I'm getting a constant 15 mpg, regardless of highway or in town driving, along with a rather rough idle. I've looked at my O2 sensor output at idle, and it stays consistently lean, bouncing around wildly (not a nice sine curve!). My short term fuel trim is about 4.25% when idling, and around -3.50% when driving. Long term fuel trim is consistently at -4.68%. Since this started I've replaced the bank 1 (exhaust manifold) O2 sensor (it was original with 120000 miles), removed and cleaned the throttle body, installed a new base gasket, TPS, and MAP sensor, and checked for vacuum leaks. Back in November I was getting 34 mpg on the highway and now I'm starting to suspect the fuel pressure regulator, but I need to do a more through job of troubleshooting. Can anyone help me out? Thanks.
2002 Pontiac Sunfire (LN2)
Oh, and I almost forgot, calculated engine load at idle is around 30%, which sounds wayyyy too high!
2002 Pontiac Sunfire (LN2)
I seen o2 sensors go bad and read lean . test the 02 sensor to see if it wil read rich , or you can just replace it .you mite check to see if the 02 sensor has a warranty .
Sounds like you could have a defective O2 sensors. If youre fuel reg is old replace it and see if it corrects the issue, Are you getting more Cel codes?
what brand 02 u buy? ive heard very bad things about Bosch 02s for our cars
95LsCoupe wrote:what brand 02 u buy? ive heard very bad things about Bosch 02s for our cars
This sounds like what my car did with Bosch o2 sensors. Get any other brand and you shouldn't have a problem.
I would start there, otherwise, it might be an ECU issue. Have you tried resetting it?
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Update. I just started using autoenginuity, so I mistakenly read the o2 output as being constantly below crossover voltage (.45V). What it is actually doing is crossing back and forth between rich and lean as is should, but spending more time in the rich region than the lean. The waveform looks like crap, but I'm tending to think that's due to a problem elsewhere, not a bad o2 sensor. So, I have an o2 sensor showing consistently rich exhaust, and a short and long term trim trying to correct a rich condition when driving. So the question is why isn't the system correcting the over rich? One other thing I noticed is the manifold vacuum at idle is 11 in. Hg, and I have never seen a stock electronically controlled engine with that low a vacuum before. Some please let me know what their idle vacuum is. Now I'm starting to think that I have a severe vacuum leak somewhere, that or severe valve seat damage. I don't think it is engine damage, as it runs well on the road, but does have a little valve train noise. Does anyone think I'm on the right track, or am I just lost?
My plan is to pull the intake manifold this weekend, replace the gasket and inspect the intake manifold for cracks/leaks, and inspect/replace the fuel pressure regulator. I'll also replace the fuel injector o-rings and try to take a look at the intake valves with an inspection mirror.
Any other suggestions?
Thanks.
2002 Pontiac Sunfire (LN2)
