I managed to fix an almost unsolvable problem by doing water injection for combustion chamber carbon cleaning. This is harder than it might sound, because the design of the intake manifold, plus the tilt of the motor, makes injecting water precisely and evenly almost impossible. The intake manifold has a tub on the bottom that holds whatever fluid you are trying to inject, releasing it unpredictably, so intake injections like water or Seafoam spray will have unreliable results.

The solution is to tap the intake manifold with pipe threads, so water can be applied directly to each intake port.


The intake runners are clear from the intake port up to the top, so any point from the intake port up to the horizontal area at the top will do. There is a bunch of junk in the way, wiring harnesses, the ECU, the fuel rail, so you have to pick your tap locations carefully. Do the first stage of wire removal (unbolting the harnesses) and mark locations that would be good for the holes. Closer to the intake is probably better, all else being equal.

It is almost certainly necessary to remove the manifold to tap the holes. While the aluminum alloy shavings might go through the motor harmlessly, it is not a chance i was willing to take. Lift out the intake manifold with the fuel rail still attached.

Before removing the manifold, clean the gap between the cylinder heads and the intake manifold. Hose it out with a strong jet of water. Mine had trapped sand and debris, and I spent quite a bit of time cleaning out the intake ports after removing the manifold.

I tapped 1/16 NPT threads. The aluminum alloy is soft and chippy, and is very easy to work. You definitely do not need HSS; carbon steel will work fine. Probably the biggest hassle was getting the tap to go in straight; the material is thin enough to only carry a couple of thread pitches. I used a step drill bit up to 1/4", fearing that the aluminum-magnesium-zinc allow would prove brittle. The thinness of the manifold material is part of why it is important to go with the smallest possible NPT, so the thread pitch allows multiple threads in the tapped hole.

Turning the tap is awkward; I used a 1/4" socket extension backwards.

1/16 NPT plugs are readily available. I got a ten pack of stainless from Amazon for ten bucks. They have a 4mm hex head.


I tapped deep enough to get a couple of threads emerging inside the runner:


Then clean up all the metal shavings, install the 1/16" plugs, and it's off to the races. Be extremely cautious when installing the plugs; the manifold material is thin and soft. Just snug should do the trick.

Watch out for the vacuum solenoid switches on the front. The tubing fittings on them are fragile. I broke one and had to band-aid it in place.

A 1/16 NPT x 1/4" barb fitting allows the water injection to be remote. The fitting needs to be at best finger tight when installed. I chose 1/4" tubing because I liked having air mixing in with the water to visually identify the rate at which water was being injected, but I suppose other sizes would work well too. It is also a large enough diameter to allow the hose to be used to twirl the fitting in.

 

It is amazing how much water can clean up the inside of a motor. I use water/methanol injection in one of my other cars, and the intake manifold looks clean enough to eat off.

 

Running water injection at idle was very slow and questionable efficacy. The system that finally worked well was powered - to provide water injection at full throttle, with zero manifold vacuum - and with a solenoid shutoff valve to prevent vacuum from drawing water in, e.g. at idle. In other words, it injects water exactly the opposite of simply hooking up to a vacuum line.

Here is the rig I set up:


Cigarette lighter adapter => momentary pushbutton switch => pump and normally closed solenoid valve
Water line: jug => pump => solenoid valve => junction => metering valve => 1/16 NPT barb fitting

I am sure this routine could be fine tuned, but here is what I settled on:
Engage the pump (press the pushbutton) after opening the throttle (giving combustion chamber a moment to warm up)
Either accelerate or drag the brakes until they get hot
Release the pushbutton before releasing the throttle. Let the brakes cool or the vehicle coast to a stop. Repeat.

It would be great to do this on a dyno, but oh well. Can't have everything.