Why is my engine throwing lean burn errors?

July 3, 2019 3:33 PM

My '02 Toyota Corolla keeps throwing lean burn errors. It seems like my local garage can't handle anything without a Cray under the hood, so it looks like I need some help from you guys...

My fleet of ancient Toyotas includes an '02 Corolla SR with the 3ZZ-FE VVTI engine in it. Annoyingly, they weren't made in the UK like all other Corollas, and everyone from local garage to Toyota main dealer is confused by them.

A while ago it started throwing lean burn error codes. I can clear them in the OBD-II and the warning goes out until it next hits about 4,500 rpm (on a 6,500 red line). The engine doesn't go into limp mode, and the power seems about normal. I don't think it misfires (although I can't promise) although it seems to dip in power between 4,500 and 5,000.

What's likely to be causing the problem?

(Sadly all my googling brings up is talk about Toyota lean burn engines and people messing about with aftermarket induction mods).

posted by sodium lights the horizon to Travel & Transportation (13 answers total)

Misfire codes in my Camry were fixed with new wires and spark plugs, a relatively inexpensive parts replacement and possibly worth doing anyway.

Lean burn would be too much air for the fuel I believe, which would make me think of clogged fuel filter or marginal fuel pump. Reason thinking it's starving for fuel at higher rpms. I think the fuel filter should be cheap, but I don't know. But I am not your mechanic.

What repairs have been attempted so far?
posted by TheAdamist at 3:40 PM on July 3, 2019

Response by poster: Unhelpfully, I don't know. :/
posted by sodium lights the horizon at 3:54 PM on July 3, 2019

Checking fuel pressure under load would be a good diagnostic step.
posted by Larry David Syndrome at 4:10 PM on July 3, 2019

It sounds like it is time for a different local garage. Some garages are good at diagnostic work, but most of them seem to be ... more challenged by it. If you trust the one you're going to, ask if they can recommend someone that specializes in either older vehicles or better yet, older Toyotas. If you were in Minnesota I would have a recommendation for you, but I wouldn't help across the pond. Good luck.
posted by Gilgamesh's Chauffeur at 6:28 PM on July 3, 2019

When you rev the engine, the engine moves. A cracked intake hose might let unmetered air in when it twists in a certain way. I see on this engine it's long and makes two sharp turns. I think you should check all your intake for cracks, loose clamps, a badly sealed air filter housing, and so on. The whole intake must have no leaks, because all air that is sucked into the engine must go past the MAF sensor and through the air cleaner.

Also check your MAF sensor (it's on top near the air filter housing) and its wiring and connector. If a wire is broken inside, it might disconnect when it moves too. It might not hurt to pull the sensor and clean it. It's simple and just has a little part that hangs in the air flow to sense it. If that's greasy or dusty, clean it with some spray brake cleaner or similar.
posted by fritley at 7:35 PM on July 3, 2019 [1 favorite]

I don't know about the 3zz, but in the 1zz-fe (another fine VVTI four) this is often caused by a leaky intake manifold gasket. The original rubber ones harden and crack, but there's an orange silicone (?) replacement that holds up better.

But do what fritley said first.
posted by transitional procedures at 9:43 PM on July 3, 2019

Also, what specific codes are you getting?
posted by transitional procedures at 9:45 PM on July 3, 2019

Response by poster: Just the generic P0171
posted by sodium lights the horizon at 8:02 AM on July 4, 2019

I second changing the wires and plugs, fuel filter, and intake hose. That’s all cheap. If it continues it’s either too little fuel or too much air so check the fuel pump next (my instinct).
posted by jeffamaphone at 9:50 AM on July 4, 2019

Here's a better description of the IM gasket issue than my coffee-deprived brain could ever hope to write.
posted by transitional procedures at 12:03 PM on July 4, 2019

The fact that the code is thrown at higher RPMs seems consistent with a minor leak on an intake manifold gasket or some other component where there’s a pressure difference between intake air and ambient atmosphere. There’s greater vacuum at higher RPMs, which could break or exacerbate a weak or failing seal (whether at an interface between two components, or along e.g. fractured plastic in a single component). But I don’t know that it necessarily rules out any possible causes. Changing the fuel filter seems like a low-cost experiment, and makes a lot of sense if you’re overdue for that maintenance anyway.
posted by compartment at 1:03 PM on July 4, 2019

Bear in mind that the IM gasket issue happened on enough Corollas that Toyota issued a TSB about it.

I'm in a Vibe/Matrix group (which are themselves Corolla variants) and *every* time somebody asks about a p0171, that's almost always the answer that fixes it.

It's under $20 for the part, and even for an inexperienced wrencher, it's not a bad time. Plenty of YouTube videos on how to do it.
posted by transitional procedures at 12:15 AM on July 5, 2019

Response by poster: Right. The inlet hose *seems* okay. So I've got a new inlet gasket on its way. I'll report back when it's fitted.

Thanks all.
posted by sodium lights the horizon at 3:13 AM on July 5, 2019 [1 favorite]