Hey forum members,

As the title reads I lean codes for both cylinder banks. I am taking the truck to my mechanic (reputable, know him well, only works on LR) Monday.

I wanted some advice as to how to communicate to him most efficiently.

Issue/ attempted fixes:

- Have has the codes since I bought the truck, about 6 weeks ago. The motor runs smoothly (98’ P38 4.6 in an 04’ Disco), has just had HG’s done, starts up easily, revs beautifully. Idle is a tiny bit rough. Does not overheat at all, temps are perfect. No coolant or oil loss. IACV is working properly. I can hear a exhaust leak type noise (subtle, only noticeable in the truck Or under the truck, not from engine bay). It’s highly possible there is another exhaust leak - is it likely for this to cause these codes?

- Fuel MPG is worse than it should be. Can barely manage 12 with mostly highway driving on a tank. Manually calculated. Usual MPG is around 11. Since my commute is really short the truck is only at operating temp by the time I arrive - these MPG numbers are from longer trips, not daily driving where the truck will certainly run rich while style cold.

- Fixed an exhaust leak at the manifold, and replaced the center muffler which also was leaking. I am confident there’s another exhaust leak due to the noise, but again, could this be solely responsible for the codes?

- Tightened up all hose clamps and easily accessible spots that could cause a vacuum leak.

- Have NOT done a smoke test. Therefore I have not eliminated something in the intake tract as the issue entirely.

- Truck had bad fuel in it while it sat before I bought it. Ran rough until the fuel was pumped out and replaced with good fuel.

- PO suggested doing the injectors due to this. I have a set of reman Bosch injectors. I do not have time to install them myself, hence the mechanic.

Heres the question - What information will be most helpful to provide my mechanic with?

He’s not concerned about my Mpg’s. He daily’s a disco himself and doesn’t drive it out of town much. I stressed the fuel mileage being poor as a way to say that the fuel system might not be functioning properly, and he dismissed it somewhat.

How can I stress that to him so it does not Come across as a complaint about mpg’s but a helpful point for diagnostics?

I am aware a vacuum leak is the likely cause - but will a vacuum leak cause reduced mpg’s? To me it seems likely the injector seals were hurt by the bad fuel, or maybe it even now has a clogged or damaged injector. Thoughts?


If you’ve read this far, thank you.

 

A vacuum leak will absolutely cause gas mileage to be poor. Think about it -- if the ECM is seeing exhaust gas readings that are too lean it will dump more and more fuel into the system to try to correct the lean condition.

As for what you should tell your mechanic, why not tell him everything you shared in your post? I think the key is that you have not done a smoke test. The problem is very likely on the intake side. And yes, it could be the injectors. Others have reported that leaking fuel injector O-rings admit unmetered air in the system that causes the lean codes.

 

Thanks @mln01, I will put together some notes just like in my post for him.

I have read that some folks have had a brake booster issue causing the same codes. Very interesting.

 

I had similar codes and it ended up being a bad Mass Air Flow unit. Good luck on getting it sorted out.

 

@MrHSE7 males complete sense. I had the same codes with the original MAF. Clean it with MAF cleaner, no change. Bought a used Bosch MAF and same codes still appeared.

I went ahead and bought a new Bosch MAF to ensure that is not the issue.

I am fairly confident it still has a small exhaust leak. Does anyone know if that will throw these codes? I imagine it would have to be leaking on both sides to get the codes for both cylinder banks.

 

1st step is to determine if it is actually lean, then determine why.

1. Elm 327 with torque app - chart the 02's. They should start near 1 v on cold start and then start cycling. Lean codes suggest they are going full lean. However 02's fail indicating lean, so it could be bad 02's.
   1. If it does as I described above, it is actually lean. Given it is both banks, that suggest big vacuum leak or bad maf. Since you have changed the MAF, guessing that is not it. Could be low fuel pressure. You can check that at the rail at the back of the manifold. Could be a leaking injector o-ring, or an injector not opening, but it woudl have to be one on each bank which is improbable. If it does this post up and we can go from there.
   2. If it does not do as above, it is not actually lean, so likely bad 02's. Disconnect both O2's to see if it cures the lean codes and goes to no signal codes.

 

I would assume that since you are getting the lean code on both sides it's not an exhaust leak (unless you have a leak on both sides). Most likely something more upstream. As member Extinct noted above in #1

As I said previously my outcome was a bad MAF but here is a list of some common issues I've found while searching these codes that come up.

• Leaking fuel injector o-rings
• Loose upper manifold bolts
• Vacuum leaks
• Bad O2 sensors

Im no expert but wanted to share my experience chasing these codes.

 

Awesome info @Extinct and @MrHSE7 . Thank you.

@Extinct, here are some responses to the questions you posed.

I did not mention this before: The vehicles runs very smooth under load, but once it warms up the idle is a little rough. The motor shakes a little more than you would expect. Smooth when cold, smooth when at RPM, but a little rough at hot idel. You can feel the motor rocking a little through the seat. It's a V8, and older truck, so some of that is certainly normal, it's not a straight six or V12 that are naturally balanced, but it's not as smooth as it should be.

• O2 sensor Voltage seems normal. Using an Ultraguage I can track Voltage and they work as you described - start near 1V and then cycle.
• My mechanic had the truck about a month ago. I have two new O2's and I asked him to check O2's and replace if in his opinion, they should be. He came back with the same results I did. They appear to function normally and recommended not replacing them.

Which O2's do you recommend unplugging? I assume the upstream O2's, but they seem a little tricky to get to to unplug. I think this is my next step in diagnosis.

• @MrHSE7 Big Vacuum leak is not out of the question. I tightened up the bolts on the upper intake after I found one was only hand tight. They all needed tightening, I tightened them and codes came right back.
• I replaced the hose clamp on the intake elbow/throttle body connection after I realized it was not tightening enough to create a good clamping force. Again, codes came right back.
• Since the truck had bad fuel in it, I suspected the Injectors and/ or injector seals might be the issue. Not sure if that is off base or not. I bought a refurb set of Bosch injectors just in case ($70, why not have them just in case?). I have not installed them - and likely won't dive into that on my own.
• The PO had the motor out of the truck replacing motor mounts (and doing HG's) a few weeks before I bought it. I think the issue is most likely related to motor reinstallation and not necessarily buttoning everything up perfectly. At least Murphy's law would tell us that, right? If something was touched, that is more likely to be the problem than something that was not.
  • Another likely cause is a combination of things. Maybe an injector seal is bad on one bank, and there is an exhaust leak on the other? Would this cause the codes?
  • There is a subtle exhaust leak noise, I am confident in that. I've heard too many exhaust leaks in my life... haha

I can do a better job of tracking and documenting O2's readings and post them here if we think that is helpful. Thanks again for all the thoughts. It helps having feedback to get all of this organized!

- Andrew

 

I would like to see the oxygen sensor graphs, cold and hot. Yes, I am thinking unplug the upstream sensors. The ECU will default to open loop programming using only MAF sensor, ECT, and rpm for fuel and timing. That should allow the engine to run smooth if a little rich. The disconnected sensors will also rule them out from being the problem with the engine runs the same.

 

@Extinct here's my plan.

- Take pictures, maybe video, of O2 sensor readings while plugged in. Post those here.
- Unplug upstream 02's and monitor to see if engine runs the same. If it does, this rules out O2's as the issue. Do I need to monitor O2 voltages with them unplugged? Assuming not since they will be 0?

I received a new Bosch MAF today. I will install it before doing these tests to rule that out completely.

How does that sound? Thanks guys.