Warning long post: For a year I've been trying to cure my intermittent rough idle and poor shifting/acceleration issues. My NAS 2003 Disco II w/ SAI has 87k on it. I have replaced all of the following: Plugs and 8mm magnecor wires, new Bosch MAF, new alternator, new hoses, water pump and viscous (engine runs at perfect temp at all times, no leaks), new serp belt and all pulleys, new PAS. I have cleaned the stepper motor and tested and cleaned the TPS, cleaned the purge valve and the breather valve, drained and refilled the tranny w/filter change, I have stripped down the top of the engine and replaced both intake gaskets, valve cover gaskets, numerous oil and filter changes (oil always comes out clean), I've done a sea foam treatment twice.

Never have thrown a code but still had intermittent rough idle (usually when warm) and the intermittent acceleration issues (intermittent because sometimes this thing runs like a rocket, even when warm and has power like a brand new 4.6) so I decided to just bite the bullet and replace all 4 O2 sensors. It ran great at first but after awhile started having the same symptoms. Then I realized what went wrong - months earlier, I stopped shy of replacing the valley pan gasket even though I knew it was leaking as it just seemed too hard for my skill level. Yesterday I realized that leak was sending oil into the top of the RH front O2 sensor plug. I opened up the O2 connector and sure enough, filled with oil. I blew it out, cleaned it with electronics cleaner, it is now bone dry and clean. I rerouted the cable so it is no longer in the path of the leaking oil from the valley pan but upon restarting and running it, I get a continual P0150 code after the truck gets to operating temp. Note: The P0150 comes up as 'pd' a 'PENDING' code each time, even after I erase it. But the truck runs pretty damn good - not perfect but a smooth idle most of the time.

So two questions:

1) Even though I cleaned this brand new O2 sensor could the oil have shorted it out permanently, thereby triggering the P0150? How would I test for that, short of replacing the sensor? (I did disconnect it but got another code, P0155, I think - but actually the truck ran surprisingly well).

2) When replacing the O2 sensors do I have to reset the adaptives with a Testbook to really reset all the codes rather than just my generic scanner? (I can erase the code, but it always comes back.

Thanks in advance for any help!

 

Clean the connectors with contact cleaner and use die electric on them, then clear your codes and you will be fine.

 

Thanks, Mike. Will do.

Update: Recleaned connectors with dielectric grease. Codes came back within 5 minutes. I am now going to swap the sensors to see if the codes pop up on the other side. If they do, then it's the sensor and I'll replace it.

 

I just replaced the RH front oxygen sensor with ANOTHER new sensor thinking the valley pan oil leak fouled it for good (please read my first long post above) and the brand new oxygen sensor is giving me the same codes!

First I get:

P0150 Stoichiometric ratio outside operating band
P0174 Multiplication fuelling adaption (Max.) exceeded rich limit ‐ RH bank
P1174 Additive fuelling adaption (Max.) exceeded rich limit ‐ RH bank

I clear all those codes, then in about five minutes the P0150 comes back as a pending and eventually so does the P0174

When I removed the Rh Front O2 sensor to replace it (mind you this Oem Bosch sensor was new 2 weeks ago) the tip was covered with black soot. When I replaced my original sensors two weeks ago, the tips on all of them were fairly clean. I regret replacing any of them now, as I never threw any of these codes, but I was searching for anything to fix my rough idle.

I am totally stumped. I have checked thoroughly for vacuum leaks. None. As listed above, my MAF is factory Bosch and new, my purge valve is clean, my stepper is clean, the O2 sensor top connectors are clean and all 4 O2 sensors are new.

I started having these codes about a day after I replaced both rear O2 sensors with Bosch factory sensors.

Any ideas? I have rough idle because of this and I smell fuel at idle. What can I check now?

 

Leaking fuel injector?

 

How would I check for that? Remember, my car runs pretty strong once I hit 1500 rpm. When I say I have poor acceleration I mean it feels like the tranny is shifting early, almost like the timing has been retarded to some default mode because the ecu is getting a lean reading.

 

It WAS the injectors. My local mechanic performed a full BG Fuel injection system service (disconnecting the fuel pump and pumping the BG straight into the engine). No more codes. Disco runs like a new vehicle. I highly recommend this treatment over simply pouring the product in the gas tank which I tried twice and it did nothing. This is a must do for any high mileage RR or Disco IMHO. Cost me $150 and worth every penny. I have been trying to solve this issue since I bought the car. Should have tried this first. Idles smooth like a Lexus now. Acceleration is incredible and MPG is way up. I'll probably add a can of the BG 4400 to the tank from now on at every oil change just to make sure the injectors don't clog up in the future.

 

I just want to thank you for this post. I have a 2003 disco and i was getting the exact same codes and having the same issues. Thanks to your post and constant updates you have saved me alot of money and time. Thank you so much

 

Ever smell that BG stuff? Its like the most toxic smelling stuff on the planet, If it smells that toxic its gotta work good.

 

Great to hear you found the problem. I would not guessed that fix. I use BG44k every other oil change or every 3 month, which ever comes first. I'm curious what brand of gas did you use? And have you always run premium (91 octane)?

I may attempt the manual BG44k delivery when I replace my fuel pump top seal. I bet it does not take long for a V8 to burn through that small can