Anyone anygood at dechiphering 02 Sensor readouts want to help me solve a puzzle?

Iamzod

Iamzod

Iamzod

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I have no experience with potential O2 Sensor issues so hoping one of you chaps might have some input.

Bottom four are the O2 Sensor output voltages, top pair being Bank 1 pre & post cat, bottom pair the same for bank 2.

Other rows are RPM, Throttle Position Sensor, MAF Voltage, Cylinder Head Temp & then Coolant Temp.

Symptoms are that the car seems to drive just great but the exhaust smells and is leaving a sooty deposit on my shiny tailpipes, long term fuel trims on both banks are about -12 to -14 and it's guzzling fuel like the tank has a hole in it, I have no EML and no pending fault codes on the car.

I know some of you guys are knowledgeable so here is a nice puzzle for you to solve :)
 
The fuel trims seem like the ECU knows there's too much fuel and is trying to reduce it, and it's not working.

I take it the thermostat is working and the car is getting warm? Is your scan tool saying it's in closed loop when warm?
 
Brookert said:
I take it the thermostat is working and the car is getting warm? Is your scan tool saying it's in closed loop when warm?
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Temp sensors are reading 90 - 100c, both coolant & cyl head. It does appear to be staying closed loop though, saw it flick to open a couple of times but not for long.
 
Iamzod said:
Temp sensors are reading 90 - 100c, both coolant & cyl head. It does appear to be staying closed loop though, saw it flick to open a couple of times but not for long.
Click to expand...

Did it flick to open loop when already warmed up? It's common for for some cold engines to start in open loop and then change into closed loop a few seconds later once the sensors have warmed up.

If you're getting open loop readings on a warm engine I'd say that's a starting point for fault finding, checking all sensors are on tight and no leaks in the exhaust introducing air.
 
vanpeebles said:
Is the MAF a new one?
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As far as i'm aware it's the original 15 year old one, i've never changed it. 3rd row down on the graph is the MAF reading.

bainbridge said:
Did it flick to open loop when already warmed up? It's common for for some cold engines to start in open loop and then change into closed loop a few seconds later once the sensors have warmed up.

If you're getting open loop readings on a warm engine I'd say that's a starting point for fault finding, checking all sensors are on tight and no leaks in the exhaust introducing air.
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It was closed permanently from start up, the only time I spotted it switch to open was when I dropped to 2nd at 30mph and blipped the throttle to match the revs to the speed as I was coming to a junction. It may have gone open at other times that I missed due to watching the road but that's the one I spotted.
 
What kind of car / engine is it ?

You can see it running closed loop fine until you get to about the 67second mark where the engine revs start to kick upwards, but the throttle isn't really doing the same kick up. It seems open loop at that point, and never really coming back out of it afterwards.

Then again, the engine doesn't look like its fully warm, its still climbing throughout that graph ... Some cars can given themselves quite a long warm up period ( irresepctive of whether the engine temp is warm or not ). I would have expected to see the graphs return to what the 1st half looks like with both upstream and downstream sensors fluttering eitherside of correct air/fuel ratio mix values. Is it possible that its:

- starting normally from cold
- going closed loop early during its warm up cycle period
- going open loop due to the hard acceleration
- not defaulting back to closed loop because it still thinks its in warm up period, so leaving it open loop and running rich ?

But as others have said ... MAF errors go a long way to puting readings out. Even if its slightly out, it puts the rest of the system out of whack.

Oh, and only buy a genuine MAF / genuine Bosch .... its one of the few things I strongly recommend going genuine only for.
 
That doesn't tell you all that much really. Pre-cat affects fuelling, post cat is only there to tell the car that the catalytic converter is working.

On the precat ones, you can see it is flipping between low and high during closed loop operation - that's what you expect to see with a narrowband O2 sensor. It stays high when in open loop.
 
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