[Malcolm Towrie]

We’ve got the twin 6-71 Detroit’s out of Bart, one of our M4A2 Sherman’s. After replacing a bunch of seals and gaskets to fix horrible oil leakage, we test ran one of the engines on a stand and got lots of fuel knock and rough running. The injectors on Detroit 2-strokes are activated by additional lobes on the cam that operate rocker arms that depress a plunger in each cylinder injector that generates the 2000-3000 psi pressure required to spray atomized fuel into each cylinder at TDC.
There’s a good test you can do on these engines where you use a screw driver with the engine running to hold each injector plunger down away from the rocker arm so it’s doesn’t move up and down to inject fuel. It’s exactly like pulling a plug wire off a gas engine to monitor the rpm drop at idle.
We did that test and found 2 injectors that made virtually no difference to idle rpm when held down. And a third that made little difference, in other words, there was a problem with those injectors.
We are lucky enough to have a Detroit injector tester tool here, so we tested the worst of the injectors. It was pathetic. Instead of spraying an intense jet of atomized diesel out of each of the six tiny orifices in the tip, it squirted an anaemic solid jet out of each. I suspect that is why we get a lot of fuel knock from this engine as it’s warming up.
Even the injector that gave the most rpm drop was not atomizing the fuel like a new injector did.
I have a bunch of spare parts from N65 injectors but I’m not sure what would cause this. Worn plunger? Plugged tip orifices?
This is the needle-type injector with the needle in the tip that pops open about 2500 psi.
Malcolm