If you have been reading the 944 forums you may have noticed a pretty regular mention of injector cleaning. Our injectors are very reliable but our cars are getting pretty old too. The injectors are a little tricky to get out, mostly because of the care needed to ensure they don't leak once re-installed.
If you do have them out, a simple visual inspection might show some surprises. If you look closely into the fuel inlet, you can see a fine screen. That is the last line of defense for stopping contamination from entering into the injector. What happens if dirt enters the injector? Maybe nothing, maybe a steady miss or maybe just a subtle change to idle. In my case, I had a steady miss. Anyway, the inlet screen should be clean but I noticed the screen was covered with a smooth surface of silt.
Now the best thing to do would be to send all four injectors out to a cleaning service. The cost varies quite a lot from $5 each to $25 each and a surprising number of auto repair shop have the equipment to do the cleaning. Its easy to do and only takes a few minutes; the only catch is, you have to have the equipment. I didn't.
Before we go further, there is a side story that I always think about when the subject of 944 injectors comes up. A friend of mine in Texas rebuilt a 944 engine a few years ago. He took great care and patience with every detail - except the injectors. Once everything was back together and the engine was running he took it out for it's first test drive. He barely got out of the driveway when the engine quit with a bang. The engine was locked up solid and he feared the worst. Once he had the head off, he could see that one of the cylinders had broken. He provided pictures and there was a lengthy Rennlist discussion about what could have caused this problem. We may never really know but the leading theory was that an injector had "stuck" in the open position and allowed too much fuel to enter the cylinder. We're guessing a little here because its a hard to imagine that enough liquid could get past an injector during one or two revolutions of the engine to cause a hydro-lock situation but the broken cylinder was proof that something very serious had happened.
The lesson I took away from this was that injectors should be approached with a wary eye.
With all that said; here's the shade tree version of injector cleaning that was suggested on one of the Pelican forums. I gave it a try and was surprised at how well it worked.
Its pretty easy. First, you'll need a 9 volt transistor battery. You will also need a vacuum gun and some common fuel hose. If you have some old connector pins from repairing your injector harness, those will help too. Basically this is a simple back-flush operation. The vacuum gun is connected to the inlet of the injector and a hose is slipped snugly over the outlet. That hose is then used to draw in clean fuel. Once I had vacuum, the 9 volt connection allowed the injector to open and the magic happened. The vacuum gun's reservoir catches the back-flushed fuel, along with the dirt.
I wish I had taken pictures of the first injector that I did, because it was by far the worst, but this next image should give you an idea of how this simple trick can work.
