Electronic Fuel Injection and the 22R-TE engine

Part 2: Modifying the stock EFI system

What the hell *is* an AFM?

In stock form, the 22RTE uses an air flow meter, aka AFM, flapper door, or vane-type meter, to measure the amount of air entering the engine. Please get this bit of terminology right, as an AFM is not a MAF (mass air flow meter), nor is it a MAP (manifold absolute pressure) sensor.

An AFM measures air flow through what is essentially a mechanical process, where incoming air deflects a flap in an enclosed space. More air coming in = more deflection of the flap. Crudely speaking, a potentiometer is connected to the flap, and as the flap moves, the potentiometer moves, and this shapes the electrical signal that is sent to the ECU.

To control the movement of this flap/aramture, the AFM is fitted with a flat-wound spring that is used to apply tension to the flap. This spring is held in place, and adjusted, via a plastic gear and a wire "holder". When this gear is tightened (turned clockwise) relative to the flat wound spring inside of it, it becomes harder for the door to open, and this translates to a leaner mixture (more air needs to flow in to move the door by a given amount, meaning theat the mixture is leaner relative to its starting point before adjustment). Turning the gear counter-clockwise has just the opposite effect, as the looser gear lets more air in relative to "stock".

    Remember:
    Turning the gear CLOCKWISE LEANS OUT THE MIXTURE.
    Turning the gear COUNTER-CLOCKWISE RICHENS THE MIXTURE

Generally speaking, each tooth of the gear is equal to about a 2% change in the transient mixture (with stock injectors).

Why would you want to adjust this thing?
The AFM is a curious device. It is simple to adjust, but difficult to adjust correctly. Most people seem to believe that their engines *must need more fuel*, so they quickly head to the AFM and begin dialing in more fuel...and then their engine bogs down and bellows black smoke and they wonder why it doesn't run better. In my opinion, the key to properly adjusting your AFM is go slowly and don't make the engine burn more fuel than it's capable of burning.

Two other rules of thumb for dealing with the AFM:

  1. **ALWAYS** mark the stock starting position of the gear. Since all adjustments are relative, failure to mark the starting position makes it impossible to know where you are relative to stock.
  2. If you don't know what you're doing, you're more likely to screw things up than make them better.

Things that improve the engine's ability to breathe, like a header, a free flow exhaust, and a cam, and, (most especially) more boost generally cause the engine to run leaner with the stock AFM setting, because more air is able to move through the engine on a given combustion cycle. Consequently, if you've made these changes, you can gain better performance and realize a better air/fuel ratio by richening the mixture by a few "clicks", or gear teeth. How many really depends on the exact nature of your mods, the size of your injectors,and your altitude above sea level, though I offer a few rough guidelines below.

One piece of advice that can't be overstated is to ALWAYS pull the EFI fuse for a couple of minutes after making any adjustment, so that the ECU has the opportunity to reset itself, and then drive around and see how things run. If the AFM is set too rich, the engine will bog down and mileage will suck. If it's set too lean, it will perform better up to the point where the in-cylinder temps get too hot and the mixture too volatile, and then you may experience detonation. If you experience ANY knocking or pinging, STOP and enrichen the mixture. So take small steps, and don't equate a rich mixture with better performance, as the engine can only burn so much fuel at a time. If you haven't made changes to the engine's compression ratio, head, cam, throttle body, or boost, your AFM adjustments are going to be modest at best.

Here's a quick cheat sheet of some possible AFM adjustments, based on some common modifications. Remember, this is intended as a rough guide, so use your own best judgement and always proceed with caution. I take *absolutely no responsibility* for anything you decide to do as a result of reading this page, and it will be SOLEY YOUR PROBLEM if you melt your engine.

The following are intended to be additive, and follow what I see as a common order of modification. Your mileage may vary **greatly** from what's listed below!!

  1. If you've added a free flowing exhaust and a K&N (or similar) filter, you can probably adjust your AFM 1 tooth richer.
  2. If you've changed cams, you can probably adjust your AFM 1-2 teeth richer.
  3. If you've installed a big bore throttle body, you can probably adjust your AFM 1-2 teeth richer.
  4. If you've ported and polished your head and installed bigger valves, you can probably adjust your AFM 1 tooth richer.
  5. If you've modestly increased boost, you can probably adjust your AFM 1-2 teeth richer.
  6. If you've added an intercooler and aggressively increased boost (say, in the 13.5 - 15 psi range), you can probably adjust your AFM 2-3 teeth richer, although you may hit the ECU's fuel cut parameter.
  7. If you've added an intercooler and a bigger turbo, and are running over 12 psi of boost...you had better get some bigger injectors in that puppy, as you won't be able to increase the fuel delivery via the AFM enough to meet demands without hitting fuel cut!

The AFM looks like it's really restrictive...is there anything I can do to make it flow more air?
For many engines, an AFM is a perfectly adequate, albeit old fashioned, way of measuring the air entering an engine. It is fairly simple, it is reasonably accurate, and it works well much of the time. But it is restrictive. On a turbocharged engine, any restriction seems to become magnified, because the restriction literally compounds itself as engine load increases.

