How much would an intake leak affect mpg?

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Twinkie, Twinkie, Cupcake
Jul 4, 2010
I've been getting 11.5mpg pretty consistantly since last summer. Last spring we bolted on my ARB and I leveled the truck with 30mm spacers and a torsion bar re-index, and I was still seeing mid 13s - and that was with my old 305s that were dragging on the steering knuckle. I was having a tough time figuring out what could have caused a 2mpg drop over a year's time, so I dug into the truck today.

After checking the usual stuff, I found that it needs a PCV valve, it could probably use new spark plugs, and the throttle body gasket has a little corrosion and is deteriorating. Other than that, the top end is in really, really good shape. What surprised me was that the intake tube that attaches to the outside of the TB was completely loose. It all but fell off when I unbolted the upper airbox assembly from the engine. I must have forgotten to tighten it back up when I was messing around with it last year. How much of an impact would that have on AFR/fuel trim/etc?

The TB itself was pretty clean except for a little carbon behind the butterfly, and the inside of the intake manifold was, incredibly, completely spotless (these are CLEAN running motors!). The only thing out of whack was the loose intake what say ye, gurus of 'Mud? Unmetered air getting in past the MAF = jacked up mileage? Truck runs fine so I'm not sure how much air was even getting's not like the intake fit is all that sloppy to begin with.

For now it's all tightened up, but I'll be doing the plugs, oil, TB gasket, air filter and fuel filter next weekend. Time will tell from there, but any thoughts in the mean time would be appreciated as I only drive ~5000mi/yr right now and burning through a tank of gas can take a painfully long time.
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I've been reading lots of conflicting information about this, and it seems that the results could be very different depending on the type of vehicle and how the ECU handles fueling.

I've heard everything from "the vehicle will run lean due to the extra air in the mixture" to "the vehicle will run rich from the computer compensating" to "it makes no difference".

Does anyone have an empirical data on this, or have any idea how the ECU on the land cruiser deals with this type of stuff?

Air getting into the intake behind the MAF will definitely give you worse mileage.
Any reason, aside that it may be your only set of wheels, that you don't just take everything off and thoroughly clean everything up, new plugs, wires, gaskets, clean out the throttle body, etc?

Just a thought...
The answer is that it depends.

The car will initially run lean before its feedback loop via the O2 sensors will effect long term fuel trims. Depending on the size of the leak, the fuel trims may be able to adequately adapt as in your case and not throw a CEL. Once the long term fuel trims stabilize, the car will stay reasonable happy.

Happy does not mean optimal though. The nature of the leak is unlikely linear and impacts the various fuel trims across the power band and transient conditions differently. The ECU will err on the side of rich generally however.

BTW, I tune piggyback and standalone ECU's for turbo Lexus cars, which have much in common with OBD-II systems in toyota's if you're wondering. ;)
Regarding running lean: I've burnt out a couple of catalytic converters on my last expo (F350 CrewCab) from it running too lean.

Just saying...
Any reason, aside that it may be your only set of wheels, that you don't just take everything off and thoroughly clean everything up, new plugs, wires, gaskets, clean out the throttle body, etc?

Just a thought...

That's actually what I was doing when I discovered the loose intake tube - I was inspecting everything to see if something was amiss since my mileage is so crappy. New plugs, TB gasket, fuel filter, air filter, PCV valve and serp belt are on order from Rock Auto. It should all be here by Saturday.

My question wasn't necessarily about fixing the intake leak, that's easy enough. I was just trying to figure out if the loose intake tube could have some kind of direct corrolation with my piss-poor gas mileage.

TecKis, thanks for chiming in! That's the info I was looking for. I kinda figured my hunch was correct but with so many different stories floating around out there, I had to ask. Some of the Camaro/LS1 guys actually reported an intake leak causing INCREASED gas mileage due to the lean condition, so that's where my confusion came in. Clearly, 'Yota ECU's compensate differently than GMs (probably to help save downstream components such as cats, like R2M was mentioning). Thanks again.

As an aside, I ordered a cheapo Purolator clearance fuel filter ($3) along with everything else, and plan to add it to my power steeing return line after I change the fluid. Depending on how that works out, I might do the same with the trans (but with a better filter).
Update: Wow.

Alright - I cleaned the TB/MAF, ran a little seafoam through the intake manifold, replaced the spark plugs and air filter, and made sure that everything was buttoned up tight when finished.

The result? 15.4mpg mixed! That's the highest I've seen since buying the truck almost 2 years ago.

Based on the fact that most people don't report huge gains when replacing the plugs (and the old ones weren't horrible), and the air filter wasn't horribly dirty, I'm operating under the assumption that the intake leak was responsible for my low mileage. All in all, I'm very happy.
Just saw this I was thinking Sr. Gringo had a similar leak which he fixed with good results.

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