Fuel economy/p0430/420 codes

[clg4th]

I've got a 2006 v6 with 169k miles on it. I purchased it knowing it was throwing a code for the emissions system, however mileage was acceptable given the tire it was running, averaging 16.5mpg in town. Fast forward a year, mileage has significantly decreased (13.5-14mpg) and experiencing a very strange symptom of the gas gauge reading 5-10% lower every morning I get in it than when I put it to bed. Went from getting 280-290 miles per tank until light down to 220-240.

Changed both rear o2 sensors at the suggestion of the mechanic, as both cats were reading to be working on the multi meter. Code gone for 400ish miles, came back

Changed my well overdue plugs the PO was less than honest about, code gone for about 1000 miles with a significant increase in economy.

Now the code is back and economy has tanked once again. Has a valve cover gasket leak on the right sight, PCV has been changed, no stumbling or hesitation issues to report.

Thoughts?

 

[m85476585]

Are you calculating MPG by hand when you fill up? If the gas gauge and/or range isn't accurate then the MPG indicated on the computer might not be accurate.

Dozens of things could cause low MPG (I think there is a thread about some possibilities), so there's a chance it might not be related to emissions problems.

 

[clg4th]

I am not, but the gauge reflects the distance I'm covering between fill ups.

 

[blackoutt]

I'm not speaking for the Toyota controls system but I know that on other automotive engines when a fault code for catalyst efficiency or O2 sensor is active the fueling tables can change. For an upstream O2 sensor issue it must revert to open loop tables, but for downstream (not related to running condition, strictly for emissions) they can still go to an offset fueling table causing poor fuel economy. Not to mention if you truly have a bad catalyst, misfires from the spark plugs can cause melted catalyst material = high exhaust restriction also leading to poor fuel economy. Might just look into replacing the catalysts.

 

[pjku79]

I am not sure if you can rent these, but when we inspect cars at work, we use something called a borescope. It is basically a small camera that you can put in through the O2 sensor hole and it will allow you to look at the front and rear face of the catalyst. This is an easy way to check the integrity of the cat.

 

[alia176]

P0420/P0430 are all about the cats getting dirty or not doing their job efficiently. Changing out the O2 sensors after the cat is almost like shooting the messenger. The O2 sensors are just doing what they're told, which is the exhaust isn't clean enough. But, you gotta try at least!

I just went through this scenario on my GF's '08 V8. We were going to clean her cats in an acid bath because she too was getting the same codes. Unfortunately, one of her cat was missing some of the matrix material inside so she ended up purchasing both new manifolds with the cats from Amazon. It hurt like hell both in terms of the cost as well working with that VVTI engine.

Our original plan was to drop the manifolds, make a bath of citric acid + oxallic acid over a single burner stove on the driveway. Soak both cats for 12 hours, then flush out with water. Reinstall and hopefully no more codes. But, we did compare the old cats internal matrix to the new cats and yup, old ones were very dirty due to the engine running lean for few months while we sorted out her fuel pump starvation issue.

Anyway, good luck with what you decide.

 

[clg4th]

bumping this for an update.

Through patterns of driving, I've found getting it on the highway for a few hours sustained is enough to clean up the system somewhere and the code will go away for 1000ish miles. Most of my driving is in town, and consists of short 5-10 minute trips, with 30min+ use only every few months. My exhaust tip is quite dark and even damp, while after an extended trip it's shining exposed metal.

Thoughts on what causes this? I realize short trips aren't ideal, but there must be something shitting the bed to cause it.