure out why my engine is running richer than it should be. I drove from ottawa to northbay ON, and it usually takes me a little less than half a tank to get there. But this time it took almost a full tank, which is quite a difference. Just prior to this change in fuel economy I was having issues with idle air control solenoid(inconsistent and jumpy idle). So my dad sprayed some sensor cleaner into the intake tract(with the engine running). and it helped a little bit right before i left, but this fuel economy problem started right after this! I took the idle air control solenoid out and cleaned it with brake cleaner really good and the idle is steady at 1000rpm when at normal running temp. which is much much better than it was but still a tad high(probably because i just need a new one). Just put new plugs in, they are black now. PCV is leaking oil into the intake tract(enough for me to glare at it). MAP sensor seems to be doing its job, voltages are consistent with vacuum and it runs full rich when i unplug it. and i of course get a code when its unpluged as well as an engine that runs like crap. So thats whats up so far,any ideas would be awesome!

tech2

the vehicle uses speed density full metering. any drop in manifold vacuum will cause an increase in injector on time....and make it richen the fuel mixture.

change the pcv valve as you either has a pcv restriction or excessive blowby causing the oil to pass into the air intake system. I am wondering if this pressure is dropping your map reading and creating the rich mixture. It might be time for a compression check dry and then wet. if its low, the engine might be tired. how many km?



if the above is ok, I would look for manifold vacuum leaks. do you have a scan tool that will show fuel trim data.

the only other causes of rich fuel trims that I can think of are...bad o2 sensor, skewed map, high fuel pressure, leaking injectors, bad ect sensor or stuck open thermostat.

littlegurlbigtruck

Thank you Tech2 , about 228,000km on the engine, yeah i have a bluetooth obd2 scanner for my phone, its not like snapon shop scanner but it does let me review and log realtime data from every sensor. What kinds of numbers do i want to see for my fuel trim? if i remeber, it shows fuel trim as a percent for STFT and LTFT. and how would the pcv leaking oil affect my map sensor?

tech2

blowby gases have to go somewhere. if the pcv valve is plugged or overwhelmed by xs blowby volume it feeds into the air cleaner. this higher air flow may drop the manifold pressure. the map sensor will enrichen the air fuel ratio as its see's this as a request for more power. the map sensor determines engine load...ie you stepping on the accelerator....throttle plate opens...more air in and less vacuum or drop in manifold pressure.



fix the pcv stuff first otherwise you will be chasing your tail. put a $2 pcv in it

the st and lt ft should both be within +/- 10%. What are you reading in closed loop operation at idle and at 2500rpm.