[Solved] Mass Air Flow Sensor - Bank 1 or Bank 2 Running Lean

Okay. I'm in the process of checking for vacuum leaks. When I first started the cold engine, I could (at least I thought I could) hear a slight "whistle", or singing. Hard to tell because the engine rpms were around 1300-1400 at the time, and I couldn't hear the whistle very well. The whistle/singing would come and go. When the engine had warmed up and the idle had dropped to around 850 rpm, I couldn't hear the whistle anymore, so I began spraying starter fluid here and there. At first I sprayed about 1 to 2 seconds worth, later I cut that down to short, 1/4 to 1/2-second spurts.

When I sprayed behind the throttle body where three rather large vacuum hoses were, the idle speed increased noticeably so I concentrated my efforts there.

There is a vacuum large (1/2 inch or so) hose that comes straight out from the front of the throttle body near the bottom. It makes a 180 degree turn, goes underneath the throttle body to the back of the engine, then dives down to a pipe next to the firewall. It is held to that pipe by a clamp. Spraying around that clamp area did not increase the idle speed. Spraying where the hose leaves the throttle body did increase the idle speed, but only a little bit. When I pulled on the hose it did not get disconnected from anything. But as near as I could tell, the leak must be somewhere around there.

When I replaced the injectors, I had to reuse the new plenum gaskets I had installed, but I don't think those are the problem.

Spraying elsewhere gave these results:

1. All around the junction of the upper and lower plenums: no change in idle speed.

2. In front of the throttle body and around the outside of the throttle body: idle speed increased somewhat, but at that time I was spraying between 1 and 2 seconds, enough time for the ether to be sucked in by just about anything.

3. Behind the throttle body and in the injector area: idle speed increased noticeable.

I can't do any more until the engine cools off, but I thought I would post my progress so far. When things are cooler I'll pull the throttle body and take a good look at the hose that passes underneath it.

 

You said "upper and lower plenums" so I will guess it's 3.4.

If so, the t-body, specifically the IAC mounted to the bottom will have three hoses. The upper two smaller hoses including the u-turn one are coolant hoses. The third larger hose, slightly lower and coming off and running to the rear is a vacuum line. It splits, enters both sides of the lower intake and eventually shoots at each injector. If it has a leak it could account for a lean condition, maybe even the slightly high 850 rpm idle. This hose could easily have been dealt some trauma in the plenum r&r process.

More on that "air assist" hose, and what it does in this recent thread.
https://www.tacomaworld.com/threads...stall-diagnosis-3-4-5vz.616967/#post-20989443
Getting back there and dealing with that hose, the plastic tee and it's two branches on an assembled engine is a wrist/knuckle skinner to say the least, but it can be done.
There is also a small vac line on the rear of the t-body in close proximity to the air assist line that could be catching some of the fluid. The shiny plated circular device it's connected to on the t-body could also be leaking but that's just theory. Never seen/heard of one leaking (called the Throttle Opener).

 

Where did you get the injectors?

 

check the easy thing first... the brake vacuum booster. When you get in in the morning is the pedal hard or is there still vacuum reserve from the night before?

Did you use toyotas metal gaskets for the upper/lower intake and lower to intake manifold? Anything less has a tendency to pull through.

 

I bought my injectors from:

MotorWest Performance
7851 Alabama Ave, Ste 16
Canoga Park, CA 91304
1-747-888-9768

In the morning before I start the engine I always press the brake pedal, and it is always hard. Also, I checked the connection by spraying starting fluid on the vacuum hose connection to the power brake thingy and to the other end of that hose.

I used new gaskets from Camelback Toyota.

I just finished checking underneath the throttle body. The image below tells what I did:

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When the engine was running, it seemed to be much more sensitive to the starting fluid when squirted near the hose to which I added a clamp.

So there it is. I put everything back together, started the engine, and did not hear the whistle/singing that I heard before. I cleared the codes and will report again when and if the P0171 code returns.

 

You've done a NICE job of documenting and researching this issue. I think you found the vacuum leak. Here's a vacuum hose diagram that I used to order hoses for my 1999 model. Near the center of the picture, there are the 17341T, 17342L, 17343H, and 17343H vacuum hoses with the tee that I think @Dirty Pool references in his post. I just replaced them all. Camelback can help you get those hoses based on the part number prefix.

I've had cases where specially shaped vacuum hoses leak vacuum that can't be substituted with a regular hose, in those cases I'll wrap the hose in "Wrapit Repair Self Fusing" silicone tape. That seals the hose until I can get a new one.

 

Nice! The ol brake cleaner/starting fluid spray trick is great for vacuum leak detection. Thank you for documenting everything! I hope you are good to go.

 

I’ve got a similar issue it started with a lean code because of long and short term fuel trim then it turned into misfires on all four cylinders (3rz). I’ve replaced plugs and wires; new iac, fuel injectors, fuel filter, tons of vacuum hoses. The lean code came immediately after installing a K&n cold air intake. Initially I had a throttle body spacer in there but it would hardly run with it installed. Anyway I just replaced the iac gasket tonight and I still am experience the same thing you test for. When I spray carburetor cleaner on the iac it get. 200-300 rpm engine surge. I can’t figure out where the leak is.

https://youtu.be/F7c3Bc3OclQ

 

Compression test?

 

Steeprock, disconnect the decel valve vacuum hose, plug it and spray again... the diaphram could have a small split.

 

Well it’s embarrassing to say but I think it was that I had the throttle body gasket turned the wrong way and it was leaking like a sieve along the bottom. Here is a before and after video.

https://youtu.be/uQE_uxbSdw4

https://youtu.be/VHTVY_-breo

 

Can you explain the function of the valve; I got the big leak fixed. But now I am having very hard starts.