I've been trouble shooting a rough idle with my 2006 LR3, and I am puzzled. Here's the facts:

• starts up fine, but when idle drops below 1,500 RPM on startup it starts to stumble, and idle is choppy between 650 RPM and 750 RPM.
• voltage drops from > 14 to >13 voltes in moments of "idle stumble".
• fuel trims show stability on bank 1, but are somewhat elevated / more variable on bank 2. Not high enough to throw a lean code.
• MAF sensor checks out ok and a swap shows no change.
• no CELs / error codes of any kind.
• extensive smoke testing reveals no vacuum leaks. Changed bank 2 valve cover gaskets anyway because of an oil leak, but no change and a follow-up smoke test continued to reveal no vacuum leak.
• swapped out throttle body with a spare; no change.

I'm left with a decreasing list of possibilities:

1. a strange partial failure of the bank 2 O2 sensor that is enough to through off the idle.
2. a clogged injector in bank 2.
3. other guesses: MAP sensor issue?

My suspicion from here is number 2, a partially clogged injector. I happen to have a fresh set of intake gaskets and injector seals, and am thinking of just pulling the intake and hand testing each injector using intake/valve cleaner and an injector testing apparatus I picked up on ebay year's back when I did the head gaskets on a Jaguar X350.

Any thoughts on other causes, or ideas for things to try? Thanks,
- scooter

 

Quick update, I'm going to pull the intake manifold and clean and reseal all the injectors. I put in about 30 minutes before work today and undid the fuel line, pulled the electrical connectors out of the way and would have pulled the full manifold but I want to blow out the engine surface with compressed air before I do that. And ordered some new knock sensors. While the LR3 intake manifold is easier to pull than on the supercharged AJV8 (a job I've done 3 times and can do now in about 3 hours, first time took me probably double that), once I have it off the knock sensors are right there and the plastic sensor housings do crack and fail over time, I might as well replace them. And with the manifold off I can also replace the valley coolant hose.

Anyway, I hope to have another update -- hopefully positive -- this weekend.

 

Did you clean the throttle body?

 

Yeah, cleaned it and swapped in a new one. No change.

 

OK sticking with the easy / cheap tests:

1. What do the spark plugs look like? How old are they? How many miles on them?
2. Try swapping the coil packs around and see if it clears up. A coil/plug combo change may make a difference.

You have done most, if not all of the things I would have done, so I guess it is time to start looking at the less obvious.

Jeff

 

I'll check, I haven't done it yet only because I'm not getting misfire codes and they are ~ 3 years old with around 20,000 miles.

Yesterday I pulled the manifold. All in probably took me 70 minutes to do, and the hardest part was getting the MAP sensor electrical connector off the back without having eyes on it.

I ran out of throttle body cleaner so I haven't cleaned and tested the injectors yet but here's a picture of my setup. The injectors all remain seated on the fuel rail and I will tape the cleaner spray to the fuel input port. I have an injector pulse tester from ebay wired to an injector connector (same connector as with Jaguar's and I have some spare from repairing a Jaguar wiring harness a few years back).

Intake ports look on the dirty side of things so I might try to clean out some of the carbon before everything is reassembled, but we'll see.

 

Wow, I took too long to respond, you already have it torn down.

I suppose you did a lot more investigating before pulling the intake such as fuel rail pressure, fuel trims, O2 sensor voltage, and more.

I was wondering how brown the plugs were. This may have lead you to cylinder #2 injector before the teardown. This could given a clue as to how lean it is running. You may have headed down that rabbit hole thinking lean without looking at the plugs. No worries, I think you may heading in the right direction.

Since you are this deep into injectors, you may as well get them cleaned or replaced I guess.

Jeff

 

I got impatient with the issue unresolved, and the fact that I was unable to replace the valley coolant hose when I have replaced every other coolant hose in the truck was bothering me. So off with the intake. It is a much easier job than on the supercharged platforms, and next time I could probably do it in 45 minutes now that I know how things route and out the plugs / tubing is undone.

Your suggestion to pull the plugs for lean condition is a good one, I'll do that in the next day or two. I have to wait until the weekend for the new knock sensors so I'm not in a rush. Thank you.

 

Pulled and inspected the injectors this weekend. Bank 1 looked ok, but bank 2 was much more messy. Clear signs of oil/carbon buildup around the injectors, some of the pintle caps were split, definitely possible that that was an air intake point. I should have everything cleaned up and reinstalled by next weekend.

 

Update... problem has been most likely identified but not quite resolved. It appears it was.... the alternator. I know, wasn't on my list of problems.

I have come to this conclusion after having replaced the intake manifold seals and gaskets, the MAP sensor, and all fuel injectors and seals. I pulled the bank 2 spark plugs and they looked pretty good. I also took the opportunity to replace the knock sensors and the valley coolant hose I had skipped before for lack of access.

The idle stumble remained, and as it stumbled the voltage dropped. I had seen this before but I had thought that this was an 'effect' and not a 'cause'. Well, on startup today the voltage stopped dropping intermittently, instead it dropped right to 12.0 while running and the battery warning came on.

My alternator has less than 1k miles on it, I suspect it has a bad diode from the manufacturer. The good news is... the one I removed was actually fine (just a lot of miles on it) and I had replaced it because I had all the coolant hoses (save the valley hose ) out of the car, so... I'll swap the old one back in.

I did get a peak at my fuel trips with the new seals and injectors, and they did look much better. Short term trims were pulling 7-8% fuel from the mix, so I do believe the new seals and injectors either solved mild vacuum leaks or improved the injector flow.

Anyway, I'll post some picks and do an update once I can swap the alternator. At least that's not too bad a job on the LR3, though I do miss this job on a bug-eye WRX where the alternator is sitting right on top pretty much.

Edit: I do think I'm closer to resolving this but I suspect the alternator was just coincident vs. related. I swapped in the old alternator last night and voltage is back to normal. I finally threw my first error code, a cylinder 8 misfire, which gives me something concrete to run down. When I have time tonight I'll swap ignition coils around and see if the error travels.