At some point (usually after you've fitted a bigger turbo and have cranked the boost up to around 15 psi), the AFM is simply flowing as much air as it can -- the opening is too small, you're trying to draw in too much air, and the meter door is as open as wide as it's going to go. This is called saturating the meter, and it is a true limit on engine performance: your engine would be capable of burning more fuel if only it could get a sufficient amount of air into the cylinders...

So, what to do? There are essentially three approaches you can take at this point:

  1. You can fit a bigger AFM from another vehicle.
  2. You can fit an AFM replacement device, like the Link ElectroSystems "AFM Link" or the Split Second MAF kit.
  3. You can dump the whole stock EFI setup -- AFM and ECU -- and replace it with a standalone, MAP based system, like those made by SDS, Haltech, Electromotive, SpeedPro, Accel/DFI, Motec, etc.

Let's look at the first of these options (the other two will be dealt with in Part 3: Aftermarket EFI systems):

Installing a bigger AFM
Luckily for 22RTE owners, Toyota made AFMs in two sizes, and for some reason, the 22RTE was fitted with the smaller of these. Swapping over to a larger AFM isn't particulary hard, but it does require some modifications to the new (and possibly old) AFM.

Obviously, in order to put one of the bigger AFM's into your 22RTE, you will need to first acquire one. The bigger AFMs can be found in most junkyards fairly cheaply. The one I used came from a 5M-GE Supra, but they can also be found on Cressidas and 3SGE MR2s, too.

There are a few nice bonuses about getting the AFM from a Supra, though: the Supra AFMs have a built-in 82 MM cone filter adapter ring, so you can put a free-flowing air filter on without having to buy another adapter. Additionally, you can also pilfer the two hoses on either side of the AFM: the short 45° angles hose, and the slightly longer (5"?) straight hose. You will need one or both of these hoses to mate the bigger AFM to the stock turbo hose, although this might be a good time to rectify that situation , too.

OK, I've got the larger AFM; now what?

Even though Toyota essentially used two sizes of AFMs, they custom tailored the electronic "traceboard" or resistor ladder that the AFM's armature skates over for each application. This means that your new AFM is not plug-and-play.

To make the larger AFM work on my engine, I removed the 5MGE AFM's traceboard and replaced it with the one from my 22RTE. I had a couple of reasons for doing it this way: the 5MGE AFM puts out a signal that is the inverse of the 22RTE AFM's signal (greater-to-smaller voltage instead of smaller-to-greater voltage), and I had been told that the 22RTE AFM signal was non-linear, and had a "hump" in it to accommodate boost. I don't know if the last point is true, but the traceboard swap worked fine for me, and my air/fuel ratios (and performance) have been very good. This is the "crude" method of doing this swap. Someone one the Old Celica Club mailing list did some research and has come up with a more sophisticated method of performing this modification, which I list here. Please don't ask me questions about his method; if you use it, you're on your own ;-)

Once you've swapped over the internal electronics, you have the matter of plumbing up the new AFM to the old system. Here's where I found the pilfered 5MGE hoses to be helpful. I literally stuffed the old hose into the new hose and sealed with a clamp and some silicone sealant. Not good enough for NASA, but it worked OK on the street.

A few interesting notes: The 22RTE AFM's traceboard was designed to provide a signal to the ECU to fire four (4) 295 cc/min injectors at 6 psi of boost, for a total load of 1180 cc/min (@100% duty cycle). The 5MGE AFM was designed to provide a signal to the ECU to fire six (6) 195 cc/min injectors, normally aspirated, for a total load of 1170 cc/min (@100% duty cycle). In other words, these two AFM's are each designed for virtually identical fuel volumes, so it's interesting to me that Toyota spec'd the smaller AFM for the 22RTE.

One of the historic problems with the 22RTE is that, as you increase boost, you raise the likelihood of hitting fuel cut with the stock ECU. Because the 22RTE ECU is such a crude device, it uses only two parameters to determine fuel cut: injector pulse width and throttle plate angle. If injector pulse width is too great, and throttle angle increases too suddenly, you will trigger fuel cut. One of the tricks that I discovered for fooling the stock ECU is to fit much larger injectors and tighten the tension on the AFM's spring. This has the effect of allowing you to deliver more fuel in less time, at greater boost, without triggering fuel cut. By using the larger AFM, you can let in more air for a given AFM vane-angle than you can with the smaller AFM, which makes this trick work. However, and this is important, tightening down that spring also limits the amount of air coming in, so you have to strike a balance between injector size and spring tension. I've been running 470 cc/min injectors and fairly high spring tension, and I believe that 420 cc/min injectors and less spring tension would be better. If you didn't have to concern yourself with fuel cut, you could run even smaller injectors and take further advantage of the bigger AFM. As I said, it's something of a balancing act to get this right.

Still under construction...more to come :-)

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