Spark Plugs in the back area blackened, engine light on WTF

[Geocentricity]

I took my 2001 Sienna to Autozone to get the diagnostic and it said that the A/F sensor was off with possible cause of misconnection or something.

The car doesn't accelerate as it use to and sometimes the engine shakes the car very noticeably. I just changed the damn sensors 3 months ago too, so after I checked them started tinkering.

1) I take out the PCV and then the engine runs fine. I put it back in and it goes back to FUBAR mode.

2) I check the spark plugs and the front 3 are clean as a whistle, but the 3 in the back are blackened. I switch them to the front and the reverse happens. The 3 dirty ones are now clean and the clean ones that I switched to the back and now dirty.

Whats the conclusion? I reckon its the A/F sensor misreading and choking the engine with too much fuel. Any thoughts?

EDIT: The code was initially P1130. As the week went on, it became P1130 (A/F sensor circuit range/performance) and P0171 (A/F mixture system lean bank 1)
I inspected the testing wires on the MAF/VAF sensor and cleaned it. PCV valve was replaced yesterday, cost me $3 and I was praying it was the problem but no. Fuel injectors are working, ignition coils swapped out too.

 

[EightySix Four]

Depends on the exact code. There's a code which says the O2 sensor failed diagnostics, which your guess would then be right, but there's also a code saying the O2 sensor is picking up an improper A/F ratio, which would mean something else is going on and you need to figure out why the motor can't maintain a stoich ratio.

 

[T2urtle]

what is a A/F sensor? you stated SENSORS!

is this a MASS AIR FLOW (MAF) sensor. If so there is only one and its like a 6 inch tube with a connector on it. I'm thinking maybe you mean Coilpacks, car has 6 all together that go on top of the spark plugs but that would never be mistaking for A/F sensor. So lastly i think maybe o2 sensors on a 6 cylinder, you should have least 3.

if you take the PCV valve out and it runs fine why dont you replace the PCV valve. They often go bad and its a cheap item to replace.


You are giving symptoms of a misfire. Do you remember what exact codes autozone got for you a P0xxx code. Something in the lines of MAF,o2, cylinder misfire, lean or rich condition.

 

[T2urtle]

my 1st google hit for "p1133 sienna" shows

http://www.tundrasolutions.com/forums/sienna/111497-99-sienna-p1130-p1133/

he solved it by replacing sensor 1 of the o2. There are a couple so you need to replace the one that is bad.

a LEAN fault could be caused by simple vaccum lines broken, your intake pipe having a crack. Try to listen for any type of hissing noise. Does it run rough/shake only at idle or all of the RPM's

I would want to start looking into your " BLACK spark plugs" maybe a leaking valve cover/spark plug tube gasket is causing it. Its not a hard job but not the most basic job either. in most v6's you have to pull the plenum/manifold, this way you can also see if any other lines are broken or damaged. But this would be my GUESS and shot in the dark with given information.

 

[Howard]

Spark plug wires, distributor (don't know if your car has one)? I'd check just in case.

 

[exdeath]

Bad o2 for that bank is causing a false lean reading and causing the computer to add fuel when it doesn't need to, causing it to run rich and foul the plugs. That or an exhaust leak near the o2 sensor, anything that can create a false lean reading for the whole bank.

It runs better when you disconnect the pcv because you're causing a vacuum leak that introduces more air to cancel out the excess fueling in the affected bank. The working bank continues to work also because the functioning o2 on the unaffected side can accommodate it dynamically.

The key here is that an entire bank is equally affected all at once. The fact that the code says lean but your plugs are black tells you that the lean code is false, the plugs tell the truth.

You'll often get MAF and O2 codes together almost every time since the two rely on each other to range check one another. It just means the maf and O2 aren't agreeing and the computer doesn't know which one is wrong. Those codes appear when the maximum allowed trim % has been reached and the problem remains.

 

[bruceb]

I agree, bad O2 sensor in the rear bank. It should be easy to spot, right in the exhaust manifold of that bank.