Need help running rich!
 

I have a 2008 HHR Chevy with the 2.2. Original it was 21/30 MPG. I added larger tires, larger exhaust, better intake and filter and did a Trifecta tune on it. Moved the mileage up to 26/38 and it ran better to. Most of the mods were done 10,000 to 15,000 miles. Now at 30,000 miles the car is running real rich and mileage is back down to 21/30???
Did a Data log on it and it said the O2 sensor was reading lean causing ECM to richen it up. Tail pipe is black and sooty.
With the Trifecta tune I really do not want to take it back to dealer unless I have to but I want it fixed. Does anyone have any ideas on how to lean it back out?
Thanks in advance, Bruce

Replace the sensor.

Definately sounds like the O2 sensor needs replacing. Did a Service Engine Soon light come on?

You can try to swap it with the post cat sensor to see if things improve until you can get a new one. Hopefully the new data log will show the same one not reading accurately.

FWIW, my 4.3L was acting like you are describing and once I swapped position of the sensors, the truck started running right again. The SES light was still on, but the code followed the bad sensor. Now I just gotta break down a get another one...

No service light has come on. There are 2 sensors one before the cat and one after. I do think they are different. I guess it would be the first one that is bad?

Probably. Replace it and see if it gets better.

The O2 sensor after the cat is not used for fuel trim, its sole purpose in life is to make you fail a smog check when your cat is busted.

If your vehicle is consistently reading lean, look into clogged fuel filter FIRST. If you have never replaced yours, and your vehicle has more than 15k miles or so, replace it. Your fuel pump will thank you, as will your performance and mileage.


AFTER replacing your fuel filter, if you still have enrichment trim that is consistently more than +3, rent a fuel pressure gauge with purge valve from your local Vato Zone or Checker etc. (it's like $70 but you get 100% of your money back when you bring back the rental) and make sure fuel pressure at the rail is within spec for your vehicle. If it is within spec, also check flow by pressing the purge valve while idling. The engine should not die easily, if it does you may still have a flow problem with your fuel pump.

Does the vehicle tend to "run out of energy" on hard acceleration above a certain RPM? Like for example does it accelerate strongly when floored, up to 2500 rpm and then kinda not get any more powerful as rpm's increase?



FWIW - gasoline burns most completely at a 14.7:1 ratio at sea level, but releases the most btu's per pound at about 12:1 - so under the RIGHT circumstances, running slightly rich can actually increase your mpg.

oh no. attempting lean on an inline AND demanding more torque..this could go all the way to bad cylinder walls.
try fuel cleaner, expensive fuel and a few redlines while torquing. The o2 sensor myth about not doing anyhting is an old hoax, they really function today. if it does not go away, you could doubt reaidng and manually check with ohm meters the other stuff beyond o2 sensor.

You might talk to Trifecta. Their website says you can send them the data from their scanner and they'll verify that the tune isn't the problem.

You added bigger tires, and got 8mpg improvement?

I wish I was running rich :(

Oh wait, we're talking about engines here :D

I'd replace the sensor. also do whatever maintenance needs to be done at 30k

Replacing the o2 sensor just because the vehicle is reporting lean and subsequently enriching is silly, there are a dozen different problems that could cause this condition and just replacing the sensor that reports the condition is literally "shooting the messenger"

The post-cat o2 sensor really, genuinely, no-myth-about-it is not used for fuel trim. Period. In fact it CAN'T be used for fuel trim. Think about it: there's a frickin catalyst right before the sensor, how would the sensor know whether o2 readings are skewed by the effectivenes of the catalyst at that moment?

Replace your fuel filter first, it needs it anyway by 30k miles. See if that corrects the problem. Then use a fuel pressure gauge on the port that exists for that very reason to diagnose the problem rather than just randomly throwing expensive parts at the problem in hopes of accidentally curing it.

Expensive parts? my o2 was like 40 bucks.

I'm not talking about the post cat sensor. I'm talking about the pre cat sensor. And you don't have to replace it, you just do a test, and if it tests bad, you replace the sensor, if it tests good, then you look elsewhere.

check for vacuum leaks and evap system, either one will cause similar problems that you are seeing. One more thing, do you have an exhaust leak by any chance? that would do it too, even the smallest one.