Fuel Trim Diagnosis

[ellzeez]

Hi all, trying to diagnose an issue with my new-to-me '05 Disco 3, petrol V6. It's done 240k k's.

Bought it with CEL on, figured it was running really nicely so couldn't be anything too drastic. Plugged in my OBD scan tool (a cheapy, wasn't intended for this car) and pulled the P0172 & 175 codes for rich mixture. I've cleared these a few times, but it returns after about 30k's. Doing some more research and looking at the live data today while driving I'm getting fairly normal readings under throttle and silly ones at idle. For example, one snapshot I took reads the following:

STFT1 1.56
LTFT1 -6.25
STFT2 1.56
LTFT2 -3.91

However at idle..:

STFT1 3.13
LTFT1 -17.19
STFT2 3.91
LTFT2 -14.84

I have cleaned the MAF sensor, no change (maybe need a new one?) and have a new air filter coming and o2 sensor data appears reasonable (0.64V, confirming rich?) but starting to suspect its more of a vacuum problem? Would I be close to the mark here?.. If so, what's the suggestion in your experience? Can provide more data if it helps, let me know what to look for.. Cheers!

[DiscoJeffster]

You need to get a better diag tool onto it and check the injector trim values. You’ll probably find them at max limit, leading to injector replacement.

[Pedro_The_Swift]

On a D2 V8----
The O2 value should be constantly changing .1 up to .9v.....

afaik thats how all O2 sensors work

[PhilipA]

The most common problem with the Ford V6 is timing chain stretch.

Have a google of it.
Regards PhilipA

[DiscoJeffster]

On a D2 V8----
The O2 value should be constantly changing .1 up to .9v.....

afaik thats how all O2 sensors work

Narrow band o2 sensors work that way as they are effectively a switch - lean or rich and so to get the perfect economy, when in cruise they would adjust the mix to sit between the two flicking back and forward.
Wide band o2 sensors are able to read values throughout the operating range and constantly, thus they can be used for tuning and far better fuel economy management. These days cars tend to use wide band as emissions are key.

When I used to tune cars in the 90’s I paid a lot for a wide band sensor out of the US to stop my engines going bang.

[Pedro_The_Swift]

and the 4L uses wideband?
and "should" read 50?
if its reading 60 shouldnt the STFT's all be negative?
and how do you tell if a wideband's gone bad? [bighmmm][smilebigeye]

[ellzeez]

So I disconnected the MAF and ran her for about 20 seconds.. these were the results:

STFT1 16.41
LTFT1 -17.97
STFT2 17.97
LTFT2 -15.63

Does this confirm a bad MAF?

[DiscoJeffster]

So I disconnected the MAF and ran her for about 20 seconds.. these were the results:

STFT1 16.41
LTFT1 -17.97
STFT2 17.97
LTFT2 -15.63

Does this confirm a bad MAF?

You can’t just disconnect the MAF and expect it to run well. No it doesn’t

[ellzeez]

Haha yeah might be being a bit naive and clutching at straws, but trying to eliminate some stuff. Just going with something I saw on another forum. Didn’t change the way it ran apart from a bunch of gearbox warnings

[Cambo_oldjaguar]

You need to smoke test it to find the leaks.

However, the o-ring gaskets on the intake manifold, where it meets the heads, are known to deteriorate and cause vacuum leaks, common enough 4.0 problem.

[ellzeez]

You need to smoke test it to find the leaks.

However, the o-ring gaskets on the intake manifold, where it meets the heads, are known to deteriorate and cause vacuum leaks, common enough 4.0 problem.

These ones? 2005-2009 For Landrover Discovery Series 3 4000 Cologne Intake Manifold Gasket | eBay (https://www.ebay.com.au/itm/201510248443)

[Cambo_oldjaguar]

Yep

[ellzeez]

Welp.. took the MAF out, saw some greenish corrosion on one of the wires up in the belly of it and scratched it off with a pin. Also put a new air filter in and after a couple short trips the CEL cleared by itself. LTFT still showing slightly rich at idle but max -10%. Not sure whether it was the filter or MAF, but have a new sensor coming anyway